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GENERAL FEATURES OF BLOOD VESSEL STRUCTURE Pattern of Blood Flow Heart ----> Aorta ----> Arteries ----> Arterioles ----> Capillaries Superior and inferior vena cava <---- Veins <---- Venules <--- Functions 1. The heart acts as a pump to move blood through the vessel system. 2. The arterial system is the vessels before the capillaries. The arterial system carries blood to

the capillaries. 3. Capillaries are the site of exchange of substances between the blood and tissues. 4. The venous system is the vessels after the capillaries. The venous system returns blood to

the heart. Structure of Arteries and Veins General Features FIGURE 21.4 1. The tunica intima is the innermost layer and consists of endothelium (simple, squamous

epithelium) with some connective tissue and smooth muscle. The smooth surface of the tunica intima functions to reduce friction.

2. The tunica media is the middle layer and consists of connective tissue fibers (elastic and

collagenic) and smooth muscle. A. The elastic fibers allow the vessel to expand and recoil. B. The smooth muscle regulates blood flow through the blood vessel.

1) Contraction of the smooth muscle produces vasoconstriction or decreased diameter of the blood vessel. Vasoconstriction decreases blood flow through the blood vessel.

2) Relaxation of the smooth muscle produces vasodilation or increased diameter of the

blood vessel. Vasodilation increases blood flow through the blood vessel.

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3. The tunica adventitia is connective tissue (dense or loose) that forms the outer boundary of the vessel.

4. Vasa vasorum are small blood vessels that supply the tunica media and tunica adventitia. Vasa vasorum are necessary for blood vessels that are greater than 1 millimeter in diameter.

Explain.

5. The sympathetic division controls vasoconstriction and vasodilation in almost all blood

vessels. The parasympathetic division causes vasodilation in a few locations (e.g., the penis or clitoris).

Large Elastic Arteries FIGURE 21.6a 1. The tunica media has large amounts of elastic tissue and a small amount of smooth muscle.

This allows the artery to expand with systolic pressure and recoil (contributing to diastolic pressure).

2. Elastic arteries have a large lumen that allows transport of large amounts of blood. They are

also called conducting arteries. 3. Examples: aorta, brachiocephalic, carotid, subclavian, and common iliac arteries. Muscular Arteries (medium-sized and small arteries) FIGURE 21.6b 1. The tunica media has a large amount of smooth muscle. 2. Vasoconstriction and vasodilation allows partial regulation of blood flow through the vessels

and thus control of blood delivery to different parts of the body. Muscular arteries are also called distributing arteries.

3. Examples: medium sized arteries (e.g., brachial, femoral, and mesenteric arteries) and small

sized arteries.

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Arterioles 1. The tunica media is reduced to 1 to 2 layers of smooth muscle. 2. Vasoconstriction and vasodilation of the arterioles regulates blood flow into the capillaries. Capillaries FIGURE 21.1 1. Capillaries consist of just the tunica intima. This allows exchange of materials between the

blood and tissues. 2. Capillaries arise from arterioles or metarterioles. FIGURE 21.3

A. Metarterioles structurally are intermediate between arterioles and capillaries. Isolated smooth-muscle cells, instead of a layer of smooth muscle, are found in metarterioles, which are larger in diameter than capillaries.

B. Movement of blood into the capillaries is controlled by precapillary sphincters (smooth

muscle). These are NOT regulated by the sympathetic division, but by local mechanisms (more later).

C. In addition to giving rise to capillaries, metarterioles can continue as thoroughfare

channels to venules. The thoroughfare channels allow blood to pass through a tissue without entering into the capillaries.

Venules 1. Venules receive blood from capillaries and thoroughfare channels. Venules are much like

capillaries, except they are larger in diameter. 2. As venules increase in size, a few smooth muscle cells surround them. Some exchange of

materials between blood and tissues takes place in venules. Veins FIGURE 21.6c 1. Small, medium and large-sized veins have all three tunics. 2. The walls of veins are thin compared to arteries (less smooth muscle and elastic fibers).

Veins are subjected to lower pressure than arteries and don't need to be as strong.

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3. Because the pressure in veins is low, they have valves which prevent backflow of blood, especially in the limbs. Contraction of surrounding skeletal muscle and increased thoracic pressure during respiration can compress the veins and cause blood movement.

FIGURE 21.6d Where else have we seen similar types of valves?

4. Veins are distensible. As a result of smooth muscle relaxation or contraction they can

greatly change their volume. Thus, they act as a "reservoir" for blood. 5. Examples: The veins seen in dissection are medium and large-sized veins. PHYSICS OF CIRCULATION Blood Flow 1. Blood flows from one place to another because of a pressure difference (P1 - P2). That is,

blood flows from a place of higher pressure (P1) to a place of lower pressure (P2). 2. The flow of blood is opposed by a resistance (R) to flow. (P1 - P2) Flow = ------------- R 3. The resistance to blood flow in a blood vessel can be defined: R = 8vl/r4 where r = radius of the blood vessel raised to the fourth power v = viscosity of blood. Water has a viscosity of 1 and blood has a viscosity of

3 to 4.5. This means it takes 3 to 4.5 times as much pressure to overcome the resistance of blood to flow as it does for water to flow.

