The human circulatory system is termed the cardiovascular system, from the Greek word kardia, meaning heart, and the Latin vasculum, meaningsmall vessel. The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. The work done by the cardiovascular system is astounding. Each year, the heart pumps more than 1,848 gal (7,000 l) ofblood through a closed system of about 62,100 mi (100,000 km) of blood vessels. This is more than twice the distance around the equator of the Earth. Asblood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands, and delivers these to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these to the lungs and kidneys, where they are excreted. Substances pass out of blood vessels to the cells through the interstitial or tissue fluid that surrounds cells.
The adult heart is a hollow cone-shaped muscular organ located in the centerof 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 to2.5 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 heartmoves.
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 lowerchamber. The upper chambers, atria (singular atrium), are thin-walled. Theyreceive 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 thelungs 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 ofthe 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 hearsthrough 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 heart cycle refers to the events associated with a single heartbeat. Thecycle 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 endof a heartbeat all four chambers rest. The rate of heartbeat averages about75 beats per minute, and each cardiac cycle takes about 0.8 seconds.
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 outto various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, thearterioles branch into a network of microscopic capillaries. Substances movein 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 eventuallyreturn 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 hepaticportal 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 sizeof 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 wallof the vessel. The inner and middle coats of veins are very thin compared tothose of arteries. The thick walls of arteries make them elastic and capableof contracting. The repeated expansion and recoil of arteries when the heartbeats 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 walls of veins keep blood flowing in onedirection. Skeletal muscles also help blood return to the heart by squeezingthe veins as they contract. The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allowthe 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.
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 fromthe 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 stickierthan 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 (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, which help maintain water balance; globulins, which help fight disease; and fibrinogen, whichaids 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 thecapillaries to form the interstitial fluid (tissue fluid) that surrounds thebody cells and keeps them moist and supplied with nutrients.
The cells in the blood are erythrocytes (red blood cells), leukocytes (whiteblood cells), and thrombocytes (platelets). More than 99% of all the blood cells are erythrocytes, or red blood cells. Red blood cells look like flexiblebiconcave discs about 8 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 ablood 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 organismsby 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.
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.