Why is there a layer of moist lining the inner walls of alveoli?

Why is there a layer of moist lining the inner walls of alveoli?

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I'm taught that the walls of the alveoli are moist, so gaseous oxygen molecules can dissolve into this water. This then allows the dissolved oxygen (liquid state) to diffuse faster from the alveoli into the bloodstream through the basement membrane.

However, why is it faster for gaseous oxygen to first dissolve in water (transitioning into liquid state) then diffuse through the basement membrane and dissolve into the blood stream than for gaseous oxygen to directly diffuse through the basement membrane and then dissolving into the blood stream?

The second mechanism would eliminate the need for the moist layer lining the alveoli wall.

Is there some physical or chemical explanations to this phenomenon?

The fluid in the alveoli contains phospholipoproteins that act as pulmonary surfactant, which keeps the alveoli open during breathing.

Villi in the Small Intestine

To absorb nutrients and the complete breakdown of food.


Villi in the small intestine absorbs nutrients and completes the breakdown of food. Factors of its structure that help it function include

  • Large surface area (provides more surface area for exchange to take place)
  • Thin wall (reduces the distance that materials need to move)
  • Moist (assists the transport of materials across the exchange surface)

The process that the nutrients move into the villi is diffusion.

Source, TommyIX, 2013

The picture above is a diagram of what is inside the villus. It explains what kind of nutrients is absorbed by the blood capillary which is glucose, amino acids (and can also be nucleotides) and by the lacteal which is fatty acids and glycerol.


They sense the presence of food, complete the digestion process and absorb the digested food. They contract to push the undigested food to the large intestine.



The mucosa of organs are composed of one or more layers of epithelial cells that secrete mucus, and an underlying lamina propria of loose connective tissue. [1] The type of cells and type of mucus secreted vary from organ to organ and each can differ along a given tract. [2]

Mucous membranes line the digestive, respiratory and reproductive tracts and are the primary barrier between the external world and the interior of the body in an adult human the total surface area of the mucosa is about 400 square meters while the surface area of the skin is about 2 square meters. [3] : 1 They are at several places contiguous with skin: at the nostrils, the lips of the mouth, the eyelids, the ears, the genital area, and the anus. [1] Along with providing a physical barrier, they also contain key parts of the immune system and serve as the interface between the body proper and the microbiome. [2] : 437

Examples Edit

Development Edit

Developmentally, the majority of mucous membranes are of endodermal origin. [4] Exceptions include the palate, cheeks, floor of the mouth, gums, lips and the portion of the anal canal below the pectinate line, which are all ectodermal in origin. [5] [6]

One of its functions is to keep the tissue moist (for example in the respiratory tract, including the mouth and nose). [2] : 480 It also plays a role in absorbing and transforming nutrients. [2] : 5,813 Mucous membranes also protect the body from itself for instance mucosa in the stomach protects it from stomach acid, [2] : 384,797 and mucosa lining the bladder protects the underlying tissue from urine. [7] In the uterus, the mucous membrane is called the endometrium, and it swells each month and is then eliminated during menstruation. [2] : 1019

Nutrition Edit

Niacin [2] : 876 and vitamin A are essential nutrients that help maintain mucous membranes. [8]

Cell types

Type I pneumocytes

The major cell type found on the alveolar surface, covering about 95% of the surface area, are thin, broad cells known as squamous (type I) alveolar cells, also known as type I pneumocytes. The thin walls of these cells allow for rapid gas diffusion between the air and blood, and therefore allow for gas exchange to occur. The other 5% of the surface area of an alveolus is covered by round to cuboidal great (type II) alveolar cells. Although type II alveolar cells cover less surface area, they greatly outnumber the squamous alveolar cells.

Histological slides illustrating type I pneumocytes (left) and type II pnemocytes (right)

Type II pneumocytes

The type II alveolar cells (also known as type II pneumocytes) have two functions: (1) to repair the alveolar epithelium when squamous cells are damaged, and (2) to secrete pulmonary surfactant. Surfactant is composed of phospholipids and protein, and coats the alveoli and smallest bronchioles, which prevents the pressure buildup from collapsing the alveoli when one exhales. Without surfactant, the walls of a deflating alveolus would tend to cling together like sheets of wet paper, and it would be very difficult to re-inflate them on the next inhalation.

Alveolar macrophages

The most numerous of all cells in the lung are the alveolar macrophages (dust cells), which drift through the alveolar lumens and the connective tissue between them clearing up debris through phagocytosis. These macrophages “eat” the dust particles that escape from mucus in the higher parts of the respiratory tract, as well as other debris that is not trapped and cleared out by your mucus. If your lungs are infected or bleeding, the macrophages also function to phagocytize bacteria and loose blood cells. At the end of each day, as many as 100 million of these alveolar macrophages will expire as they ride up the mucociliary escalator to be swallowed at the esophagus and digested—this is how debris from the lungs is removed.

Why is there a layer of moist lining the inner walls of alveoli? - Biology

A bronchus is a passage of airway in the respiratory tract that conducts air into the lungs and divides into terminal bronchioles.

