Parts of the Cardiovascular System
The cardiovascular system and the lymphatic system form what is collectively known as the circulatory system. Together these systems transport oxygen, nutrients, cell wastes, hormones, and many other substances to and from all cells in the body. The trillions of cells in the human body take up nutrients and excrete wastes every minute of every day. Although the pace of this exchange may increase with activity or slow with rest, it happens continuously. If it stops, so does life. Of the two systems, the cardiovascular system is the primary transport operator; the lymphatic system aids it in its function.
Cardiovascular comes from the Greek word cardia, meaning "heart", and the Latin vasculum, meaning "small vessel". The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. The system can be compared to a large muscular pump (the heart) that sends a fluid (blood) through a series of large and small tubes (blood vessels). As blood circulates through the increasingly intricate system of vessels, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands. It delivers these substances to the cells, picking up carbon dioxide (formed when cells use sugars or fats to produce energy) and other wastes in return. The blood then takes these waste products to the lungs and kidneys, where they are excreted.
The cardiovascular systems composes of 5 primary components: the heart, blood vessels, pulmonary and systematic circulations, blood, and plasma.
The Heart:
The heart is a hollow cone-shaped muscular organ located behind and slightly to the left of the sternum or breastbone. Nestled between the lungs, the heart sits within a protective bony cage formed by the sternum, ribs, and spine. The lower tip of the heart, called the apex, points toward the left hip and rests on the diaphragm (a membrane of muscle separating the chest cavity from the abdominal cavity). The upper portion of the heart, called the base, points toward the right shoulder and lies beneath the second rib. It is from the base that the major blood vessels of the body emerge. The heart is about the size of a clenched fist. At birth, an infant's heart and fist are about the same size. As a human body develops, the heart and fist grow at about the same rate. In adults, an average-sized heart weighs between 9 and 12 ounces (255 and 350 grams). It is slightly larger in males than in females (Body by Design 2007). The pericardium is a tough fibrous membranous sac that surrounds, protects, and anchors the heart. It is composed of three layers. The thin inner layer tightly hugs the outer surface of the heart and is actually a part of the heart wall. The fibrous outer layer protects the heart and anchors it to surrounding structures such as the sternum and diaphragm. The inner portion of this outer layer is lined by another layer that produces serous fluid. This watery lubricant between the inner and outer layers of the pericardium allows the layers to slide smoothly across each other, reducing friction when the heart beats. The heart wall is made up of three layers: the epicardium, the myocardium, and the endocardium. The outer layer, the epicardium, is actually the thin inner layer of the pericardium. The middle layer, the myocardium, is a thick layer of cardiac muscle that contracts to force blood out of the heart. The inner layer, the endocardium, is a thin, glistening membrane that allows blood to flow smoothly through the chambers of the heart.
Heart Chambers:
The heart is divided into four chambers. A muscular septum or partition divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, the atria, are thin-walled. They are the receiving chambers of the heart. Blood flows into them from the body. The atria then pump the blood downward to the ventricles, the lower heart chambers. The ventricles are the discharging chambers of the heart. Their walls are thicker and contain more cardiac muscle than the walls of the atria. This extra thickness enables the ventricles to contract and pump blood out of the heart to the lungs and the rest of the body (2007).
Cardiovascular comes from the Greek word cardia, meaning "heart", and the Latin vasculum, meaning "small vessel". The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. The system can be compared to a large muscular pump (the heart) that sends a fluid (blood) through a series of large and small tubes (blood vessels). As blood circulates through the increasingly intricate system of vessels, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands. It delivers these substances to the cells, picking up carbon dioxide (formed when cells use sugars or fats to produce energy) and other wastes in return. The blood then takes these waste products to the lungs and kidneys, where they are excreted.
The cardiovascular systems composes of 5 primary components: the heart, blood vessels, pulmonary and systematic circulations, blood, and plasma.
The Heart:
The heart is a hollow cone-shaped muscular organ located behind and slightly to the left of the sternum or breastbone. Nestled between the lungs, the heart sits within a protective bony cage formed by the sternum, ribs, and spine. The lower tip of the heart, called the apex, points toward the left hip and rests on the diaphragm (a membrane of muscle separating the chest cavity from the abdominal cavity). The upper portion of the heart, called the base, points toward the right shoulder and lies beneath the second rib. It is from the base that the major blood vessels of the body emerge. The heart is about the size of a clenched fist. At birth, an infant's heart and fist are about the same size. As a human body develops, the heart and fist grow at about the same rate. In adults, an average-sized heart weighs between 9 and 12 ounces (255 and 350 grams). It is slightly larger in males than in females (Body by Design 2007). The pericardium is a tough fibrous membranous sac that surrounds, protects, and anchors the heart. It is composed of three layers. The thin inner layer tightly hugs the outer surface of the heart and is actually a part of the heart wall. The fibrous outer layer protects the heart and anchors it to surrounding structures such as the sternum and diaphragm. The inner portion of this outer layer is lined by another layer that produces serous fluid. This watery lubricant between the inner and outer layers of the pericardium allows the layers to slide smoothly across each other, reducing friction when the heart beats. The heart wall is made up of three layers: the epicardium, the myocardium, and the endocardium. The outer layer, the epicardium, is actually the thin inner layer of the pericardium. The middle layer, the myocardium, is a thick layer of cardiac muscle that contracts to force blood out of the heart. The inner layer, the endocardium, is a thin, glistening membrane that allows blood to flow smoothly through the chambers of the heart.
