The functions of the circulatory system are related to transportation. Nearly every substance made or used in the body is transported in the blood – hormones, products of digestion, metabolic wastes and immune system cells. The blood also carry heat. Stimulated by the hormones of thermoregulation, blood flow can disperse heat to the environment at the body surface or conserve heat for essential functions in the core.
The circulatory system, or cardiovascular system, consists of the heart and the blood vessels. The heart is basically a pump system, that squeezes blood out of the heart and the pressure generated forces the blood through the blood vessels.
Heart Position and Structure
The heart is shaped like a cone is only about the size of a fist. It lies between the lungs just behind the sternum and the tip of the cone points to the left. In most persons, the heart is situated somewhat to the left of the center of the chest.
In a typical day, the heart beats 100,000 times. And in a lifetime, it beats more than 2.5 billion times. Even as we rest, the heart is working twice as hard as our leg muscles would if we were running at full speed.
There a four hollow spaces, the chambers, in the heart. The heart is divided anatomically and functionally into left and right sides, with each side having one atrium and one ventricle, each with separate functions. The contraction of the heart muscle pumps blood, rhythmically into and out of the chambers.
Between the chambers there are valves that allow measured quantities of blood to flow in the right direction.
The blood vessels form a network of channels through which the blood flows. The vessels are active organs that assist the heart in circulating the blood and influence the blood’s constitution. The vessels that move blood away from the heart are arteries, and the vessels bringing blood to the heart are called veins. The arteries decrease in diameter as they spread throughout the body, eventually ending in the capillaries, the tiny vessels that connect the arterial and venous systems. The capillaries rely on diffusion to do their work; moving oxygen and nutrients from the blood to the cells and waste materials from the cells to the blood.
Blood consists of many different types of cells in a matrix called plasma. Plasma is around 90 percent water. The remaining part is made up of plasma proteins, salt ions, oxygen and carbon dioxide gases, glucose, fats, amino acids, waste and other substances such as hormones and enzymes.
The red blood cells bind oxygen that is released in the capillaries. At the same time as oxygen diffuses into a cell, carbon oxide diffuses out making its way to the venous system.
The beating heart pushes blood around a double-circuit:
- Out through the arteries,
- into the capillary beds,
- across the capillary beds and into the veins,
- back through the veins,
- passes through the heart to the lungs,
- back to the heart again,
- and out through the arteries.
A complete circuit takes around one minute.
Blood pressure is a term used to describe the force of blood pushing against the wall of an artery. The pressure is measured in millimeters of mercury (mm Hg) at both the highest point, systole when the heart is contracted, and the lowest point, diastole when the heart is relaxed. The higher systolic and diastolic value, the more pressure on the walls.
The circulatory system endures constant mechanical stress, physical blockages, large and small, and other physical and chemical assaults.
Myocardial Infarction – Heart Attack
Myocardial infarction is the irreversible damage of myocardial tissue (on the heart) caused by a blockage of blood flow to a section of heart muscle. The blockage results in oxygen not reaching the blocked section, whereby parts of the heart die. Depending on the size of the infarct and its location, the severity of the follow-on effects on cardiac rhythm and integrity of the heart muscle itself can range from relatively mild to fatal.
A stroke is damage to the brain caused by reduced blood flow (ischemia) or bleeding (hemorragia). Whether ischemic or hemorragic, some brain tissue dies or is irreversibly damaged. Disability following a stroke may be mild or damage may be severely disabling, both physically and mentally. Stroke is a common cause of death among elderly.
Aging and the Circulatory System
Age is the major risk factor for cardiovascular disease. Heart disease and stroke incidence rises steeply after age 65, accounting for more than 40 percent of all deaths among people age 65 to 74 and almost 60 percent at age 85 and above. People age 65 and older are much more likely than younger people to suffer a heart attack, to have a stroke, or to develop coronary heart disease and high blood pressure leading to heart failure.
The heart has a natural pacemaker system that controls the heartbeat. Some of the pathways of this system may develop fibrous tissue and fat deposits. The natural pacemaker loses some of its cells. These changes may result in a slightly slower heart rate.
A slight increase in the size of the heart, especially the left ventricle, is not uncommon. The heart wall thickens, so the amount of blood that the chamber can hold may decrease despite the increased overall heart size. The heart may fill more slowly.
The capillary walls thicken somewhat. This may cause a slightly slower rate of exchange of nutrients and wastes.
The main artery from the heart becomes thicker, stiffer and less flexible. This is probably related to changes in the connective tissue of the blood vessel wall. As a result, the blood pressure increases and the heart has to work harder, which may lead to thickening of the heart muscle (hypertrophy). The other arteries also thicken and stiffen. In general, most elderly people have a moderate increase in blood pressure.
The speed with which red blood cells are produced in response to stress or illness is reduced. This creates a slower response to blood loss and anemia.
Most of the white blood cells stay at the same levels, although certain white blood cells important to immunity (neutrophils) decrease in their number and ability to fight off bacteria. This reduces the ability to resist infection.
Schwartz JB, Zipes DP. Cardiovascular disease in the elderly. In: Bonow RO, Mann DL, Zipes DP, Libby P, eds. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 9th ed.