Circulatory System Design BIO 40 by Owen Borville August 9, 2025
The circulatory system in humans and animals is a network of cylindrical vessels that carry blood through the arteries, veins, and capillaries and are powered by a pump known as the heart. In most animals and humans, the circulatory system is a closed-loop system, where blood does not exist freely in a cavity.
In a closed circulatory system, blood is kept inside blood vessels and circulates unidirectionally from the heart around the systematic circulatory route, then returns to the heart again. The closed circulatory system is found in all vertebrates and some invertebrates.
In an open circulatory system blood is not enclosed in vessels but is pumped into a cavity called a hemocel and is called hemolymph because the blood mixes with the interstitial fluid. Invertebrates like insects, crustaceans, and mollusks have an open circulatory system. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then re-enters the hearts through openings called ostia. This movement allows for gas and nutrient exchange.
Some invertebrate animals do not have a circulatory system like sponges and rotifers because diffusion enables the exchange of water, nutrients, waste, and dissolved gasses. Jellies also use diffusion through their epidermis and internally through their gastrovascular compartment. Diffusion occurs both internally and externally as the animal lives in water.
Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated, called gill-circulation. The blood then continues through the rest of the body before arriving back at the atrium, called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish's systemic circuit. The oxygen reach in the body is limited and reduces the metabolic ability of the fish.
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, called pulmonary circulation. The other is throughout the rest of the body and its organs including the brain. In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.
Amphibians have a three chambered heart that has two atria and one ventricle instead of the two-chambered heart of fish. The two atria, or superior heart chambers, receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart's ventricle or inferior heart chamber, which reduces the efficiency of oxygenation. The advantage of this system is that high pressure in the vessels pushes blood to the lungs and body. Amphibians also have double circulation as oxygen-rich blood is diverted to the systemic circular system and deoxygenated blood to the pulmocutaneous circuit.
Reptiles usually have a three-chambered heart like the amphibian heart that directs blood to the pulmonary and systemic circuits. The ventricle is divided more effectively by a partial septum, which prevents mixing of oxygenated and deoxygenated blood.
Some reptiles have a four-chambered heart. Crocodiles can divert blood from the lungs to the stomach and other organs during long periods of submergence. There are two main arteries, where one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. A hole in the heart between the two ventricles, the foramen of Panizza, allows blood to move from one side of the heart to the other and specialized connective tissue that slows the blood flow to the lungs.
In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation.
Hemoglobin, Hb, is the protein responsible for distributing oxygen and carbon dioxide throughout the circulatory systems of humans and animals. In addition to hemoglobin protein, blood also contains plasma, the liquid portion that contains water, proteins, salt, electrolytes, lipids, and glucose. Also in the blood is cells (red and white blood cells) and cell fragments called platelets. The red cells are responsible for carrying the gases (oxygen and CO2) and the white cells are responsible for immune response. The platelets are responsible for blood clotting. Interstitial fluid that surrounds the cells is separate from the blood, but in hemolymph, they are combined.
Blood regulates body systems and homeostasis by stabilizing pH, temperature, osmotic pressure, and by eliminating excess heat. Blood helps growth by distributing nutrients and hormones in the body and removes waste. Blood clotting by platelets prevents blood loss and white blood cells transport disease-fighting agents to sites of infection.
Red blood cells are specialized cells that circulate throughout the body delivering oxygen to cells. Red blood cells are formed from stem cells in the bone marrow. The red coloring of red blood cells comes from iron in the protein hemoglobin. The main job is to carry oxygen and also carbon dioxide. Each hemoglobin molecule has four oxygen atoms and there are 250 million molecules of hemoglobin per cell. So each red blood cell contains one billion molecules of oxygen.
Not all organisms use hemoglobin for oxygen transport. Invertebrates that use hemolymph instead of blood use different pigments to bind the oxygen, such as copper or iron. Other respiratory pigments that invertebrates use include hemocyanin, a blue-green protein which contains copper. Chlorocruorin is a green, iron-containing pigment. Hemerythrin is a red, iron containing protein.
Red blood cells have a large surface area and small size which enables rapid diffusion of oxygen and carbon dioxide across the plasma membrane. In the lungs, carbon dioxide is released and oxygen is taken in by the blood. In the tissues, oxygen is released from the blood and carbon dioxide is sent back to the lungs.