Heart Chambers:
The heart is divided into four chambers. A muscular septum or partition divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, the atria, are thin-walled. They are the receiving chambers of the heart. Blood flows into them from the body. The atria then pump the blood downward to the ventricles, the lower heart chambers. The ventricles are the discharging chambers of the heart. Their walls are thicker and contain more cardiac muscle than the walls of the atria. This extra thickness enables the ventricles to contract and pump blood out of the heart to the lungs and the rest of the body (2007).
The Gale Encyclopedia of Science 2010
As blood flows from one chamber to the next, one-way valves prevent the blood from flowing backward. The valves located between the atria and ventricles are called atrioventricular or AV valves. The left AV valve (between the left ventricle and left atrium) is the mitral or bicuspid valve. It is called bicuspid because it consists of two triangle-shaped flaps of tissue. The right AV valve (between the right atrium and right ventricle) is the tricuspid valve, so called because it has three flaps of tissue. The valves located between the ventricles and the major arteries into which they pump blood are called semilunar valves. The pulmonary semilunar valve is located between the right ventricle and the pulmonary trunk. The aortic semilunar valve is located between the left ventricle and the aorta (2007).
Blood Vessels:
The blood vessels form a closed transport system of tubes measuring about 62,000 miles (99,780 kilometers) in length--two and a half times the distance around the equator of Earth. The entire blood vessel system can be thought of as a series of connected roads and highways. Blood leaves the heart through large vessels (highways) that travel outward into the body. At various points these large vessels divide to become smaller vessels (secondary roads). In turn, these vessels continue to divide into smaller and smaller vessels (one-lane roads). On its return trip, the blood travels through increasingly larger and larger vessels (one-lane roads merging into secondary roads merging into highways) before eventually reaching the heart (2007).
Blood Vessels:
The blood vessels form a closed transport system of tubes measuring about 62,000 miles (99,780 kilometers) in length--two and a half times the distance around the equator of Earth. The entire blood vessel system can be thought of as a series of connected roads and highways. Blood leaves the heart through large vessels (highways) that travel outward into the body. At various points these large vessels divide to become smaller vessels (secondary roads). In turn, these vessels continue to divide into smaller and smaller vessels (one-lane roads). On its return trip, the blood travels through increasingly larger and larger vessels (one-lane roads merging into secondary roads merging into highways) before eventually reaching the heart (2007).
An image of the main components of the human circulatory system. The heart (placed between the lungs) delivers blood to the lungs, where it picks up oxygen and circulates it throughout the body by means of blood vessels (UXL Complete Life Science Resource 2010).
Arteries, capillaries, and veins are the main parts of this transport system. Arteries are the vessels that carry blood away from the heart. Large arteries leave the heart and then branch into smaller ones that reach out to various parts of the body. These divide even further into smaller vessels called arterioles. Within the tissues, arterioles divide into microscopic vessels called capillaries. The exchange of materials between the blood and the cells occurs through the walls of the capillaries. Before leaving the tissues, capillaries merge to form venules, which are small veins. As these vessels move closer to the heart, they merge to form larger and larger veins (2007).
The Pulmonary and Systematic Circulations:
There are two main circuits or routes in the body for the circulation of the blood: the pulmonary circulation and the systemic circulation. Vessels involved in the pulmonary circulation transport blood between the heart and the lungs. Vessels in the systemic circulation transport blood to all other body parts. The main artery of the systemic circulation is the aorta. In adults, the aorta is about the same size as a standard garden hose. It emerges upward out of the left ventricle for about an inch, then curves leftward over the heart (a portion called the aortic arch) before plunging downward to divide into branches that carry blood to the major parts of the body (2007). Branches of the aorta include the carotid arteries, which carry blood to the head; the coronary arteries, which supply blood to the muscles of the heart; the brachial arteries, which carry blood downward in the arms; and the femoral arteries, which carry blood downward through the thighs.The vena cava is the largest vein in the body. It has two branches: the superior vena cava, which accepts blood drained from the head and arms; and the inferior vena cava, which receives blood from the lower body. Both sections (collectively called the venae cavae) empty into the right atrium. The veins that drain into the venae cavae include the jugular veins, which drain the head; the brachial and cephalic veins, which drain the arms; the femoral veins, which drain the thighs; and the iliac veins, which drain the pelvic or hip region.