Learning Objectives

Illustrate the anatomical structure of the bronchi and their subdivisions

Key Takeaways

Key Points

  • The human trachea (windpipe) divides into two main bronchi (also called mainstem bronchi), at the anatomical point known as the carina.
  • The right main bronchus is wider and shorter than the left main bronchus. The right main bronchus subdivides into three lobar bronchi and the left main bronchus divides into two.
  • The lobar bronchi divide into tertiary bronchi, also known as segmentalinic bronchi, each of which supplies a bronchopulmonary segment.
  • The segmental bronchi divide into many primary bronchioles that divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into and terminate in tiny air sacs called alveoli.
  • The mucous membrane of the primary bronchi is initially lined by ciliated pseudostratified columnar epithelium, but eventually the lining transitions to simple cuboidal epithelium, and then to simple squamous epithelium.
  • The bronchi are part of the conducting zone and contribute to anatomical dead space.
  • Bronchoconstriction is the tightening of the smooth muscle of the bronchi from a variety of causes, which makes it more difficult to breathe.

Key Terms

  • bronchus: Either of the two airways that are the primary branches of the trachea, leading directly into the lungs.
  • bronchoconstriction: The tightening of the smooth muscle of the bronchi due to parasympathetic nervous system stimulation, excess mucus production, inflammation, or allergic reactions.
  • bronchopulmonary segment: A distinct functional region of the lung that is separated from the rest of the lung by connective tissue.

A bronchus (plural bronchi, adjective bronchial) is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes called bronchioles.

The bronchi and bronchioles are considered anatomical dead space, like the trachea and upper respiratory tract, because no gas exchange takes place within this zone.

Anatomy of the Bronchi

The human trachea divides into two main bronchi (also called mainstem bronchi), that extend laterally (but not symmetrically) into the left and right lung respectively, at the level of the sternum. The point where the trachea divides into the bronchi is called the carina.

The right main bronchus is wider, shorter than the left main bronchus, which is thinner and longer. The right main bronchus subdivides into three lobar bronchi, while the left main bronchus divides into two. The lobar bronchi (also called secondary bronchi) divide into tertiary bronchi, each of which supplies air to a different bronchopulmonary segment.

A bronchopulmonary segment is a distinct region of the lung separated from the rest of the lung by connective tissue. Each bronchopulmonary segment forms a discrete functional unit in the lung that is independent of the other segments. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments.

There are 10 segments in the right lung and 8 to 9 segments in the left lung due to anatomical differences. The segmental bronchi divide into many primary bronchioles that divide into terminal bronchioles. Each terminal bronchiole then gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts.

There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the smallest anatomical unit of the lung, and the site of gas exchange between the lung and the bloodstream.


The histology of the bronchi are largely similar to that of the trachea. There is hyaline (transparent and consisting of collagen) cartilage present in the bronchi, in rings that are more irregular than those in the trachea.

There are also small plates and islands of hyaline cartilage in the primary and terminal bronchioles. Smooth muscle is present continuously around the bronchi (similar to the trachealis muscle of the trachea) and is innervated with the parasympathetic nervous system.

The amount of bronchial smooth muscle increases as the amount of hyaline cartilage decreases as the bronchi become smaller further into the lungs. The mucous membrane lining the bronchi also undergoes a transition—from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium further into the lungs.

Physiology of the Bronchi

Like the trachea, the bronchi and bronchioles are part of the conducting zone, so they moisten and warm air and contribute to the volume of anatomical dead space. The bronchi and bronchioles are also part of the mucociliary escalator that removes mucus and pathogens from the lungs.

A unique characteristic of the bronchi and bronchioles is bronchoconstriction, in which the smooth muscle of the bronchi or bronchioles tightens. This leads to coughing, wheezing, and dyspnea (shortness of breath).

It is caused by activation of the parasympathetic nervous system and release of acetylcholine in the bronchi, as well as by overproduction of mucus or allergic reactions and inflammation. It is a symptom of diseases such as bronchitis (chronic inflammation and mucus production in the bronchi) and asthma (an acute attack of bronchoconstriction, often allergic). Both cause obstruction of the airways and make it more difficult to breathe.

Bronchoconstriction is treated with anti-inflammatory drugs, such as corticosteroids, and prevented by maintaining lung health, such as through avoiding smoking, air pollution, and airborne allergens.

The complete respiratory system: This figure details the respiratory system including the bronchi and its many subdivisions.

Swim-Bladder: Development, Structure and Types | Fishes

In this article we will discuss about Swim-Bladder:- 1. Introduction to Swim-Bladder 2. Development of Swim-Bladder 3. Basic Structure 4. Gas Composition 5. Types 6. Modifications 7. Shape and Size 8. Weberian Ossicles 9. Functions 10. Hydrostatic Organ 11. Adjustable Float 12. Maintains Proper Centre of Gravity 13. Respiration 14. Resonator.


  1. Introduction to Swim-Bladder
  2. Development of Swim-Bladder
  3. Basic Structure of Swim-Bladder
  4. Gas Composition of Swim-Bladder
  5. Types of Swim-Bladder
  6. Modifications in Swim-Bladder
  7. Shape and Size of Swim-Bladder
  8. Weberian Ossicles
  9. Functions of Swim-Bladder
  10. Hydrostatic Organ
  11. Swim-Bladder acts as Adjustable Float
  12. Maintains Proper Centre of Gravity
  13. Swim-Bladder helps in Respiration
  14. Swim-Bladder as Resonator

1. Introduction to Swim-Bladder:

In most of the fishes a characteristic sac­like structure is present between the gut and the kidneys. This structure is called by various names, viz., swim-bladder, or gas-bladder, or air-bladder. In our present discussion, the name of the bladder is followed as the swim-bladder to avoid confusion.