Red blood cells also have glycolipid and glycoprotein coating, which have carbohydrate molecules attached, and determine the various blood types.
White blood cells (also called leukocytes) make up a small percentage of the blood and their function is very different than red blood cells. White blood cells help initiate the immune response to identify and target pathogens, such as bacteria, viruses, and foreign organisms. White blood cells have nuclei but don't have hemoglobin.
The two main types of white blood cells are granulocytes and agranulocytes. Granulocytes contain granules in their cytoplasm. Agranulocytes lack granules in their cytoplasm.
Blood clotting prevents bleeding and is performed by small cell fragments called platelets that are attracted to the wound site and inject materials into the wound site that enable and activate blood clotting.
The liquid component of blood is plasma, which is more than 90 percent water, and contains substances that maintain the body's pH, osmotic load, and protect the body. Plasma also contains coagulation factors and antibodies.
Serum is the part of the plasma other than the coagulation factors. Serum is similar to interstitial fluid and has key ions as electrolytes helps the function of muscles and nerves in the body. Serum also includes proteins that maintain pH, osmotic balance, and give viscosity to the blood. Serum also contain antibodies, which are proteins that fight bacteria and viruses. Lipids, including cholesterol, are transported in the serum, along with nutrients, hormones, waste, drugs, viruses, and bacteria. Human serum albumin is the most abundant protein in human blood plasma and it is made in the liver.
The heart in mammals pumps blood through the circulatory system: the coronary vessels that serve the heart, the pulmonary heart and lungs, and systems of the body. The heart muscle is asymmetric and about the size of a fist. The heart is divided into four chambers: two atria and two ventricles. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood.
The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava, which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself.
This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens only in one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the bicuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the semilunar valve (pulmonic valve) to the pulmonary arteries and on to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and to the aorta, the aortic semilunar valve (aortic valve) closes preventing blood from flowing backwards into the left ventricle. This pattern of pumping is called double circulation and is found in all mammals.
The heart is composed of three layers: the epicardium, the myocardium, and the endocardium. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart. It allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.
The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again to the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain (called angina) and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack. Marie M. Daly was a biochemist that first identified the link between cholesterol, high blood pressure, and the causes of atherosclerosis.
The cardiac cycle is a repeating sequence and the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body. This is followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, which pushes the blood through the atrioventricular valves into the ventricles. The closing of the atrioventricular valves produces the "lup" sound. Soon the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs. Closing of the semilunar valves produces the "dup" sound.
The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. Cardiomyocytes are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntary like smooth muscle. They are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.
The autonomous beating of cardiac muscle cells is regulated by the heart's internal pacemaker that uses electric signals to time the beating of the heart. The electric signals and mechanical functions are closely connected. The internal pacemaker starts at the sinoatrial (SA) node located near the wall of the right atrium. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes and observed on an ECG (electrocardiogram), which is a recording of the electrical impulses of the cardiac muscle.
Arteries are blood vessels that take blood away from the heart. The main artery is the aorta that branches into major arteries and transport blood to different limbs and organs. The carotid artery carries blood to the brain, the brachial arteries take blood to the arms, the thoracic artery carries blood to the thorax then to the hepatic, renal, and gastric arteries for the liver, kidney, and stomach respectively. The iliac artery takes blood to the lower limbs. Arteries branch into smaller arteries, then smaller vessels called arterioles.
Arterioles branch into capillary beds, which contain a large number of capillaries that branch among the cells and tissues of the body. Capillaries are narrow diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries merge again into venules that connect minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart by way of the lymphatic system.
Three distinct layers or tunics form the walls of blood vessels. The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of blood vessels, and vasodilation, the widening of blood vessels. This is important in the overall regulation of blood pressure.
Veins and arteries have two more tunics that surround the endothelium. The middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves that prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.
Blood pressure is the pressure exerted by the blood on the walls of a blood vessel that helps push blood through the body. Systolic blood pressure measures the amount of pressure that blood exerts on vessels while the heart is beating. Diastolic blood pressure measures the pressure in the vessels between heartbeats. Factors that affect blood pressure are hormones, stress, exercise, eating, sitting, and standing. Blood flow in the body is regulated by the size of blood vessels, by the action of smooth muscle, by one-way valves, and by the fluid pressure of blood itself.
