Physiology 5th Ed.

Chapter 4. Cardiovascular Physiology

 

The primary function of the cardiovascular system is to deliver blood to the tissues, providing essential nutrients to the cells for metabolism and removing waste products from the cells. The heart serves as the pump, which, by contracting, generates the pressure to drive blood through a series of blood vessels. The vessels that carry blood from the heart to the tissues are the arteries, which are under high pressure and contain a relatively small percentage of the blood volume. The veins, which carry blood from the tissues back to the heart, are under low pressure and contain the largest percentage of the blood volume. Within the tissues, thin-walled blood vessels, called capillaries, are interposed between the arteries and veins. Exchange of nutrients, wastes, and fluid occurs across the capillary walls.

The cardiovascular system also is involved in several homeostatic functions: It participates in the regulation of arterial blood pressure; it delivers regulatory hormones from the endocrine glands to their sites of action in target tissues; it participates in the regulation of body temperature; and it is involved in the homeostatic adjustments to altered physiologic states such as hemorrhage, exercise, and changes in posture.

CIRCUITRY OF THE CARDIOVASCULAR SYSTEM

HEMODYNAMICS

CARDIAC ELECTROPHYSIOLOGY

CARDIAC MUSCLE CONTRACTION

CARDIAC CYCLE

RELATIONSHIPS BETWEEN CARDIAC OUTPUT AND VENOUS RETURN

REGULATION OF ARTERIAL PRESSURE

MICROCIRCULATION

SPECIAL CIRCULATIONS

TEMPERATURE REGULATION

INTEGRATIVE FUNCTIONS OF THE CARDIOVASCULAR SYSTEM

SUMMARY

ent The cardiovascular system is composed of the heart and blood vessels. The heart, by contracting, pumps blood through the systemic and pulmonary vasculatures. Blood vessels act as conduits that deliver blood to the tissues. The thin-walled capillaries serve as the site of exchange of nutrients and waste products.

ent Hemodynamics are the principles that govern blood flow: velocity of flow; flow, pressure, and resistance relationships; and compliance of blood vessels.

ent Velocity of blood flow is proportional to the rate of volume flow and inversely proportional to the cross-sectional area. Velocity is lowest in the capillaries, which have the largest cross-sectional area.

ent Blood flow is proportional to the size of the pressure gradient and inversely proportional to the resistance of the blood vessels.

ent Resistance to blood flow is proportional to the viscosity of blood and vessel length and inversely proportional to vessel radius to the fourth power. The arterioles are the site of highest resistance in the vasculature. Resistances can be arranged in series or in parallel.

ent Compliance is the relationship between volume and pressure: The higher the compliance of a blood vessel, the greater the volume contained at a given pressure. Veins have high compliance and hold large volumes of blood (the unstressed volume) at low pressure. Arteries have low compliance and hold small volumes of blood (the stressed volume) at high pressure.

ent The cardiac action potential is initiated in the SA node, which depolarizes spontaneously. The action potential spreads in a specific sequence throughout the myocardium via a specialized conducting system. Conduction is rapid, except through the AV node, where slow conduction ensures ample time for ventricular filling prior to contraction.

ent In atria and ventricles, the upstroke of the action potential is the result of an inward Na+ current. The action potential in the atria and ventricles exhibits a plateau, which is the result of an inward Ca2+current. This plateau accounts for the action potential’s long duration and long refractory period.

ent In the SA node, the upstroke of the action potential is the result of an inward Ca2+ current. The SA node exhibits slow, spontaneous depolarization during phase 4, which brings the cells to threshold to fire action potentials. Slow depolarization is the result of an inward Na+ current (If).

ent Excitation-contraction coupling in myocardial cells is similar to that in skeletal muscle. In myocardial cells, however, Ca2+ entering the cell during the plateau of the action potential serves as a trigger for the release of more Ca2+from the sarcoplasmic reticulum. Ca2+ then binds to troponin C to allow cross-bridge formation.

ent Inotropism or contractility is the ability of the myocardial cell to develop tension at a given cell length: Intracellular [Ca2+] determines the degree of inotropism, with positive inotropic agents increasing intracellular [Ca2+] and contractility.

ent Myocardial cells and the myocardium exhibit a length-tension relationship based on the degree of overlap of contractile elements. The Frank-Starling law of the heart describes this relationship between cardiac output and end-diastolic volume. End-diastolic volume reflects venous return. Therefore, cardiac output is determined by venous return, and in the steady state, cardiac output and venous return are equal.

ent Pa is the product of cardiac output and TPR. Pa is carefully monitored and maintained at a normal value of 100 mm Hg. The baroreceptor reflex is a fast, neural mechanism that detects changes in Pa and orchestrates changes in sympathetic and parasympathetic outflow to the heart and blood vessels to restore Pa back to normal. The renin–angiotensin II–aldosterone system is a slower, hormonal mechanism that detects changes in Pa and, via aldosterone, restores Pa to normal through changes in blood volume.

ent The exchange of fluid across capillary walls is determined by the balance of Starling forces. The net Starling pressure determines whether there will be filtration out of the capillary or absorption into the capillary. If filtration of fluid exceeds the ability of the lymphatics to return it to the circulation, then edema occurs.

ent The blood flow to the organ systems is a variable percentage of the cardiac output. Blood flow is determined by arteriolar resistance, which can be altered by vasodilator metabolites or by sympathetic innervation.


