Vasoconstriction and blood flow

Vasoconstriction and blood flow

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The resistance in a blood vessel is equal to the pressure difference divided by the blood flow. Let us now say that a sympathomimetic causes vasoconstriction which increases the resistance. Does this primarily increase the pressure in the blood vessel? Does it primarily reduce the blood flow in the blood vessel?

In context, the filtration fraction of the kidneys is the GFR divided by the plasma blood flow through the afferent arteriole. The sympathomimetic would in general increase the filtration fraction by reducing blood flow. However, the GFR is also dependent on hydrostatic pressure which may increase if there is a rise in resistance. Therefore, shouldn't the filtration fraction change minimally due to the rise in GFR counteracting the fall in blood flow?

  • As blood is pumped away from the heart, it travels through the aorta to arteries, aterioles, and the capillary beds.
  • Blood flow through the capillary beds reaches almost every cell in the body and is controlled to divert blood according to the body&rsquos needs.
  • After oxygen is removed from the blood, the deoxygenated blood flows to the lungs, where it is reoxygenated and sent through the veins back to the heart.
  • arteriole: one of the small branches of an artery, especially one that connects with capillaries
  • vein: a blood vessel that transports blood from the capillaries back to the heart
  • artery: an efferent blood vessel from the heart, conveying blood away from the heart regardless of oxygenation status
  • vena cava: either of the two large veins that take oxygen depleted blood from the upper body and lower body and return it to the right atrium of the heart

Endotherms and Ectotherms

Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that do not control their body temperature are ectotherms instead they rely on external energy to dictate their body temperature. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. Poikilotherms are animals with constantly varying internal temperatures, while an animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm.

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure 2). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

Figure 2. Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction. (credit b: modification of work by “Kullez”/Flickr credit c: modification of work by Chad Rosenthal credit d: modification of work by “stacey.d”/Flickr)

Heat Conservation and Dissipation

Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.

Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.

Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

Mechanisms of Vasodilation

Vasodilation can occur by a couple different cellular mechanisms. It can be a result of a lower concentration of calcium within cells or by dephosphorylation (removal of a phosphate group from) the protein myosin, which is found in muscle cells. Either of these mechanisms will result in the relaxation of smooth muscle cells in blood vessels. Vasodilators can work through affecting calcium channel blockers or the levels of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP).

Treating vasoconstriction or constricted blood vessels

Responding immediately to signs and symptoms of vasoconstriction will reduce chances of further illnesses. Vasoconstriction treatment varies from person-to-person however, here we endeavor to explain the most common approaches.

  • Medications: There are medications that act as vasodilators to increase blood flow. They work by blocking calcium channels and inhibiting the activity of alpha-adrenoceptors, which are a class of important G-protein receptors.
  • Exercise: Cardio exercises for an hour each day can help combat vasoconstriction. Exercising can enhance blood flow and help dilate the blood vessels.
  • Avoid cold: Being exposed to too much cold can constrict blood vessels so it is important to not stay in the cold for too long.
  • Healthy diet: Maintain a healthy diet &ndash one that does not include processed foods, canned foods, or items that contain too much salt.
  • Limit alcohol and drugs: Avoid alcohol intake, as well as drugs like cocaine.
  • Avoid stress: The less stress, the better. Avoiding stress can help prevent acute vasoconstriction.
  • Treat underlying diseases: When constriction is due to another disease, it is crucial to get proper treatment for that disease, as it can only help with the vasoconstriction.
  • Massage: Some people find that massaging the area can increase blood flow so that vasoconstriction is at least temporarily reversed. Some people report that just a 10-minute massage is all it takes.

As with any condition, it is important to pay attention to your overall health too. If you are generally in good health, it will make fighting vasoconstriction easier. You should eat well-balanced meals, maintain a healthy weight, and get ample sleep.

What is Vasodilation

Vasodilation refers to the dilation of blood capillaries near the skin while constricting the deeper blood vessels to lose heat from the body. The relaxation of smooth muscles of the blood capillaries near the skin causes vasodilation. This leads to the widening of those blood capillaries, reducing the vascular resistance to the blood flow inside the blood vessel. On that account, the blood flow through the blood capillaries near the skin increases. Other than that, the blood pressure in these blood capillaries is also reduced. The effect of vasodilation and vasoconstriction on blood capillaries is shown in figure 1.

Figure 1: Cross Section of Blood Vessels

As vasodilation increases the flow of blood to the skin, it brings the internal body heat near the skin, cooling the body in high environmental temperatures. Vasodilation also enhances the entry of clotting factors and white blood cells into the damaged tissues. It increases the delivery of nutrients and oxygen throughout the body during energy-consuming activities.