The swim-bladder occupies- the same position as the lungs of higher verte­brates and is regarded as homologous to the lungs. It differs from the lungs of higher forms mainly in origin and blood supply.

The swim bladder arises from the dorsal wall of the gut and gets the blood supply usually from the dorsal aorta, while the vertebrate lung origi­nates from the ventral wall of the pharynx and receives blood from the sixth aortic arch.

The swim-bladder is present in almost all the bony fishes and functions usually as a hydrostatic organ. Starting as a very insignificant cellular extension from the gut, the swim-bladder in fishes leads the whole group through an evo­lutionary channel.

2. Development of Swim-Bladder:

Opinions differ as regards the development of swim bladder in fishes. In teleosts, it originates as an unpaired dorsal or dorsolateral diverticulum of the oesophagus. It starts as a small pouch budded off from the oesophagus. The diverti­culum with an opening in the oesophagus becomes subsequently divided into two halves.

Of these two, the left one often atrophies except in a few primitive forms. The right half becomes well-developed and take a median position. In dipnoans and Polypteridae, the swim-bladder is modified into the ‘lungs’ and originates as the down-growths from the floor of the pharynx.

These out-growths have been rotated around the right side of the alimentary canal to occupy the dorsal position. As a con­sequence of shifting of the position, the original right ‘lung’ becomes the left one. Spengel advocates the view that the swim-bladder in fishes originates from the posterior pair of the gill-pouches, but definite embryological evi­dence in support of this idea is lacking.

3. Basic Structure of Swim-Bladder:

The swim-bladder in fishes varies greatly in struc­ture, size and shape.

a. It is essentially a tough sac-like structure with an overlying capillary network.

b. Beneath the capillary system there is a connective tissue layer called tunica externa.

c. Below this layer lies the tunica interna consisting primarily of smooth muscle fibres and epithelial gas-gland.

d. The swim bladder lies below the kidneys, between the gonads and above the gut.

e. The connection with the oesophagus may be retained through­out life or may be lost in the adult.

4. Gas Composition of Swim-Bladder:

Biot (1807) and Morean (1876) have shown that the gas secreted by the swim-bladder is mostly oxygen. Nitrogen, and little quantity of carbon-dioxide are also present. Generally the gas composition varies in different species. In salmonids, the maximum amount of gas in the swim-bladder is Nitrogen. Again in many species the composition includes mostly a mix­ture of oxygen and carbon dioxide.

5. Types of Swim-Bladder:

Depending on the presence of the duct (ductus pneumaticus) between the swim-bladder and the oesophagus, the swim-bladder in fishes can be divided into two broad categories: Physostomous [Gk. physi = a bladder stomata, mouth] and Physoclistous types [Gk. clistic = enclosed].

Depending on the condition of the swim-bladder, the teleosts are classified by older taxonomists into two groups Physostomi and Physoclisti. A transi­tional condition is observed in eels.

A. Physostomous Condition:

The swim-bladder develops from the oesopha­gus. When the ductus pneumaticus is present between the swim-bladder and the oesopha­gus, the swim-bladder is called physostomous type (Fig. 6.85A).

A vessel emerging from the coeliacomesenteric artery supplies the swim bladder and the blood from it is conveyed to the heart through a vein joining the hepatic portal vein. This condition is observed in bony ganoid fishes, the dipnoans and soft-rayed teleosts.

B. Physoclistous Condition:

In this condition the ductus pneumaticus is either closed or atrophied (Fig. 6.85B). This type of swim bladder is observed in spiny-rayed fishes. In this type of swim-bladder, there lies an anteroventral secretory gas gland (containing retia mirabilia) and a posterodorsal gas absorbing region called the oval. The oval develops out of the degenerating ductus pneu­maticus.

The rete mirabilis of the gas gland, the oval and the walls of the bladder are sup­plied by the coeliacomesenteric artery and also by arteries from the dorsal aorta. But the blood from the different parts of the swim bladder is returned by two routes.

The blood from the gas gland is returned to the heart by the hepatic portal vein, while from the rest of the bladder by the posterior cardinal veins. The bladder, specially the gas gland, gets the lateral branches from the vagus, while the oval is innervated by sympathetic nerves.

C. Transitional Condition:

In Eel (Anguilla), a transitional condition between the physosto­mous and physoclistous type is present. The swim-bladder retains the ductus pneumaticus which becomes enlarged to form a separate chamber containing the oval (Fig. 6.85C). The gas glands are also present.

The swim-bladder is supplied with the blood through a branch from the coeliacomesenteric artery while the blood is returned to the heart by a vessel joining the post cardinal vein. The condition represents an intermediate stage when a physostomous condition is on the verge of transformation into the physoclistous state.

6. Modifications in Swim-Bladder:

In fishes a great diversity in size, shape and function of the swim-bladder is observed. In elasmobranchs, bottom dwelling and deep-sea teleosts the swim-bladder is absent in an adult but a transitory rudiment during development may be present.

In flat fishes (Pleuronectidae) swim-bladder is present in the early life when the animals maintain a vertical position. As they tip over one side and assume the lazy adulthood, the swim-bladder becomes atrophied.