Most of the increased viscosity of blood results from red blood cells.

Hematocrit is the percent of the total blood volume composed of red blood cells. For example, a hematocrit of 45 means that 45% of the blood volume results from red blood cells.

l = length of the blood vessel (for our use, the length of the blood vessel is

constant)

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Poiseuille's Law 1. The rate of flow of blood in a blood vessel can be described by Poiseuille's law, (P1 - P2) (P1 - P2) Flow = -------------- = -------------- R 8vl/r4

Rearranging, (P1 - P2)r4 Flow = ------------- Explain how to interpret equations. 8vl Use Poiseuille's Law to explain what happens to blood flow when:

1. Blood vessels in the skin vasoconstrict in response to cold.

Reducing diameter 50% results in a 94% decrease in blood flow.

2. The hematocrit of the blood increases in erythrocytosis.

3. Systolic blood pressure decreases. Assume that P1 is systolic pressure in the aorta, and P2 is

the pressure in the right ventricle during diastole. Therefore blood flows from the aorta to the right ventricle, accounting for 70% of ventricular filling.

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Critical Closing Pressure and the Law of LaPlace 1. According to the law of LaPlace, the force (F) exerted on the wall of the blood vessel to

keep it open is equal to the diameter of the blood vessel (D) times the blood pressure (P). F = D x P F 2. Blood vessels are elastic and tend to collapse. According to the law of LaPlace, blood

pressure keeps the blood vessels expanded. 3. If blood pressure drops below the critical closing pressure of a blood vessel, the force

acting on the vessel wall decreases to the point that the blood vessel collapses and blood flow through the blood vessel stops. For example, during shock decreased blood pressure results in the shut down of blood flow to some tissues.

4. An aneurysm is a bulge in a weaken blood vessel wall. An aneurysm in the aorta or arteries

of the brain can be fatal if it ruptures, because the clotting mechanism cannot stop blood loss into the surrounding cavity or tissues.

Once an aneurysm begins, it is likely to get worse and worse, i.e., a positive-feedback cycle

leading to rupture of the aneurysm is in operation. Explain how this positive-feedback mechanism operates.

Vascular Compliance 1. Compliance is the tendency for blood vessels to expand as a result of an increase in blood

pressure. The diameter (D) of a blood vessel is proportional to the compliance (C) of the vessel and the pressure (P) within the vessel.

D = C x P

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2. The more compliant a blood vessel, the more elastic the blood vessel, and the easier the blood vessel can expand. For example, a balloon is more compliant than the inner tube of a car tire. It is easier to blow up the balloon than the inner tube.

3. Veins are approximately 24 times as compliant as arteries. Remember that veins are thin-

walled compared to arteries. A small increase in blood pressure produces a large increase in the diameter of veins.

D = C x P in an artery 24D = 24C x P in a vein 4. As the diameter of veins increases, they expand and hold more blood, even though pressure

in the veins is lower than in the arteries. Remember that veins are larger in diameter than comparable arteries.

5. The venous system can function as a blood reservoir. Veins contain 64% of the total blood

volume. PHYSIOLOGY OF SYSTEMIC CIRCULATION Cross-sectional Area of Blood Vessels FIGURE 21.32 1. The cross-sectional area of a single blood vessel is the space through which blood must flow

to pass through the blood vessel. 2. The cross-sectional area of a blood vessel type is the space through which blood must flow to

pass through all blood vessels of that type. 3. The cross-sectional area increases going from the aorta to the capillaries, and decreases going

from the capillaries to the vena cava. 4. As cross-sectional area increases, the rate of blood flow decreases. Why is it advantageous for total cross-sectional area to increase in the capillaries?

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Pressure and Resistance 1. Blood pressure in the vascular system is generated by contractions of the heart.

A. Blood pressure in the aorta varies between systolic and diastolic pressures (120/80 mm Hg). Mean arterial pressure (MAP) is the average pressure (approximately 100 mm Hg).

B. Blood pressure drops to a value near zero in the relaxed ventricle.

2. Blood pressure drops because of the resistance (see Poiseuille’s law) to flow produced in the vascular system.

FIGURE 21.33

A. The sum of all the resistances in the vascular system is called peripheral resistance (PR).

B. The greatest part of peripheral resistance occurs in the arterioles, and this is also where

the greatest drop in blood pressure occurs. The next greatest component of peripheral resistance occurs in the capillaries.

C. The lowest resistance to flow occurs in the veins.

Pulse Pressure 1. Pulse pressure is the difference between systolic and diastolic pressure. For example, if

systolic pressure is 120 mm Hg and diastolic pressure is 80 mm Hg, then pulse pressure is 40 mm Hg.

2. Two factors affect pulse pressure.

A. When stroke volume increases, pulse pressure increases. When stroke volume decreases, pulse pressure decreases.

B. When vascular compliance decreases, pulse pressure increases. For example, with age

there is a decrease in vessel elasticity, which results in decreased compliance. With decreased compliance in the aorta, when a given volume of blood is pushed into the aorta, systolic pressure increases, resulting in an increase in pulse pressure.

