Circulatory System

Circulatory System I INTRODUCTIO . N Circulatory System, or cardiovascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.

II COMPONENTS OF THE CIRCULATORY . SYSTEM The heart, blood, and blood vessels are the three structural elements that make up the circulatory system. The heart is the engine of the circulatory system. It is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. The walls of these chambers are made of a special muscle called myocardium, which contracts continuously and rhythmically to pump blood. The pumping action of the heart occurs in two stages for each heart beat: diastole, when the heart is at rest; and systole, when the heart contracts to pump deoxygenated blood toward the lungs and oxygenated blood to the body. During each heartbeat, typically about 60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If the heart stops pumping, death usually occurs within four to five minutes. Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats. Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat. Arteries have thicker walls than veins to withstand the pressure of blood being pumped from the heart. Blood in the veins is at a lower pressure, so veins have one-way valves to prevent blood from flowing backwards away from the heart. Capillaries, the smallest of blood vessels, are only visible by microscope—ten capillaries lying side by side are barely as thick as a human hair. If all the arteries, veins, and capillaries in the human body were placed end to end, the total length would equal more than 100,000 km (more than 60,000 mi)—they could stretch around the earth nearly two and a half times.

The arteries, veins, and capillaries are divided into two systems of circulation: systemic and pulmonary. The systemic circulation carries oxygenated blood from the heart to all the tissues in the body except the lungs and returns deoxygenated blood carrying waste products, such as carbon dioxide, back to the heart. The pulmonary circulation carries this spent blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart before transferring to the systemic circulation.

I OPERATION AND II. FUNCTION Only in the past 400 years have scientists recognized that blood moves in a cycle through the heart and body. Before the 17th century, scientists believed that the liver creates new blood, and then the blood passes through the heart to gain warmth and finally is soaked up and consumed in the tissues. In 1628 English physician William Harvey first proposed that blood circulates continuously. Using modern methods of observation and experimentation, Harvey noted that veins have one-way valves that lead blood back to the heart from all parts of the body. He noted that the heart works as a pump, and he estimated correctly that the daily output of fresh blood is more than seven tons. He pointed out the absurdity of the old doctrine, which would require the liver to produce this much fresh blood daily. Harvey’s theory was soon proven correct and became the cornerstone of modern medical science.

Systemic A. Circulation The heart ejects oxygen-rich blood under high pressure out of the heart’s main pumping chamber, the left ventricle, through the largest artery, the aorta. Smaller arteries branch off from the aorta, leading to various parts of the body. These smaller arteries in turn branch out into even smaller arteries, called arterioles. Branches of arterioles become progressively smaller in diameter, eventually forming the capillaries. Once blood reaches the capillary level, blood pressure is greatly reduced. Capillaries have extremely thin walls that permit dissolved oxygen and nutrients from the blood to diffuse across to a fluid, known as interstitial fluid, that fills the gaps between the cells of tissues or organs. The dissolved oxygen and nutrients then enter the cells from the interstitial fluid by diffusion across the cell membranes. Meanwhile, carbon dioxide and other wastes leave the cell, diffuse through the interstitial fluid, cross the capillary walls, and enter the blood. In this way, the blood delivers nutrients and removes wastes without leaving the capillary tube. After delivering oxygen to tissues and absorbing wastes, the deoxygenated blood in the capillaries then starts the return trip to the heart. The capillaries merge to form tiny veins, called venules. These veins in turn join together to form progressively larger veins. Ultimately, the veins converge into two large veins: the inferior vena cava, bringing blood from the lower half of the body; and the superior vena cava, bringing blood from the upper half. Both of these two large veins join at the right atrium of the heart. Because the pressure is dissipated in the arterioles and capillaries, blood in veins flows back to the heart at very low pressure, often running uphill when a person is standing. Flow against gravity is made possible by the one-way valves, located several centimeters apart,

in the veins. When surrounding muscles contract, for example in the calf or arm, the muscles squeeze blood back toward the heart. If the one-way valves work properly, blood travels only toward the heart and cannot lapse backward. Veins with defective valves, which allow the blood to flow backward, become enlarged or dilated to form varicose veins.