The Pulmonary and Systematic Circulations:
There are two main circuits or routes in the body for the circulation of the blood: the pulmonary circulation and the systemic circulation. Vessels involved in the pulmonary circulation transport blood between the heart and the lungs. Vessels in the systemic circulation transport blood to all other body parts. The main artery of the systemic circulation is the aorta. In adults, the aorta is about the same size as a standard garden hose. It emerges upward out of the left ventricle for about an inch, then curves leftward over the heart (a portion called the aortic arch) before plunging downward to divide into branches that carry blood to the major parts of the body (2007). Branches of the aorta include the carotid arteries, which carry blood to the head; the coronary arteries, which supply blood to the muscles of the heart; the brachial arteries, which carry blood downward in the arms; and the femoral arteries, which carry blood downward through the thighs.The vena cava is the largest vein in the body. It has two branches: the superior vena cava, which accepts blood drained from the head and arms; and the inferior vena cava, which receives blood from the lower body. Both sections (collectively called the venae cavae) empty into the right atrium. The veins that drain into the venae cavae include the jugular veins, which drain the head; the brachial and cephalic veins, which drain the arms; the femoral veins, which drain the thighs; and the iliac veins, which drain the pelvic or hip region.
A scanning electron micrograph (SEM) of the aortic valve. It lies between the left ventricle of the heart and the aortic arch and consists of a fibrous ring with three semilunar pockets attached to it (The Gale Encyclopedia of Science 2010).
The vessels involved in the pulmonary circulation carry blood to the lungs for gas exchange, in which carbon dioxide is unloaded and oxygen is picked up. The vessels then return the oxygen-carrying blood to the heart. The main vessels of the pulmonary circulation are the pulmonary arteries and the pulmonary veins. The two pulmonary arteries branch off from the pulmonary trunk, which originates from the right ventricle. The right pulmonary artery goes to the right lung, the left pulmonary artery to the left lung. After gas exchange occurs in the lungs, the oxygenated (carrying oxygen) blood is transported back to the left atrium of the heart by four pulmonary veins.
Blood:
Blood is the fluid pumped by the heart through the blood vessels to all parts of the body. It is considered a form of connective tissue. As the name suggests, connective tissue in general connects body parts, providing support, storage, and protection. Found everywhere in the body, connective tissue is the most abundant type of the four types of tissues (the other three are epithelial, muscle, and nervous). Of all the tissues in the body, blood is unique--it is the only one that is fluid under normal circumstances. Blood has many functions in the body. It carries everything that must be transported from one place to another within the body: oxygen and nutrients to the cells, hormones to the tissues, and waste products to the organs responsible for removing them from the body. Blood helps to protect the body by clotting to seal minor cuts and by acting as a defense against foreign microorganisms (2007). It also keeps the body at a constant temperature by carrying heat away from cells, and it regulates the acid/alkaline balance (pH) of the body. Stickier and heavier than water, blood ranges in color from scarlet to dull red depending on the amount of oxygen it is carrying (the brighter the color, the greater the amount of oxygen). Inside the body, blood has a temperature of about 100.4°F (38°C). It makes up approximately 7 percent of a person's body weight. A man of average weight has about 6 quarts (5.6 liters) of blood in his body; a woman of average weight has about 4.8 quarts (4.5 liters). Men tend to have more blood than women of the same height and weight due to the presence of testosterone, the male sex hormone that also stimulates blood formation (2007). Blood is composed of both solid and liquid elements. Red blood cells, white blood cells, and platelets are the solid components that are suspended in plasma, a watery straw-colored fluid. The living blood cells make up about 45 percent of the blood; the nonliving plasma makes up the remaining 55 percent.
Blood:
Blood is the fluid pumped by the heart through the blood vessels to all parts of the body. It is considered a form of connective tissue. As the name suggests, connective tissue in general connects body parts, providing support, storage, and protection. Found everywhere in the body, connective tissue is the most abundant type of the four types of tissues (the other three are epithelial, muscle, and nervous). Of all the tissues in the body, blood is unique--it is the only one that is fluid under normal circumstances. Blood has many functions in the body. It carries everything that must be transported from one place to another within the body: oxygen and nutrients to the cells, hormones to the tissues, and waste products to the organs responsible for removing them from the body. Blood helps to protect the body by clotting to seal minor cuts and by acting as a defense against foreign microorganisms (2007). It also keeps the body at a constant temperature by carrying heat away from cells, and it regulates the acid/alkaline balance (pH) of the body. Stickier and heavier than water, blood ranges in color from scarlet to dull red depending on the amount of oxygen it is carrying (the brighter the color, the greater the amount of oxygen). Inside the body, blood has a temperature of about 100.4°F (38°C). It makes up approximately 7 percent of a person's body weight. A man of average weight has about 6 quarts (5.6 liters) of blood in his body; a woman of average weight has about 4.8 quarts (4.5 liters). Men tend to have more blood than women of the same height and weight due to the presence of testosterone, the male sex hormone that also stimulates blood formation (2007). Blood is composed of both solid and liquid elements. Red blood cells, white blood cells, and platelets are the solid components that are suspended in plasma, a watery straw-colored fluid. The living blood cells make up about 45 percent of the blood; the nonliving plasma makes up the remaining 55 percent.