Assume that Ima Buster has an aortic aneurysm. As Ima gets older she develops

arteriosclerosis (hardening of the arteries). What effect will this have on her vascular compliance?

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What effect will the change in vascular compliance have on her pulse pressure?

What effect will the change in pulse pressure have on the likelihood that the aneurysm

will rupture? Whose law explains how this happens?

3. Pulse pressure produces a pressure wave that travels through the vascular system.

A. The pressure wave is monitored as the pulse, e.g., the radial pulse. B. Intense sympathetic stimulation, as occurs in shock, causes vasoconstriction of the

arterial system. The pulse pressure wave is suppressed and a weak pulse results.

Capillary Exchange FIGURE 21.34 1. Nutrients and gases are exchanged, mostly by diffusion, between blood and tissues across the

walls of capillaries. 2. There is net movement of fluid from the blood into tissues at the arterial ends of capillaries,

and net movement of fluid back into the blood at the venous ends of capillaries. 3. More fluid moves into the tissues than moves back into the blood. The excess fluid is

removed by the lymphatic system. A. If more fluid enters the tissues than moves back into the blood or is removed by the

lymphatic system, there is a buildup of fluid in the tissues. This accumulation of fluid is called edema.

B. If more fluid leaves the tissues into the blood or lymphatic system than enters the tissues,

then the tissues become dehydrated.

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4. Net filtration pressure moves fluid out of the capillary into the tissue. Net filtration pressure is equal to net hydrostatic pressure minus net osmotic pressure.

Net filtration pressure = Net hydrostatic pressure - Net osmotic pressure.

A. Net hydrostatic pressure moves fluid out of the capillary into the tissue, whereas net

osmotic pressure moves fluid from the tissue into the capillary. The difference between these two forces determines how much fluid moves.

B. When NFP > 0, fluid moves from the capillary into the tissue. This occurs at the arterial

end of the capillary. C. When NFP < 0, fluid moves from the tissue into the capillary. This occurs at the venous

end of the capillary. 5. Net hydrostatic pressure is the pressure difference between the blood and the interstitial

fluid. It is equal to blood pressure (BP) minus interstitial fluid pressure (IFP).

Net hydrostatic pressure = BP - IFP

A. Blood pressure results from the heart beating, but the effect of gravity on blood when standing or lying down can affect blood pressure as well. Most of net hydrostatic pressure results from blood pressure.

B. Interstitial fluid pressure is a slight negative pressure produced by the lymphatic vessels

as they pump fluid out of the tissue spaces (discussed in more detail in chapter 22). 6. Net osmotic pressure is the difference in osmotic pressure between the blood and the

interstitial fluid. Blood colloid osmotic pressure (BCOP) is the osmotic pressure in the blood and interstitial colloid osmotic pressure (ICOP) is the osmotic pressure in the interstitial fluid.

Net osmotic pressure = BCOP - ICOP

A. Colloid osmotic pressure is mainly due to the concentration of proteins. BCOP is higher

than ICOP because of albumin and other proteins in the plasma. B. Remember that fluid tends to move from a solution with a lower osmotic pressure into a

fluid with a higher osmotic pressure. C. Fluid moves by osmosis from the tissue into the capillary because BCOP is higher than

ICOP.

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7. How to determine fluid movement between the capillary and the blood at the arterial and venous ends of a capillary.

Arterial End Venous End of Capillary of Capillary Calculate Net Hydrostatic Pressure

Blood pressure 30 10 Interstitial fluid pressure -(-3) -(-3) Net hydrostatic pressure 33 13

Calculate Net Osmotic Pressure Blood colloid osmotic pressure 28 28 Interstitial colloid osmotic pressure -8 -8 Net osmotic pressure 20 20 Calculate Net Filtration Pressure Net hydrostatic pressure 33 13 Net osmotic pressure -20 -20 Net filtration pressure 13 -7

In cirrhosis of the liver, there is often abdominal swelling as the peritoneal cavity fills with

fluid (ascites). We are going to explain how that happens. Cirrhosis of the liver results in reduced blood flow through the liver. What effect does this

generally have on blood pressure in the capillaries of the intestine? (Hint: think of the hepatic portal system). What effect does this change in capillary blood pressure have on net hydrostatic pressure in the intestinal capillaries?

Cirrhosis of the liver results in reduced albumin (a plasma protein) production by the liver.

What effect does this have on blood colloid osmotic pressure and net osmotic pressure?

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What effect do these changes in net hydrostatic pressure and net osmotic pressure have on net filtration pressure (You add arrows)?

Net filtration pressure = Net hydrostatic pressure - Net osmotic pressure

Summary:

How does the change in net hydrostatic pressure affect the movement of fluid from the capillary into the tissue?

Increases Decreases

How does the change in net osmotic pressure affect the movement of fluid from the tissue into the capillary?

Increases Decreases

As a result, there is a net movement of fluid.

