Chapter 1. Overview of the Cardiovascular System

The student understands the homeostatic role of the cardiovascular system, the basic principles of cardiovascular transport, and the basic structure and function of the components of the system:

• Defines homeostasis.
• Identifies the major body fluid compartments and states the approximate volume of each.
• Lists 3 conditions, provided by the cardiovascular system, that are essential for regulating the composition of interstitial fluid (ie, the internal environment).
• Diagrams the blood flow pathways between the heart and other major body organs.
• States the relationship among blood flow, blood pressure, and vascular resistance.
• Predicts the relative changes in flow through a tube caused by changes in tube length, tube radius, fluid viscosity, and pressure difference.
• Uses the Fick principle to describe convective transport of substances through the CV system and to calculate a tissue’s rate of utilization (or production) of a substance.
• Identifies the chambers and valves of the heart and describes the pathway of blood flow through the heart.
• Defines cardiac output and identifies its 2 determinants.
• Describes the site of initiation and pathway of action potential propagation in the heart.
• States the relationship between ventricular filling and cardiac output (Starling’s law of the heart) and describes its importance in the control of cardiac output.
• Identifies the distribution of sympathetic and parasympathetic nerves in the heart and lists the basic effects of these nerves on the heart.
• Lists the 5 factors essential to proper ventricular pumping action.
• Lists the major different types of vessels in a vascular bed and describes the morphological differences among them.
• Describes the basic and functions of the different vessel types.
• Identifies the major mechanisms in vascular resistance control and blood flow distribution.
• Describes the basic composition of the fluid and cellular portions of blood.

Three compartments of watery fluids, known collectively as the total body water, account for approximately 60% of body weight. This water is distributed among the intracellular, interstitial, and plasma compartments, as indicated in Figure 1–1. Note that about two-thirds of our body water is contained within cells and communicates with the interstitial fluid across the plasma membranes of cells. Of the fluid that is outside cells (ie, extracellular fluid), only a small amount, the plasma volume, circulates within the cardiovascular system. Total circulating blood volume is larger than that of blood plasma, as indicated in Figure 1–1, because blood also contains suspended blood cells that collectively occupy approximately 40% of its volume. However, it is the circulating plasma that directly interacts with the interstitial fluid of body organs across the walls of the capillary vessels.

As blood passes through capillaries, solutes exchange between plasma and interstitial fluid by the process of diffusion. The net result of transcapillary diffusion is always that the interstitial fluid tends to take on the composition of the incoming blood. If, for example, potassium ion concentration in the interstitium of a particular skeletal muscle was higher than that in the plasma entering the muscle, then potassium would diffuse into the blood as it passes through the muscle’s capillaries. Because this removes potassium from the interstitial fluid, its potassium ion concentration would decrease. It would stop decreasing when the net movement of potassium into capillaries no longer occurs, that is, when the concentration of the interstitial fluid reaches that of incoming plasma.

Three conditions are essential for this circulatory mechanism to effectively control the composition of the interstitial fluid: (1) there must be adequate blood flow through the tissue capillaries; (2) the chemical composition of the incoming (or arterial) blood must be controlled to be that which is optimal in the interstitial fluid; and (3) diffusion distances between plasma and tissue cells must be short. Figure 1–1 shows how the cardiovascular transport system operates to accomplish these tasks. Diffusional transport within tissues occurs over extremely small distances because no cell in the body is located farther than approximately 10 μm from a capillary. Over such microscopic distances, diffusion is a very rapid process that can move huge quantities of material. Diffusion, however, is a very poor mechanism for moving substances from the capillaries of an organ, such as the lungs, to the capillaries of another organ that may be 1 m or more distant. Consequently, substances are transported between organs by the process of convection, by which the substances easily move along with blood flow because they are either dissolved or contained within blood. The relative distances involved in cardiovascular transport are not well illustrated in Figure 1–1. If the figure were drawn to scale, with 1 in. representing the distance from capillaries to cells within a calf muscle, then the capillaries in the lungs would have to be located about 15 miles away!

