Circulatory System

Living things require a circulatory system to deliver food, oxygen, and other needed substances to all cells, and to take away waste products. Materials are transferred between individual cells and their internal environment through the cell membrane by diffusion, osmosis, and active transport. During diffusion and osmosis, molecules move from a higher concentration to a lower concentration. During active transport, carrier molecules push or pull substances across the cell membrane, using adenosine triphosphate (ATP) for energy. Unicellular organisms depend on passive and active transport to exchange materials with their watery environment. More complex multicellular forms of life rely on transport systems that move material-containing liquids throughout the body in specialized tubes. In vascular plants, tubes transport food and water. Some invertebrates rely on a closed system of tubes, while others have an open system. Humans and other higher vertebrates have a closed system of circulation.

The human heart

The adult heart is a hollow cone-shaped muscular organ located in the center of the chest cavity. The lower tip of the heart tilts toward the left. The heart is about the size of a clenched fist and weighs approximately 10.5 oz (300 g). Remarkably, the heart beats more than 100,000 times a day and close to 2.5 billion times in the average lifetime. A triple-layered sac, the pericardium, surrounds, protects, and anchors the heart. A liquid pericardial fluid located in the space between two of the layers, reduces friction when the heart moves.

The heart is divided into four chambers. A partition or septum divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, atria (singular atrium), are thin-walled. They receive blood entering the heart, and pump it to the ventricles, the lower heart chambers. The walls of the ventricles are thicker and contain more cardiac muscle than the walls of the atria, enabling the ventricles to pump blood out to the lungs and the rest of the body. The left and right sides of the heart function as two separate pumps. The right atrium receives oxygen-poor blood from the body from a major vein, the vena cava, and delivers it to the right ventricle. The right ventricle, in turn, pumps the blood to the lungs via the pulmonary artery. The left atrium receives the oxygen-rich blood from the lungs from the pulmonary veins, and delivers it to the left ventricle. The left ventricle then pumps it into the aorta, a major artery that leads to all parts of the body. The wall of the left ventricle is thicker than the wall of the right ventricle, making it a more powerful pump able to push blood through its longer trip around the body.

One-way valves in the heart keep blood flowing in the right direction and prevent backflow. The valves open and close in response to pressure changes in the heart. Atrioventricular (AV) valves are located between the atria and ventricles. Semilunar (SL) valves lie between the ventricles and the major arteries into which they pump blood. The "lub-dup" sounds that the physician hears through the stethoscope occur when the heart valves close. The AV valves produce the "lub" sound upon closing, while the SL valves cause the "dup" sound. People with a heart murmur have a defective heart valve that allows the backflow of blood.

The rate and rhythm of the heartbeat are carefully regulated. We know that the heart continues to beat even when disconnected from the nervous system. This is evident during heart transplants when donor hearts keep beating outside the body. The explanation lies in a small mass of contractile cells, the sino-atrial (SA) node or pacemaker, located in the wall of the right atrium. The SA node sends out electrical impulses that set up a wave of contraction that spreads across the atria. The wave reaches the atrio-ventricular (AV) node, another small mass of contractile cells. The AV node is located in the septum between the left and right ventricle. The AV node, in turn, transmits impulses to all parts of the ventricles. The bundle of His, specialized fibers, conducts the impulses from the AV node to the ventricles. The impulses Colorized image of the main components of the human circulatory system. The heart (placed between the lungs) delivers blood to lungs to pick up oxygen and circulates it throughout the body by means of a system of blood vessels (red). Photograph by Howard Sochurek. The Stock Market. Reproduced with permission. stimulate the ventricles to contract. An electrocardiogram, ECG or EKG, is a record of the electric impulses from the pacemaker that direct each heartbeat. The SA node and conduction system provide the primary heart controls. In patients with disorganized electrical activity in the heart, surgeons implant an artificial pacemaker that serves to regulate the heart rhythm. In addition to self-regulation by the heart, the autonomic nervous system and hormones also affect its rate.

The heart cycle refers to the events associated with a single heartbeat. The cycle involves systole, the contraction phase, and diastole, the relaxation phase. In the heart, the two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The heart cycle consists of a systole and diastole of both the atria and ventricles. At the end of a heartbeat all four chambers rest. The rate of heartbeat averages about 75 beats per minute, and each cardiac cycle takes about 0.8 seconds.