Out of the blood Into the blood

What effect would standing still for a long period of time have on fluid movement into or out

of the tissues of the lower limbs? Explain.

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Functional Characteristics of Veins 1. The state of contraction of the veins, which is called venous tone, greatly influences cardiac

output. 2. Remember that according to Starling's law of the heart, cardiac output is equal to venous

return. During exercise, sympathetic stimulation causes vasoconstriction of the veins. What effect

does this have on cardiac output? Why is this beneficial?

3. Although the veins have great compliance, they cannot expand indefinitely. What effect would a transfusion (i.e., an increase in blood volume) have on cardiac output?

Explain.

CONTROL OF BLOOD FLOW IN TISSUES Introduction 1. At rest, blood flow through some tissues (thyroid gland) is greater than through other tissues

(skin). 2. During activity, blood flow to some tissues (skeletal and cardiac muscle) increases while

flow to other tissues (GI tract) decreases. 3. Mechanisms exists that ensure adequate delivery of blood to tissues with different metabolic

needs. A. Local control mechanisms operate within tissues at the capillary level. B. Nervous control regulates blood flow to tissues through distributing (muscular) arteries

and arterioles.

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Local Control of Blood Flow by the Tissues FIGURE 21.35 1. Blood flow through capillaries is cyclic, that is, the flow speeds up, slows down, speeds up,

etc. The variation in blood flow results from vasomotion, which is a change in vessel diameter caused by the periodic contraction and relaxation of the precapillary sphincters.

2. Regulation of the precapillary sphincters.

A. Regulation is NOT the result of nervous control, because the precapillary sphincters are not innervated by the nervous system.

B. Local factors.

1) Decrease of nutrients. Lack of oxygen and other nutrients causes relaxation of the precapillary sphincters.

2) Increase in vasodilator substances. A buildup of vasodilator substances such as

carbon dioxide or lactic acid causes relaxation of the precapillary sphincters.

3. The metabolic needs of a tissue determine the blood flow to the tissue. A. Local control ensures that when metabolic rate goes up (oxygen decreases, carbon

dioxide and lactic acid increases), the precapillary sphincters relax and blood flow increases.

B. Local control is normally the most important factor governing blood flow through tissues.

4. When blood flow to a tissue has been blocked for a time, the blood flow through that tissue can increase as much as 5 times its normal value after the removal of the block. This response is called reactive hyperemia.

Give an explanation for reactive hyperemia based on your knowledge of local control of

blood flow. Give an example.

Autoregulation of Blood Flow 1. Even if blood pressure goes up or down, the precapillary sphincters adjust blood flow

through the tissue to normal values. That is, local control mechanisms maintain blood flow through the tissues.

2. Despite changes in blood pressure between 75 mm Hg and 175 mm Hg, blood flow remains

within 10 to 15% of normal. This is called autoregulation.

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Long Term Local Blood Flow 1. If the metabolic needs of a tissue increase and remain elevated, the diameter and number of

capillaries in the tissue increases. For example, an athlete has more capillaries in skeletal muscle than does a couch potato.

2. The availability of oxygen appears to be the factor responsible for stimulating these changes

in the capillaries. Nervous and Hormonal Regulation of Local Circulation FIGURE 21.36 1. The sympathetic division regulates vasoconstriction and vasodilation of most blood vessels

(the parasympathetic division is involved with the external genitalia). 2. The vasomotor center, located in the medulla oblongata and pons, controls sympathetic

output. 3. Control of the vasomotor center.

A. Reflexes (more later). B. Higher brain centers. Emotions, temperature regulation (through the hypothalamus) or

exercise (through the motor areas of the cerebral cortex) can stimulate or inhibit the vasomotor center.

4. Most blood vessels are innervated by sympathetic fibers that secrete norepinephrine.

A. Normally, the vasomotor center is tonically active, producing partial vasoconstriction of blood vessels. This is called vasomotor tone.

B. Increasing sympathetic stimulation increases norepinephrine release and produces

vasoconstriction. Decreasing sympathetic stimulation decreases norepinephrine release and produces vasodilation.

More Tonic Less Stimulation Stimulation Stimulation More Tonic Less NE NE NE Vasoconstriction Normal Vasodilation Vasomotor Tone

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6. The effect of sympathetic stimulation on blood delivery to tissues. A. Control of distributing (muscular) arteries and arterioles allows routing of blood to areas

where it is needed and restricts blood flow to areas where blood is not needed. For example, during exercise blood flow to skeletal muscle increases while blood flow to the GI tract decreases.

B. Control of veins determines the amount of blood held within the venous reservoir.

Increased sympathetic stimulation of the veins decreases the venous "space" and increases venous return. The increased venous return increases cardiac output (Starling's law).

C. The hypothalamus controls vasodilation and vasoconstriction of blood vessels for the

purpose of regulating body temperature. If connection to the hypothalamus is interrupted (e.g., spinal cord injury), the ability of the isolated body part to respond to temperature is for all practical purposes lost. This includes changes in blood flow, sweating, and shivering.