Challenge Yourself

Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are provided at the end of the book.

1 What are the units of hemodynamic resistance?

2 If heart rate is 75 beats/minute, what is the R-R interval in units of milliseconds?

3 What is the correct order of the following events: Ca2+ binding to troponin C, tension, Ca2+ release from sarcoplasmic reticulum, ventricular action potential, Ca2+ accumulation by sarcoplasmic reticulum?

4 If heart rate is 85 beats/minute, end-diastolic volume is 150 mL, and stroke volume is 75 mL, what is the ejection fraction?

5 Which portion of the cardiac cycle has the lower ventricular volume: atrial systole or isovolumetric ventricular relaxation?

6 According to the cardiac and vascular function curves, an increase in blood volume leads to _______ right atrial pressure and _______ cardiac output.

7 If cardiac output is 5.2 L/min, heart rate is 76 beats/minute, and end-diastolic volume is 145 mL, what is the end-systolic volume?

8 In a capillary, if Pc is 35 mm Hg, πc is 25 mm Hg, Pi is 2 mm Hg, and πi is 1 mm Hg, is there net absorption or filtration, and what is the magnitude of the driving force?

9 When a person moves quickly from a lying to a standing position, which of the following decrease(s): venous return, cardiac output, arterial pressure (Pa)?

10 What is the name of the volume contained in the left ventricle immediately before it contracts?

11 Which of the following produce(s) an increase in contractility: decreased heart rate, increased phosphorylation of phospholamban, increased action potential duration?

12 During which phase of the ventricular action potential, phase 0 or phase 4, is inward current greater than outward current?

13 Which term best applies to the absolute refractory period of the ventricular action potential: automaticity, excitability, conduction velocity, maximum diastolic potential?

14 If, simultaneously, there is an increased rate of phase 4 depolarization and hyperpolarization of the threshold potential, will there be an increase, decrease, or no change in heart rate?

15 Among the responses that occur following hemorrhage, which of the following increase(s): unstressed volume, heart rate, resistance of cutaneous vascular beds, firing rate of carotid sinus nerves, angiotensin II levels?

16 In the myogenic mechanism of autoregulation, according to the law of Laplace, does an increase in pressure lead to an increase, decrease, or no change in the radius of the blood vessel?

17 Of the following, which circulation receives the highest percentage of the cardiac output: renal, pulmonary, coronary, skeletal muscle during intense exercise, skin during intense exercise?

18 Which of the following cause(s) an increase in stroke volume from the left ventricle: increased contractility, decrease in end-diastolic volume, increase in aortic pressure?

19 During which portion(s) of the cardiac cycle is the aortic valve open: atrial systole, rapid ventricular ejection, diastasis?

20 According to the cardiac and vascular function curves, an increase in TPR leads to _______ venous return and _______ cardiac output.

21 Which situation is associated with the higher efficiency of myocardial oxygen consumption: increased cardiac output secondary to increased heart rate or decreased cardiac output secondary to increased aortic pressure?

22 Three resistors, each with a value of 10, are arranged in parallel. How much does total resistance change if a fourth resistor with a value of 10 is added in parallel?

23 Blood vessel “A” has a cross-sectional area of 1 cm2, and blood vessel “B” has a cross-sectional area of 10 cm2. If blood flow through the two vessels is the same, in which vessel is velocity of blood flow higher?

24 Where am I? For each item in the following list, give its correct location in the cardiovascular system. The location may be anatomic, a graph or portion of a graph, an equation, or a concept.

Dicrotic notch

β1 receptors

Lmax

Radius to the fourth power

Phospholamban

Negative dromotropic effect

Pulse pressure

Normal automaticity

Ejection fraction

25 During which portions(s) of the cardiac cycle is the mitral valve closed? Atrial systole, rapid ventricular ejection, isovolumetric ventricular relaxation, diastasis.

26 During exercise, which of the following decrease(s)? Heart rate, venous return, stroke volume, diameter of splanchnic arterioles, TPR.

27 According to the ventricular pressure-volume loop, an increase in afterload produces an increase in which of the following? End-diastolic volume, end-diastolic pressure, end-systolic volume, stroke volume.

28 Which of the following is/are mediated by an increase in ICa? Sympathetic effect to increase heart rate, parasympathetic effect to decrease heart rate, sympathetic effect to increase contractility, parasympathetic effect to decrease conduction velocity in AV node.


SELECTED READINGS

Berne RM, Levy MN: Cardiovascular Physiology, 8th ed. St Louis, Mosby, 2001.

Guyton AC, Hall JE: Textbook of Medical Physiology, 9th ed. Philadelphia, WB Saunders, 1996.

Smith JJ, Kampine JP: Circulatory Physiology, 3rd ed. Baltimore, Williams & Wilkins, 1990.