The overall functional arrangement of the cardiovascular system is illustrated in Figure 1–2. Because a functional rather than an anatomical viewpoint is expressed in this figure, the role of heart appears in three places: as the right heart pump, as the left heart pump, and as the heart muscle tissue. It is common practice to view the cardiovascular system as (1) the pulmonary circulation, composed of the right heart pump and the lungs, and (2) the systemic circulation, in which the left heart pump supplies blood to the systemic organs (all structures except the gas exchange portion of the lungs). The pulmonary and systemic circulations are arranged in series, that is, one after the other. Consequently, both the right and left hearts must pump an identical volume of blood per minute. This amount is called the cardiac output.

As indicated in Figure 1–2, most systemic organs are functionally arranged in parallel (ie, side by side) within the cardiovascular system. There are two important consequences of this parallel arrangement. First, nearly all systemic organs receive blood of identical composition—that which has just left the lungs and is known as arterial blood. Second, the flow through any one of the systemic organs can be controlled independently of the flow through the other organs. Thus, for example, the cardiovascular response to whole-body exercise can involve increased blood flow through some organs, decreased blood flow through others, and unchanged blood flow through yet others.

Many of the organs in the body help perform the task of continually reconditioning the blood circulating in the cardiovascular system. Key roles are played by organs, such as the lungs, that communicate with the external environment. As is evident from the arrangement shown in Figure 1–2, any blood that has just passed through a systemic organ returns to the right heart and is pumped through the lungs, where oxygen and carbon dioxide are exchanged. Thus, the blood’s gas composition is always reconditioned immediately after leaving a systemic organ.

Like the lungs, many of the systemic organs also serve to recondition the composition of blood, although the flow circuitry precludes their doing so each time the blood completes a single circuit. The kidneys, for example, continually adjust the electrolyte composition of the blood passing through them. Because the blood conditioned by the kidneys mixes freely with all the circulating blood and because electrolytes and water freely pass through most capillary walls, the kidneys control the electrolyte balance of the entire internal environment. To achieve this, it is necessary that a given unit of blood pass often through the kidneys. In fact, the kidneys normally receive about one-fifth of the cardiac output under resting conditions. This greatly exceeds the amount of flow that is necessary to supply the nutrient needs of the renal tissue. This situation is common to organs that have a blood-conditioning function.

Blood-conditioning organs can also withstand, at least temporarily, severe reduction of blood flow. Skin, for example, can easily tolerate a large reduction in blood flow when it is necessary to conserve body heat. Most of the large abdominal organs also fall into this category. The reason is simply that because of their blood-conditioning functions, their normal blood flow is far in excess of that necessary to maintain their basal metabolic needs.

The brain, heart muscle, and skeletal muscles typify organs in which blood flows solely to supply the metabolic needs of the tissue. They do not recondition the blood for the benefit of any other organ. Normally, the blood flow to the brain and the heart muscle is only slightly greater than that required for their metabolism; hence, they do not tolerate blood flow interruptions well. Unconsciousness can occur within a few seconds after stoppage of cerebral flow, and permanent brain damage can occur in as little as 4 min without flow. Similarly, the heart muscle (myocardium) normally consumes approximately 75% of the oxygen supplied to it, and the heart’s pumping ability begins to deteriorate within beats of a coronary flow interruption. As we shall see later, the task of providing adequate blood flow to the brain and the heart muscle receives a high priority in the overall operation of the cardiovascular system.

As outlined above, the task of maintaining interstitial homeostasis requires that an adequate quantity of blood flow continuously through each of the billions of capillaries in the body. In a resting individual, this adds up to a cardiac output of approximately 5 to 6 L/min (approximately 80 gallons/h!). As people go about their daily lives, the metabolic rates and therefore the blood flow requirements in different organs and regions throughout the body change from moment to moment. Thus, the cardiovascular system must continuously adjust both the magnitude of cardiac output and how that cardiac output is distributed to different parts of the body. One of the most important keys to comprehending how the cardiovascular system operates is to have a thorough understanding of the relationship among the physical factors that determine the rate of fluid flow through a tube.

or

where Q̇ = flow rate (volume/time),

ΔP = pressure difference (mm Hg1), and

R = resistance to flow (mm Hg × time/volume).

