The Circulatory System

Types of Circulatory Systems
Living things must be capable of transporting nutrients, wastes and gases to and form cells, Single-celled organisms use their cell surface as a point of exchange with the outside environment. Multicellular orgainsms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes. Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by ciliary action. Simple animals, such as the hydra and planaria lack specialized organs such as hearts and blood vessels, instead using thrir skin as an exchange point for materials, This, however, limits the size an animal can attain. To become larger, they need specialized organs and organs systems.
Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide and metabolic wastes. Components of the circulatory system include
- blood: a connective tissue of liquid plasma and cells
- heart: a muscular pump to move the blood
- blood vessels: arteries, capillaries and veins that deliver blood to all tissues
There are several types of circulatory systems. The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded· by the blood. The resulting blood flow is sluggish
Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems (evolved in echinoderms and vertebrates) have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Blood flow is not sluggish. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly hemoglobin molecules in blood cells transport oxygen. The human closed circulatory system is sometimes called the cardiovascular system.
A secondary circulatory system, the lymphatic circulation, collects fluid and cells and returns them to the cardiovascular system.
Vertebrate Cardiovascular System
The vertebrate cardiovascular system includes a heart, which is a muscular pump that contracts to propel blood out to the body through arteries, and a series of blood vessels. The upper chamber of the heart, the atrium (pI. atria), is where the blood enters the heart. Passing through a valve, blood enters the lower chamber, the ventricle. Contraction of the ventricle forces blood from the heart through an artery. The heart muscle is composed of cardiac muscle cells.
Arteries are blood vessels that carry blood away from heart. Arterial walls are able to expand and contract. Arteries have three layers of thick walls. Smooth muscle fibers contract, another layer of connective tissue is quite elastic, allowing the arteries to carry blood under high pressure.
Heart at rest
The aorta is the main artery leaving the heart. The pulmonary artery is the only artery that carries oxygen-poor blood. The pulmonary artery carries deoxygenated blood to the lungs. In the lungs, gas exchange occurs, carbon dioxide diffuses out, oxygen diffuses in. Arterioles are small arteries that connect larger arteries with capillaries. Small arterioles branch into collections of capillaries known as capillary beds
Capillaries are thin-walled blood vessels in which gas exchange occurs. In the capillary, the wall is only one cell layer thick. Capillaries are concentrated into capillary beds. Some capillaries have small pores between the cells of the capillary wall, allowing materials to flow in and out of capillaries as well as the passage of white blood cells. Changes in blood pressure also occur in the various vessels of the circulatory system. Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries. Capillaries are microscopic in size, although blushing is one manifestation of blood flow into capillaries. Control of blood flow into capillary beds is done by nerve-controlled sphincters.
The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes. Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary
The extensive network of capillaries in the human body is estimated at between 50,000 and 60,000 miles long. Thoroughfare channels allow blood to bypass a capillary bed. These channels can open and close by the action of muscles that control blood flow through the channels
Blood leaving the capillary beds flows into a progressively larger series of venules that in turn join to form veins. Veins carry blood from capillaries to the heart. With the exception of the pulmonary veins, blood in veins is oxygen-poor. The pulmonary veins carry oxygenated blood from lungs back to the heart. Venules are smaller veins that gather blood from capillary beds into veins. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along. The veins have valves that prevent back-flow of blood
Ventricular contraction propels blood 'into arteries under great pressure. Blood pressure is measured in mm of mercury; healthy young adults should have pressure of ventricular systole of 120mm, and 80 mm at ventricular diastole. Higher pressures (human 120/80 as compared to a 12/1 in lobsters) mean the volume of blood circulates faster (20 seconds in humans, 8 minutes in lobsters).
As blood gets farther from the heart, the pressure likewise decreases. Each contraction of the ventricles sends pressure through the arteries. Elasticity of lungs helps keep pulmonary pressures low.
Systemic pressure is sensed by receptors in the arteries and atria. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure.
Vertebrate Vascular Systems
Humans, birds, and mammals have a four-chambered heart that completely separate oxygen-rich and oxygen-depleted blood. Fish have a two-chambered heart in which a single-loop circulatory pattern takes blood from the heart to the gills and then to the body. Amphibians have a three-chambered heart with two atria and one ventricle. A loop from the heart goes to the pulmonary capillary beds, where gas exchange occurs. Blood then is returned to the heart. Blood exiting the ventricle is diverted. some to the pulmonary circuit, some to systemic circuit. The disadvantage of the three-chambered heart is the mixing of oxygenated and deoxygenated blood. Some reptiles have partial separation of the ventricle. Other reptiles, plus, all birds and mammals, have a fourchambered heart, with complete separation of both systemic and pulmonary circuits.
The Heart
The human heart is a two-sided, four-chambered structure with muscular walls. An atrioventricular (AV), valve separates each auricle from ventricle. A semilunar (also known as arterial) valve separates each ventricle from its connecting artery.