Vasodilators refer to the bodily natural signals that cause vasodilation. They include parasympathetic nerve impulses, the release of hormones and bradykinin, and drugs. The drugs that cause vasodilation may be given for angina, congestive heart failure, hypertension or pulmonary hypertension.


Chronic hypertension is a common pathological process related to the cardiovascular system. This condition is significant because, with hypertension, there is an increase in afterload. A long-term increase in afterload leads to concentric hypertrophy of the heart and eventual left-sided diastolic heart failure. Also, an S4 heart sound will be audible at the apex of the heart. Another type of heart disease is alcoholic cardiomyopathy, which occurs in alcoholics and causes dilated cardiomyopathy, which means the ventricles become dilated, leading to systolic failure. It can be reversible if the patient stops drinking alcohol.

Heart failure or cardiac tamponade can cause cardiogenic shock. In cardiogenic shock, there is an increase in PCWP because there is a back up of blood the heart is not able to pump blood forward because it is not able to overcome the afterload. Subsequently, there is a decrease in CO. In response to low CO, the SVR increases.

In hemorrhagic shock, there is a loss of blood, thus a loss in total volume. Because there is a loss of volume, there is a decrease in pressure and, therefore, a decrease in PCWP. Also, there is an increase in cardiac output because there is a need for more blood in the periphery. While there is an increase in CO, there is also an increase in SVR to maintain MAP.


The mean arterial pressure (MAP), HR, MBV and VR at baseline are shown in Table 1. There MAP and HR did not differ significantly between the trials measuring the RA and SMA, whereas the MBV was significantly greater in the RA than in the SMA (P < 0.05).

RA trial SMA trial
MAP (mmHg) 75 ± 2 74 ± 2
HR (beats min −1 ) 64 ± 3 65 ± 3
MBV (m s −1 ) 0.41 ± 0.02 0.24 ± 0.02
VR (a.u.) 187 ± 10 337 ± 31

The MAP, HR and VR increased significantly from their baseline levels throughout the CWT (Fig. 1 and Table 2), There was no substantial difference in HR and MAP responses to CWT during either SMA or RA measuring trial. This was supported by the significant positive correlations in each MAP and HR between both SMA and RA measuring trials (r= 0.74 and r = 0.90, respectively).

Mean arterial pressure (MAP), heart rate (HR), mean blood velocity (MBV) and vascular resistance (VR) responses to 3 min of the colour word conflict test (CWT) relative to the baseline values in the superior mesenteric artery (SMA ○) and renal artery (RA •) measurement. The subjects performed the CWT from 180 to 360 s. Significant increases in MAP, HR and VR were observed throughout the CWT. Two-way ANOVA revealed that the relative changes in MBV and VR from the baseline values differed significantly between the RA and SMA. *Significantly different from baseline (P < 0.05)

RA trial SMA trial
MAP (mmHg) 84 ± 2 83 ± 2
HR (beats min −1 ) 74 ± 3 74 ± 3
MBV (m s −1 ) 0.37 ± 0.04 0.22 ± 0.02
VR (a.u.) 224 ± 12 395 ± 42
  • Data are means ± s.e.m . Individual data were obtained at the time point where maximal change was observed in 1-min averaged data. a.u., arbitary units.

MAP, HR and VR reached their peak responses at the first minute, although the real peak was not always exactly at the first minute, maintaining almost steady values. During the recovery period, the values returned to baseline within 1 min. The CWT significantly decreased the MBV only in the RA at the third minute (Fig. 1), whereas it did not significantly change the MBV in the SMA.

The two-way ANOVA revealed that the relative changes in the MBV and VR from the baselines differed significantly between the RA and SMA, whereas they did not vary over time.

This video was in response to a question asked by one of the students in my Anatomy & Physiology Academy. However, I thought it would be useful for you to, so I decided to include it here on this website.

It&rsquos a simple concept and I use the idea of water flowing through a hose as an analogy to help you understand. It&rsquos quite simple. Here are the main points:

  • If peripheral resistance increases, blood flow decreases
  • The diameter of the blood vessel is inversely proportional to the amount of resistance
  • If a blood vessel gets clogged, that increases resistance
  • Vasoconstriction increases peripheral resistance
  • Vasodilation decreases peripheral resistance
  • The body uses these processes to make sure that blood goes to the right place at the right time

So I hope that helps you understand the concepts of peripheral resistance and blood flow a little better. There&rsquos much more


All authors (B.K.A., J.W.C. and N.C.) designed and outlined the work, performed literature reviews and interpreted findings, and drafted and revised the manuscript. All authors approved the final version of the manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Blood Velocity: Definition and Factors (With Diagram) | Humans | Biology

In this article we will discuss about:- 1. Definition of Blood Velocity 2. Factors Effecting Blood Velocity 3. Methods of Measurement.