The basic flow equation may be applied not only to a single tube but also to complex networks of tubes, for example, the vascular bed of an organ or the entire systemic system. The flow through the brain, for example, is determined by the difference in pressure between cerebral arteries and veins divided by the overall resistance to flow through the vessels in the cerebral vascular bed. It should be evident from the basic flow equation that there are only two ways in which blood flow through any organ can be changed: (1) by changing the pressure difference across its vascular bed or (2) by changing its vascular resistance. Most often, it is changes in an organ’s vascular resistance that cause the flow through the organ to change.

From the work of the French physician Jean Leonard Marie Poiseuille (1799–1869), who performed experiments on fluid flow through small glass capillary tubes, it is known that the resistance to flow through a cylindrical tube depends on several factors, including the radius and length of the tube and the viscosity of the fluid flowing through it. These factors influence resistance to flow as follows:

where r = inside radius of the tube,

L = tube length, and

η = fluid viscosity.

Note especially that the internal radius of the tube is raised to the fourth power in this equation. Thus, even small changes in the internal radius of a tube have a huge influence on its resistance to flow. For example, halving the inside radius of a tube will increase its resistance to flow by 16-fold.

The preceding two equations may be combined into one expression known as the Poiseuille equation, which includes all the terms that influence flow through a cylindrical vessel:

Again, note that flow occurs only when a pressure difference exists. (If ΔP = 0, then flow = 0.) It is not surprising then that arterial blood pressure is an extremely important and carefully regulated cardiovascular variable. Also note once again that for any given pressure difference, tube radius has a very large influence on the flow through a tube. It is logical, therefore, that organ blood flows are regulated primarily through changes in the radii of vessels within organs. Although vessel length and blood viscosity are factors that influence vascular resistance, they are not variables that can be easily manipulated for the purpose of moment-to-moment control of blood flow.

In regard to the overall cardiovascular system, as depicted in Figures 1–1 and 1–2, one can conclude that blood flows through the vessels within an organ only because a pressure difference exists between the blood in the arteries supplying the organ and the veins draining it. The primary job of the heart pump is to keep the pressure within arteries higher than that within veins. Normally, the average pressure in systemic arteries is approximately 100 mm Hg, and the average pressure in systemic veins is approximately 0 mm Hg.

Therefore, because the pressure difference (ΔP) is nearly identical across all systemic organs, cardiac output is distributed among the various systemic organs, primarily on the basis of their individual resistances to flow. Because blood preferentially flows along paths of least resistance, organs with relatively low resistance naturally receive relatively high flow.

1 Although pressure is most correctly expressed in units of force per unit area, it is customary to express pressures within the cardiovascular system in millimeters of mercury. For example, mean arterial pressure may be said to be 100 mm Hg because it is same as the pressure existing at the bottom of a mercury column 100 mm high. All cardiovascular pressures are expressed relative to atmospheric pressure, which is approximately 760 mm Hg.

Transport rate = Flow rate × Concentration

where Ẋ = rate of transport of X (mass/time),

Q̇ = blood flow rate (volume/time), and

[X] = concentration of X in blood (mass/volume).

It is evident from the preceding equation that only two methods are available for altering the rate at which a substance is carried to an organ: (1) a change in the blood flow rate through the organ or (2) a change in the arterial blood concentration of the substance. The preceding equation might be used, for example, to calculate how much oxygen is carried to a certain skeletal muscle each minute. Note, however, that this calculation would not indicate whether the muscle actually used the oxygen carried to it.

The Fick Principle

where Ẋtc = transcapillary efflux rate of X,

Q̇ = blood flow rate, and

[X]a,v = arterial and venous concentrations of X.

The Fick principle is useful because it offers a practical method to deduce a tissue’s steady-state rate of consumption (or production) of any substance. To understand why this is so, one further step in logic is necessary. Consider, for example, what possibly can happen to a substance that enters a tissue from the blood. It can either (1) increase the concentration of itself within the tissue, or (2) be metabolized (ie, converted into something else) within the tissue. A steady state implies a stable situation wherein nothing (including the substance’s tissue concentration) is changing with time. Therefore, in the steady state, the rate of the substance’s loss from blood within a tissue must equal its rate of metabolism within that tissue.

2 This notation implies that the gain or loss of a substance from blood as it passes through an organ happens because of substance movement across capillary walls. Although this is a reasonable assumption, it is not a necessary one. The basic Fick principle is valid regardless of where or how substances enter or leave the blood as it passes through an organ.