In elasmobranchs, the swim-bladder is represented by the transitory rudiment in the embryonic stages. Miklucho-Maclay (1867) has observed a rudimentary dorsal diverticu­lum from the foregut in the embryos of Squalus, Mustelus ana Caleus. In many fishes, viz., Heptranchias, Scyllium, Squatina, Pristiurus, Carcharius and many Rays, small pits are recorded in the oesophageal wall.

Wassnezow (1932) has observed one to six similar oesophageal pits in Pristiurus, Torpedo and Trygon. These pits are located posterior to the fifth pouch. In sharks the swim-bladder is absent in adults, but a hint of a rudimentary swim-bladder is observed during embryonic development. But almost all the teleosts pos­sess the swim-bladder and extreme modifica­tions of the same are encountered because of adaptation to the different modes of living.

Modifications of Physostomous Condition:

The typical physostomous pattern becomes modified in different fishes and the basic trends are:

(1) The formation of paired sacs and

(2) The gradual acquisition of two chambers— an anterior and a posterior.

The swim-bladder in Polypterus (bichir) (Fig. 6.87A, B) represents the primitive condi­tion. It is a bilobed sac with two unequally developed lobes. The left lobe is shorter and the right lobe is longer. The bilobed sac opens on the floor of the pharynx through a slit-like glottis. The glottis is provided with muscular sphincter. The internal lining of the bladder is smooth and partly ciliated.

The lack of alveolar sacculations and the pre­sence of muscular walls are the two noted feature in the swim-bladder of Polypterus. The walls of the bladder are highly vascular and are lined by two layers of striated muscle fibres.

The bladder is supplied by a pair of pulmonary arteries arising from the last pair of pulmonary arteries arising from the last pair of epibranchial arteries and the corres­ponding veins enter into the hepatic vein below the sinus venosus.

In the dipnoans, the swim-bladder is called the lung and the inner walls are produced into numerous alveoli. The swim-bladder resem­bles the tetrapod lungs both structurally as well as functionally. In Neoceratodus it is single- lobed, while in Protopterus and Lepidosiren it is bilobed (Fig. 6.87C, D, E).

Other details regarding the structural construction, blood’ and nerve supplies have already been dealt in the biology of the lung-fishes.

In Sturgeons (Acipenser), the swim-blad­der is short and oval in shape. The ductus pneumaticus enters the bladder ventrally and it opens into the gut posterior to the pharynx. The glottis is lacking and the opening into the oesophagus is closed by the simple constric­tion of the ductus pneumaticus.

The walls of the bladder are fibrous and thick but the inner walls are smooth (Fig. 6.87H). In Acipenser, both the left and right lobes develop from the dorsal side of the oesophagus in the embryon­ic stage, but the left one becomes completely obliterated and right one gives rise to the adult swim-bladder.

In Amia and Lepisosteus, the swim bladder is an unpaired sac extending nearly the entire length of the body cavity. In both the cases rudiment of the left lobe appears during development but persists only for a short time. The ductus pneumatics opens into the oesoph­agus posterior to the pharynx through a dorsal slit-like glottis.

The walls are highly vascular and exhibit sacculations resembling the pul­monary alveoli (Fig. 6.87G). The sacculations or the respiratory pouches are arranged in two lateral rows. As regards the development of sacculations the swim-bladder of Lepisosteus is more advanced than that of Amia. There are some more minor differences regarding the supply of blood.

The swim-bladder in Amia gets arterial blood from the pulmonary arter­ies, while that of Lepisosteus gets arterial branches from the dorsal aorta. The blood from the bladder is returned by the left ductus Cuvieri in Amia and by the right post-cardinal in Lepisosteus.

Gymnarchus presents an inter­mediate stage where the efferent branchial arteries from the third and fourth gill-arches join to form a common root for the emergence of the pulmonary and coeliacomesenteric arteries (Fig. 6.87F). Amongst the dipnoans, the swim-bladder of Neocertatodus resembles that of Lepisosteus. The walls are sacculated and act as the lung’.

In Clupea harengus, the ductus pneumati­cus opens into “the fundus of the stomach and there is a second duct from the posterior part of the swim-bladder opening to the exterior near the anus (Fig. 6.87 I). Similar posterior opening is present in Pellona, Caranx, Sardinella.

Modifications of Physoclistous Condition:

The swim-bladder in all teleosts begins as a physostomous type but in an adult condition the ductus pneumaticus gets degenerated to become a physoclistous type. A typical physo­clistous swim-bladder consists of a closed sac having two compartments—an anterior and a posterior. These two compartments are inter­communicated through an aperture called ductus communicans.

The opening and closure of this aperture is regulated by circular and radiating muscles which act as the sphicter. The anterior chamber is formed by circular and radiating muscles which act as the sphincter. The anterior chamber is formed by the enlargement and forward growth of the budding swim-bladder, while the posterior chamber develops as an enlargement of the ductus pneumaticus.

This typical structural plan is modified in certain forms. The poste­rior chamber with retia Mirabella becomes flat­tened almost to the point of obliteration and is designated ‘oval’ as seen in the families like Myctophidae, Percidae, Mugilidae.

The oval is a thin-walled highly muscular area specialised for the reabsorption of gases (see Fig. 6.86D). The opening of the oval is guarded by circular and longitudinal muscles. This device is of great significance for the fishes undergoing rapid vertical movements.