Definition of Blood Velocity:

It is the rate of blood flow through a given vessel. Blood flow is the volume of blood flowing through a particular vessel in given interval of time. Difference between the blood velocity and blood flow is that, the flow remaining constant, the velocity is inversely proportional to the cross-sectional area of the blood vessel.

But blood flow is directly proportional to the cross-sectional area of the vessel. But velocity in local blood vessel, if constricted, is increased and sometimes it reaches the critical velocity producing sound (turbulent flow).

Factors Effecting Blood Velocity:

Velocity of blood depends upon the following factors:

i. Lateral Pressure and Kinetic Energy for Flow:

In a tube of varying diameter, the lateral pressure varies directly and velocity inversely with the cross-sectional area of the tube. That is where there is any increase of velocity the lateral pressure will be decreased at that unit area due to conversion of certain amount of potential energy into kinetic energy for flow.

ii. Total Cross-Sectional Area of the Vascular Bed:

It is inversely proportional to the total cross-sectional area of the vascular bed. As one proceeds to the periphery the total vascular bed enlarges and blood velocity falls. Hence, in the aorta and larger arteries, it is about 0.5-1 metre per sec., whereas in the capillaries, about 0.5-1 mm per sec. As the capillaries join up to form bigger and bigger veins, the vascular bed shortens and the velocity increases. Near the heart, the total cross-sectional area of the great veins is nearly double that of the aorta. Hence, velocity is about half.

iii. Pumping Action of the Heart:

It is directly proportional to the force with which blood is propelled. Other factors remaining constant the velocity will, therefore, depend upon the minute output of the heart. Blood velocity thus increases during systole and diminishes during diastole.

iv. Peripheral Resistance:

Blood flow is inversely proportional to the peripheral resistance. Vascular dilatation will reduce the resistance and increase blood flow. While vasoconstriction will increase the resistance and reduce the blood flow. But the velocity of blood flow is just the reverse. If blood vessel is locally dilated or constricted then the velocity of blood flow is decreased or increased, respectively.

This principle is very helpful in maintenance of blood flow in the locally constricted blood vessels due to atherosclerotic invasion (Bernoulli’s principle). Because due to constriction at that area the kinetic energy for blood flow is increased but the lateral pressure is decreased. If the peripheral resistance is increased in general then there is possibility of decreasing the both (blood velocity and blood flow).

Methods of Measurement of Blood Velocity:

The blood velocity can be measured by noting the rate of progress of red blood corpuscles under the microscope.

ii. In the Larger Vessels (in Animals):

a. Ludwig’s Stromuhr (Fig. 7.94):

With this instrument the amount of blood passing through a vessel per unit time is measured. The velocity is then calculated by dividing the total volume flow (V) with the cross-sectional area of the vessel (πr 2 = area of circle).

b. Differential Stromuhr (Fleischl):

This is a more accurate instrument by which the change of blood velocity during each heart beat can be recorded.

iii. Thermostromuhr (Rein):

The principle is to heat a particular spot on the vessel by high frequency current at a known rate and then the rise of temperature of blood is noted a little down the stream by a thermo­couple. The rise of temperature is inversely proportional to the velocity.

iv. Electromagnetic Method:

A more advanced method based on electromagnetic principle has also been de­vised.

Mean Volume Flow:

Instrument used—Plethysmograph (Fig. 7.95). The total volume of blood passing through an organ or any other part does not depend upon the blood velocity flow through the corresponding artery.

It depends upon three factors:

1. The total cross-sectional area of the vascular bed in the organ.

2. The rate of metabolism in the organ.

3. The degree of vasodilatation or vasoconstriction in the locality.

The following figures give the mean vol­ume flow per minute for each 100 gm:

iv. Liver – 150 ml (1/3rd arterial, 2/3rds portal).

vi. Heart (Coronary) – 100 ml.

Alteration of volume of a limb is recorded by means of an instrument called Plethysrmograph (Fig. 7.95). The cuff of the Sphygmomanometer is applied to the arm and inflated up to a certain pressure which is less than the arterial pressure. The veins are occluded but the arterial blood flow in the limb remains unchanged. The volume of the limb gradually increases as no blood can leave due to occlusion of the veins.

The limb is kept in a water-tight glass vessel which is connected with the volume recorder. Changes in the volume of the limb will displace water from the glass vessel and this displacement of water will be recorded in a sensitive electronic apparatus through transducer system.