Pumping Action

The heart lies in the center of the thoracic cavity and is suspended by its attachments to the great vessels within a thin fibrous sac called the pericardium. A small amount of fluid in the sac lubricates the surface of the heart and allows it to move freely during contraction and relaxation. Blood flow through all organs is passive and occurs only because arterial pressure is kept higher than venous pressure by the pumping action of the heart. The right heart pump provides the energy necessary to move blood through the pulmonary vessels, and the left heart pump provides the energy to move blood through the systemic organs.

The pathway of blood flow through the chambers of the heart is indicated in Figure 1–4. Venous blood returns from the systemic organs to the right atrium via the superior and inferior venae cavae. This “venous” blood is deficient in oxygen because it has just passed through systemic organs that all extract oxygen from blood for their metabolism. It then passes through the tricuspid valve into the right ventricle and from there it is pumped through the pulmonic valve into the pulmonary circulation via the pulmonary arteries. Within the capillaries of the lung, blood is “reoxygenated” by exposure to oxygen-rich inspired air. Oxygenated pulmonary venous blood flows in pulmonary veins to the left atrium and passes through the mitral valve into the left ventricle. From there it is pumped through the aortic valve into the aorta to be distributed to the systemic organs.

When the ventricular muscle cells are contracting, they generate a circumferential tension in the ventricular walls that causes the pressure within the chamber to increase. As soon as the ventricular pressure exceeds the pressure in the pulmonary artery (right pump) or aorta (left pump), blood is forced out of the chamber through the outlet valve, as shown in Figure 1–5. This phase of the cardiac cycle during which the ventricular muscle cells are contracting is called systole. Because the pressure is higher in the ventricle than in the atrium during systole, the inlet or atrioventricular (AV) valve is closed. When the ventricular muscle cells relax, the pressure in the ventricle falls below that in the atrium, the AV valve opens, and the ventricle refills with blood, as shown on the right side in Figure 1–5. This portion of the cardiac cycle is called diastole. The outlet valve is closed during diastole because arterial pressure is greater than intraventricular pressure. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.

It should be evident from this relationship that all influences on cardiac output must act through changes in either the heart rate or the stroke volume.

An important implication of the above is that the volume of blood that the ventricle pumps with each heartbeat (ie, the stroke volume, SV) must equal the blood volume inside the ventricle at the end of diastole (end-diastolic volume, EDV) minus ventricular volume at the end of systole (end-systolic volume, ESV). That is:

Thus, stroke volume can only be changed by changes in EDV and/or ESV. The implication for the bigger picture is that cardiac output can only be changed by changes in HR, EDV, and/or ESV.

Cardiac Excitation

Efficient pumping action of the heart requires a precise coordination of the contraction of millions of individual cardiac muscle cells. Contraction of each cell is triggered when an electrical excitatory impulse (action potential) sweeps over its membrane. Proper coordination of the contractile activity of the individual cardiac muscle cells is achieved primarily by the conduction of action potentials from one cell to the next via gap junctions that connect all cells of the heart into a functional syncytium (ie, acting as one synchronous unit). In addition, muscle cells in certain areas of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction, and the rate of the impulse propagation through various regions of the heart. The major components of this specialized excitation and conduction system are shown in Figure 1–6. These include the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and the right and left bundle branches made up of specialized cells called Purkinje fibers.

The SA node contains specialized cells that normally function as the heart’s pacemaker and initiate the action potential that is conducted through the heart. The AV node contains slowly conducting cells that normally function to create a slight delay between atrial contraction and ventricular contraction. The Purkinje fibers are specialized for rapid conduction and ensure that all ventricular cells contract at nearly the same instant. The overall message is that HR is normally controlled by the electrical activity of the SA nodal cells. The rest of the conduction system ensures that all the rest of the cells in the heart follow along in proper lockstep for efficient pumping action.