Red blood cells flowing through blood vessels. Also known as erythrocytes, red blood cells are the most prevalent of the three types of blood cells (Body by Design 2010).
Plasma:
Blood plasma is approximately 92 percent water. Over 100 different substances are dissolved in this fluid, including nutrients, respiratory gases, hormones, plasma proteins, salts, and various wastes. Of these dissolved substances, plasma proteins are the most abundant, comprising about 8 percent of blood plasma. These proteins, most of which are produced by the liver, serve a variety of functions (2007). Fibrinogen is an important protein that aids in blood clotting. Albumins help to keep water in the bloodstream. Proteins called gamma globulins act as antibodies, which are substances produced by the body to help protect it against foreign substances. The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. They are involved in many important body functions, including muscle contraction, the transmission of nerve impulses, and the regulation of the body's pH (acid-base) balance.
Blood plasma is approximately 92 percent water. Over 100 different substances are dissolved in this fluid, including nutrients, respiratory gases, hormones, plasma proteins, salts, and various wastes. Of these dissolved substances, plasma proteins are the most abundant, comprising about 8 percent of blood plasma. These proteins, most of which are produced by the liver, serve a variety of functions (2007). Fibrinogen is an important protein that aids in blood clotting. Albumins help to keep water in the bloodstream. Proteins called gamma globulins act as antibodies, which are substances produced by the body to help protect it against foreign substances. The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. They are involved in many important body functions, including muscle contraction, the transmission of nerve impulses, and the regulation of the body's pH (acid-base) balance.
How the Cardiovascular System Functions
In its continuous work, the average heart contracts more than 100,000 times a day to force blood through the thousands of miles of blood vessels to nourish each of the trillions of cells in the body. With each contraction, the heart forces about 2.5 ounces (74 milliliters) of blood into the bloodstream. At an average adult heart rate of 72 beats per minute, this output equals about 1.4 gallons (5.3 liters) of blood every minute, 84 gallons (318 liters) every hour, and 2,016 gallons (7,631 liters) every day. During exercise, this amount may be increased by as much as five times. The heart of a newborn baby beats faster than that of an adult--between 130 and 150 beats per minute, and a normal heart rate for a toddler is also higher than an adult's, about 100-130 beats per minute.
The cardiovascular system operates with 6 different functions/parts: the cardiac cycle, blood pressure, regulating the heart rate, exchanges between capillaries and general body tissues, capillary exchange in the lungs, and hepatic portal circulation.
Cardiac Cycle:
Cardiac cycle refers to the series of events that occur in the heart during one complete heartbeat. Each complete cardiac cycle takes about 0.8 second. During this brief moment, blood enters the heart, passes from chamber to chamber, then is pumped out to all areas of the body. Each cardiac cycle is divided into two phases. The two atria contract while the two ventricles relax. Then the two ventricles contract while the two atria relax. The contraction phase, especially of the ventricles, is known as systole; the relaxation phase is known as diastole. The cardiac cycle consists of a systole and diastole of both the atria and ventricles (2007). The cycle begins as deoxygenated (carrying very little oxygen) blood returns to the right atrium of the heart via the venae cavae. At the same time, oxygenated blood transported from the lungs by the four pulmonary veins empties into the left atrium. The AV valves open, and as blood flows into the atria it also flows passively into the ventricles. The semilunar valves, however, are closed to prevent blood from flowing out of the ventricles into the arteries. When the ventricles are about 70 percent full, the SA node sends out an impulse that spreads through the atria to the AV node. The atria contract, pumping out the remaining 30 percent of blood into the ventricles.