The heart pumps blood through the body. As blood moves through smaller vessels, the flow rate slows down significantly. Fluids normally flow faster within smaller diameter tubes. However, because the overall diameter of all capillaries in the body combined together is much greater than the diameter of the aorta, the flow rate in the capillaries is slower.
The slower rate of blood flow through capillaries allows for gas and nutrient exchange and diffusion of fluid into interstitial space. Blood primarily moves in the veins by the rhythmic movement of smooth muscle in the vessel wall and by the action of skeletal muscle as the body moves. Blood is prevented from moving backward by way of gravity and one-way valves.
Blood flow through the body is regulated by the body to address needs and is directed by nerve and hormone signals. Blood flow can change during and after eating, and during exercise.
Some blood plasma enters the lymphatic vessels and flows through lymph nodes, which filter the lymph liquid by percolation through connective tissue filled with white blood cells, which remove bacteria and viruses, and clean the lymph liquid before it returns to the bloodstream by the action of smooth and skeletal muscles and one-way valves.
Blood pressure is exerted by the blood on the walls of the blood vessels, specifically hydrostatic pressure. Fluid will move from areas of high pressure to areas of low pressure. Hydrostatic pressure near the heart is very high. The rate of flow is slowed by the smaller openings of the arterioles.
Artery walls can stretch temporarily to accommodate an increase in blood pressure. Blood continues to enter the arterioles at a constant or even rate, caused by the resistance to blood flow called peripheral resistance.
Cardiac output is the volume of blood pumped by the heart in one minute and is calculated by multiplying the number of heart contractions that occur per minute or heart rate times the stroke volume, which is the volume of blood pumped into the aorta per contraction of the left ventricle.
Cardiac output can be increased by increasing heart rate, such as during exercise. In addition, cardiac output can be increased by increasing stroke volume, such as when the heart contracts with greater strength or faster blood circulation.
Blood vessels can increase in diameter to offset the increase in heart rate. Stress decreases the diameter of blood vessels, and increases blood pressure. Nerve signals and hormones can also affect blood pressure, along with body position.
The circulatory system in humans and animals is a network of cylindrical vessels that carry blood through the arteries, veins, and capillaries and are powered by a pump known as the heart. In most animals and humans, the circulatory system is a closed-loop system, where blood does not exist freely in a cavity.
In a closed circulatory system, blood is kept inside blood vessels and circulates unidirectionally from the heart around the systematic circulatory route, then returns to the heart again. The closed circulatory system is found in all vertebrates and some invertebrates.
In an open circulatory system blood is not enclosed in vessels but is pumped into a cavity called a hemocel and is called hemolymph because the blood mixes with the interstitial fluid. Invertebrates like insects, crustaceans, and mollusks have an open circulatory system. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then re-enters the hearts through openings called ostia. This movement allows for gas and nutrient exchange.
Some invertebrate animals do not have a circulatory system like sponges and rotifers because diffusion enables the exchange of water, nutrients, waste, and dissolved gasses. Jellies also use diffusion through their epidermis and internally through their gastrovascular compartment. Diffusion occurs both internally and externally as the animal lives in water.
Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated, called gill-circulation. The blood then continues through the rest of the body before arriving back at the atrium, called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish's systemic circuit. The oxygen reach in the body is limited and reduces the metabolic ability of the fish.
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, called pulmonary circulation. The other is throughout the rest of the body and its organs including the brain. In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.
Amphibians have a three chambered heart that has two atria and one ventricle instead of the two-chambered heart of fish. The two atria, or superior heart chambers, receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart's ventricle or inferior heart chamber, which reduces the efficiency of oxygenation. The advantage of this system is that high pressure in the vessels pushes blood to the lungs and body. Amphibians also have double circulation as oxygen-rich blood is diverted to the systemic circular system and deoxygenated blood to the pulmocutaneous circuit.
Reptiles usually have a three-chambered heart like the amphibian heart that directs blood to the pulmonary and systemic circuits. The ventricle is divided more effectively by a partial septum, which prevents mixing of oxygenated and deoxygenated blood.