Control of Cardiac Output

Autonomic Neural Influences

Cholinergic parasympathetic nerve fibers travel to the heart via the vagus nerve and innervate the SA node, the AV node, and the atrial muscle. When active, these parasympathetic nerves release acetylcholine on cardiac muscle cells. Acetylcholine interacts with muscarinic receptors on cardiac muscle cells to decrease the heart rate (SA node) and decrease the action potential conduction velocity (AV node). Parasympathetic nerves may also act to decrease the force of contraction of atrial (not ventricular) muscle cells. Overall, parasympathetic activation acts to decrease cardiac pumping. Usually, an increase in parasympathetic nerve activity is accompanied by a decrease in sympathetic nerve activity, and vice versa.

Diastolic Filling: Starling’s Law of the Heart

Requirements for Effective Operation

For effective efficient ventricular pumping action, the heart must be functioning properly in five basic respects:

1. The contractions of individual cardiac muscle cells must occur at regular intervals and be synchronized (not arrhythmic).

2. The valves must open fully (not stenotic).

3. The valves must not leak (not insufficient or regurgitant).

4. The muscle contractions must be forceful (not failing).

5. The ventricles must fill adequately during diastole.

In the subsequent chapters, we will study in detail how these requirements are met in the normal heart.

Blood that is ejected into the aorta by the left heart passes consecutively through many different types of vessels before it returns to the right heart. As illustrated in Figure 1–8, the major vessel classifications are arteries, arterioles, capillaries, venules, and veins. These consecutive vascular segments are distinguished from one another by differences in their physical dimensions, morphological characteristics, and function. One thing that all these vessels have in common is that they are lined with a contiguous single layer of endothelial cells. In fact, this is true for the entire circulatory system including the heart chambers and even the valve leaflets.

Vessel Characteristics

Some representative physical characteristics of these major vessel types are shown in Figure 1–8. It should be realized, however, that the vascular bed is a continuum and that the transition from one type of vascular segment to another does not occur abruptly. The total cross-sectional area through which blood flows at any particular level in the vascular system is equal to the sum of the cross-sectional areas of all the individual vessels arranged in parallel at that level. The number and total cross-sectional area values presented in Figure 1–8 are estimates for the entire systemic circulation.

Arteries are thick-walled vessels that contain, in addition to some smooth muscle, a large component of elastin and collagen fibers. Primarily because of the elastin fibers, which can stretch to twice their unloaded length, arteries can expand to accept and temporarily store some of the blood ejected by the heart during systole and then, by passive recoil, supply this blood to the organs downstream during diastole. The aorta is the largest artery and has an internal (luminal) diameter of approximately 25 mm. Arterial diameter decreases with each consecutive branching, and the smallest arteries have diameters of approximately 0.1 mm. The consecutive arterial branching pattern causes an exponential increase in arterial numbers. Thus, although individual vessels get progressively smaller, the total cross-sectional area available for blood flow within the arterial system increases to several fold that in the aorta. Arteries are often referred to as conduit vessels because they have relatively low and unchanging resistance to flow.

Capillaries are the smallest vessels in the vasculature. In fact, red blood cells with diameters of 7 μm must deform to pass through them. The capillary wall consists of a single layer of endothelial cells that separate the blood from the interstitial fluid by only approximately 1 μm. Capillaries contain no smooth muscle and thus lack the ability to change their diameters actively. They are so numerous that the total collective cross-sectional area of all the capillaries in systemic organs is more than 1000 times that of the root of the aorta. Given that capillaries are approximately 0.5 mm in length, the total surface area available for exchange of material between blood and interstitial fluid can be calculated; it exceeds 100 m2. For obvious reasons, capillaries are viewed as the exchange vessels of the cardiovascular system. In addition to the transcapillary diffusion of solutes that occurs across these vessel walls, there can sometimes be net movements of fluid (volume) into and/or out of capillaries. For example, tissue swelling (edema) is a result of net fluid movement from plasma into the interstitial space.

After leaving capillaries, blood is collected in venules and veins and returned to the heart. Venous vessels have very thin walls in proportion to their diameters. Their walls contain smooth muscle, and their diameters can actively change. Because of their thin walls, venous vessels are quite distensible. Therefore, their diameters change passively in response to small changes in transmural distending pressure (ie, the difference between the internal and external pressures across the vessel wall). Venous vessels, especially the larger ones, also have one-way valves that prevent reverse flow. As will be discussed later, these valves are especially important in the cardiovascular system’s operation during standing and during exercise. It turns out that peripheral venules and veins normally contain more than 50% of the total blood volume. Consequently, they are commonly thought of as the capacitance vessels. More importantly, changes in venous volume greatly influence cardiac filling and therefore cardiac pumping. Thus, peripheral veins actually play an extremely important role in controlling cardiac output.