Histological Modifications:

The morphologi­cal modifications of the swim-bladder are accompanied by histological modifications in different fishes, the swim-bladder acts as a hydrostatic organ. It helps fishes to sink or ascend to various depths by altering the gas content in the bladder. In fishes having open ductus pneumaticus, the volume of gas con­tent in the bladder can be changed by swal­lowing or removing air from the bladder.

But in some physostomous and all physoclistous fishes this process of gas transference is done directly from the blood stream. Inside the bladder there is an oxygen-producing device and an oxygen-absorbing device. The swim bladder is a vascular structure but the degree of vascularization varies in different teleosts.

In some species of the families Clupeidae and Salmonidae the capillaries are uniformly pre­sent all over the swim-bladder, but in most cases these highly vascular interlacing and tightly packed capillaries form a mass called rete mirabilis. The anterior chamber of swim bladder shows the tendency to become diffe­rentiated into oxygen-producing area called red body.

The oxygen is produced by the reduction of the oxyhaemoglobin in the ery­throcytes when brought into close contact with the secreting epithelial cells of the gas gland. The red body consists of internal oxygen-secreting cells (gas gland) and sup­plied by the blood vessels from the retia Mirabella (sing, rete mirabilis).

It forms a com­plicated structure where the arterial and venous capillaries communicate only after reaching the gas gland. The most primitive condition is observed in Pickerel where the gland is covered by thick glandular epithelium which is thrown into a number of folds. In eels and some other fishes, the red bodies are non- glandular in nature but serve the same physio­logical function.

The red gland is supplied with blood from the coeliac artery and is returned to the portal vein. The activity of the red gland is controlled by the vagus nerve. In the fishes with functional ductus pneumaticus the gas glands are absent but in eels this func­tion is taken up by the red gland.

In the physoclistous fishes, the anterior region is modified for gas production and the posterior region or chamber is specialised for the absorption of gas into the blood. The pos­terior chamber becomes excessively thin- walled to facilitate gas diffusion.

Beneath the walls, the gas is absorbed directly into the blood. The formation of the oval in some fishes, is a special development for the absorption of gas. The wall of the oval is very thin and highly vascular. Through this epithe­lial lining oxygen can easily pass to the net­work of vessels. This gas absorbing region receives blood supply from the dorsal aorta and the blood is returned to the post cardinal vein. The activities are governed by the sym­pathetic nerves.

The histological differentiation for the gas production and gas absorption is a very signi­ficant achievement in fishes. The gas pro­duced by the red body is mostly oxygen and this oxygen is readily absorbed or diffused from the swim-bladder directly into the capi­llaries. The oval is modified for gas absorption in many fishes.

By the alternate process of gas production and gas absorption, the internal pressure and volume of the gas content inside the swim-bladder can be increased or decreased. The red body is usually confined to the anterior chamber, but in fishes where the anterior chamber becomes secondarily associated with the auditory function, the gas gland may be confined to the posterior chamber.

7. Shape and Size of Swim-Bladder:

The swim-bladder varies extensively in shape and size. In Umbrina (Fig. 6.88A), it is oval shaped and without any appendage. In Atractoscion (Fig. 6.88B), it gives off only one pair of simple diverticula that extends from the anterior side. In Kathala (Fig. 6.88C), the swim-bladder develops a pair of appendage extending in front of transverse septum into head.

In some forms it gives off many branched diverticula. In many fishes, the ante­rior prolongations of the swim-bladder come into close contact with the wall of the space containing the internal ear. In Clupea, the narrow anterior end of the swim-bladder enters into a canal in the basioccipital of the skull and divides into two slender branches.

The anterior end of each branch dilates to form a round swelling and lies in close contact with the internal ear. A more or less similar condition is observed in Tenualosa ilisha. In many fishes finger-like diverticula develop from the swim-bladder.

In Gadus, a pair of diverticula originating from the anterior part of the bladder project into the head region. In Otolithus, each anterolateral end of the swim-bladder gives rise to an outgrowth which sends one anterior and a posterior horn.

In Otolithoides (Fig. 6.88D), the appendages attached to posterior end of bladder and at least the main part lying parallel to the blad­der. In Corvina lobata, many such branched diverticula develop from the lateral walls of the swim-bladder. In Johnius (Fig. 6.88E), it is hammer-shaped with 12 to 15 pairs arbores­cent appendages, the first branching in the head and the posterior tip are highly pointed.

Usually in most cases, the swim-bladder is divided transversely into an anterior and a posterior chamber as seen in cyprinoids (Fig. 6.87K), Esox (Fig. 6.87J), Catostomus, Pangassius, Corvina, etc. But the longitudinal division of the swim-bladder is rare.

In Arius the swim-bladder is splitted longitudinally. In Notopterus, a longitudinal septum divides the swim-bladder into two lateral chambers. Due to the presence of septum or septa, the internal cavity of the swim-bladder is either completely or partially divided.

8. Weberian Ossicles:

The perilymphatic sac and the anterior end of the swim-bladder are connected by a series of four ossicles (Fig. 6.89), which are articulated as a conducting chain.

Of the four, the tripus, intercalarium and scaphium actu­ally form the chain, while the fourth one, claustrum lies dorsal to the scaphium and lies in the wall of posterior prolongation of the perilymphatic sac. The function of these ossi­cles is controversial.

It is regarded that the Weberian ossicles either help to intensify sound vibrations and convey these waves to the internal ear of help to understand the state of tension of air pressure in the bladder and transit changes of such pressure to the peri­lymph to set up a reflex action. There are various views regarding the actual process of derivation of these ossicles.