The heart beats or contracts approximately 70 times per minute. The human heart will undergo over 3 billIon contraction cycles during a normal lifetime. The cardiac cycle consists of two parts: systole (contraction of the heart muscle) and diastole (relaxation of the heart muscle). -Atria contract while ventricles relax. The pulse is a wave of traction transmitted along the arteries. Valves in the heart open and close during the cardiac cycle. Heart muscle contraction' is due to the presence of nodal tissue in two regions of the heart. The SA node (sinoatrial node) initiates heartbeat. The A V node (atrioventricul.at node) causes ventricles to contract. The A V node is sometimes called the pacem~ker since it keeps heartbeat regular. Heartbeat is also controlled by nerve messages originating from the autonomic nervous system.
Blood flows through the heart from veins to atria to ventricles out by arteries. Heart valves limit flow to a single direction. One heartbeat, or cardiac cycle, includes atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause. Normal cardiac cycles (at rest) take 0.8 seconds. Blood from the body flows into the vena cava, which empties into the right atrium. At the same time, oxygenated blood from the lungs flows from the pulmonary vein into the left atrium. The muscles of both atria contract, forcing blood downward through each AV valve into each ventricle.
Diastole is the filling of the ventricles with blood. Ventricular systole opens the SL valves, forcing blood out of the ventricles through the pulmonary artery or aorta. The sound of the heart contracting and the valves opening and closing produces a characteristic "lub-dub" sound. Lub is associated with closure of the AV valves, dub is the closing of the SL valves.
Human heartbeats' originate from the sinoatrial node (SA node) near the right atrium. Modified muscle cells contract, sending a signal to other muscle cells in the heart to contract. The signal.spreads to the atrioventricular node (AV node). Signals carried from the AV node, slightly delayed, through bundle of His fibers and Purkinjie fibers cause the ventricles to contract simultaneously.
An electrocardiogram (ECG) measures changes in electrical potential across the heart, and can detect the contraction pulses that pass over the surface of the heart. Positive deflections are the Q and S waves. The P wave represents the contraction impulse of the atria, the T wave the ventricular contraction. ECGs are useful in diagnosing heart abnormalities.
Diseases of the Heart and Cardiovascular System
Cardiac muscle cells are serviced by a system of coronary arteries. During exercise the flow through these arteries is up to five times normal flow. Blocked flow in coronary arteries can result in death of heart muscle, leading to a heart attack.
Blockage of coronary arteries is usually the result of gradual buildup of lipids and cholesterol in the inner wall of the coronary artery. Occasional chest pain, angina pectoralis, can result during periods of stress or physical exertion. Angina indicates oxygen demands are greater than capacity to deliver it and that a heart attack may occur in the future. Heart muscle cells that die are not replaced since heart muscle cells do not divide.
The Vascular System
Two main routes for circulation are the pulmonary (to and from the lungs) and the systemic (to and from the body). Pulmonary arteries carry blood from the heart to the lungs. In the lungs gas exchange occurs. Pulmonary veins carry blood from lungs to heart. The aorta is the main artery of systemic circuit. The vena cavae are the main veins of the systemic circuit. Coronary arteries deliver oxygenated blood, food, etc. to the heart. Animals often have a portal system, which begins and ends in capillaries, such as between the digestive tract and the liver.
Fish pump blood from the heart to their gills, where gas exchange occurs, and then on to the rest of the body. Mammals pump blood to the lungs for gas exchange, then back to the heart for pumping out to the systemic circulation. Blood flows in only one direction.
Blood
Plasma is the liquid component of the blood. Mammalian blood consists of a liquid (plasma) and a number of cellular and cell fragment components as shown in Figure 21. Plasma is about 60 % of a volume of blood; cells and fragments are 40%. Plasma has 90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and gases. It acts as a buffer, maintaining pH near 7.4. Plasma contains nutrients, wastes, salts, proteins, etc. Proteins in the blood aid in transport of large molecules such as cholesterol.
Red blood cells, also known as erythrocytes, are flattened, doubly concave cells about 7 um in diameter that carry oxygen associated in the cell's hemoglobin. Mature erythrocytes lack a nucleus. They are small, 4 to 6 million cells per cubic millimeter of blood, and have 200 'million hemoglobin molecules per cell. Humans have a total of 25 trillion red blood cells (about 1/3 of all the cells, in the body). Red blood cells are continuously manufactured in red marrow of long bones, ribs, skull, and vertebrae. Life-span of an erythrocyte is only 120 days, after which they are destroyed in liver and spleen. Iron from hemoglobin is recovered and reused by red marrow. The liver degrades the heme units and secretes them as pigment in the bile, responsible for the color of feces. Each second two million red blood cells are produced to replace those thus taken out of circulation.
