Route of Blood Flow Through the Heart
Figure 22-3 illustrates the pathway for normal blood flow through the heart’s chambers and valves. The right atrium receives venous (deoxygenated) blood from the body via the superior and inferior vena cavae and the coronary sinus. Blood then passes through the tricuspid valve into the right ventricle. It moves on through the pulmonic valve (pulmonary semilunar valve) during ventricular contraction to enter the pulmonary artery and then the lungs, where it exchanges carbon dioxide for oxygen.
Oxygenated blood returns to the left atrium via four pulmonary veins. It travels through the mitral (bicuspid) valve into the left ventricle. During ventricular contraction, the blood from this chamber exits through the aortic semilunar valve into the aorta and out to the systemic circulation (Fig. 22-4).
Key Concept The pulmonary arteries are the only arteries in the body that carry deoxygenated blood. The pulmonary veins are the only veins that carry oxygenated blood.
The oxygen saturation of the hemoglobin within the arteries is normally about 95%-I00%. Within the veins, it is about 75%.
Blood Vessels of the Heart
Coronary Arteries
Heart muscle requires its own blood supply because heart tissue does not absorb blood from its chambers. Therefore, two coronary arteries (right and left) branch off the ascending aorta to provide blood to heart muscle (Fig. 22-5). Their openings (orifices) lie behind two cusps of the aortic valve. They receive their blood supply during ventricular relaxation, when the valves are closed. The right and left coronary arteries extend over the heart’s surface and divide into smaller branches that penetrate the myocardium to supply heart tissue with oxygen and nourishment. They are called the coronary arteries because they fit over the heart like a crown (corona). Their patterns and branches vary among individuals more than any other part of the cardiovascular anatomy.
FIGURE 22-3 · Blood flow through the heart.
FIGURE 22-4 · The circle of blood flow in the body.
Left Coronary Artery. The left coronary artery (LCA), also known as the left main coronary artery (LMCA), passes along the left atrium and divides into two branches: the anterior interventricular branch or left anterior descending (LAD) artery and the left circumflex (LCX) artery.
The LAD artery descends along the anterior intraventric-ular groove to provide blood to most of the ventricular septum and anterior portion of the left ventricle. The LAD artery and its branches also supply blood to the anterior papillary muscles, the apex of the left ventricle, and the right and left bundle branches.
The LCX artery extends around the left side of the heart, along the groove between the left atrium and left ventricle, to supply blood to the left atrium and the lateral and posterior portions of the left ventricle. It supplies blood to the sinoatrial (SA) node in approximately 40% of the population and to the AV node in approximately 10% of the population.
Right Coronary Artery. The right coronary artery (RCA) branches out along the right AV groove to supply blood to the right atrium and right ventricle. It also provides blood to the SA node in approximately 60% of the population and to the AV node in 90% of the population.
The main branch of the RCA that supplies the heart’s right side is called the marginal branch. In most people, a second branch travels down the posterior intraventricular septum to supply blood to the posterior septum, the inferior and posterior portions of the left ventricle, and the posterior left papillary muscle. This branch is called the posterior descending artery (PDA). In some people the PDA comes off the LCX artery to supply blood to these areas. This arrangement is known as “left coronary dominance.”
Collateral Circulation. An important factor in heart physiology is that the large coronary arteries join in very few places. Consequently, if one of these arteries becomes plugged, the blood has no way to detour. A blockage of these arteries causes either myocardial insufficiency ischemia (reversible cell injury due to decreased blood and oxygen supply), or myocardial infarction (MI), which is a localized area of dead tissue caused by a lack of blood supply (commonly known as a “heart attack”). Over time, collateral circulation may help to supply blood to “at risk”tissue. (Collateral circulation occurs when two vessels that nourish the same area interconnect.) Very small interconnections are normally found among microscopic branches of coronary arteries throughout the heart. When coronary obstruction occurs gradually (as with atherosclerosis), these vessels can enlarge to nourish endangered heart muscle.
FIGURE 22-5 · Coronary arteries and cardiac veins. Left, Anterior view. Right, Posterior view. It is important to note that coronary circulation patterns vary among people.
Coronary Veins
The coronary arteries drain into capillaries in the myocardium, where delivery of oxygen and nutrients occurs, along with waste removal. Blood then leaves the capillaries, and most of it enters the venous system via two main veins. These two principal vessels are the great cardiac vein, which drains blood from the anterior surface of the heart, and the middle cardiac vein, which drains the heart’s posterior surface. These vessels transport blood into an opening called the coronary sinus, which returns blood to the right atrium (see Fig. 22-4).
SYSTEMIC BLOOD VESSELS
Blood is carried through the body in a set of blood vessels: arteries, capillaries, and veins. The arteries carry blood away from the heart, the capillaries serve as “in-between” channels, and the veins carry blood toward the heart.