Some reptiles have a four-chambered heart. Crocodiles can divert blood from the lungs to the stomach and other organs during long periods of submergence. There are two main arteries, where one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. A hole in the heart between the two ventricles, the foramen of Panizza, allows blood to move from one side of the heart to the other and specialized connective tissue that slows the blood flow to the lungs.
In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation.
Hemoglobin, Hb, is the protein responsible for distributing oxygen and carbon dioxide throughout the circulatory systems of humans and animals. In addition to hemoglobin protein, blood also contains plasma, the liquid portion that contains water, proteins, salt, electrolytes, lipids, and glucose. Also in the blood is cells (red and white blood cells) and cell fragments called platelets. The red cells are responsible for carrying the gases (oxygen and CO2) and the white cells are responsible for immune response. The platelets are responsible for blood clotting. Interstitial fluid that surrounds the cells is separate from the blood, but in hemolymph, they are combined.
Blood regulates body systems and homeostasis by stabilizing pH, temperature, osmotic pressure, and by eliminating excess heat. Blood helps growth by distributing nutrients and hormones in the body and removes waste. Blood clotting by platelets prevents blood loss and white blood cells transport disease-fighting agents to sites of infection.
Red blood cells are specialized cells that circulate throughout the body delivering oxygen to cells. Red blood cells are formed from stem cells in the bone marrow. The red coloring of red blood cells comes from iron in the protein hemoglobin. The main job is to carry oxygen and also carbon dioxide. Each hemoglobin molecule has four oxygen atoms and there are 250 million molecules of hemoglobin per cell. So each red blood cell contains one billion molecules of oxygen.
Not all organisms use hemoglobin for oxygen transport. Invertebrates that use hemolymph instead of blood use different pigments to bind the oxygen, such as copper or iron. Other respiratory pigments that invertebrates use include hemocyanin, a blue-green protein which contains copper. Chlorocruorin is a green, iron-containing pigment. Hemerythrin is a red, iron containing protein.
Red blood cells have a large surface area and small size which enables rapid diffusion of oxygen and carbon dioxide across the plasma membrane. In the lungs, carbon dioxide is released and oxygen is taken in by the blood. In the tissues, oxygen is released from the blood and carbon dioxide is sent back to the lungs.
Red blood cells also have glycolipid and glycoprotein coating, which have carbohydrate molecules attached, and determine the various blood types.
White blood cells (also called leukocytes) make up a small percentage of the blood and their function is very different than red blood cells. White blood cells help initiate the immune response to identify and target pathogens, such as bacteria, viruses, and foreign organisms. White blood cells have nuclei but don't have hemoglobin.
The two main types of white blood cells are granulocytes and agranulocytes. Granulocytes contain granules in their cytoplasm. Agranulocytes lack granules in their cytoplasm.
Blood clotting prevents bleeding and is performed by small cell fragments called platelets that are attracted to the wound site and inject materials into the wound site that enable and activate blood clotting.
The liquid component of blood is plasma, which is more than 90 percent water, and contains substances that maintain the body's pH, osmotic load, and protect the body. Plasma also contains coagulation factors and antibodies.
Serum is the part of the plasma other than the coagulation factors. Serum is similar to interstitial fluid and has key ions as electrolytes helps the function of muscles and nerves in the body. Serum also includes proteins that maintain pH, osmotic balance, and give viscosity to the blood. Serum also contain antibodies, which are proteins that fight bacteria and viruses. Lipids, including cholesterol, are transported in the serum, along with nutrients, hormones, waste, drugs, viruses, and bacteria. Human serum albumin is the most abundant protein in human blood plasma and it is made in the liver.
The heart in mammals pumps blood through the circulatory system: the coronary vessels that serve the heart, the pulmonary heart and lungs, and systems of the body. The heart muscle is asymmetric and about the size of a fist. The heart is divided into four chambers: two atria and two ventricles. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood.
The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava, which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself.
This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens only in one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the bicuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the semilunar valve (pulmonic valve) to the pulmonary arteries and on to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and to the aorta, the aortic semilunar valve (aortic valve) closes preventing blood from flowing backwards into the left ventricle. This pattern of pumping is called double circulation and is found in all mammals.
The heart is composed of three layers: the epicardium, the myocardium, and the endocardium. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart. It allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.