The AV node slows the impulse briefly, allowing the atria time to complete their contraction. The impulse then travels through the AV bundle, the bundle branches, and the Purkinje fibers to the apex of the heart. As the contraction of the ventricles is initiated at this spot, pressure begins building rapidly in the ventricles and the AV valves close (the "lub" sound heard through a stethoscope) to prevent blood from flowing back into the atria. When the pressure in the ventricles becomes higher than the pressure in the large arteries leaving the heart, the semilunar valves are forced open and blood is pumped out of the ventricles. Deoxygenated blood in the right ventricle is pumped to the lungs via the pulmonary arteries; oxygenated blood in the left ventricle is pumped to the rest of the body via the aorta (2007). While the ventricles are contracting (systole), the atria are at rest (diastole) and are filling with blood once again. When all the blood has been pumped from the ventricles, the semilunar valves close (the "dup" sound heard through a stethoscope) to prevent the backflow of blood into the heart. For a moment, the ventricles are empty closed chambers. When the pressure in the atria increases above that in the ventricles, the AV valves are forced open and blood begins to flow into the ventricles, starting a new cardiac cycle that will take less than one second to complete.
In short, during the cardiac cycle, the upper half of the heart (the two atria) receives blood. The lower half (the ventricles) then pumps out the blood. The right side of the heart (right atrium and right ventricle) receives and pumps out deoxygenated blood; the left side (left atrium and left ventricle) receives and pumps out oxygenated blood.
The cardiovascular system operates with 6 different functions/parts: the cardiac cycle, blood pressure, regulating the heart rate, exchanges between capillaries and general body tissues, capillary exchange in the lungs, and hepatic portal circulation.
Cardiac Cycle:
Cardiac cycle refers to the series of events that occur in the heart during one complete heartbeat. Each complete cardiac cycle takes about 0.8 second. During this brief moment, blood enters the heart, passes from chamber to chamber, then is pumped out to all areas of the body. Each cardiac cycle is divided into two phases. The two atria contract while the two ventricles relax. Then the two ventricles contract while the two atria relax. The contraction phase, especially of the ventricles, is known as systole; the relaxation phase is known as diastole. The cardiac cycle consists of a systole and diastole of both the atria and ventricles (2007). The cycle begins as deoxygenated (carrying very little oxygen) blood returns to the right atrium of the heart via the venae cavae. At the same time, oxygenated blood transported from the lungs by the four pulmonary veins empties into the left atrium. The AV valves open, and as blood flows into the atria it also flows passively into the ventricles. The semilunar valves, however, are closed to prevent blood from flowing out of the ventricles into the arteries. When the ventricles are about 70 percent full, the SA node sends out an impulse that spreads through the atria to the AV node. The atria contract, pumping out the remaining 30 percent of blood into the ventricles.
The AV node slows the impulse briefly, allowing the atria time to complete their contraction. The impulse then travels through the AV bundle, the bundle branches, and the Purkinje fibers to the apex of the heart. As the contraction of the ventricles is initiated at this spot, pressure begins building rapidly in the ventricles and the AV valves close (the "lub" sound heard through a stethoscope) to prevent blood from flowing back into the atria. When the pressure in the ventricles becomes higher than the pressure in the large arteries leaving the heart, the semilunar valves are forced open and blood is pumped out of the ventricles. Deoxygenated blood in the right ventricle is pumped to the lungs via the pulmonary arteries; oxygenated blood in the left ventricle is pumped to the rest of the body via the aorta (2007). While the ventricles are contracting (systole), the atria are at rest (diastole) and are filling with blood once again. When all the blood has been pumped from the ventricles, the semilunar valves close (the "dup" sound heard through a stethoscope) to prevent the backflow of blood into the heart. For a moment, the ventricles are empty closed chambers. When the pressure in the atria increases above that in the ventricles, the AV valves are forced open and blood begins to flow into the ventricles, starting a new cardiac cycle that will take less than one second to complete.
In short, during the cardiac cycle, the upper half of the heart (the two atria) receives blood. The lower half (the ventricles) then pumps out the blood. The right side of the heart (right atrium and right ventricle) receives and pumps out deoxygenated blood; the left side (left atrium and left ventricle) receives and pumps out oxygenated blood.
Biology Guide (n.d.)
Blood Pressure:
When the ventricles contract, they force or propel blood from the heart into the large, elastic arteries that expand as the blood is pushed through them. The pressure the blood exerts against the inner walls of the blood vessels is known as blood pressure. This pressure is necessary to keep the blood flowing to all areas of the body and then back to the heart. Blood pressure is greatest in the large arteries closest to the heart. Because their walls are elastic, the arteries are able to recoil and keep most of the pressure on the blood as it flows away from the heart. As the blood courses through the system in less elastic vessels--arterioles into capillaries into venules into veins--blood pressure drops. When the blood finally returns to the right atrium via the venae cavae, the pressure behind it is almost zero (2007). Since the heart contracts and relaxes during a cardiac cycle, blood pressure rises and falls during each beat. It is higher during systole (left ventricle contracting) and lower during diastole (left ventricle relaxing). Blood pressure is measured in millimeters of mercury (mmHg) with a sphygmomanometer. A blood pressure reading is most often taken on the brachial artery in the arm. The systolic pressure is recorded first, followed by the diastolic pressure. Average young adults have a blood pressure reading of about 120 mmHg for systolic pressure and 80 mmHg for diastolic pressure (written as 120/80 and read as "one-twenty over eighty"). Depending on age, sex, weight, and other factors, normal blood pressure can range from 90 to 135 mmHg for the systolic pressure and 60 to 85 mmHg for the diastolic pressure. Blood pressure normally increases with age because the arteries are less flexible in older people. Normal blood pressure is also usually lower in children than in adults (2007).
