Pulmonary Circulation

Pulmonary circulation begins with the right side of the heart, sending blood to the lungs to absorb oxygen (O₂) and to release carbon dioxide (CO₂). Note in Figure 9-2 that the vessels that carry blood to the lungs from the heart are blue—to show the blood as being deoxygenated, or oxygen deficient. Once the oxygen is absorbed, the blood is considered oxygenated, or oxygen rich. Note also in Figure 9-2 that the vessels traveling away from the lungs are red—to show oxygenation. The blood then progresses back to the left side of the heart, where it is pumped out to begin its route through the systemic circulatory system.

Systemic Circulation

Systemic circulation carries blood from the heart to the cells of the body, where nutrient and waste exchange takes place. Certain organs of the body are key to the process of waste removal. During systemic circulation, blood passes through the kidneys. This part of systemic circulation is known as renal circulation. In this phase, the kidneys filter much of the waste from the blood to be excreted in the urine. Blood also passes through the small intestine during systemic circulation. This phase is known as portal circulation. Here, the blood from the small intestine collects in the portal vein, which passes through the liver. The liver filters sugars from the blood and stores them for use as needed. On return to the right side of the heart, the blood is pushed out to the lungs to dispose of its CO₂, absorb O₂, and repeat the cycle. Figure 9-2 shows the oxygenated/deoxygenated status of blood.

In systemic circulation, the blood traveling away from the heart first passes through the largest artery in the body called the aorta. From the aorta, the vessels branch into conducting arteries, then into smaller arterioles, and finally to the capillaries. Note in Figure 9-2 that the color has changed from the red of oxygenated blood to a purple color at the capillaries. This is the site of exchange between the cells’ fluids and the plasma of the circulatory system. Oxygen and other substances are supplied, and carbon dioxide is collected, along with a number of other wastes. Once the blood begins its journey back to the heart, it first goes through venules, then veins, and finally into one of the two largest veins, the superior or inferior vena cava. The great vessels include the pulmonary arteries and veins, the superior vena cava, and the thoracic aorta. The inferior vena cava is classified as a lower vein.

All of the vessels of the cardiovascular system (including the heart) share a lining of endothelial cells. While all carry blood, each type of vessel has a slightly different, but significant structure. Figure 9-3 illustrates these differences. The muscular, thick arteries are composed of three tunics, or coats: the outer layer is called the tunica externa (adventitia), the muscle layer and elastic layer are called the tunica media, and the inner layer is called the tunica interna (intima). Compare the thickness and structure of an artery to the thinner, valvular nature of veins. Veins do not have the thick muscle coat of the arteries to propel the blood on its journey through the circulatory system but instead rely on one-way valves that prevent the backflow of blood. In addition, skeletal muscle contraction provides pumping action. The capillaries have no coats, and their diameters are so tiny that only one blood cell at a time can pass through them.

Coronary Circulation and Blood Flow Through the Heart

Using Figure 9-5 as a guide, follow the route of the blood through the heart. The picture and arrows in this diagram are shaded red and blue to represent oxygenated and deoxygenated blood. Deoxygenated blood is returned to the heart through the venae cavae. The superior vena cava returns blood from the upper body, whereas the lower body is drained by the inferior vena cava. Blood is squeezed from the right atrium (RA) to the right ventricle (RV) through the tricuspid valve (TV). Valves are considered to be competent (capable) if they open and close properly, letting through or holding back an expected amount of blood. Once in the right ventricle the blood is squeezed out through the pulmonary semilunar valve into the pulmonary arteries (PA), which carry deoxygenated blood to the lungs from the heart. These are the only arteries that carry deoxygenated blood. The main pulmonary artery (pulmonary trunk) divides into right and left arteries to supply each lung. The conus arteriosus is the cone-shaped extension of the right ventricle into the pulmonary trunk. In the capillaries of the lungs, the CO₂ is passed out of the blood and O₂ is taken in. The now-oxygenated blood continues its journey back from the lungs to the left side of the heart through the pulmonary veins (PV). These are the only veins that carry oxygenated blood. The blood then enters the heart through the left atrium (LA) and has to pass the mitral valve (MV), also termed the bicuspid valve, to enter the left ventricle (LV). When the left ventricle contracts, the blood finally pushes out through the aortic semilunar valve into the aorta (the largest artery in the body) and begins yet another cycle through the body. The first part of the aorta, the ascending aorta, rises toward the head, then bends into the aortic arch and continues downward through the chest as the descending thoracic aorta. Once it passes the diaphragm, it is termed the abdominal aorta.

Each valve has a fibrous ring at its base called the annulus. The bicuspid valve has two leaflets (cusps) that are attached to two nipple-like papillary muscles by the chordae tendineae, cordlike tendons. The papillary muscles open and close the heart valves. The tricuspid valve has three leaflets attached to three papillary muscles, connected again by chordae tendineae. When a writer refers to heartstrings being tugged at in sentimental situations, he/she is referring to the chordae tendineae.

The amount of blood expelled from the left ventricle compared with the total volume of blood filling the ventricle is referred to as the stroke volume and is a measure of the ejection fraction of cardiac output. Typically around 65%, this amount is reduced in certain types of heart disease.

If a woman’s heart rate is 80 beats per minute (BPM), that means her heart contracts almost 5000 times per hour and more than 100,000 times per day, every day, for a lifetime. Truly an amazing amount of work is accomplished by an individual’s heart without a bit of conscious thought!

The heart muscle has its own dedicated system of blood supply, the coronary arteries (Fig. 9-6). The two main coronary arteries are called the left and right coronary arteries (LCA, RCA). The right coronary artery branches to form the posterior (interventricular) descending artery, which divides to form the atrioventricular artery and finally the posterior septal artery. The other main branch of the RCA is the marginal artery that divides to form the acute marginal artery. The left coronary artery branches to form the anterior (interventricular) descending branch, which continues to the anterior septal artery. It also forms a circumflex branch that divides into anterior and posterior ventricular branches. They supply a constant, uninterrupted blood flow to the heart muscle. Table 9-1 illustrates the path of blood through the coronary arteries. The return to the circulatory system is via the coronary veins, which deposit the deoxygenated blood into the coronary sinus, a small cavity where the blood is collected before it empties into the right atrium. The shallow grooves that hold these arteries on the surface of the heart are termed the left and right coronary sulci (sing. sulcus). In the human embryo, the sinus venosus is a hollow space that holds the blood as it returns to the heart. In normal development, the right side of the space becomes part of the right atrium. On the left side it becomes the coronary sinus

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