Heart disease is the number one cause of death among people living in the industrial world. In coronary heartdisease (CHD), a clot or stoppage occurs in a blood vessel of the heart. Deprived of oxygen, the surrounding tissue becomes damaged. Education about prevention of CHD helps to reduce its occurrence. We have learned to prevent heart attacks by eating less fat, preventing obesity, exercising regularly, and by not smoking. Medications, medical devices and techniques also help patients with heartdisease. One of these, the heart-lung machine, is used during open-heart and bypass surgery. This device pumps the patient's blood out of the body, and returns it after having added oxygen and removed carbon dioxide. For patients with CHD, physicians sometimes use coronary artery bypass grafting (CABG). This is a surgical technique in which a blood vessel from another part of the body is grafted into the heart. The relocated vessel provides a new route for blood to travel as it bypasses the clogged coronary artery. In addition, cardiologists can also help CHD with angioplasty. Here, the surgeon inflates a balloon inside the aorta. This opens the vessel and improves the blood flow. For diagnosing heartdisease, the echocardiogram is used in conjunction with the ECG. This device uses high frequency sound waves to take pictures of the heart.

Blood vessels

The blood vessels of the body make up a closed system of tubes that carry blood from the heart to tissues all over the body and then back to the heart. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries connect small arteries (arterioles) and small veins (venules). Large arteries leave the heart and branch into smaller ones that reach out to various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, the arterioles branch into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which are small veins. The venules merge to form larger and larger veins that eventually return blood to the heart. The two main circulation routes in the body are the pulmonary circulation, to and from the lungs, and the systemic circulation, to and from all parts of the body. Subdivisions of the systemic system include the coronary circulation, for the heart, the cerebral circulation, for the brain, and the renal circulation, for the kidneys. In addition, the hepatic portal circulation passes blood directly from the digestive tract to the liver.

The walls of arteries, veins, and capillaries differ in structure. In all three, the vessel wall surrounds a hollow center through which the blood flows. The walls of both arteries and veins are composed of three coats. The inner coat is lined with a simple squamous endothelium, a single flat layer of cells. The thick middle coat is composed of smooth muscle that can change the size of the vessel when it contracts or relaxes, and of stretchable fibers that provide elasticity. The outer coat is composed of elastic fibers and collagen. The difference between veins and arteries lies in the thickness of the wall of the vessel. The inner and middle coats of veins are very thin compared to arteries. The thick walls of arteries make them elastic and capable of contracting. The repeated expansion and recoil of arteries when the heart beats creates the pulse. We can feel the pulse in arteries near the body surface, such as the radial artery in the wrist. The walls of veins are more flexible than artery walls and they change shape when muscles press against them. Blood returning to the heart in veins is under low pressure often flowing against gravity. One-way valves in the walks of veins keep blood flowing in one direction. Skeletal muscles also help blood return to the heart by squeezing the veins as they contract. Varicose veins develop when veins lose their elasticity and become stretched. Faulty valves allow blood to sink back thereby pushing the vein wall outward. The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allow the exchange of materials between cells and the blood. Their extensive branching provides a sufficient surface area to pick up and deliver substances to all cells in the body.

Blood pressure is the pressure of blood against the wall of a blood vessel. Blood pressure originates when the ventricles contract during the heartbeat. In a healthy young adult male, blood pressure in the aorta during systole is about 120 mm Hg, and approximately 80 mm Hg during diastole. The sphygmomanometer is an instrument that measures blood pressure. A combination of nervous carbon and hormones help regulate blood pressure around a normal range in the body. In addition, there are local controls that direct blood to tissues according to their need. For example, during exercise, reduced oxygen and increased carbon dioxide stimulate blood flow to the muscles.