Regulating the Heart Rate:
Under normal circumstances, the heart controls the rate at which it contracts or beats. But another body system--the nervous system--can and does affect heart rate to help the body adapt to different situations. The medulla oblongata is a mass of nerve tissue at the top of the spinal cord and at the base of the brain that controls such involuntary processes as breathing and heart rate. Inside the medulla are two cardiac centers, the accelerator center and the inhibitory center. These centers send nerve impulses to the heart to regulate its beating (2007). The autonomic nervous system is a division of the nervous system that affects such internal organs as the heart, lungs, stomach, and liver. It functions involuntarily, meaning the processes it controls occur without conscious effort on the part of an individual. The autonomic nervous system is divided into two parts, the parasympathetic and sympathetic systems. The parasympathetic system is active primarily in normal, restful situations; the sympathetic system is most active during times of stress or when the body needs energy. The accelerator center in the medulla sends impulses along sympathetic nerves to the heart to increase heart rate and the force of contraction. The inhibitory center sends impulses along parasympathetic nerves to the heart to slow down the heart rate. The centers act in response to changes in blood pressure and the level of oxygen in the blood. These changes are often brought about by such factors as exercise, a rise in body temperature, and emotional stress. Such changes are detected by receptors located in the carotid arteries and the aortic arch (2007). Receptors in the carotid arteries detect a decrease in blood pressure; those in the aortic arch detect a decrease in the level of oxygen in the blood. Both send out impulses along sensory nerves to the accelerator center, which in turn sends impulses along nerves to the SA node of the heart to speed up the heart rate. When blood pressure or blood oxygen level has been restored to normal, the inhibitory center sends out impulses along nerves to the SA node to slow heart rate to a normal resting pace.
Exchanges Between Capillaries and General Body Tissues:
Arteries, arterioles, venules, and veins: the only function of these vessels is to transport blood from or to the heart. The exchange of blood, gases, and other materials--oxygen, carbon dioxide, nutrients, and wastes--between the blood and interstitial fluid occurs through the capillaries. The movement of these materials is variously brought about by three processes: diffusion, filtration, and osmosis.
When the ventricles contract, they force or propel blood from the heart into the large, elastic arteries that expand as the blood is pushed through them. The pressure the blood exerts against the inner walls of the blood vessels is known as blood pressure. This pressure is necessary to keep the blood flowing to all areas of the body and then back to the heart. Blood pressure is greatest in the large arteries closest to the heart. Because their walls are elastic, the arteries are able to recoil and keep most of the pressure on the blood as it flows away from the heart. As the blood courses through the system in less elastic vessels--arterioles into capillaries into venules into veins--blood pressure drops. When the blood finally returns to the right atrium via the venae cavae, the pressure behind it is almost zero (2007). Since the heart contracts and relaxes during a cardiac cycle, blood pressure rises and falls during each beat. It is higher during systole (left ventricle contracting) and lower during diastole (left ventricle relaxing). Blood pressure is measured in millimeters of mercury (mmHg) with a sphygmomanometer. A blood pressure reading is most often taken on the brachial artery in the arm. The systolic pressure is recorded first, followed by the diastolic pressure. Average young adults have a blood pressure reading of about 120 mmHg for systolic pressure and 80 mmHg for diastolic pressure (written as 120/80 and read as "one-twenty over eighty"). Depending on age, sex, weight, and other factors, normal blood pressure can range from 90 to 135 mmHg for the systolic pressure and 60 to 85 mmHg for the diastolic pressure. Blood pressure normally increases with age because the arteries are less flexible in older people. Normal blood pressure is also usually lower in children than in adults (2007).
