The Cardiovascular, or Circulatory System, is made up of the heart and the blood vessels, including the arteries, veins, and capillaries. The blood carries food and oxygen to the cells and removes wastes from them. It is also where the body fights infections.

The Heart

            The human heart is about the size of a fist. A wall of muscle divides the right side from the left. Each of these two sides is divided into the atrium, or top part, and the ventricle, or bottom part. The human heart beats an average of 72 times a minute. We use our fingers to measure this pulse.

            Blood pressure is measure with a sphygmomanometer. The blood pressure is usually 100 to 120 over 60 to 80 in a resting, healthy person. The higher number represents the minimum pressure needed to stop the flow of blood. This is called the systolic blood pressure. The lower number represents the maximum pressure when full blood flow is restored. This is called the diastolic blood pressure.

Circulation

            Pulmonary circulation carries blood from the heart to the lungs. Gases, mostly carbon dioxide, are taken out of the blood, and oxygen is taken on by hemoglobin in the red blood cells. The blood then returns to the heart and is pumped to other parts of the body in the systemic circulation. Bright red, oxygen-rich blood from the pulmonary circulation flows from the left atrium into the left ventricle. With each beat of the heart, blood is forced through the large artery of the heart, called the aorta, and throughout the body.

            Many smaller arteries branch off the major ones, and each branch leads to smaller and smaller arteries. The blood finally passes from the smallest arteries into the capillaries. In the capillaries the blood gives its oxygen, food, and fluid to the tissues, and the tissues give carbon dioxide and fluid wastes to the blood.

            Several organs work with the circulatory system. Nutrients and fluids are absorbed from the digestive organs. The spleen then filters the blood. It removes old blood cells and microorganisms. White blood cells surround them and destroy them. The liver adds chemicals that are necessary for clotting to the blood. The liver also filters the blood. For instance, it changes the waste ammonia into less toxic urea. The kidneys pull water, minerals, urea, and other waste products from the blood. These materials are later excreted in the urine.

            The blood that travels into the veins from the capillaries is oxygen-poor and dark red. The veins, like the arteries, branch into larger and larger vessels, and eventually return to the heart. Here the blood enters the right atrium and then the right ventricle, where it is again pumped to the lungs, and the trip starts all over again.

Blood

            Red blood cells are not the only kind of blood cells. White blood cells fight against disease. The white blood cells can help our immune system fight infection. Platelets are small blood cells that help our blood to clot. All the blood cells are carried by the watery part of the blood called plasma.

            Sometimes an injured or sick person doesn’t have enough blood or needs more of one kind of blood cells. A blood donor can give blood to that person. This is called a transfusion. Before anyone can get a transfusion, that person's red cells must be typed, or grouped, to see if they are immune to the donor's blood.

            Tests on the blood can tell how well the body is working. Analysis of blood can tell us what the body takes in and what substances the body makes. Abnormal substances in the blood are clues that something is wrong in the body.

Diseases

            Because the circulatory system is necessary for life, diseases that affect any part are serious. The force of the blood being pumped through the arteries puts pressure on the artery walls. When this pressure is too high it is an unhealthy condition called hypertension, or high blood pressure. If left untreated, it can lead to heart attack, stroke, kidney disease, or other illnesses.

            Arteriosclerosis is another group of diseases of the arteries. One of the most common types of arteriosclerosis is a condition in which fatty deposits of cholesterol form in the arteries. These deposits can break away and travel to the heart or brain, causing heart attacks or strokes. Diets that are high in certain fats called saturated fats increase the chance of having these deposits.