Chapter 16 - Circulation

Section 1 - The Body's Transport System

  The cardiovascular system-which consists of the heart, blood vessels, and blood-carries needed substances to cells and carries waste products away from cells. In addition, blood contains cells that fight disease. The heart is a hollow, muscular organ that pumps blood throughout the body. Each time the heart beats, it pushes blood through the blood vessels of the cardiovascular system. The heart has a right side and a left side. The right side of the heart is completely separated from the left side by a wall of tissue called the septum. Each side has two chambers. Each upper chamber, or atrium, receives blood that comes into the heart. Located in the right atrium is the pacemaker, a group of cells that send out signals that make the heart muscle contract. Each lower chamber, or ventricle, pumps blood out of the heart. The atrium and ventricle are separated by a valve, a flap of tissue that prevents blood from flowing backward.

  Blood travels through three kinds of blood vessels. Arteries carry blood away from the heart and into the capillaries. Capillaries are tiny vessels where substances are exchanged between the blood and body cells. Veins carry blood back to the heart.

  The overall pattern of the body's blood flow consists of two loops. In the first loop, blood travels from the heart to the lungs and then back to the heart. In the second loop, blood is pumped from the heart through the body and then returns again to the heart. When blood leaves the heart, it travels through arteries. The right ventricle pumps blood into arteries that go to the lungs. The left ventricle pumps blood into the aorta, the largest artery in the body. Every organ receives blood from arteries that branch off the aorta. The coronary arteries carry blood to the heart itself. The walls of arteries are generally very thick. In fact, artery walls consist of three cell layers. The pulse you feel on the inside of your wrist is caused by the alternating expansion and relaxation of the artery wall.

  Blood eventually flows from arteries into capillaries. In the capillaries, materials are exchanged between the blood and the body's cells. Capillary walls are only one cell thick. One way in which materials are exchanged is by diffusion. In diffusion, molecules move from an area where they are highly concentrated to an area where they are less concentrated.

  After blood moves through capillaries, it enters larger blood vessels called veins, which carry blood back to the heart. The walls of veins, like those of arteries, have three layers with muscle in the middle layer.

  Blood exerts a force, called blood pressure, against the walls of blood vessels. It is caused by the force with which the ventricles contract.

Section 2 - Blood and Lymph

  Blood is made up of four components: plasma, red blood cells, white blood cells, and platelets.

  Plasma is the liquid part of blood. Plasma is mostly water, but 10 percent is made of dissolved materials. Plasma carries nutrients, such as glucose, fats, vitamins, and minerals.

  Red blood cells carry oxygen from the lungs to the body cells. A red blood cell is made mostly of hemoglobin. Hemoglobin is an iron-containing protein that binds chemically to oxygen molecules. Red blood cells are produced in the bone marrow and have no nuclei.

  White blood cells are the body's disease fighters. They are bigger than red blood cells and have nuclei. Some white blood cells alert the body when disease-causing organisms invade. Others produce chemicals to fight the invaders. Some surround and kill the disease-causing organisms.

  Platelets are cell fragments that help form blood clots. They collect and stick to any site where a blood vessel is cut. Platelets then release chemicals that cause the production of the chemical fibrin. Fibrin weaves a net of fibers across the wound. The net traps blood cells and a clot is formed.

  There are four major types of blood. Each type has different marker molecules. Blood type A has the A marker and blood type B has the B marker. Blood type AB has both the A and B markers. Blood type O has no markers. Your plasma contains clumping agents that makes cells with foreign markers clump together. The marker molecules on your red blood cells determine your blood type and the type of blood that you can safely receive in transfusions. People with type A blood can receive transfusions of blood that does not have a B marker: type A or O blood. People with type B blood can receive transfusions of blood that does not have an A marker: type B or O. People with type AB blood have no clumping proteins. They can receive all blood types. People with type O blood have both anti-A and anti-B clumping proteins. They can only receive type O blood.

  Red blood cells also contain another marker, called the Rh factor. A person's blood type can either be Rh negative or rh positive.

  In the capillaries, some fluid moves out of the cardiovascular system and into the surrounding tissues. The fluid moves into the body's drainage system, called the lymphatic system.The lymphatic systemis a network of veinlike vessels that returns the fluid to the bloodstream. When the fluid enters the lymphatic system, it is called lymph. Lymph nodes are small knobs of tissue that filter the lymph as it passes through.

Section 3 - Cardiovascular Health

  Cardiovascular health is important to all people. Cardiovascular disease is the leading cause of death in the United States. Diseases of the cardiovascular system include atherosclerosis and hypertension.

  Atherosclerosis is a condition in which an artery wall thickens due to the buildup of fatty materials. One of these materials is a waxy, fatlike substance called cholesterol. Atherosclerosis restricts the flow of blood in the affected arteries.

  If atherosclerosis develops in the coronary arteries that supply the heart, the heart muscle receives less blood. This condition may lead to a heart attack. A heart attack occurs when the blood flow to part of the heart muscle is blocked. Cells die in the part of the heart that does not receive blood. This permanently damages the heart.

  Treatment for mild atherosclerosis usually includes a low-fat diet and a moderate exercise program. Medication that lowers the levels of cholesterol and fats in the blood may be prescribed. People with severe atherosclerosis may need surgery or other procedures to unclog blocked arteries.

  Hypertension is a disorder in which a person's blood pressure is consistently higher than normal. Hypertension makes the heart work harder. It also may damage the walls of the blood vessels over time. Hypertension is sometimes called the "silent killer" because people with hypertension often have no obvious symptoms to warn them.

  Hypertension is closely related to atherosclerosis. As the arteries narrow, blood pressure increases. Other factors that increase the risk of hypertension are being overweight and failing to get enough exercise.

  For mild hypertension, treatment usually includes regular exercise and careful food choices. People with hypertension need to limit their intake of sodium, which can increase their blood pressure. Sodium is found in salt and in processed foods such as soups and packaged snack foods. For some people, medication is needed to reduce their blood pressure.

  To help maintain cardiovascular health, people can exercise regularly; eat a balanced diet that is low in saturated fats and trans fats, cholesterol, and sodium; and avoid smoking. Exercise strengthens your heart muscle and also helps prevent atherosclerosis. Avoid eating foods that are high in saturated fat, trans fat, or cholesterol. These foods can cause fatty deposits on artery walls. Smokers are more than twice as likely to have a heart attack than are nonsmokers. If smokers quit, their risk of death from cardiovascular disease decreases.

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