Regulating the Heart Rate:
Under normal circumstances, the heart controls the rate at which it contracts or beats. But another body system--the nervous system--can and does affect heart rate to help the body adapt to different situations. The medulla oblongata is a mass of nerve tissue at the top of the spinal cord and at the base of the brain that controls such involuntary processes as breathing and heart rate. Inside the medulla are two cardiac centers, the accelerator center and the inhibitory center. These centers send nerve impulses to the heart to regulate its beating (2007). The autonomic nervous system is a division of the nervous system that affects such internal organs as the heart, lungs, stomach, and liver. It functions involuntarily, meaning the processes it controls occur without conscious effort on the part of an individual. The autonomic nervous system is divided into two parts, the parasympathetic and sympathetic systems. The parasympathetic system is active primarily in normal, restful situations; the sympathetic system is most active during times of stress or when the body needs energy. The accelerator center in the medulla sends impulses along sympathetic nerves to the heart to increase heart rate and the force of contraction. The inhibitory center sends impulses along parasympathetic nerves to the heart to slow down the heart rate. The centers act in response to changes in blood pressure and the level of oxygen in the blood. These changes are often brought about by such factors as exercise, a rise in body temperature, and emotional stress. Such changes are detected by receptors located in the carotid arteries and the aortic arch (2007). Receptors in the carotid arteries detect a decrease in blood pressure; those in the aortic arch detect a decrease in the level of oxygen in the blood. Both send out impulses along sensory nerves to the accelerator center, which in turn sends impulses along nerves to the SA node of the heart to speed up the heart rate. When blood pressure or blood oxygen level has been restored to normal, the inhibitory center sends out impulses along nerves to the SA node to slow heart rate to a normal resting pace.
Exchanges Between Capillaries and General Body Tissues:
Arteries, arterioles, venules, and veins: the only function of these vessels is to transport blood from or to the heart. The exchange of blood, gases, and other materials--oxygen, carbon dioxide, nutrients, and wastes--between the blood and interstitial fluid occurs through the capillaries. The movement of these materials is variously brought about by three processes: diffusion, filtration, and osmosis.
Simple diffusion (top) and carrier-facilitated diffusion (bottom) in a red blood cell (The Gale Encyclopedia of Science, 2010).
Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration. Diffusion takes place because molecules have free energy, meaning they are always in motion. This is particularly true of molecules in a gas, which move more rapidly than those in a solid or liquid. Oxygen and carbon dioxide, the gases that pass between the capillaries and the interstitial fluid, move by diffusion. As blood courses through a capillary, the oxygen carried by the hemoglobin in red blood cells exists in a greater amount and thus moves into the surrounding interstitial fluid to be taken up by the cells. Conversely, carbon dioxide exists in a greater amount in the interstitial fluid and so moves into the capillary to be carried away. This exchange of gases between the blood and the interstitial fluid is called internal respiration (2007).
Filtration is the movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure. When blood enters the capillaries, it has a pressure reading of about 33 mmHg; the pressure of the interstitial fluid is only about 2 mmHg. Thus plasma and such nutrients as amino acids, glucose, and vitamins are forced by filtration through the capillary walls into the surrounding interstitial fluid.
Osmosis is the diffusion of water through a semipermeable membrane (a membrane that allows some materials but not others to flow through it). It is the movement of water from an area where it is abundant to an area where it is scarce or less abundant. Directly related to this movement is osmotic pressure, which is the tendency of a solution to "pull" water into it. The strength of this pressure is determined by the amount of dissolved material, called solutes, in the solution. The greater the amount of solutes, the lower the amount of water in that solution. A solution containing a high amount of solutes has a high osmotic pressure, and water has a greater tendency to move into the solution.
Capillary Exchange in the Lungs:
After blood has flowed through the tissues of the body, exchanging oxygen and nutrients for carbon dioxide and wastes, it heads back to the heart. The deoxygenated blood empties into the right atrium via the venae cavae, then into the right ventricle. From here it is pumped into the pulmonary trunk, which then divides into the right and left pulmonary arteries. These arteries transport the deoxygenated blood to each lung. In the lungs, the arteries branch out into successively smaller arteries and successively smaller arterioles. Finally, the smallest arterioles branch into capillaries. These pulmonary capillaries surround the alveoli, the air sacs in the lungs. The exchange of oxygen and carbon dioxide in the lungs, known as external respiration, takes place across the walls of the alveoli and nearby capillaries (2007). As in internal respiration, the exchange of gases in external respiration occurs through diffusion. Air in the alveoli has a high concentration of oxygen. The blood in the pulmonary capillaries has a high concentration of carbon dioxide. Following diffusion, the oxygen in the alveoli moves into the capillaries while the carbon dioxide in the capillaries moves into the alveoli. Now the freshly oxygenated blood flows from the capillaries into venules, which merge to form larger and larger veins. Finally, the blood leaves each lung through two large pulmonary veins and is carried to the left atrium to be pumped back into the systemic circulation once again. The movement of blood from the lungs to the heart is a special occurrence in the body: it is the only time and location in the body in which veins carry oxygenated blood.