Two disorders that involve blood vessels are hypertension and atherosclerosis. Hypertension, or high blood pressure, is the most common circulatory disease. For about 90% of hypertension sufferers, the blood pressure stays high without any known physical cause. Limiting salt and alcohol intake, stopping smoking, losing weight, increasing exercise, and managing stress help reduce blood pressure. Medications also help control hypertension. In atherosclerosis, the walls of arteries thicken and lose their elasticity. Fatty material such as cholesterol accumulates on the artery wall forming plaque that obstructs blood flow. The plaque can form a clot that breaks The human circulatory system. Illustration by Argosy. The Gale Group. off, travels in the blood, and can block a smaller vessel. For example, a stroke occurs when a clot obstructs an artery or capillary in the brain. Treatment for atherosclerosis includes medication, surgery, a low-fat, high-fiber diet, and exercise. The type of cholesterol carried in the blood indicates the risk of atherosclerosis. Low density lipoproteins (LDLs) deposit cholesterol on arteries, while high density lipoproteins (HDLs) remove it.

Blood

Blood is liquid connective tissue. It transports oxygen from the lungs and delivers it to cells. It picks up carbon dioxide from the cells and brings it to the lungs. It carries nutrients from the digestive system and hormones from the endocrine glands to the cells. It takes heat and waste products away from cells. The blood helps regulate the body's base-acid balance (pH), temperature, and water content. It protects the body by clotting and by fighting disease through the immune system.

When we study the structure of blood, we find that it is heavier and stickier than water, has a temperature in the body of about 100.4°F (38°C), and a pH of about 7.4. Blood makes up approximately 8% of the total body weight. A male of average weight has about 1.5 gal (5-6 l) of blood in his body, while a female has about 1.2 gal (4-5 l). Blood is composed of a liquid portion (the plasma), and blood cells.

Plasma is composed of about 91.5% water which acts as a solvent, heat conductor, and suspending medium for the blood cells. The rest of the plasma includes plasma proteins produced by the liver, such as albumins, that help maintain water balance, globulins, that help fight disease, and fibrinogen, that aids in blood clotting. The plasma carries nutrients, hormones, enzymes, cellular waste products, some oxygen, and carbon dioxide. Inorganic salts, also carried in the plasma, help maintain osmotic pressure. Plasma leaks out of the capillaries to form the interstitial fluid (tissue fluid) that surrounds the body cells and keeps them moist, and supplied with nutrients.

The cells in the blood are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). More than 99% of all the blood cells are erythrocytes, or red blood cells. Red blood cells look like flexible biconcave discs about 8 nm in diameter that are capable of squeezing through narrow capillaries. Erythrocytes lack a nucleus and therefore are unable to reproduce. Antigens, specialized proteins on the surface of erythrocytes, determine the ABO and Rh blood types. Erythrocytes contain hemoglobin, a red pigment that carries oxygen, and each red cell has about 280 million hemoglobin molecules. An iron ion in hemoglobin combines reversibly with one oxygen molecule, enabling it to pick up, carry and drop off oxygen. Erythrocytes are formed in red bone marrow, and live about 120 days. When they are worn out, the liver and spleen destroy them and recycle their breakdown products. Anemia is a blood disorder characterized by too few red blood cells.

Leukocytes are white blood cells. They are larger than red blood cells, contain a nucleus, and do not have hemoglobin. Leukocytes fight disease organisms by destroying them or by producing antibodies. Lymphocytes are a type of leukocyte that bring about immune reactions involving antibodies. Monocytes are large leukocytes that ingest bacteria and get rid of dead matter. Most leukocytes are able to squeeze through the capillary walls and migrate to an infected part of the body. Formed in the white/yellow bone marrow, a leukocyte's life ranges from hours to years depending on how it functions during an infection. In leukemia, a malignancy of bone marrow tissue, abnormal leukocytes are produced in an uncontrolled manner. They crowd out the bone marrow cells, interrupt normal blood cell production, and cause internal bleeding. Treatment for acute leukemia includes blood transfusions, anticancer drugs, and, in some cases, radiation.

Thrombocytes or platelets bring about clotting of the blood. Clotting stops the bleeding when the circulatory system is damaged. When tissues are injured, platelets disintegrate and release the substance thromboplastin. Working with calcium ions and two plasma proteins, fibrinogen and prothrombin, thromboplastin converts prothrombin to thrombin. Thrombin then changes soluble fibrinogen into insoluble fibrin. Finally, fibrin forms a clot. Hemophilia, a hereditary blood disease in which the patient lacks a clotting factor, occurs mainly in males. Hemophiliacs hemorrhage continuously after injury. They can be treated by transfusion of either fresh plasma or a concentrate of the deficient clotting factor.