Circulation and Blood Early Theories ‐
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Galen: believed that blood does not circulate, but ebbs and flows back and forth William Harvey: based his work on Galileo’s theories of fluid movement and concluded that blood must flow Blood Vessels Arteries – vessels that carry blood away from the heart ‐
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Three distinct layers: outer and inner are connective tissue, middle layer is muscle fiber and elastic connective tissue As the heart contracts, blood surges through the arteries and they stretch; this produces a pulse Muscle layer in the artery contracts after blood passes by, helping to push blood along Arterioles – smaller arteries ‐
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Arterioles branch off from main arteries Diameter of arterioles can change dramatically to direct the transfer of blood to specific tissues o Vasodilatation – arterioles dilate and blood flow increases – seen in the blushing response o Vasoconstriction – arterioles constrict and blood flow decreases – fright response Atherosclerosis: narrowing of the arteries due to deposits of plaque (fat droplets with deposits of calcium and other minerals) – can completely block an arteriol and lead to heart attack (heart) or a stroke (brain). Capillaries – microscopic vessels, with walls a single cell thick ‐
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Transfer of fluids and gases occur here between the cells and the blood Oxygen diffuses out of blood cells, through thin capillary wall, into extracellular fluid and then into body tissues Carbon dioxide will also diffuse in the opposite direction Glucose, vitamins and ions and waste molecules are also exchanged Veins – vessels that carry blood toward the heart ‐
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Coming from capillaries, blood enters small veins called venules and then into larger veins – middle layer of cells are much thinner than arteries, so cannot withstand the same levels of pressure Veins also lack muscular layer, so do not push blood along as do arteries Veins rely on muscular action of surrounding muscle tissue to help push blood back to the heart One way valves also exist to prevent backflow of blood in the veins Veins also serve as large reservoirs of blood – as much as 50% of your total blood volume can be found in the veins at any one time. Damaged valves can result in varicose veins: local pooling of blood in the veins which begin to bulge outward and become highly visible under the skin ‐
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The Heart The heart can be thought of as a double‐pump: once to the lungs, and again to the tissues of the body. These circuits are called the pulmonary circulation and the systemic circulation respectively Blood is carried to the heart by the superior and inferior vena cava These flow into the right atrium, a thin muscular chamber At the same time, blood enters the left atrium from the lungs via the pulmonary veins Both of the atria contract, pushing blood through a set of valves (atrio‐ventricular or AV valves) which lead in to the larger, thick muscle‐walled chambers called the ventricles The AV valves are sometimes referred to as tricuspid valves and are supported by tendons that help prevent back‐flow into the atria The ventricles then contract and push blood through a set of valves called semi‐lunar valves or bicuspid valves. The right ventricle sends blood to the lungs via the pulmonary artery, and the left ventricle sends blood to the body tissues through the aorta. The aorta is the largest artery in the body, and branches off into several other arteries: o carotid – to the head and brain o brachial – to the arms o coronary – to the heart muscle tissue the coronary artery provides blood to the working heart muscle – if a problem develops here, a heart attack can occur – coronary bypass may be necessary The Heart Beat the heart must contract in a coordinated fashion – the cardiac muscle is different from skeletal muscle because it is able to contract without external nerve stimulation the heart rate is controlled by a small patch of tissue on the upper end of the right atrium – this is called the sino‐atrial node or SA node. Nerve impulses are carried from the SA node to the other heart muscle cells by the specialized cardiac conduction system and the myocardial cells When the SA node sends a signal, it travels across the muscular walls of the right atrium – a set of conducting fibers carry the signal a little faster toward the left atrium, which allows the right atrium to contract at the same time the heart beat signal is delayed slightly at the AV node before it is sent on toward down the bundle branches to the ventricles ‐
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here the Purkinje fibers cause the ventricles to contract in an upside‐down milking motion the contraction of the ventricles is referred to as systole (at this time, the atria have finished contracting are re‐filling) the relaxation of the ventricles is referred to as diastole the pressure of the blood against the inside walls of the arteries changes during these parts of the heart’s stroke cycle blood pressure can vary in several ways – over time and over the position of the body the systolic and diastolic pressures determine the change in pressure over time. The greater the cardiac output, the greater the pressure (as after exercise) Since arterioles can dilate and constrict, they can have an effect on the volume of blood in the arteries and thus the blood pressure The diameter of the arterioles can change with nervous or hormonal control, or can also respond to metabolic products such as carbon dioxide and lactic acid – which would cause vasodilation and a drop in blood pressure Components of Blood 55% fluid: plasma – which is 90% water, and 10% dissolved materials (glucose, proteins, gases, minerals, vitamins, waste products) large plasma proteins can be albumins (osmotic balance), globulins (immunity), fibrinogens (clotting) 45% of the blood is composed of blood cells: red blood cells, white blood cells, platelets red blood cells (RBCs) – also called erythrocytes primary function is to transport oxygen – uses