Physiology of the Cardiovascular System
A Primary vehicle of transport of substances in the body.
1. Respiratory gases (e.g., oxygen and carbon dioxide)
2. Circulating antibodies and leukocytes involved in the body’s defense mechanisms
3. Platelets and clotting factors involved in hemostasis
4. Cellular nutrients to all of the cells
5. Cellular waste products away from the cells
6. Electrolytes, proteins, water, and hormones, all of which contribute to the numerous complex functions of blood.
II Anatomic Classification of the Vascular Bed (Figure 10-1)
A The typical vascular bed begins with the aorta or pulmonary artery.
B Branches from either of these main arteries are called large arteries.
C The larger arteries continue to branch to medium arteries.
D The medium arteries branch further to the arterioles.
E The end of the arteriolar bed is marked by a thick band of smooth muscle called the precapillary sphincter, which marks the initial portion of the microcirculation.
F The arterioles branch to metarterioles or directly to capillaries.
G Distal to the precapillary sphincter are the capillaries.
H Many capillaries join to form venules.
I Numerous venules join to form small veins, which in turn join to form large veins.
J Large veins join the major veins of the body, either the superior vena cava, inferior vena cava, or pulmonary veins.
III Functional Divisions of the Vascular Bed
A Distribution, resistance, exchange, and capacitance vessels
1. Distribution vessels begin with the major arteries and include the large and medium arteries.
a. These vessels distribute the cardiac output (CO) to the various organ systems.
b. These vessels typically are under an elevated pressure, contain a relatively small percentage of total blood volume, and are highly elastic.
2. Resistance vessels begin with the arterioles and end with the precapillary sphincter.
a. These vessels have the largest proportion of smooth muscle constituting the vascular wall of any of the blood vessels.
b. Through contraction and relaxation of the smooth muscle, the resistance vessels can regulate the distribution of blood to the various capillary beds.
c. The resistance vessels are the major source of peripheral resistance and predominantly function in arterial blood pressure regulation.
3. The exchange vessels are the capillaries.
a. Fluid, gas, nutrient, and waste exchange occurs in these vessels.
b. Exchange of these substances occurs between capillary blood and interstitial fluid. Exchange then occurs from the interstitial fluid to the cells that make up the tissue.
c. The major process underlying exchange in the capillaries is diffusion.
d. Because of the vast distribution of capillary beds, the process of diffusion is sufficiently fast enough to maintain cellular metabolism.
4. Capacitance vessels include the venules through the large veins and encompass the total venous system.
a. Capacitance vessels serve as channels for blood return to the heart from the various capillary beds.
b. These vessels are called capacitance, or reservoir, vessels because they contain most (70% to 75%) of the total blood volume.
c. The capacitance vessels are typically under low pressure, contain a large blood volume, and are relatively inelastic compared with their arterial counterparts.
5. Distribution (volume) of blood in the components of the vascular system varies widely depending on the function of the component (Table 10-1).
TABLE 10-1
Estimated Distribution of Blood in Vascular System of the Hypothetical Adult Man
| Volume | ||
| Region | ml | % |
| Heart (diastole) | 360 | 7.2 |
| Pulmonary | ||
| Arteries | 130 | 2.6 |
| Capillaries | 110 | 2.2 |
| Veins | 200 | 4.0 |
| Subtotal | 440 | 8.8 |
| Systemic | ||
| Aorta and large arteries | 300 | 6.0 |
| Small arteries | 400 | 8.0 |
| Capillaries | 300 | 6.0 |
| Small veins | 2300 | 46.0 |
| Large veins | 900 | 18.0 |
| Subtotal | 4200 | 84.0 |
| Grand total | 5000 | 100 |
Age, 40 years; weight, 75 kg; surface area, 1.85 m2.
From Mountcastle VB: Medical Physiology, ed 14. St. Louis, Mosby, 1980.
IV Vascular System: Systemic and Pulmonary Circulations (Figure 10-2)
1. Systemic circulation begins with the systemic pump, the left ventricle, and continues to a typical vascular bed, ending with the right atrium.
a. Functions of systemic circulation:
(1) To distribute left ventricular CO so that each region of the body receives an adequate volume of blood per unit time.
(2) To perfuse individual tissues so that cellular metabolism is maintained.
(3) To return venous blood to the right side of the heart to maintain right ventricular output.
b. The velocity of the blood flow varies inversely with the total cross-sectional area through which blood flows at a given time (Figure 10-3). This physical law, coupled with the architecture of the vascular system, nicely accomplishes the three functions of the systemic circulation.
(1) The velocity of the blood flowing through the aorta is fast.