D. Although local control is normally the most important factor in regulating blood flow

through tissues, there are times when nervous control can dominate local control. For example, during shock, vasoconstriction through the sympathetic division shuts down blood flow to "nonessential" areas of the body (limbs, viscera) and routes the blood to "essential" areas (brain, heart). As shock progresses, the lack of blood flow can cause tissue damage that promotes further development of shock.

7. The sympathetic division also stimulates the adrenal medulla to release epinephrine and

norepinephrine, which usually cause vasoconstriction (more later). REGULATION OF MEAN ARTERIAL PRESSURE Introduction 1. Blood pressure in the vascular system is generated by the contraction of the heart.

A. Blood pressure in the aorta varies between systolic and diastolic pressures (120/80 mm Hg). Mean arterial pressure (MAP) is the average pressure (about 100 mm Hg).

B. Blood pressure drops to a value near zero in the relaxed ventricle. C. Blood flows through the vascular system because of the difference in pressure between

the aorta (100 mm Hg) and the relaxed ventricle (0 mm Hg). Therefore maintenance of blood pressure is absolutely essential for life.

2. Blood pressure drops because of the resistance (friction) to flow produced in the vascular

system. The sum of all the resistances in the vascular system is called peripheral resistance (PR).

3. Mean arterial pressure can be defined as follows:

MAP = CO x PR MAP = HR x SV x PR where CO = cardiac output HR = heart rate SV = stroke volume PR = peripheral resistance

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FOR YOUR INTEREST

Derivation of the Relationship of Blood Pressure to Cardiac Output and Peripheral Resistance

1. The flow of blood in blood vessels results from a pressure difference divided by a resistance

to flow. Flow = Pressure Difference/Resistance to flow. 2. Flow, pressure difference, and resistance to flow can be defined as follows:

A. The flow of blood to the relaxed right ventricle is the venous return (VR). B. The pressure difference responsible for blood flow is the difference between mean

arterial pressure (MAP) in the aorta and the pressure in the relaxed right ventricle during diastole.

C. The resistance to flow in the vascular system is called peripheral resistance (PR).

3. The flow of blood, or venous return can therefore be defined as: VR = (MAP - Relaxed Right Ventricle Pressure)/PR

where VR = venous return MAP = mean arterial pressure in the aorta PR = peripheral resistance

4. Because the relaxed right ventricle pressure is essentially zero: VR = (MAP - 0)/PR VR = MAP/PR 5. According to Starling's law of the heart, all of the blood than enters the heart is pumped out.

Therefore cardiac output (CO) is equal to venous return. CO = VR 6. Because VR = MAP/PR substitution in equation 5 yields: CO = MAP/PR 7. Rearranging the terms of the equation yields: MAP = CO x PR 8. Cardiac output is equal to heart rate (HR) times stroke volume (SV). CO = HR x SV 9. Substituting HR x SV for CO in equation 7 yields:

MAP = HR x SV x PR

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4. Blood pressure is maintained by: A. Changing heart rate. B. Changing stroke volume.

1) Stroke volume can be changed by changing the force of contraction of the heart. For example, increased force of contraction increases stroke volume by decreasing end systolic volume.

2) Stroke volume can be changed by changing venous return.

a. Vasoconstriction of veins increases venous return, whereas vasodilation decreases venous return.

b. For example, if blood volume increases, venous return increases, which increases

end-diastolic volume. Therefore stroke volume increases (Starling’s law).

C. Changing peripheral resistance. 1) Vasodilation decreases peripheral resistance and vasoconstriction increases peripheral

resistance. 2) The most significant changes in peripheral resistance result from vasodilation and

vasoconstriction of arterioles. Veins offer little resistance to blood flow, and vasoconstriction of veins does not significantly affect peripheral resistance.

What effect on mean arterial pressure is produced by the following:

Value Changed Effect on MAP Decrease HR

Increases Decreases

Increase SV

Increases Decreases

Increase PR

Increases Decreases

Short-Term Regulation of Blood Pressure 1. If blood pressure changes, short-term mechanisms rapidly respond to return blood pressure to

normal levels (homeostasis). 2. Neural mechanisms are primarily responsible for short-term regulation of blood pressure. 3. Epinephrine and norepinephrine from the adrenal gland also are short-term regulators of

blood pressure.

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Baroreceptor Reflex FIGURES 21.37 and 21.38 1. Recall from chapter 20 that baroreceptors are sensory receptors that respond to stretch.

They are located in the carotid sinuses and aortic arch. 2. The baroreceptor reflex can change HR and SV through the cardioregulatory center and PR

through the vasomotor center. If MAP , then HR , SV , PR in response If MAP , then HR , SV , PR in response 3. The baroreceptor reflex is very effective and fast acting. Even transient events, such as

changing from a lying to a standing position, cause minor changes in arterial pressure due to hydrostatic pressure.

4. The baroreceptor reflex is the most important short-term regulator of blood pressure. Assume that you have just changed positions from sitting to standing. Does this increase or

decrease blood pressure in the lower limbs?

As a result of the change in blood pressure in the lower limbs, does the force acting on the

vessel wall increase or decrease? Whose law describes this relationship?

