The cardiovascular system - function and disease
The cardiovascular system circulates BLOOD through the body to deliver nutrients and collect wastes from cells. Physician specialists who treat conditions of the HEART and blood vessels are cardiologists. This section, “The Cardiovascular System,” presents an overview of the structures and functions of the cardiovascular system, a discussion of cardiovascular health and disorders, and entries about the health conditions that can affect the cardiovascular system.
Functions of the Cardiovascular System
The cardiovascular system circulates blood through the body to supply cells with nutrients, notably oxygen and GLUCOSE, and to remove the waste byproducts of METABOLISM. The centerpiece of this system is the heart, a muscular organ about the size and shape of a closed fist that beats 70 to 90 times a minute in a healthy adult at rest. The body’s circulation is a closed, pressurized system that contains a constant blood volume of about 10 liters (2.6 gallons). An extensive network of blood vessels–which, if stretched end to end, would traverse 100,000 miles—transports that blood through the body.
Cardiovascular function defines life and death. The cardiovascular system is among the first of the body systems to become functional. The rudimentary heart begins beating at three weeks gestational age, and by eight weeks the heart’s formation is complete. Doppler ULTRASOUND, a noninvasive procedure that uses sound waves to detect motion, can detect the fetal heartbeat at 10 to 12 weeks into PREGNANCY.
PULSE and BLOOD PRESSURE—the rate and force of blood as it flows through the arteries—are among the basic vital signs health-care providers assess to determine core health status and indeed life or death. When the heart stops beating, pulse and blood pressure cease. Cells in the BRAIN and the MYOCARDIUM, deprived of oxygen, begin to die. Though myocardial cells are capable, to an extent, of regenerating, brain cells are not. Only two to three minutes without oxygen can result in permanent neurologic damage and even death.
The heart
The heart lies behind the protective enclosure of the rib cage, its left side beneath the STERNUM (breastbone) and its right side extending about to the center of the right BREAST. A tough sac, the PERICARDIUM, encases the heart. A thin layer of fluid between the pericardium and the myocardium (heart MUSCLE) allows the heart to move continuously without friction in much the same way motor oil permits pistons to glide freely within an engine. A crownlike network of arteries, the CORONARY ARTERIES, encircles the outer surface of the myocardium. This network directs roughly 20 percent of the body’s blood supply and 70 percent of the blood’s oxygen content to the heart with each heartbeat. A thin membrane, the ENDOCARDIUM, lines the chambers of the heart. The endocardium is so smooth not even platelets, the blood’s clotting cells, can stick to it.
The heart’s four chambers collect and expel bood through rhythmic, synchronized contractions. The two upper chambers, the atria, receive blood into the heart. The two lower chambers, the ventricles, pump blood from the heart. One-way valves regulate the flow and volume of blood into and out of each chamber. The four chambers of the heart contract and relax in precise coordination. The walls of the heart become progressively thicker from the atria to the ventricles, reaching their greatest density and STRENGTH in the left ventricle to support its work to contract with enough force to pump blood to the most distant cells in the body. A thick inner wall, the septum, separates the heart’s chambers.
A cluster of specialized NERVE cells, the SINOATRIAL (SA) NODE, generates a “pacing” electrical impulse that starts with the right atrium and builds momentum as it courses through the cells and fibers of the myocardium. Other electrical structures—the BUNDLE OF HIS, right BUNDLE BRANCH and left bundle branch, ATRIOVENTRICULAR (AV) NODE, and the Purkinje fibers—amplify and focus the electrical impulses so they reach maximum intensity when they arrive at the left ventricle. This collective effort forms the CARDIAC CYCLE, the sequential and coordinated contraction and relaxation of the atria and the ventricles.
The right atrium and right ventricle, known collectively as the right heart, handle deoxygenated blood returning from the body. Blood flows from the superior VENA CAVA (bringing blood from the upper body) and inferior vena cava (bringing blood from the lower body) into the right atrium, which pumps it through the tricuspid valve into the right ventricle. The right ventricle pumps the blood through the pulmonary valve into the pulmonary ARTERY, which carries it to the LUNGS for OXYGENATION.
The left atrium and left ventricle, known collectively as the left heart, handle oxygenated blood. The PULMONARY VEINS (right and left, from the right lung and left lung respectively) deliver oxygenated blood from the lungs to the left atrium. The left atrium pumps the blood through the mitral valve into the left ventricle. The left ventricle propels blood through the aortic valve into the AORTA, the body’s largest blood vessel, and on its circulatory journey.