Key Concept Arteries carry blood away from the heart. Veins carry blood toward the heart. Except in the pulmonary circuit, arteries carry oxygenated blood and veins carry deoxygenated blood.
Special Considerations :LIFESPAN
Physiologic Differences and Coronary Artery Disease
Anatomic and physiologic differences can make coronary heart disease potentially a more dangerous threat for women than men. Women’s hearts are on the average 10% smaller than men’s hearts, with corresponding smaller coronary arteries. Smaller vessel size leads to decreased perfusion (circulation), especially in the presence of atherosclerosis. Clots can form more easily in women’s coronary arteries owing to higher fibrinogen levels and greater fibrinolytic activity with advancing age. Increased clot formation, coupled with decreased coronary artery size, can increase the risk of vessel occlusion. Female hormones also appear to influence the development of coronary artery disease.
Arteries and Arterioles
Arteries are elastic and smooth (involuntary) muscular tubes that, with the exception of the pulmonary artery, carry oxygenated blood to body cells. They are known as “resistance vessels” because they can support high pressures and hold large volumes of blood. Table 22-1 lists major arteries, which are also illustrated in Figure 22-6. The largest artery is the aorta (about 1 inch—2.54 cm in diameter). The aorta is about 3,000 times larger than the capillaries. It is divided into the ascending aorta, aortic arch, thoracic aorta, and abdominal aorta. From the aorta, arteries branch into smaller vessels, like branches from the central trunk of a tree. The smallest arteries are called arterioles. Arterioles contain less elastic tissue and more smooth muscle than arteries. Constriction and dilation of arterioles regulate blood pressure and flow. By changing vessel diameter, the volume of blood supplied to tissues increases or decreases.
TABLE 22-1. Major Arteries
|
NAME |
DISTRIBUTION |
|
Branches of the Ascending Aorta |
|
|
Left and right coronary arteries |
Heart muscle |
|
Branches of the Aortic Arch |
|
|
Brachiocephalic (innominate) branches into Right subclavian Right common carotid |
Right upper extremity Right side of head and neck |
|
Left common carotid |
Left side of head and neck |
|
Left subclavian |
Left upper extremity |
|
Each subclavian artery extends into Axillary |
Axilla |
|
Brachial |
Arm proper |
|
Radial |
Thumb side of forearm and wrist |
|
Ulnar |
Medial side of hand |
|
Branches of the Thoracic Aorta |
|
|
Bronchial |
Lungs |
|
Esophageal |
Esophagus |
|
Intercostals |
Muscles and other structures of chest wall |
|
Superior phrenic |
Posterior and superior surfaces of diaphragm |
|
Branches of Abdominal Aorta |
|
|
Celiac trunk branches into |
|
|
Left gastric |
Stomach |
|
Splenic |
Spleen |
|
Hepatic |
Liver |
|
Superior mesenteric |
Small intestine, first half of large intestine |
|
Inferior mesenteric |
Second half of large intestine |
|
Phrenic |
Diaphragm |
|
Suprarenal (adrenal) |
Adrenal glands |
|
Renal |
Kidneys |
|
Ovarian (female) or testicular (male; formerly spermatic arteries) |
Sex glands |
|
Lumbar |
Musculature of the abdominal wall |
|
Common iliac branches into internal iliac |
Pelvic muscles, bladder, rectum, prostate, reproductive organs |
|
External iliac branches into |
|
|
Femoral |
Thigh |
|
Popliteal |
Knee |
|
Tibial |
Leg, ankle, heel |
|
Dorsalis pedis |
Foot |
Capillaries
From the arterioles, blood flows into the smallest vessels of all, capillaries. A capillary is about 8 μm (micrometers— 1/8 millionth of a m [meter]). Blood flow through capillaries is known as microcirculation. Capillaries are so small that tiny red blood cells must pass through them in single file.
FIGURE 22-6 · Principal systemic arteries. The arterial system carries blood away from the heart. Major pressure points (pulse points) are used to count the pulse or to stop hemorrhage.
Capillaries make up most of the great length of the body’s blood vessels. In the capillaries, overall resistance is very low, so blood flows very slowly, allowing time for oxygen and nutrients to leave blood vessels and enter body tissues.The relatively high osmotic pressure of albumin (a plasma protein) within capillaries pulls interstitial fluid into them. The fluid that is pulled back into capillaries contains cellular waste products now on their way to the kidneys for excretion. (Ecchymotic areas [black and blue marks] result from ruptured capillaries.)
Key Concept The capillaries are very important.They are known as "exchange vessels" because it is here that the exchange of nutrients and wastes occurs.