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As a result of the change in force, do the veins in the lower limb expand or get smaller? As a result of this change, what happens to venous return?

As a result of the change in venous return, what happens to blood pressure in the aorta and in

the carotid arteries?

As a result of this change in blood pressure, what effect does the baroreceptor reflex have on:

(see figure 21.42):

Heart rate Increases Decreases

Stroke volume

Increases Decreases

Peripheral resistance

Increases Decreases

Peripheral resistance increases primarily because of vasoconstriction of arterioles. How does

increased peripheral resistance contribute to an increase in blood pressure?

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Peripheral resistance also increases slightly because of vasoconstriction of veins in the lower limbs. How does vasoconstriction of lower limb veins contribute to an increase in blood pressure?

4. The baroreceptor reflex does not change blood pressure on a long-term basis. The

baroreceptors adapt to an elevated blood pressure within 1 to 3 days. Thereafter the baroreceptor reflex maintains blood pressure at the elevated level. Thus, the baroreceptor reflex cannot control hypertension.

Adrenal Medullary Mechanism FIGURES 21.38 and 21.39 1. Stimuli, such as a decrease in blood pressure, can result in increased sympathetic stimulation

of the adrenal gland. 2. Sympathetic stimulation of the adrenal gland results in the release of epinephrine (80%) and

norepinephrine (20%) from the adrenal gland. 3. Heart rate and stroke volume increase, resulting in an increase in blood pressure. 4. Epinephrine and norepinephrine affect blood vessels, but their effect is small compared to

that produced by local control and the nervous system. The overall effect of these hormones is to causes vasoconstriction in the skin and viscera and vasodilation in skeletal muscle and the heart.

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Chemoreceptor Reflex FIGURES 21.40 and 21.41 1. Recall from chapter 20 that peripheral chemoreceptors are located in the carotid and aortic

bodies. 2. The peripheral chemoreceptor reflex responds primarily to low blood oxygen levels. It also

responds to decreased blood pH (often caused by increased blood carbon dioxide levels). A. Very low blood oxygen levels result when blood pressure is too low to delivery adequate

amounts of blood to the lungs and to the tissues. B. The peripheral chemoreceptor reflex can increase blood pressure. Consequently, there is

increased movement of blood between the tissues and the lungs. The increased blood movement helps to raise oxygen levels and restore homeostasis.

C. The peripheral chemoreceptor reflex functions under emergency conditions or high

altitudes. They are not important for normal regulation of blood pressure.

3. Through the peripheral chemoreceptor reflex the vasomotor center causes increased vasoconstriction, which increases peripheral resistance.

Oxygen , PR , MAP 4. The peripheral chemoreceptor reflex causes an increase in respiration rate. When respiration

rate increases, sympathetic stimulation of the heart increases and parasympathetic stimulation decreases. As a result, heart rate and stroke volume increase.

Oxygen , RR , HR , SV , MAP Central Nervous System Ischemic Response FIGURES 21.40 and 21.41 1. Recall from chapter 20 that central chemoreceptors are located in the medulla oblongata. 2. Central chemoreceptors in the vasomotor center of the medulla oblongata respond to changes

in pH. Because carbon dioxide levels affect blood pH, it indirectly affects the central chemoreceptors. Higher than normal carbon dioxide levels result when blood pressure is too low and carbon dioxide is not effectively removed from tissues and eliminated in the lungs.

CO2 + H2O H2CO3 H+ + HCO3

- (higher than normal) Lowers pH as H+ ions accumulate

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3. Decreased pH activates the CNS ischemic response from the vasomotor center, resulting in vasoconstriction that increases PR and thus MAP.

carbon dioxide , pH , vasoconstriction , PR , MAP 4. As blood pressure increases, blood delivery to the lungs increases, carbon dioxide levels

decrease, and pH increases. CO2 + H2O H2CO3 H+ + HCO3

- (eliminated in lungs) Increases pH as H+ ions decrease 5. The CNS ischemic response functions under emergency conditions (blood pressure falls

below 50 mm Hg). If blood pressure is not sufficiently restored, decreased blood delivery to the medulla results in inactivation of the vasomotor center. Consequently, blood pressure decreases further and death results.

6. The opposite effect of the CNS ischemic response can be observed during hyperventilation.

Hyperventilation removes carbon dioxide from the blood at a faster than normal rate. The lowered carbon dioxide levels increase pH, which inhibits the vasomotor center.

carbon dioxide , pH , vasodilation , PR , MAP Why does a person experiencing an emotional attack of hyperventilation become dizzy?

Would you expect heart rate to increase or decrease as a result of hyperventilation? Explain.

Regulation of Blood Pressure During Shock

1. Suppose a person loses a large amount of blood. What effect would this have on blood pressure? Why does this happen?

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2. Draw in the appropriate arrows to indicate the responses produced by the baroreceptor reflex (see figure 21.39).

MAP HR , SV , PR MAP As a result of As a result of the As a result of changes in blood loss baroreceptor reflex HR, SV, PR 3. What effect does the decrease in blood pressure have on blood oxygen and carbon dioxide

levels? Explain.