Control of Blood Vessels

Arteriolar smooth muscle is also very responsive to changes in the local chemical conditions within an organ that accompany changes in the metabolic rate of the organ. For reasons to be discussed later, increased tissue metabolic rate leads to arteriolar dilation and increased tissue blood flow.

Venules and veins are also richly innervated by sympathetic nerves and constrict when these nerves are activated. The mechanism is the same as that involved with arterioles. Thus, increased sympathetic nerve activity is accompanied by decreased venous volume. The importance of this phenomenon is that venous constriction tends to increase cardiac filling and therefore cardiac output via Starling’s law of the heart.

There is no important neural or local metabolic control of either arterial or capillary vessels.

Blood Cells

Blood contains 3 general types of “formed elements”: red cells, white cells, and platelets (see Appendix A). All are formed in bone marrow from a common stem cell. Red cells are by far the most abundant. They are specialized to carry oxygen from the lungs to other tissues by binding oxygen to hemoglobin, an iron-containing heme protein contained within red blood cells. Because of the presence of hemoglobin, blood can transport 40 to 50 times the amount of oxygen that plasma alone could carry. In addition, the hydrogen ion buffering capacity of hemoglobin is vitally important to the blood’s capacity to transport carbon dioxide.

A small, but important, fraction of the cells in blood is white cells or leukocytes. Leukocytes are involved in immune processes. Appendix A gives more information on the types and function of leukocytes. Platelets are small cell fragments that are important in the blood-clotting process.

Plasma

Plasma is the liquid component of blood and, as indicated in Appendix B, is a complex solution of electrolytes and proteins. Serum is the fluid obtained from a blood sample after it has been allowed to clot. For all practical purposes, the composition of serum is identical to that of plasma except that it contains none of the clotting proteins.

Inorganic electrolytes (inorganic ions such as sodium, potassium, chloride, and bicarbonate) are the most concentrated solutes in plasma. Of these, sodium and chloride are by far the most abundant and, therefore, are primarily responsible for plasma’s normal osmolarity of approximately 300 mOsm/L. To a first approximation, the “stock” of the plasma soup is a 150-mM solution of sodium chloride. Such a solution is called “isotonic saline” and has many clinical uses as a fluid that is compatible with cells.

Plasma normally contains many different proteins. Most plasma proteins can be classified as albumins, globulins, or fibrinogen on the basis of different physical and chemical characteristics used to separate them. More than 100 distinct plasma proteins have been identified and each presumably serves some specific function. Many plasma proteins are involved in blood clotting or immune/defense reactions. Many others are important carrier proteins for a variety of substances including fatty acids, iron, copper, vitamin D, and certain hormones.

Proteins do not readily cross capillary walls and, in general, their plasma concentrations are much higher than their concentrations in the interstitial fluid. As will be discussed, plasma proteins play an important osmotic role in transcapillary fluid movement and consequently in the distribution of extracellular volume between the plasma and interstitial compartments. Albumin plays an especially strong role in this regard simply because it is by far the most abundant of the plasma proteins.

Plasma also serves as the vehicle for transporting nutrients and waste products. Thus, a plasma sample contains many small organic molecules such as glucose, amino acids, urea, creatinine, and uric acid whose measured values are useful in clinical diagnosis.

This chapter has presented an overall description of the design of the cardiovascular system. Some important, basic, bottom-line principles that should help you understand many aspects of cardiovascular function have been included. (See the study questions at the end of this chapter, for example.)

Subsequent chapters will expand these concepts in much greater detail, but we urge students not to lose sight of the overall picture presented in this chapter. It may be useful to repeatedly refer back to this material. At this juncture we would also like to draw the reader’s attention to Appendix C, which is a shorthand compilation of many of the key cardiovascular relationships that we have and will encounter in due course.