(2) The velocity of blood flow progressively slows from the arteries to arterioles.
(3) As the extraordinarily large cross-sectional area of the systemic capillaries is encountered, the velocity of blood flow attains it slowest rate.
(4) The velocity of the blood flow progressively increases from venules to veins as the cross-sectional area markedly decreases.
(5) As a result blood is quickly distributed (arteries), spends the greatest amount of time in the parenchymal tissue of the circulation (capillaries) serving the metabolic needs of the tissues, and then is quickly returned (veins) for recirculation.
2. Control of systemic circulation is governed by four major mechanisms: autonomic control, hormonal control, local control, and mechanical factors.
a. The arterial portion of the systemic circulation is basically governed by three mechanisms: autonomic nervous system, hormonal control, and local control.
(1) Arteries and arterioles are innervated extensively and virtually exclusively by postganglionic fibers of the sympathetic nervous system.
(2) The arterial vasculature of different tissues varies in the degree of sympathetic innervation.
(a) The largest degree of sympathetic innervation is to the arterial vasculature perfusing the skin.
(b) The degree of sympathetic innervation steadily decreases through the arterial vasculature perfusing spleen, mesenteric vessels, and kidneys.
(c) A smaller degree of sympathetic innervation exists in the muscles.
(d) The least degree of sympathetic innervation exists in the vessels perfusing the heart and brain. Furthermore, these vessels have a small degree of parasympathetic innervation.
(3) Sympathetic stimulation of blood vessels results in smooth muscle contraction and vasoconstriction.
(a) This principally affects the resistance vessels because of their large component of smooth muscle.
(b) Tonic sympathetic stimulation of arterial blood vessels results in a given arteriolar caliber.
(i) Increased sympathetic stimulation above this tonic level results in vasoconstriction and an increase in resistance to flow through these vessels.
(ii) Decreased sympathetic stimulation below this tonic level results in vasodilation and a decrease in resistance to flow through these vessels.
(iii) Because of differing degrees of sympathetic innervation in the different tissues, general sympathetic stimulation results in varying degrees of vasoconstriction and varying vascular resistance from tissue to tissue and hence a corresponding varying amount of blood flow from tissue to tissue (Table 10-2).
TABLE 10-2
Estimated Distribution of Cardiac Output and Oxygen Consumption in Normal Human Subject* at Rest Under Usual Indoor Conditions
| Blood Flow | Oxygen Uptake | ||||
| Circulation | ml/min | % Total | Arteriovenous Oxygen Difference (vol%) | ml/min | % Total |
| Splanchnic | 1400 | 24 | 4.1 | 58 | 25 |
| Renal | 1100 | 19 | 1.3 | 16 | 7 |
| Cerebral | 750 | 13 | 6.3 | 46 | 20 |
| Coronary | 250 | 4 | 11.4 | 27 | 11 |
| Skeletal muscle | 1200 | 21 | 8.0 | 70 | 30 |
| Skin | 500 | 9 | 1.0 | 5 | 2 |
| Other organs | 600 | 10 | 3.0 | 12 | 5 |
| Total | 5800 | 100 | 4.0* | 234 | 100 |
From Mountcastle VB: Medical Physiology, ed 14. St. Louis, Mosby, 1980.
(4) Parasympathetic stimulation of the arterial vasculature of the brain and heart results in smooth muscle relaxation and vasodilation. This phenomenon results in a decrease in resistance to blood flow.
(5) The adrenomedullary hormones norepinephrine and epinephrine stimulate the α (alpha) receptors and produce vasoconstriction.
(6) Acidosis, hypoxemia, hypercarbia, and increased temperature produce local relaxation of smooth muscle in resistance vessels and resultant vasodilation.
b. The capillary bed of the systemic circulation is governed almost exclusively by local factors.
(1) In tissues where capillary blood flow is limited by arteriolar constriction, there is local accumulation of acid and carbon dioxide and a deficiency of oxygen.
(2) These local factors result in relaxation of the smooth muscle and local arteriolar dilation, which reestablishes blood flow.
(3) Blood flow removes the local accumulation of waste products and replenishes oxygen and nutrient supply, resulting in arteriolar constriction, which in turn limits blood flow.
(4) Thus the cycle repeats itself, providing blood flow to tissues intermittently to maintain cellular metabolism.
c. The veins of the systemic circulation are governed by the autonomic nervous system, hormonal factors, and mechanical factors.
(1) The veins are exclusively innervated by postganglionic fibers of the sympathetic nervous system.
(2) The veins have a less extensive innervation than do their arterial counterparts. However, unlike that of the arteries, sympathetic innervation of the venous vasculature does not vary from one tissue to the next.