B Pulmonary . Circulation In pulmonary circulation, deoxygenated blood returning from the organs and tissues of the body travels from the right atrium of the heart to the right ventricle. From there it is pushed through the pulmonary artery to the lung. In the lung, the pulmonary artery divides, forming the pulmonary capillary region of the lung. At this site, microscopic vessels pass adjacent to the alveoli, or air sacs of the lung, and gases are exchanged across a thin membrane: oxygen crosses the membrane into the blood while carbon dioxide leaves the blood through this same membrane. Newly oxygenated blood then flows into the pulmonary veins, where it is collected by the left atrium of the heart, a chamber that serves as collecting pool for the left ventricle. The contraction of the left ventricle sends blood into the aorta, completing the circulatory loop. On average, a single blood cell takes roughly 30 seconds to complete a full circuit through both the pulmonary and systemic circulation.

Additional C. Functions In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passing through the walls of the intestine. The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation. The hepatic portal circulation carries the nutrients to the liver for further metabolic processing. The liver stores a variety of substances, such as sugars, fats, and vitamins, and releases these to the blood as needed. The liver also cleans the blood by removing waste products and toxins. After hepatic portal blood has crossed the liver cells, veins converge to form the large hepatic vein that joins the vena cava near the right atrium. The circulatory system plays an important role in regulating body temperature. During exercise, working muscles generate heat. The blood supplying the muscles with oxygen and nutrients absorbs much of this heat and carries it away to other parts of the body. If the body gets too warm, blood vessels near the skin enlarge to disperse excess heat outward through the skin. In cold environments, these blood vessels constrict to retain heat. The circulatory system works in tandem with the endocrine system, a collection of hormoneproducing glands. These glands release chemical messengers, called hormones, directly into the bloodstream to be transported to specific organs and tissues. Once they reach their target destination, hormones regulate the body’s rate of metabolism, growth, sexual development, and other functions. The circulatory system also works with the immune system and the coagulation system. The immune system is a complex system of many types of cells that work together to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by the circulatory system. The coagulation system is composed of special blood cells, called platelets, and special proteins, called

clotting factors, that circulate in the blood. Whenever blood vessels are cut or torn, the coagulation system works rapidly to stop the bleeding by forming clots. Other organs support the circulatory system. The brain and other parts of the nervous system constantly monitor blood circulation, sending signals to the heart or blood vessels to maintain constant blood pressure. New blood cells are manufactured in the bone marrow. Old blood cells are broken down in the spleen, where valuable constituents, such as iron, are recycled. Metabolic waste products are removed from the blood by the kidneys, which also screen the blood for excess salt and maintain blood pressure and the body’s balance of minerals and fluids. Hypertension, or elevated blood pressure, develops when the body’s blood vessels narrow, causing the heart to pump harder than normal to push blood through the narrowed openings. Hypertension that remains untreated may cause heart enlargement and thickening of the heart muscle. Eventually the heart needs more oxygen to function, which can lead to heart failure, brain stroke, or kidney impairment. Some cases of hypertension can be treated by lifestyle changes such as a low-salt diet, maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium. If blood pressure remains high despite these lifestyle adjustments, medications may be effective in lowering the pressure by relaxing blood vessels and reducing the output of blood. What is high blood pressure? Blood pressure is a measure of how hard the blood pushes against the walls of your arteries and veins as it moves through your body. It’s normal for blood pressure to go up and down throughout the day, but if it stays up, you have high blood pressure. Another name for high blood pressure is hypertension. When blood pressure is high, it starts to damage the blood vessels, heart, and kidneys. This can lead to heart attack, stroke, and other problems. High blood pressure is called a "silent killer,'' because it doesn't usually cause symptoms while it is causing this damage. Your blood pressure consists of two numbers: systolic and diastolic. Someone with a systolic pressure of 120 and a diastolic pressure of 80 has a blood pressure of 120/80, or "120 over 80." • •