.

4. Draw in the appropriate arrows to indicate the responses produced by the peripheral

chemoreceptor reflex (see figure 21.41). Oxygen PR Oxygen As a result of As a result of the As a result of changes in blood loss chemoreceptor reflex PR that cause MAP 5. Draw in the appropriate arrows to indicate the responses produced by the CNS ischemic

response (see figure 21.41). Carbon dioxide PR Carbon dioxide As a result of As a result of the As a result of changes in blood loss CNS ischemic response PR that cause MAP 6. Sympathetic stimulation of what type of blood vessel causes most of the increase in PR

produced by the baroreceptor reflex, chemoreceptor reflex, and CNS ischemic response?

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7. Name two ways that increased sympathetic stimulation can increase stroke volume, and thus increase blood pressure. A. B. Circulatory Changes During Exercise

1. During exercise, blood flow within the capillaries of skeletal muscle greatly increases. Give two explanations, which do not include neural or hormonal mechanisms, to explain this increase.

A. B.

2. As a result of the changes in the capillaries, what must happen to peripheral resistance?

3. Decreased sympathetic stimulation of arterioles and epinephrine cause vasodilation of

arterioles in skeletal muscle and the heart. Why is this advantageous? What effect does this have on peripheral resistance?

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4. Increased sympathetic stimulation causes vasoconstriction of arterioles in the skin and viscera. Why is this advantageous? What effect does this have on peripheral resistance?

5. Peripheral resistance is decreased by the relaxation of precapillary sphincters and arterioles

in skeletal muscle. Peripheral resistance is increased by vasoconstriction in the skin and viscera. The overall effect is a decrease in peripheral resistance. Why is a decrease in peripheral resistance advantageous during exercise?

6. During exercise blood pressure increases approximately 15%. How is this possible when

peripheral resistance decreases? Why is it advantageous for blood pressure to increase?

7. At rest, an increase in blood pressure activates the baroreceptor reflex. During exercise a

strong baroreceptor reflex would inhibit an increase in heart rate and stroke volume. The resting baroreceptor reflex is reset to a higher value during exercise, allowing a beneficial increase in heart rate and stroke volume.

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Long-Term Regulation of Blood Pressure 1. Short-term mechanisms allow the body to respond rapidly to sudden changes in blood

pressure. 2. Long-term mechanisms are responsible for maintaining the baseline blood pressure around

which the short-term mechanisms operate. They operate by changing blood volume and peripheral resistance. A. Increasing blood volume increases venous return and therefore blood pressure. B. Increasing peripheral resistance increases blood pressure.

Renin-Angiotensin-Aldosterone Mechanism FIGURES 21.42 and 21.44 1. When mean arterial pressure drops, the kidneys secrete the enzyme renin [rE′nin]. Renin

coverts a plasma protein produced by the liver, angiotensinogen, into angiotensin I. Angiotensin I is converted into angiotensin II by angiotensin converting enzyme (ACE), which primarily is found in the small blood vessels of the lungs.

2. Angiotensin II causes vasoconstriction of arterioles, and to a lesser extent, veins. This

increases blood pressure by increasing peripheral resistance and increasing venous return. 3. Angiotensin II also stimulates aldosterone production by the adrenal cortex. The

aldosterone stimulates sodium and water retention by the kidneys. In other words, urine production decreases and blood volume is maintained or increases. The increased blood volume increases venous return and thus blood pressure.

4. ACE inhibitors are a class of drugs used to treat hypertension. They prevent ACE from

converting angiotensin I to angiotensin II. Thus, there is less angiotensin II, which results in less vasoconstriction and stimulation of aldosterone production.

Vasopressin Mechanism FIGURES 21.43 and 21.44 1. The baroreceptor reflex is activated by a decrease in blood pressure. The baroreceptor reflex

stimulates the hypothalamus, resulting in the release of antidiuretic hormone (ADH) from the posterior pituitary.

2. Blood pressure must drop significantly for ADH release to be stimulated. Normally, small

changes in the concentration of blood regulate ADH release (more later with the urinary system).

3. Actions of ADH.

A. ADH is also called vasopressin. ADH causes vasoconstriction, although ADH is not as powerful a vasoconstrictor as epinephrine or angiotensin II.

B. ADH decreases urine production by the kidneys, thus promoting an increase in blood

volume and blood pressure.

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Atrial Natriuretic Mechanism FIGURE 27.6 (p. 1010) 1. Increased blood pressure in the right atrium causes increased stretch of the right atrium. This

results in the increased release of atrial natriuretic [nA-trE-yU-ret′ik] hormone from specialized cardiac muscle cells. Decreased blood pressure results in decreased release of atrial natriuretic hormone.

2. Natriuresis is increased urinary excretion of sodium. Atrial natriuretic hormone increases

sodium and water loss by the kidneys, resulting in decreased blood volume and blood pressure.

