Overall Design Cardiovascular

OF THE SYSTEM

The rapid flow of blood throughout the body is produced by pressures created by the pumping action of the heart. This type of flow is known as bulk flow because all constituents of the blood move in one direction together. The extraordinary degree of branching of blood vessels ensures that almost all cells in the body are within a few cell diameters of at least one of the smallest branches, the capillaries. Nutrients and metabolic end products move between capillary blood and the interstitial fluid by diffusion. Movements between the interstitial fluid and cell interior are accomplished by both diffusion and mediated transport.

At any given moment, approximately 5 percent of the total circulating blood is actually in the capillaries. Yet it is this 5 percent that is performing the ultimate functions of the entire cardiovascular system: the

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

supplying of nutrients and the removal of metabolic end products. All other components of the system subserve the overall aim of getting adequate blood flow through the capillaries. This point should be kept in mind as we describe these components.

As discovered by the British physiologist William Harvey in 1628, the cardiovascular system forms a circle, so that blood pumped out of the heart through one set of vessels returns to the heart via a different set. There are actually two circuits (Figure 14-7), both originating and terminating in the heart, which is divided longitudinally into two functional halves. Each half contains two chambers: an upper chamber—the atrium—and a lower chamber—the ventricle. The atrium on each side empties into the ventricle on that side, but there is no direct communication between the two atria or the two ventricles in the adult heart.

Blood is pumped via one circuit, the pulmonary circulation, from the right ventricle through the lungs and then to the left atrium. It is then pumped through the systemic circulation from the left ventricle through all the organs and tissues of the body except the lungs and then to the right atrium. In both circuits, the vessels carrying blood away from the heart are called arteries, and those carrying blood from either the lungs or all other parts of the body (peripheral organs and tissues) back toward the heart are called veins.

In the systemic circuit, blood leaves the left ventricle via a single large artery, the aorta (Figure 14-8). The arteries of the systemic circulation branch off the aorta, dividing into progressively smaller vessels. The smallest arteries branch into arterioles, which branch into a huge number (estimated at 10 billion) of very small vessels, the capillaries, which unite to form larger diameter vessels, the venules. The arterioles, capillaries, and venules are collectively termed the microcirculation.

The venules in the systemic circulation then unite to form larger vessels, the veins. The veins from the various peripheral organs and tissues unite to produce two large veins, the inferior vena cava, which collects blood from the lower portion of the body, and the superior vena cava, which collects blood from the upper half of the body. It is via these two veins that blood is returned to the right atrium.

The pulmonary circulation is composed of a similar circuit. Blood leaves the right ventricle via a single large artery, the pulmonary trunk, which divides into the two pulmonary arteries, one supplying the right lung and the other the left. In the lungs, the arteries continue to branch, ultimately forming capillaries that unite into venules and then veins. The blood leaves the lungs via four pulmonary veins, which empty into the left atrium.

Pulmonary arteries

/^Pulmonary veins

/^Pulmonary veins

Systemic arteries

Pulmonary arteries

Systemic veins

Veins The Lungs

IV Right -

ventricle

Systemic circulation

Systemic veins

Systemic arteries

IV Right -

ventricle

Systemic circulation

All organs and tissue other than lungs

FIGURE 14-7

The systemic and pulmonary circulations. As depicted by the color change from blue to red, blood becomes fully oxygenated (red) as it flows through the lungs and then loses some oxygen (red to blue) as it flows through the other organs and tissues. For simplicity, the arteries and veins leaving and entering the heart are depicted as single vessels; in reality, this is true for the arteries but not for the veins (see Figure 14-8). % EO!

All organs and tissue other than lungs

FIGURE 14-7

The systemic and pulmonary circulations. As depicted by the color change from blue to red, blood becomes fully oxygenated (red) as it flows through the lungs and then loses some oxygen (red to blue) as it flows through the other organs and tissues. For simplicity, the arteries and veins leaving and entering the heart are depicted as single vessels; in reality, this is true for the arteries but not for the veins (see Figure 14-8). % EO!

As blood flows through the lung capillaries, it picks up oxygen supplied to the adjacent lung air sacs by breathing. Therefore, the blood in the pulmonary veins, left heart, and systemic arteries has a high oxygen content. As this blood flows through the capillaries of peripheral tissues and organs, some of this oxygen leaves the blood to enter and be used by cells, resulting in the lower oxygen content of systemic venous blood.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Superior Arch of aorta vena cava

Right -pulmonary artery

Superior Arch of aorta vena cava

Right -pulmonary artery

Right atrium

Right ventricle

Images Circle Pulmonary Trunk

Right pulmonary artery

Pulmonary trunk

Left pulmonary veins

Left atrium

Left ventricle

Fat deposits

Inferior vena cava

FIGURE 14-8

Blood leaves each of the ventricles via a single artery, the pulmonary trunk from the right ventricle, and the aorta from the left ventricle. [Because the aorta and pulmonary trunk cross each other before emerging from the heart (see Figure 14-12), one gets the mistaken notion that both arise from the right ventricle.] Blood enters the right atrium via two large veins, the superior vena cava and inferior vena cava; it enters the left atrium via four pulmonary veins. %

Right pulmonary veins

Right atrium

Right ventricle

Right pulmonary artery

Pulmonary trunk

Left pulmonary veins

Left atrium

Left ventricle

Fat deposits

Inferior vena cava

FIGURE 14-8

Blood leaves each of the ventricles via a single artery, the pulmonary trunk from the right ventricle, and the aorta from the left ventricle. [Because the aorta and pulmonary trunk cross each other before emerging from the heart (see Figure 14-12), one gets the mistaken notion that both arise from the right ventricle.] Blood enters the right atrium via two large veins, the superior vena cava and inferior vena cava; it enters the left atrium via four pulmonary veins. %

As shown in Figure 14-7, blood can pass from the systemic veins to the systemic arteries only by first being pumped through the lungs. Thus the blood returning from the body's peripheral organs and tissues via the systemic veins is oxygenated before it is pumped back to them.

It must be emphasized that the lungs receive all the blood pumped by the right heart, whereas each of the peripheral organs and tissues receives only a fraction of the blood pumped by the left ventricle. For reference, the typical distribution of the blood pumped by the left ventricle in an adult at rest is given in Figure 14-9.

Finally, there are several exceptions, notably the liver, kidneys, and pituitary, to the usual anatomical pattern described in this section for the systemic circulation, and these will be presented in the relevant chapters dealing with those organs.

Organ

Rest ml/min

Brain

650 (13%)

Heart

215 (4%)

Skeletal muscle

1030 (20%)

Skin

430 (9%)

Kidney

950 (20%)

Abdominal organs

1200 (24%)

Other

525 (10%)

Total 5000 (100%)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Cornelia
    When blood leaves the lungs through the pulmonary veins, it will empty into the?
    8 years ago
  • lori boyce
    Which veins empty to the left atrium?
    7 years ago
  • perry
    What color is the blood in pulmonary trunk?
    7 years ago
  • arja
    What are the 4 pulmonary veins that enter the left atrium?
    7 years ago
  • POLO
    What is the flow of blood through both the pulmonary and systemic circulation?
    7 years ago
  • aldo
    Where are the pulmonary arteries and veins?
    7 years ago

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