In this chapter you learned about the structure, functions, and disorders of the cardiovascular system. Specifically, you learned that:

• The cardiovascular system is the organ system that transports materials to and from all the cells of the body. The main components of the cardiovascular system are the heart, blood vessels, and blood.
• The cardiovascular system has two interconnected circulations. The pulmonary circuit carries blood between the heart and lungs, where blood is oxygenated. The systemic circuit carries blood between the heart and the rest of the body, where it delivers oxygen.
• The heart is a muscular organ in the chest that consists mainly of cardiac muscle. It pumps blood through blood vessels by repeated, rhythmic contractions.

• • The wall of the heart consists of three layers. The middle layer, the myocardium, is the thickest layer, and consists mainly of cardiac muscle.
  • The interior of the heart consists of four chambers, with an upper atrium and lower ventricle on each side of the heart. Blood enters the heart through the atria, which pump it to the ventricles. Then, the ventricles pump blood out of the heart. Four valves in the heart keep blood flowing in the correct direction and prevent backflow.

• • Deoxygenated blood flows into the right atrium through veins from the upper and lower body (superior and inferior vena cava, respectively), and oxygenated blood flows into the left atrium through four pulmonary veins from the lungs. Each atrium pumps the blood to the ventricle below it. From the right ventricle, deoxygenated blood is pumped to the lungs through the two pulmonary arteries. From the left ventricle, oxygenated blood is pumped to the rest of the body through the aorta.
  • The coronary circulation consists of blood vessels that carry blood to and from the heart muscle cells. There are two coronary arteries that supply the two sides of the heart with oxygenated blood. Cardiac veins drain deoxygenated blood back into the heart.
  • The cardiac cycle refers to a single complete heartbeat. It includes diastole — when the atria contract — and systole, when the ventricles contract.
  • The normal, rhythmic beating of the heart is called sinus rhythm. It is established by the heart’s pacemaker cells in the sinoatrial node. Electrical signals from the pacemaker cells travel to the atria and cause them to contract. Then, the signals travel to the atrioventricular node, and from there to the ventricles via the Purkinje fibres, causing them to contract. Electrical stimulation from the autonomic nervous system and hormones from the endocrine system can also influence heartbeat.
• Blood vessels carry blood throughout the body. Major types of blood vessels are arteries, veins, and capillaries.

• • Arteries are blood vessels that usually carry blood away from the heart (except for coronary arteries that supply the heart muscle with blood). Most arteries carry oxygenated blood. The largest artery is the aorta, which is connected to the heart and extends into the abdomen. Blood moves through arteries due to pressure from the beating of the heart.
  • Veins are blood vessels that usually carry blood toward the heart. Most veins carry deoxygenated blood. The largest veins are the superior vena cava and inferior vena cava. Blood moves through veins by the squeezing action of surrounding skeletal muscles. Valves in veins prevent backflow of blood.
  • Capillaries are the smallest blood vessels. They connect arterioles and venules. They form capillary beds where substances are exchanged between the blood and surrounding tissues.
  • The walls of arteries and veins have three layers. The middle layer is thickest in arteries, in which it contains smooth muscle tissue that controls the diameter of the vessels. The outer layer is thickest in veins and consists mainly of connective tissue. The walls of capillaries consist of little more than a single layer of epithelial cells.
  • Blood pressure is a measure of the force that blood exerts on the walls of arteries. It is expressed as a double number, with the higher number representing systolic pressure when the ventricles contract, and the lower number representing diastolic pressure when the ventricles relax. Normal blood pressure is generally defined as a pressure of 120/80 mm Hg or less.
  • Vasoconstriction (narrowing) and vasodilation (widening) of arteries can occur to help regulate blood pressure or body temperature or to change blood flow as part of the fight-or-flight response.
• Blood is a fluid connective tissue that circulates throughout the body in blood vessels. Blood supplies tissues with oxygen and nutrients, and removes their metabolic wastes. Blood helps defend the body from pathogens and other threats, transports hormones and other substances, and helps to keep the body’s pH and temperature in homeostasis. Blood consists of a liquid part (called plasma) and cells, including erythrocytes, leukocytes and thrombocytes.

• • Plasma makes up more than half of blood by volume. It consists of water and many dissolved substances. It also contains blood cells.
  • Erythrocytes are the most numerous cells in blood. They consist mostly of hemoglobin, which carries oxygen. Red blood cells also carry antigens that determine blood types.
  • Leukocytes are less numerous than red blood cells and are part of the body’s immune system. They protect the body from abnormal cells, microorganisms, and other harmful substances. There are several different types of white blood cells that differ in their specific immune functions.
  • Thrombocytes are cell fragments that play important roles in blood clotting, or coagulation. They stick together at breaks in blood vessels to form a clot and stimulate the production of fibrin, which strengthens the clot.
  • All blood cells form by proliferation of stem cells in red bone marrow in a process called hematopoiesis. When blood cells die, they are phagocytized by white blood cells and removed from the circulation.
  • Disorders of the blood include leukemia, which is cancer of the bone-forming cells; hemophilia, which is any of several genetic blood-clotting disorders; carbon monoxide poisoning, which prevents erythrocytes from binding with oxygen and causes suffocation; HIV infection, which destroys certain leukocytes and can cause AIDS; and anemia, in which there are not enough erythrocytes to carry adequate oxygen to body tissues.
• Cardiovascular disease is a class of diseases that involve the cardiovascular system. Worldwide, it is the leading cause of death. Most cases occur in people over age 60, and onset typically occurs about a decade earlier in males than in females. Other risk factors include smoking, obesity, diabetes, high blood cholesterol, and lack of exercise.

• • Two common conditions that lead to most cases of cardiovascular disease are hypertension and atherosclerosis. Hypertension is blood pressure that is persistently at or above 140/90 mm Hg. Atherosclerosis is a buildup of fatty, fibrous plaques in arteries that may reduce or block blood flow. Treating these conditions is important for preventing cardiovascular disease.
  • Coronary artery disease is a group of diseases that result from atherosclerosis of coronary arteries. Two of the most common are angina and myocardial infarction (heart attack). In angina, cardiac cells receive inadequate oxygen, which causes chest pain. In a heart attack, cardiac cells die because blood flow to part of the heart is blocked. A heart attack may cause death, or lead to heart arrhythmias, heart failure, or cardiac arrest.
  • Stroke occurs when blocked or broken arteries in the brain result in the death of brain cells. This may happen when an artery is blocked by a clot or plaque, or when an artery ruptures and bleeds in the brain. In both cases, part of the brain is damaged, and functions such as speech and controlled movements may be impaired, either temporarily or permanently.
  • Peripheral artery disease occurs when atherosclerosis narrows peripheral arteries, usually in the legs, often causing pain when walking. It is important to diagnose this disease so the underlying atherosclerosis can be treated before it causes a heart attack or stroke.

In this chapter, you learned that the cardiovascular system carries nutrients to the cells of the body. Read the next chapter about the Digestive System to learn about how your body transforms your meals into the nutrients that cells need to function.