Hepatic Portal Circulation:
Another unique circulation route is the hepatic portal circulation, a subdivision of the systemic circulation. Within this circulation pathway, blood from the digestive organs and the spleen flow through the liver before heading to the heart. The capillaries that drain the stomach, small intestine, colon, pancreas, and spleen flow into two large veins, the superior mesenteric vein and the splenic vein. These two veins then unite to form the portal vein, which carries the blood into the liver. Once in the liver, the portal vein branches to form capillaries called sinusoids. Sinusoids are larger than normal capillaries. Their walls are also more permeable, allowing proteins and blood cells to enter or leave easily. This feature is important since the blood entering the liver from the digestive organs contains large amounts of nutrients (2007). As the blood flows slowly through the sinusoids in the liver, some of these nutrients are removed from the blood and either stored in the liver for later use or changed into other materials the body needs. From the sinusoids, blood flows into the right and left hepatic veins, then into the inferior vena cava, and finally into the right atrium. The complete flow of blood from the digestive organs to the heart is unusual. Normally, arteries flow into capillaries, which flow into veins. In the hepatic portal circulation, no arteries are involved. Here the capillaries merge to form veins, which branch out again into capillaries that merge again to form veins. This strange route is necessary so that the blood may be altered by the liver. Nutrients may be stored or changed and possible poisons (such as alcohol and medicines) may be transformed into less harmful substances before the blood returns to the heart and the rest of the circulation.
Filtration is the movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure. When blood enters the capillaries, it has a pressure reading of about 33 mmHg; the pressure of the interstitial fluid is only about 2 mmHg. Thus plasma and such nutrients as amino acids, glucose, and vitamins are forced by filtration through the capillary walls into the surrounding interstitial fluid.
Osmosis is the diffusion of water through a semipermeable membrane (a membrane that allows some materials but not others to flow through it). It is the movement of water from an area where it is abundant to an area where it is scarce or less abundant. Directly related to this movement is osmotic pressure, which is the tendency of a solution to "pull" water into it. The strength of this pressure is determined by the amount of dissolved material, called solutes, in the solution. The greater the amount of solutes, the lower the amount of water in that solution. A solution containing a high amount of solutes has a high osmotic pressure, and water has a greater tendency to move into the solution.
Capillary Exchange in the Lungs:
After blood has flowed through the tissues of the body, exchanging oxygen and nutrients for carbon dioxide and wastes, it heads back to the heart. The deoxygenated blood empties into the right atrium via the venae cavae, then into the right ventricle. From here it is pumped into the pulmonary trunk, which then divides into the right and left pulmonary arteries. These arteries transport the deoxygenated blood to each lung. In the lungs, the arteries branch out into successively smaller arteries and successively smaller arterioles. Finally, the smallest arterioles branch into capillaries. These pulmonary capillaries surround the alveoli, the air sacs in the lungs. The exchange of oxygen and carbon dioxide in the lungs, known as external respiration, takes place across the walls of the alveoli and nearby capillaries (2007). As in internal respiration, the exchange of gases in external respiration occurs through diffusion. Air in the alveoli has a high concentration of oxygen. The blood in the pulmonary capillaries has a high concentration of carbon dioxide. Following diffusion, the oxygen in the alveoli moves into the capillaries while the carbon dioxide in the capillaries moves into the alveoli. Now the freshly oxygenated blood flows from the capillaries into venules, which merge to form larger and larger veins. Finally, the blood leaves each lung through two large pulmonary veins and is carried to the left atrium to be pumped back into the systemic circulation once again. The movement of blood from the lungs to the heart is a special occurrence in the body: it is the only time and location in the body in which veins carry oxygenated blood.
Hepatic Portal Circulation:
Another unique circulation route is the hepatic portal circulation, a subdivision of the systemic circulation. Within this circulation pathway, blood from the digestive organs and the spleen flow through the liver before heading to the heart. The capillaries that drain the stomach, small intestine, colon, pancreas, and spleen flow into two large veins, the superior mesenteric vein and the splenic vein. These two veins then unite to form the portal vein, which carries the blood into the liver. Once in the liver, the portal vein branches to form capillaries called sinusoids. Sinusoids are larger than normal capillaries. Their walls are also more permeable, allowing proteins and blood cells to enter or leave easily. This feature is important since the blood entering the liver from the digestive organs contains large amounts of nutrients (2007). As the blood flows slowly through the sinusoids in the liver, some of these nutrients are removed from the blood and either stored in the liver for later use or changed into other materials the body needs. From the sinusoids, blood flows into the right and left hepatic veins, then into the inferior vena cava, and finally into the right atrium. The complete flow of blood from the digestive organs to the heart is unusual. Normally, arteries flow into capillaries, which flow into veins. In the hepatic portal circulation, no arteries are involved. Here the capillaries merge to form veins, which branch out again into capillaries that merge again to form veins. This strange route is necessary so that the blood may be altered by the liver. Nutrients may be stored or changed and possible poisons (such as alcohol and medicines) may be transformed into less harmful substances before the blood returns to the heart and the rest of the circulation.