The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again to the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain (called angina) and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack. Marie M. Daly was a biochemist that first identified the link between cholesterol, high blood pressure, and the causes of atherosclerosis.
The cardiac cycle is a repeating sequence and the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body. This is followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, which pushes the blood through the atrioventricular valves into the ventricles. The closing of the atrioventricular valves produces the "lup" sound. Soon the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs. Closing of the semilunar valves produces the "dup" sound.
The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. Cardiomyocytes are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntary like smooth muscle. They are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.
The autonomous beating of cardiac muscle cells is regulated by the heart's internal pacemaker that uses electric signals to time the beating of the heart. The electric signals and mechanical functions are closely connected. The internal pacemaker starts at the sinoatrial (SA) node located near the wall of the right atrium. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes and observed on an ECG (electrocardiogram), which is a recording of the electrical impulses of the cardiac muscle.
Arteries are blood vessels that take blood away from the heart. The main artery is the aorta that branches into major arteries and transport blood to different limbs and organs. The carotid artery carries blood to the brain, the brachial arteries take blood to the arms, the thoracic artery carries blood to the thorax then to the hepatic, renal, and gastric arteries for the liver, kidney, and stomach respectively. The iliac artery takes blood to the lower limbs. Arteries branch into smaller arteries, then smaller vessels called arterioles.
Arterioles branch into capillary beds, which contain a large number of capillaries that branch among the cells and tissues of the body. Capillaries are narrow diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries merge again into venules that connect minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart by way of the lymphatic system.
Three distinct layers or tunics form the walls of blood vessels. The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of blood vessels, and vasodilation, the widening of blood vessels. This is important in the overall regulation of blood pressure.
Veins and arteries have two more tunics that surround the endothelium. The middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves that prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.
Blood pressure is the pressure exerted by the blood on the walls of a blood vessel that helps push blood through the body. Systolic blood pressure measures the amount of pressure that blood exerts on vessels while the heart is beating. Diastolic blood pressure measures the pressure in the vessels between heartbeats. Factors that affect blood pressure are hormones, stress, exercise, eating, sitting, and standing. Blood flow in the body is regulated by the size of blood vessels, by the action of smooth muscle, by one-way valves, and by the fluid pressure of blood itself.
The heart pumps blood through the body. As blood moves through smaller vessels, the flow rate slows down significantly. Fluids normally flow faster within smaller diameter tubes. However, because the overall diameter of all capillaries in the body combined together is much greater than the diameter of the aorta, the flow rate in the capillaries is slower.
The slower rate of blood flow through capillaries allows for gas and nutrient exchange and diffusion of fluid into interstitial space. Blood primarily moves in the veins by the rhythmic movement of smooth muscle in the vessel wall and by the action of skeletal muscle as the body moves. Blood is prevented from moving backward by way of gravity and one-way valves.
Blood flow through the body is regulated by the body to address needs and is directed by nerve and hormone signals. Blood flow can change during and after eating, and during exercise.
Some blood plasma enters the lymphatic vessels and flows through lymph nodes, which filter the lymph liquid by percolation through connective tissue filled with white blood cells, which remove bacteria and viruses, and clean the lymph liquid before it returns to the bloodstream by the action of smooth and skeletal muscles and one-way valves.
Blood pressure is exerted by the blood on the walls of the blood vessels, specifically hydrostatic pressure. Fluid will move from areas of high pressure to areas of low pressure. Hydrostatic pressure near the heart is very high. The rate of flow is slowed by the smaller openings of the arterioles.
Artery walls can stretch temporarily to accommodate an increase in blood pressure. Blood continues to enter the arterioles at a constant or even rate, caused by the resistance to blood flow called peripheral resistance.
Cardiac output is the volume of blood pumped by the heart in one minute and is calculated by multiplying the number of heart contractions that occur per minute or heart rate times the stroke volume, which is the volume of blood pumped into the aorta per contraction of the left ventricle.
Cardiac output can be increased by increasing heart rate, such as during exercise. In addition, cardiac output can be increased by increasing stroke volume, such as when the heart contracts with greater strength or faster blood circulation.
Blood vessels can increase in diameter to offset the increase in heart rate. Stress decreases the diameter of blood vessels, and increases blood pressure. Nerve signals and hormones can also affect blood pressure, along with body position.