De Beer (1937) and Watson (1939) regarded that these are detached or modified processes of the first three anterior vertebrae. As regards the actual mode of origin of the four ossicles there are differences of opinion.

The claustrum is regarded to be modified interspinous ossicle or modified spine of first vertebra or modified neural arch of first vertebra or modified intercalated cartilage or modified neural process of first cartilage.

The scaphium is considered to be the modified neural arch of the first ver­tebra or modified rib of the first vertebra or derived from the neural arch of the first vertebra and also from the mesenchyme.

The intercalarium is derived from the neural arch and trans­verse process of the second vertebra or from the neural arch of the second vertebra and also from the ossified ligament or from the neural arch of the second vertebra only.

The tripus is formed from the rib of the third vertebra and the ossified ligament or from the trans­verse process of the third vertebra along with ossified wall of the swim-bladder or from the transverse process of the third vertebra and the ribs of third and fourth vertebrae.

9. Functions of Swim-Bladder:

The swim-bladder in fishes performs a variety of functions.

10. Hydrostatic Organ:

It is primarily a hydro­static organ and helps to keep the weight of the body equal to the volume of the water, the fish displaces. It also serves to equilibrate the body in relation to the surrounding medium by increasing or decreasing the volume of gas content.

In the physostomous fishes the expul­sion of the gas from the swim-bladder is caused by way of the ductus pneumaticus, but in the physoclistous fishes where the ductus pneumaticus is absent the superfluous gas is removed by diffusion.

11. Swim-Bladder acts as Adjustable Float:

The swim-bladder also acts as an adjustable float to enable the fishes to swim at any depth with the least effort. When a fish likes to sink, the specific gravity of the body is increased. When it ascends the swim-bladder is distended and the specific gravity is diminished. By such adjustment, a fish can maintain equilibrium at any level.

12. Swim-Bladder Maintains Proper Centre of Gravity:

The swim bladder helps to maintain the proper centre of gravity by shifting the contained gas from one part of it to the other and this facilitates in exhibiting a variety of movement.

13. Swim-Bladder helps in Respiration:

The respiratory function of the swim-bladder is quite significant. In many fish­es living in water in which oxygen content is considerably low, the oxygen produced in the bladder may serve as a source of oxygen. In a few fishes, specially in the dipnoans, the swim bladder becomes modified into the ‘lung’. The ‘lung’ is capable of taking atmospheric air.

14. Swim-Bladder as Resonator:

The swim-bladder is regarded to act as a resonator. It intensifies the vibra­tions of sound and transmits these to the ear through the Weberian ossicles.

The swim-bladder helps in the production of sound. Many fishes, Doras, Platystoma, Malapterurus, Trigla can produce grunting or hissing or drumming sound. The circulation of the contained air inside the swim-bladder causes the vibration of the incomplete septa.

The sound is pro­duced as the consequence of vibration of the incomplete septa present on the inner wall of the swim-bladder. The vibrations are caused by the movement of the contained air of the swim-bladder.

Sound may also be produced by the compression of the extrinsic and intrinsic musculature of the swim-bladder. Polypterus, Protopterus and Lepidosiren can produce sound by compression and forceful expulsion of the contained gas in the swim­bladder. In Cynoscion male, the musculus sonorificus probably helps in compression.

Examples of Stratified Squamous Epithelia

Example I: Vaginal Epithelium

The vagina connects the cervix of the uterus to the external genital structures in mammalian females. The vaginal epithelium plays an important role in maintaining sexual and reproductive health, especially among animals that undergo menstrual and estrous cycles. The thickness and keratinization of the squamous epithelium found in this organ are subject to changes during a reproductive cycle and can influence fertility as well as susceptibility to infections. For instance, hormone levels influence the number of cell layers in this epithelium. This results in the epithelium changing morphology with the maximum thickness being achieved during estrus or ovulation. During menses or diestrus, the epithelium is at its thinnest. Additionally, research on rhesus monkeys indicates that juvenile and geriatric females have thin epithelia with minimal keratin presence when compared to mature cycling individuals. There are also major changes to this epidermal tissue during pregnancy and before childbirth to facilitate the movement of the fetus through the muscular walls of the vagina.

Example II: Masticatory Mucosa

Oral mucosa lines the inside of the mouth and consists of stratified squamous epithelium as well as the connective tissue underneath. The mouth contains both keratinized as well as non-keratinized stratified squamous epithelium. The parts of the mouth that feel a little rough such as the upper surface of the tongue and the hard palate at the roof of the mouth contain keratinized epithelia. These tissues are formed by four layers: the basal layer, the spinous layer, the granular layer and the most superficial keratinized layer. The basal layer remains in contact with the basement membrane, and keratinization begins with the spinous layer. The third layer is distinguished by the appearance of lipids and proteins that are secreted into the extracellular matrix. The keratinized or corneal layer consists of dead cells containing abundant keratin, but no nucleus or cytoplasm.

The rest of the mouth, such as the mucosa lining the cheeks, or the inner lining of the lips feel softer, and more moist. These are lined by non-keratinized epithelial tissue.

Function of Alveoli

Much of the outside surface area of lung alveoli are covered with tiny capillaries.   These capillaries and the walls of alveoli share a very thin membrane that allows oxygen from inhaled air to pass through the walls of alveoli and enter the bloodstream via the capillaries. At the same time, carbon dioxide is pushed out in the same way when the air is exhaled.