The systolic number shows how hard the blood pushes when the heart is pumping. The diastolic number shows how hard the blood pushes between heartbeats, when the heart is relaxed and filling with blood. Adults should have a blood pressure of less than 120/80. High blood pressure is 140/90 or higher. Many people fall into the category in between, called prehypertension. People with prehypertension need to make lifestyle changes to bring the blood pressure down and help prevent or delay high blood pressure. What causes high blood pressure? In most cases, doctors can't point to the exact cause. But several things are known to raise blood pressure, including being very overweight, drinking too much alcohol, having a family history of high blood pressure, eating too much salt, and getting older. Your blood pressure may also rise if you are not very active, you don't eat enough potassium and calcium, or you have a condition called insulin resistance. What are the symptoms?

High blood pressure doesn't usually cause symptoms. Most people don't know they have it until they go to the doctor for some other reason. Without treatment, high blood pressure can damage the heart, brain, kidneys, or eyes. This damage causes problems like coronary artery disease, stroke, and kidney failure. Very high blood pressure can cause headaches, vision problems, nausea, and vomiting. Malignant high blood pressure (hypertensive crisis), which is blood pressure that rises very fast, can also cause these symptoms. Malignant high blood pressure is a medical emergency. How is high blood pressure diagnosed? Most people find out they have high blood pressure during a routine doctor visit. For your doctor to confirm that you have high blood pressure, your blood pressure must be at least 140/90 on three or more separate occasions. It is usually measured 1 to 2 weeks apart. You may have to check your blood pressure at home if there is reason to think the readings in the doctor’s office aren't accurate. You may have what is called white-coat hypertension, which is blood pressure that goes up just because you're at the doctor’s office. Even routine activities, such as attending a meeting, can raise your blood pressure. So can commuting to work or smoking a cigarette. How is it treated? Treatment depends on how high your blood pressure is, whether you have other health problems such as diabetes, and whether any organs have already been damaged. Your doctor will also consider how likely you are to develop other diseases, especially heart disease. You can help lower your blood pressure by making healthy changes in your lifestyle. If those lifestyle changes don't work, you may also need to take pills. Either way, you will need to control your high blood pressure throughout your life. •

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If you have prehypertension, your doctor will likely recommend lifestyle changes. These may include losing extra weight, exercising, limiting alcohol, cutting back on salt, quitting smoking, and eating a low-fat diet that includes more fruits, vegetables, whole grains, and low-fat dairy foods. If you have high blood pressure without any organ damage or other risk factors for heart disease, your doctor may recommend that you take medicine in addition to making lifestyle changes. If you have high blood pressure and have some organ damage or other risk factors for heart disease, you may need to try various combinations of medicines in addition to making big lifestyle changes. Most people take more than one pill for high blood pressure. Work with your doctor to find the right pill or combination of pills that will cause the fewest side effects. It can be hard to remember to take pills when you have no symptoms. But your blood pressure will go back up if you don't take your medicine. Make your pill schedule as simple as you can. Plan times to take them when you are doing other things, like eating a meal or getting ready for bed. What can you do to prevent high blood pressure? There are six lifestyle changes you can make to help prevent high blood pressure:

Lose extra weight. Eat less salt. Exercise. Limit alcohol to 2 drinks a day for men and 1 drink a day for women and lighter-weight men. Get 3,500 mg of potassium in your diet every day. Fresh,

unprocessed whole foods have the most potassium. These foods include meat, fish, nonfat and low-fat dairy products, and many fruits and vegetables. Follow the DASH eating plan (Dietary Approaches to Stop Hypertension). This diet is rich in fruits, vegetables, and low-fat dairy products and is low in fat.

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