(a) Thus sympathetic stimulation causes venoconstriction of all veins of the body.
(b) Generalized venoconstriction decreases the venous vascular space, resulting in increased venous return to the heart.
(c) Conversely, decreased sympathetic stimulation results in a decrease in venous tone and venodilation.
(d) Generalized venodilation increases the venous vascular space and decreases venous return to the heart.
(3) Adrenomedullary hormones epinephrine and norepinephrine mimic sympathetic stimulation and produce venoconstriction.
(4) Mechanical factors that affect the veins of the systemic venous system are the thoracoabdominal pump, skeletal muscle pump, and semilunar valves.
(a) The thoracoabdominal pump affects the veins by aiding venous return. This is accomplished by exposing the intrathoracic veins to the fluctuating subatmospheric pressure produced by spontaneous ventilation. Coupled with the fact that extrathoracic veins are surrounded by atmospheric or supraatmospheric pressure, venous return is enhanced.
(b) The veins in the limbs contain semilunar valves that prevent retrograde flow of blood. When skeletal muscle contracts, it compresses the veins, increasing venous pressure. Because these veins have valves, compression of vessels can squeeze blood in only one direction. This mechanism also is responsible for enhancing venous return.
3. Specific regional systemic circulations
a. Coronary circulation is most influenced by local and mechanical factors.
(1) Local metabolites are major determinants of perfusion.
(2) Seventy-five percent of coronary perfusion takes place during ventricular diastole when the myocardium is in a state of mechanical relaxation. Hence diastolic arterial pressure is the value most often monitored to assess the perfusion pressure at the openings of the coronary arteries. Diastolic pressures <60 mm Hg directly reduce coronary blood flow.
b. Cerebral circulation is most influenced by local factors.
c. Gastrointestinal/splanchnic/pancreatic/hepatic circulations
(1) Sympathetic influences are dominant.
(2) Venous systems of these circulations contain significant blood volumes and respond by increasing and decreasing venous return directly as a result of the amount of venomotor tone exerted by sympathetic input.
d. Renal and epidermal circulation are principally under significant sympathetic influence.
B Pulmonary circulation (see Figure 10-2)
1. Pulmonary circulation begins with the pulmonary pump (the right ventricle) and continues to a typical vascular bed, ending with the left atrium.
a. Functions of pulmonary circulation:
(1) To distribute right ventricular output to pulmonary capillaries, matching the alveolar ventilation with an adequate volume of blood per unit time (external respiration).
(2) To perfuse the cells of the lung parenchyma with nutrients and rid them of waste products.
(3) To return blood to the left side of the heart to maintain left ventricular output.
b. The velocity of the blood flow varies inversely with the total cross-sectional area through which blood flows at a given time. This physical law, coupled with the architecture of the vascular system, nicely accomplishes the three functions of the pulmonary circulation (see Figure 10-3).
(1) The velocity of the blood flowing through the pulmonary artery is fast.
(2) The velocity of blood flow progressively slows from the arteries to arterioles.
(3) As the extraordinarily large cross-sectional area of the pulmonary capillaries is encountered, the velocity of blood flow attains it slowest rate.
(4) The velocity of the blood flow progressively increases from venules to veins as the cross-sectional area markedly decreases.
(5) As a result blood is quickly distributed (arteries), spends the greatest amount of time in the parenchymal tissue of the pulmonary circulation (capillaries) performing external respiration, and then is quickly returned (veins) to the heart for recirculation.
2. Control of pulmonary circulation is governed by the same four mechanisms that affect systemic circulation.
a. The pulmonary vasculature generally has less smooth muscle and thinner walls than its counterpart in the systemic circulation.
b. This makes pulmonary circulation susceptible to mechanical factors (e.g., intrathoracic and alveolar pressures) and the effects of gravity (secondary to alterations in bodily position) on the distribution of blood flow.
c. Pulmonary vasculature responds to sympathetic stimulation just as does the systemic circulation but to a much lesser extent.
d. Three local factors that have profound effects on pulmonary resistance vessels are decreased alveolar Po2, hypoxemia, and acidemia. All three cause pulmonary vasoconstriction, with increased resistance to blood flow.
e. Adrenomedullary hormones produce pulmonary vasoconstriction but to a milder degree than in systemic circulation.
f. Thus most of the control of pulmonary circulation depends on passive response to mechanical factors and on local factors. This is in contrast to the dominance that the sympathetic nervous system displays in controlling systemic circulation.
3. Systemic vascular resistance is normally 6 to 10 times pulmonary vascular resistance.
V Basic Functions of the Heart