The total amount of surface area available for this gas/blood exchange determines how well a person is able to breathe.   In a normal healthy adult, there is an abundance of available area for this process.

Breathing: Grade 9 Understanding for IGCSE Biology 2.46 2.47

Breathing is the movement of air in and out of the lungs. It is a small point but you must be careful with your language in answering questions in this topic. Meaning is lost if words are not used correctly: for example often candidates write than “oxygen is breathed in and carbon dioxide breathed out….” Can you see why this is not correct and actually muddles your understanding of the process?

(Please don’t confuse breathing with gas exchange which is the diffusion of oxygen and carbon dioxide in and out of the blood, nor with respiration which is a series of chemical reactions happening in all cells in which food molecules are oxidised to release energy for the cell)

So back to breathing – the movement of air in and out of the lungs…..

1) What is the pathway air follows to get from the atmosphere and into the alveoli in the lung?

The trachea is the main tube that carries air into the lungs. It has a ciliated epithelium lining – these cilia waft mucus and foreign particles up to the top of the trachea and then the mucus is swallowed into the stomach and any bacteria trapped in the mucus are killed. The trachea is also strengthened by C-shaped rings of cartilage that prevent the tube collapsing when the air pressure inside drops. The trachea branches into two tubes called bronchi, one going to each lung. The bronchi branch over and over again into smaller tubes called bronchioles and ultimately the smallest bronchioles end in a cluster of microscopic air sacs called alveoli. This whole structure is called the Bronchial Tree.

2) What causes air to move in and out of the lungs in breathing?

The movement of air into and out of the lungs is brought about by the action of two muscles: the diaphragm, a dome-shaped muscle that separates the thorax from the abdomen, and the two sets of intercostal muscles. This is an easy area to get confused as there are plenty of similar words and precision in explanation is vital to clear understanding…..

Breathing in (Inhalation) is the active stage in breathing. This means that under normal condition it is the stage in which the muscles contract. During inhalation, the diaphragm contracts. This contraction causes it to change shape from the dome-shape at rest to a flattened shape. This change in shape of the diaphragm increases the volume of the thorax (in fact it is the volume of the pleural space between the two pleural membranes that is significant but we might skip over this for simplicity….).

If the volume of a gas increases, the pressure decreases (Boyle’s Law I seem to remember from boring Physics lessons a long time ago). If the pressure in the thorax decreases, it may drop below atmospheric pressure and so air can be pushed into the alveoli through the bronchial tree by the higher atmospheric pressure.

Breathing out (Exhalation) is a passive process. The diaphragm is a most unusual muscle as it is very elastic. This means that when it relaxes, it springs back to its original dome-shape through elastic recoil. This movement decreases the volume of the thorox, thus increasing the pressure and if the pressure rises above atmospheric pressure, air will be pushed out of the alveoli.

3) What role do the Intercostal muscles play in breathing?

The intercostal muscles are two sets of muscles that are found between the ribs. Contraction of these muscles can either pull the rib cage up and out, or push the rib cage down and in. The muscles on the outside are called the external intercostal muscles and the ones on the inside are called internal intercostal muscles.

When you are breathing at rest the rib cage does not move at all. (I hope everyone reading this post is calm, relaxed and not hyperventilating in panic over upcoming exams….) As you are breathing at rest the only muscle involved is the diaphragm (see section above) as you are only moving about half a litre of air in and out with each breath. But there are situations in which this tidal volume has to increase and that is when the intercostal muscles come into their own.

The two sets of intercostal muscles are antagonistic – when one contracts the other relaxes.

If you need to take a big breath in, the external intercostals will contract at the same time as the diaphragm. The external intercostals pull the ribcage up and out, thus increasing even further the volume of the thorax, thus dropping the air pressure even more in the thorax, allowing more air to come in. When you come to breathe out, the external intercostal muscles will relax and gravity will allow the ribcage to fall back down to its original position.

But I hear you say…. “What happens if you are lying down or upside down? How can the ribcage get back to its original position without the help of gravity?” Well don’t worry – you have the internal intercostals which in extreme situations will contract during exhalation to push the ribcage down and in…

I suggest you draw up a table to summarise the process of breathing. Give inhalation and exhalation a column each, and the rows of the table should be diaphragm, external intercostals, internal intercostals… Tweet me a photo of your table if you want me to have a look…


The trachea (windpipe) divides at the carina into two main or primary bronchi, the left bronchus and the right bronchus. The carina of the trachea is located at the level of the sternal angle and the fifth thoracic vertebra (at rest).

The right main bronchus is wider, shorter, and more vertical than the left main bronchus, [3] its mean length is 1.09 cm. [4] It enters the root of the right lung at approximately the fifth thoracic vertebra. The right main bronchus subdivides into three secondary bronchi (also known as lobar bronchi), which deliver oxygen to the three lobes of the right lung—the superior, middle and inferior lobe. The azygos vein arches over it from behind and the right pulmonary artery lies at first below and then in front of it. About 2 cm from its commencement it gives off a branch to the superior lobe of the right lung, which is also called the eparterial bronchus. Eparterial refers to its position above the right pulmonary artery. The right bronchus now passes below the artery, and is known as the hyparterial branch which divides into the two lobar bronchi to the middle and lower lobes.