White blood cells, also known as leukocytes, are larger than erythrocytes, have a nucleus, and lack hemoglobin. They function in the cellular immune response. White blood cells (leukocytes) are less than 1% of the blood's volume. They are made from stem cells in bone marrow. There are five types of leukocytes, important components of the immune system. Neutrophils enter the tissue fluid by squeezing through capillary walls and phagocytozing foreign substances. Macrophages release white blood cell growth factors, causing a population increase for white blood cells. Lymphocytes fight infection. T-cells attack cells containing viruses. B-cells produce antibodies. Antigen-antibody complexes are phagocytized by a macrophage. White blo04 cells can squeeze through pores in the capillaries and fight infectious diseases in interstitial areas
Platelets result from cell fragmentation and are involved with clotting, as is shown by Figures 17 and 18. Platelets are cell' fragments that bud off megakaryocytes in bone marrow. They carry chemicals essential to blood clotting. Platelets sunive for 10 days before being removed by the liver and spleen. There are 150,000 to 300,000 platelets in each milliliter of blood. Platelets stick and adhere to tears in blood vessels; they also release clotting factors. A hemophiliac's blood cannot clot. Providing correct proteins (clotting factors) has been a common method of treating hemophiliacs. It has also led to HIV transmission due to the use of transfusions and use of contaminated blood products.
The Lymphatic System
Water and plasma are forced from the capillaries into intracellular spaces. This interstitial fluid transports materials between cells. Most of this fluid is collected in the capillaries of a secondary circulatory system, the lymphatic system. Fluid in this system is known as lymph.
Lymph flows from small lymph capillaries into lymph vessels that are similar to veins in having valves that prevent backflow. Lymph vessels connect to lymph nodes. lymph organs, or to the cardiovascular system at the thoracic duct and right lymphatic duct.
Lymph nodes are small irregularly shaped masses through which lymph vessels flow. Clusters of nodes occur in the armpits, groin, and neck. Cells of the immune system line channels through the nodes and attack bacteria and viruses traveling in the lymph.
Blood groups
There are different ways to classify blood. The two major forms of classification include the ABa system and the Rhesus (Rh) type system, characteristics that are inherited independeptly. Together, they comprise the eight main blood groups. Other blood group systems exist and, to date, researchers have identified around 300 minor factors.
The ABO group
The four different blood groups are A, B, AB and O. A person's blood group is determined by a pair of genes, one each inherited from their mother and father. Each blood group is identified by its own set of complicated chemical substances - called antigens - located on the surfaces of red blood cells. When a person needs a blood transfusion, it is important that the donated blood matches their particular blood group. A mismatch can cause serious complications
The Rhesus factor
A person's Rhesus type is also determined by a pair of genes, each one inherited from one parent. Blood is either Rh-positive or Rh-negative, depending on whether or not certain molecules are present. A person who is Rh-negative will experience a severe immune system reaction if Rh-positive blood gets into their bloodstream. This can happen during childbirth, if an Rh-negative'woman gives birth to an Rh-positive baby. Hemolytic disease of the newborn (HDN) results from Rh incompatibility between an Rh mother and Rh + fetus. If blood cells from the baby travel across the placenta, the woman's immune system will regard the Rh-positive cells as a threat. Specialised white blood cells will make antibodies designed to kill Rh-positive blood cells. If the woman subsequently conceives another Rh-positive baby, her immune system will flood her child with antibodies. These antibodies then destroy the baby's red blood cells. If left untreated, this can result in severe anaemia or even death.
Preventing Rhesus disease
Rhesus disease is now rare, sine Rh-negative who give birth to Rh-positive babies are immunised within 72 hours of giving birth. The immuneglobulin preparation works by killing the baby's red blood cells inside the mother's bloodstream before her immune system has time to react.
Blood Transfusion
A blood transfusion is the tranfer of blood form one person to another. The donated blood must match the recipient's blood type, or complications will occur. generally, both receiving and donating blood are safe medical procedures. For instance, O nagative blood can be given to anybody if necessary, but it is always preferable to match the exact blood group. The different types of blood transfusion include homologous (whole blood transfusion) and aphaerisis (only certain components - such as platelets - are transfused).
Important Facts
Blood Type frequency in percentage of total population:
Blood Type %Frequency
O 46%
A 40%
B 10%
AB 4%
Blood types are not evenly distributed throughout the human population. O+ is the most common, AB-is the rarest. There are also variations in blood-type distribution within human subpopulations:
Blood Type Abbr %Frequency
O Rh-positive O+ 38%
O Rh-negative O- 7%
A Rh-positive A+ 34%
A Rh-negative A- 6%
B Rh-positive B+ 9%
B Rh-negative B- 2%
AB Rh-positive AB+ 3%
AB Rh-negative AB- 1%
Alloimunization Most people, on average, will only need blood one time in their lives, to help fight a disease, restore blood lost during surgery or because of traumatic injury. But some patients, like sickle cell patients, may need blood many times during thrir lives. If the blood they reveive is not a very close match, they will begin to reject transfusions, and an important source of help and hope will be gone. To prevent that, blood for these patients should be closely matched. Often, this will be a rare blood type, For sickle cell patients, the best match will come form donors of African descent. Fully one third of requests for rare blood received by the Red Cross is for a blood type found excusively among African Americans.
Universal donor Type O negative donors are known as universal donors because thrir blood may be transfused to patients of any other blood type in an emeragency situation or if the specific needed blood type is unavialable. Because any patient can receive O negative blood, there is a constant need for O negative donors to give more often and shortages of type O blood can have critical consequences in national disasters. Whatever a person's blood type, they can be very important to someone in emergency crisis.