the pigment hemoglobin to carry oxygen: about 200mL of O2 in 1L of blood RBCs are enucleated, biconcave disk‐shaped cells – specializations that allow the cell to hold more hemoglobin and thus more oxygen Erythropoeisis – red blood cell production – occurs in stem cells in the marrow of long bones – stem cells are generalized cells that do have a nucleus – as the RBCs mature, they lose their nucleus Any factor that decreases blood oxygen concentration will cause an increase in erythropoeisis (exercise, high altitude, hemorrhage). White blood cells (WBCs) – also called leucocytes – main function is for fighting disease – several types and methods of fighting disease Neutrophils: 55 – 70% of leucocytes – phagocytic cell that digests bacteria and cellular debris – accumulate in large numbers in early stages of infection. Eosinophils: 1 – 4% of leucocytes – fight parasitic worms and aid in allergic responses Basophils: release chemicals (histamines) for allergic response Lymphocytes: 20 – 30% of leucocytes – B cells: release antibodies that attack specific invaders – T cells: attack invaders directly Monocytes: 2 – 8% of leucocytes – also called macrophages – accumulate in high numbers at site of infection and are highly phagocytic Platelets – help in the formation of blood clots – created from small fragments of larger cells ‐
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Blood Clotting normally, platelets flow smoothly through the blood vessels if the platelet strikes a rough surface, the platelet breaks apart and releases a protein called thromboplastin this starts a set of reactions that eventually produces a clot thromboplastin combines with the calcium in the blood → which activates another plasma protein called prothrombin → this is converted into thrombin → thrombin converts fibrinogen into fibrin → fibrin molecules form long threads, which seal the damaged area in a sticky mesh Blood Groups special markers called glycoproteins are located on the cell membrane of some red blood cells – these proteins identify a cell as belonging to a particular type: o blood type A has marker A o blood type B has marker B o blood type AB have both a and B markers o type O has no glycoprotein marker these markers are called antigens the blood also produces proteins called antibodies (also called immunoglobulins) – these attach to the antigens and cause the blood to clump (agglutinate) – see table 1 and figure 1 on page 247 a person with type A blood will have antibodies against type B blood, so a transfusion of either type B or type AB will cause clumping – a transfusion of type A or type O will not cause clumping, because there are no “foreign” antigens present on red blood cells of those types. Immune Response ‐
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the body’s first line of defense against foreign invasion is the skin in the respiratory tract, there are many areas that contain mucous layers and cilia to trap particles and remove them tears secrete an enzyme (lysozyme) that destroys the cell wall of bacteria once inside the body, foreign invaders must contend with the white blood cells leucocytes – seek out and destroy any foreign particles, mostly through phagocytosis complementary proteins – many types – can respond to the marker proteins on the cell membranes of invading cells ‐ some can coat and seal the invader, some will dissolve the cell membrane of an invader, others will attach to invader and attract phagocytes. lymphocytes – T‐cells identify invaders and alert B cells B cells produce antibodies and release them when alerted The antibodies are proteins designed to target specific foreign invaders – the antibody for measles is ineffective on HIV or the common cold virus – each invader requires its own specific antibody Therefore with the first infection of a new invader, the B cells must “learn” to make the antibody – the second infection will not be as effective because the body has already stored up the correct antibodies for that invader ‐
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Capillary Fluid Exchange capillaries provide cells with oxygen, glucose and amino acids, but also exchange fluid between the blood and the ECF (extra‐cellular fluid). Most fluids (including dissolved substances) can diffuse through the capillary membrane since it is a selectively permeable – larger molecules and some small proteins can be exchanged by endocytosis and exocytosis The capillary membrane is impermeable to large protein molecules of the blood (such as hemoglobin, albumins, immunoglobulins, and fibrinogens) – important so that these stay in the bloodstream Two forces control the movement of water between the blood and ECF: o Blood pressure: exerts approximately 35 mm Hg against the wall of capillary, forcing water and dissolved ions out of capillary into the ECF – this outward flow is called filtration o Osmotic pressure: large protein molecules in the blood cause a concentration gradient that draws fluid back into the capillaries – this is called absorption In a normal situation, these two forces are balanced During hemorrhage, the decreased blood volume affects blood pressure – now the effect of osmotic pressure is greater than that of blood pressure, so more fluid is drawn into the capillaries than necessary – as fluid moves into the capillaries, the blood volume (and thus the blood pressure) can normalize During starvation, blood plasma proteins are used as a last resort to provide energy for the cells – this decrease in plasma proteins changes the concentration in the blood, and the osmotic pressure decreases ‐ now the effect of blood pressure is greater than the effect of osmotic pressure, and fluid moves into the ECF This causes swelling of the tissues Lymphatic system normally, a small amount of protein leaks through the capillaries into the ECF if this is allowed to accumulate, it would change the concentration gradient, and effect the osmotic pressure the lymph vessels are open‐ended vessels that transport these proteins back to the circulatory system they rely on muscular contractions to move the lymph fluid enlargements called lymph nodes are involved in fighting disease as they house white blood cells that filter bacteria and foreign particles from the lymph