The left main bronchus is smaller in caliber but longer than the right, being 5 cm long. It enters the root of the left lung opposite the sixth thoracic vertebra. It passes beneath the aortic arch, crosses in front of the esophagus, the thoracic duct, and the descending aorta, and has the left pulmonary artery lying at first above, and then in front of it. The left bronchus has no eparterial branch, and therefore it has been supposed by some that there is no upper lobe to the left lung, but that the so-called upper lobe corresponds to the middle lobe of the right lung. The left main bronchus divides into two secondary bronchi or lobar bronchi, to deliver air to the two lobes of the left lung—the superior and the inferior lobe.

The secondary bronchi divide further into tertiary bronchi, (also known as segmental bronchi), each of which supplies a bronchopulmonary segment. A bronchopulmonary segment is a division of a lung separated from the rest of the lung by a septum of connective tissue. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. Initially, there are ten segments in each lung, but during development with the left lung having just two lobes, two pairs of segments fuse to give eight, four for each lobe. The tertiary bronchi divide further in another three branchings known as 4th order, 5th order and 6th order segmental bronchi which are also referred to as subsegmental bronchi. These branch into many smaller bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into two to eleven alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.

The main bronchi have relatively large lumens that are lined by respiratory epithelium. This cellular lining has cilia departing towards the mouth which removes dust and other small particles. There is a smooth muscle layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer contains seromucous glands, which secrete mucus, in its wall. Hyaline cartilage is present in the bronchi, surrounding the smooth muscle layer. In the main bronchi, the cartilage forms C-shaped rings like those in the trachea, while in the smaller bronchi, hyaline cartilage is present in irregularly arranged crescent-shaped plates and islands. These plates give structural support to the bronchi and keep the airway open. [5]

The bronchial wall normally has a thickness of 10% to 20% of the total bronchial diameter. [6]

Microanatomy Edit

The cartilage and mucous membrane of the primary bronchi are similar to those in the trachea. They are lined with respiratory epithelium, which is classified as ciliated pseudostratified columnar epithelium. [7] The epithelium in the main bronchi contains goblet cells, which are glandular, modified simple columnar epithelial cells that produce mucins, the main component of mucus. Mucus plays an important role in keeping the airways clear in the mucociliary clearance process.

As branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium, to simple cuboidal epithelium, to simple squamous epithelium in the alveolar ducts and alveoli. [7] [8]

Variation Edit

In 0.1 to 5% of people there is a right superior lobe bronchus arising from the main stem bronchus prior to the carina. This is known as a tracheal bronchus, and seen as an anatomical variation. It can have multiple variations and, although usually asymptomatic, it can be the root cause of pulmonary disease such as a recurrent infection. In such cases resection is often curative [9] [10]

The cardiac bronchus has a prevalence of ≈0.3% and presents as an accessory bronchus arising from the bronchus intermedius between the upper lobar bronchus and the origin of the middle and lower lobar bronchi of the right main bronchus. [11]

An accessory cardiac bronchus is usually an asymptomatic condition but may be associated with persistent infection or hemoptysis. [12] [13] In about half of observed cases the cardiac bronchus presents as a short dead-ending bronchial stump, in the remainder the bronchus may exhibit branching and associated aerated lung parenchyma.

The bronchi function to carry air that is breathed in through to the functional tissues of the lungs, called alveoli. Exchange of gases between the air in the lungs and the blood in the capillaries occurs across the walls of the alveolar ducts and alveoli. The alveolar ducts and alveoli consist primarily of simple squamous epithelium, which permits rapid diffusion of oxygen and carbon dioxide.

Bronchial wall thickening, as can be seen on CT scan, generally (but not always) implies inflammation of the bronchi. [14] Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23. [15]

Bronchitis Edit

Bronchitis is defined as inflammation of the bronchi, which can either be acute or chronic. Acute bronchitis is usually caused by viral or bacterial infections. Many sufferers of chronic bronchitis also suffer from chronic obstructive pulmonary disease (COPD), and this is usually associated with smoking or long-term exposure to irritants.

Aspiration Edit

The left main bronchus departs from the trachea at a greater angle than that of the right main bronchus. The right bronchus is also wider than the left and these differences predispose the right lung to aspirational problems. If food, liquids, or foreign bodies are aspirated, they will tend to lodge in the right main bronchus. Bacterial pneumonia and aspiration pneumonia may result.

If a tracheal tube used for intubation is inserted too far, it will usually lodge in the right bronchus, allowing ventilation only of the right lung.

Asthma Edit

Asthma is marked by hyperresponsiveness of the bronchi with an inflammatory component, often in response to allergens.

In asthma, the constriction of the bronchi can result in difficulty in breathing giving shortness of breath this can lead to a lack of oxygen reaching the body for cellular processes. In this case, an inhaler can be used to rectify the problem. The inhaler administers a bronchodilator, which serves to soothe the constricted bronchi and to re-expand the airways. This effect occurs quite quickly.

Bronchial atresia Edit

Bronchial atresia is a rare congenital disorder that can have a varied appearance. A bronchial atresia is a defect in the development of the bronchi, affecting one or more bronchi – usually segmental bronchi and sometimes lobar. The defect takes the form of a blind-ended bronchus. The surrounding tissue secretes mucus normally but builds up and becomes distended. [16] This can lead to regional emphysema. [17]

The collected mucus may form a mucoid impaction or a bronchocele, or both. A pectus excavatum may accompany a bronchial atresia. [16]