3. Note that the renin-angiotensin-aldosterone and the atrial natriuretic mechanisms operate in

the opposite directions. A. The renin-angiotensin-aldosterone mechanism responds to a decrease in blood pressure

by decreasing urine production, which increases blood volume and blood pressure. B. Atrial natriuretic hormone responds to an increase in blood pressure by increasing urine

production, which decreases blood volume and blood pressure. Review of Long-Term Hormonal Mechanisms

1. Indicate the effect of each hormone by placing an up arrow by the hormone name if there is an increase, and a down arrow if there is a decrease, and a zero if there is no effect.

2. Effect of a decrease in blood pressure on hormone production.

Hormone Production Angiotensin II Aldosterone ADH Atrial natriuretic hormone

3. Effect of an increase of the hormones on vasoconstriction and blood pressure. Vasoconstriction Blood Pressure

Angiotensin II ADH

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4. Effect of an increase of the hormones on urine production, blood volume, and blood pressure.

Urine Production Blood Volume Blood Pressure Aldosterone ADH Atrial natriuretic hormone

The Fluid Shift Mechanism 1. Within a few minutes to a few hours these mechanism act to regulate blood pressure. 2. The fluid shift mechanism is the movement of fluid into and out of tissues in response to

changes in blood pressure. A. An increase in blood pressure. If blood pressure increases, what effect does this have on the net movement of fluid into

tissues? Explain.

As a result of the fluid movement, what happens to blood pressure? Explain.

B. A decrease in blood pressure. If blood pressure decreases, fluid moves out of tissues into

the blood, resulting in an increase in blood pressure.

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The Stress-Relaxation Response 1. A decrease in blood pressure. When blood pressure decreases, what happens to the force exerted on the walls of blood

vessels? Whose law describes this relationship?

As a result of the pressure change smooth muscle in the blood vessel wall contracts. What

effect does this have on blood pressure? Explain.

2. An increase in blood pressure. The opposite effect occurs when blood pressure increases,

i.e., the smooth muscle in the blood vessel wall relaxes and blood pressure decreases. Explain why a rapid loss of a large amount of blood can result in death, but the loss of the

same amount of blood over a period of several hours does not.

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SUMMARY OF CARDIOVASCULAR REGULATION 1. CO = HR x SV 2. MAP = CO x PR and therefore MAP = HR x SV x PR 3. VR = Input = Output = CO (Starling's Law) If blood volume (BV) , then VR and CO . If BV , then VR and CO . 4. Short-term regulation of blood pressure (see figure 21.39, p. 769). Stimulus Mechanism Response Effect MAP

Baroreceptor reflex

HR , SV , PR

MAP

MAP

Baroreceptor reflex

HR , SV , PR

MAP

Adrenal medullary mechanism

HR , SV , PR

5. Regulation of blood pH and blood gases (see figure 21.41, p. 771) Stimulus Mechanism Response Effect pH (CO2 )

Central chemoreceptor reflex

PR (major) HR , SV (minor)

MAP , CO2 , pH

O2

Peripheral chemoreceptor reflex

PR RR , HR , SV

MAP , O2

pH (CO2 )

Central chemoreceptor reflex

HR , SV

MAP , CO2 , pH

CNS ischemic response

PR

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6. Long term regulation of blood pressure (see figure 21.44, p. 776) Stimulus Mechanism Response Effect MAP

Renin-angiotensin-aldosterone mechanism

Angiotensin II , PR Aldosterone , BV

MAP

Atrial natriuretic mechanism

ANH , BV , PR

Stress-relaxation mechanism

PR

Fluid shift mechanism

BV

MAP

Renin-angiotensin-aldosterone mechanism

Angiotensin II , PR Aldosterone , BV

MAP

Vasopressin mechanism

ADH , BV , PR

Stress-relaxation mechanism

PR

Fluid shift mechanism

BV

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☞ Practice Problems 1. Private U.P. Wright had to stand at attention in the hot sun for several hours. Eventually he

fainted. Explain what happened.

2. Just as private U.P. Wright faints, his buddies realize what is happening. To save him from

the embarrassment of fainting they held him in a standing position. Explain what is wrong with this treatment. What would you suggest they do?

3. While standing at attention for long periods of time, soldiers are told not to lock their knees

in place. Instead, they should keep the knees slightly bent. Explain why this would help to prevent fainting.

4. During an experiment in a physiology laboratory a student named Ima Gaspar breathed into

and out of a plastic bag for a few minutes. Predict what would happen to her heart rate and blood pressure. (Hint: blood levels of carbon dioxide change more rapidly than oxygen). Explain.

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5. Ima's lab partner, Johnny Uptite, became very concerned as Ima performed her experiment. He began to hyperventilate. What should happen to his heart rate and blood pressure as a result of the hyperventilation? Explain.

6. During an experiment in a physiology laboratory a student named C. Saw was placed on a

table that could be tilted. Predict what would happen to C. Saw's heart rate if the table were tilted so that her head were lower than her feet. Explain.

7. After C. Saw was tilted so that her head was lower than her feet for a few minutes, the table

was tilted so that her head was higher than her feet. Predict the effect this change would have on C. Saw's heart rate.

8. All other things being equal, does the heart beat faster or slower at 11,000 feet compared to

sea level? Explain.