• The primary role of the cardiovascular system is to maintain homeostasis in the interstitial fluid.
• The physical law that governs cardiovascular operation is that flow through any segment is equal to pressure difference across that segment divided by its resistance to flow, ie, Q̇ = ΔP/R.
• The rate of transport of a substance within the blood (X) is a function of its concentration in the blood [X] and the blood flow rate, ie,Ẋ = Q̇ [X].
• The heart pumps blood by rhythmically filling and ejecting blood from the ventricular chambers that are served by passive one-way inlet and outlet valves.
• Cardiac output (CO) is a function of the heart rate (HR) and stroke volume (SV), ie, CO = HR × SV.
• Changes in heart rate and stroke volume (and therefore cardiac output) can be accomplished by alterations in ventricular filling and by alterations in autonomic nerve activity to the heart.
• Blood flow through individual organs is regulated by changes in the diameter of their arterioles.
• Changes in arteriolar diameter can be accomplished by alterations in sympathetic nerve activity and by variations in local conditions.
• Blood is a complex suspension of red cells, white cells, and platelets in plasma that is ideally suited to carry gases, salts, nutrients, and waste molecules throughout the system.

1–1. Which organ in the body always receives the most blood flow?

1–2. Whenever skeletal muscle blood flow increases, blood flow to other organs must decrease. True or false?

1–3. When a heart valve does not close properly, a sound called a “murmur” can often be detected as the valve leaks. Would you expect a leaky aortic valve to cause a systolic or diastolic murmur?

1–4. Slowing of action potential conduction through the AV node will slow the heart rate. True or false?

1–5. Suppose the diameters of the vessels within an organ increase by 10%. Other factors equal, how would this affect the:

• a. resistance to blood flow through the organ?
• b. blood flow through the organ?

1–6. The pressure in the aorta is normally about 100 mm Hg, whereas that in the pulmonary artery is normally about 15 mm Hg. A few of your fellow students offer the following alterative hypotheses about why this might be so:

• a. The right heart pumps less blood than the left heart.
• b. The right heart rate is slower than the left heart rate.
• c. The right ventricle is less muscular than the left ventricle.
• d. The pulmonary vascular bed has less resistance than the systemic bed.
• e. The stroke volume of the right heart is less than that of the left heart.
• f. It must be genetics.

Which of their suggestions is (are) correct?

1–7. Usually, an individual who has lost a significant amount of blood is weak and does not reason very clearly. Why would blood loss have these effects?

1–8. What direct cardiovascular consequences would you expect from an intravenous injection of norepinephrine?

1–9. What direct cardiovascular effects would you expect from an intravenous injection of a drug that stimulates α-adrenergic receptors but not β-adrenergic receptors?

1–10. Individuals with high arterial blood pressure (hypertension) are often treated with drugs that block β-adrenergic receptors. What is a rationale for such treatment?

1–11. The clinical laboratory reports a serum sodium ion value of 140 mEq/L in a blood sample you have taken from a patient. What does this tell you about the sodium ion concentration in plasma, in interstitial fluid, and in intracellular fluid?

1–12. An individual has had the “flu” for 3 days with severe vomiting and diarrhea. How is this likely to influence his or her hematocrit?

1–13. Explain how it is that the water flow into your kitchen sink changes when you turn the handle on its faucet.

1–14. A common “side effect” of β-blocker therapy is decreased exercise tolerance. Why is this not surprising?

1–15.You need to determine the correct dose of an IV drug that distributes only within the extracellular space. Which of the following values would be the closest estimate of the extracellular fluid volume of a healthy young adult male weighing 100 kg (220 lb)?

• a. 3 L
• b. 5 L
• c. 8 L
• d. 10 L
• e. 20 L

• Arterial blood glucose concentration, [G]a = 50 mg/100 mL
• Muscle venous blood glucose concentration, [G]v = 30 mg/100 mL
• Muscle blood flow Q̇ = 60 mL/min

1–17. The Fick principle implies that doubling the flow through an organ will necessarily double the organ’s rate of metabolism (or production) of a substance. True or False?

1–18. Five requirements for normal cardiac pumping action were listed in this chapter. Recall that CO = HR × (EDV − ESV). Use this as a basis for explaining in detail why a lack of each of the requirements would adversely affect CO.

See Answers.