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Physiology Cardiovascular System Blood • Overview o Components Blood is the only fluid tissue in the body. It appears to be a thick, homogeneous liquid, but the microscope reveals that blood has both cellular and liquid components Blood is a specialized type of connective tissue • Collagen and elastic fibers typical of other connective tissue are absent from blood, but dissolved fibrous proteins become visible as fibrin strands during blood clotting. Major components of whole blood include: • Plasma o 55% of whole blood o Least dense component (floats to the top in a centrifuge) • Buffy Coat = a thin, whitish layer at the erythrocyte-plasma junction o Leukocytes and platelets o <1% of whole blood • Erythrocytes o 45% of whole blood Hematocrit = “blood fraction”; total blood volume occupied by erythrocytes Normal hematocrit values vary • Healthy males = 47% +/- 5% • Healthy females = 42% +/- 5% o Most dense component (sinks to the bottom in a centrifuge) o Physical Characteristics Depending on amount of oxygen it is carrying, blood color varies from scarlet (oxygen rich) to dark red (oxygen poor). Blood is more dense than water Blood is slightly alkaline (pH between 7.35 and 7.45) Blood temperature (38-degrees C or 100.4-degrees F) is higher than body temperature Blood accounts for about 8% of body weight Blood volume is about 1.5 gallons (approx. 5 L) o Functions Distribution • Delivering oxygen from lungs and nutrients from digestive tract to all body cells • Transporting metabolic waste products from cells to elimination sites • Transporting hormones from the endocrine organs to their target organs Regulation • Maintaining appropriate body temperature by absorbing and distributing heat throughout the body and to the skin surface to encourage heat loss Maintaining normal pH in body tissues. Many blood proteins and other bloodborne solutes act as buffers to prevent excessive or abrupt changes in blood pH that could jeopardize normal cell activities. • Maintaining adequate fluid volume in the circulatory system. Salts and blood proteins act to prevent excessive fluid loss from the bloodstream into the tissue spaces. Protection • Preventing blood loss. When a blood vessel is damaged, platelets and plasma proteins initiate clot formation, halting blood loss. • Preventing infection. Drifting along in blood are antibodies, complement proteins, and white blood cells, all of which help defend the body against foreign invaders such as bacteria and viruses. • • • Blood Plasma o A straw-colored, sticky fluid o Mostly water (about 90%), plasma contains over 100 different dissolved solutes o When blood starts to become too acidic (acidosis), both the respiratory system and the kidneys are called into action to restore plasma’s normal, slightly alkaline pH. o Steroids and hormones are carried by plasma proteins Formed Elements o Erythrocytes Red blood cells that transport oxygen Cells with no nuclei or organelles Shaped like biconcave discs—flattened discs with depressed centers They are little more than “bags” of hemoglobin (Hb) • Hemoglobin = RBC protein that makes the RBC red and functions in gas transport by binding easily to oxygen Disorders of Erythrocytes: • Anemias = “lacking blood” o A condition where blood has abnormally low oxygen-carrying capacity o Anemic individuals are fatigued, often pale, short of breath, and chilly o Common Causes An insufficient number of red blood cells • i.e., hemorrhagic anemia resulting from blood loss Low hemoglobin content • i.e., iron-deficiency anemia Abnormal hemoglobin • i.e., sickle-cell anemia o Leukocytes White blood cells that protect the body in various ways Only complete cells with nuclei and all the usual organelles in the blood Diapedesis = “leaping across”; ability of WBCs to slip out of the capillary blood vessels and go to areas where they are most needed to mount inflammation or immune responses. o Whenever WBCs are mobilized for action, the body speeds up their production and twice the normal number may appear in the blood within a few hours. • Leukocytosis = a WBC count of over 11,000 cells/ul; a normal homeostatic response to an infection in the body. Platelets Cell fragments that help stop bleeding Essential for the clotting process They stick to the damaged site forming a temporary plug that helps seal the break. The Heart • Anatomy o About the size of a fist, the heart is less than a pound. o Heart rests on the superior surface of the diaphragm lying anterior to vertebral column and posterior to sternum. o Situated between the 2nd and 5th intercostal space • Pathway of Blood through the Heart o Heart has four chambers Left Atrium • Receive blood from the lungs back to the heart Left Ventricle • Pumps oxygen-rich blood to the rest of the body via the systemic circuit o Systemic Circuit = blood vessels that functionally carry blood to and from all body tissues. Right Atrium • Receives blood from the body Right Ventricle • Pumps oxygen-poor blood into the lungs via the pulmonary circuit o Pulmonary Circuit = serves gas exchange; blood vessels that carry blood to and from the lungs • Physiology o The Cardiac Cycle Includes all events associated with the blood flow through the heart during one complete heartbeat. Systole = contraction period Diastole = relaxation period Blood Vessels • Arterial System o Arteries can be divided into three groups Elastic Arteries • Thick-walled arteries near the heart • Largest in diameter; approx. 2.5cm to 1 cm and most elastic • They are pressure reservoirs—expanding and recoiling as blood is ejected from the heart. This provides a continuous blood flow without stopping or becoming intermittent. Muscular Arteries Aka, distributing arteries Deliver blood to specific body organs and account for most of the named arteries studied in the lab. • Internal diameter ranges from that of a little finger (1 cm) to a pencil lead (0.3 mm). Arterioles • Smallest of all the arteries • Minute-to-minute blood flow into the capillary beds is determined by arteriole diameter which varies in response to changing neural, hormonal and local chemical influences. • When arterioles constrict, the tissues served are largely bypassed. • When arterioles dilate, blood flow into the local capillaries increases dramatically. • • • • • Capillaries o The smallest blood vessels o They have a very thin wall o Given their location and thinness of their walls, capillaries are ideally suited for their role—exchange of materials (gases, nutrients, hormones, and so on) between blood and the interstitial fluid o Capillary Beds Capillaries do not function independently They form interweaving networks Microcirculation = flow of blood from an arteriole to a venule through a capillary bed. Amount of blood entering a capillary bed is regulated by local chemical conditions and arteriolar vasomotor nerve fibers. A bed may be flooded with blood or almost completely bypassed, depending on conditions in the body or in that specific organ. Venous System o Blood is carried from the capillary beds toward the heart by veins. o Venules = formed when capillaries unite o Veins = venules join to form veins Physiology of Circulation o Any fluid driven by a pump through a circuit of closed channels operates under pressure, and the nearer the fluid is to the pump, the greater the pressure exerted on the fluid. o Blood flows through the blood vessels along a pressure gradient, always moving from higher- to lower-pressure areas. o The pumping action of the heart generates blood flow; pressure results when flow is opposed by resistance. o Arterial Blood Pressure Reflects two factors • How much the elastic arteries close to the heart can be stretched • The volume of blood forced into them at any time o If the amount of blood entering and leaving the elastic arteries in a given period were equal, arterial pressure would be constant Systolic pressure = as the left ventricle contracts and expels blood into the aorta, it imparts kinetic energy to the blood which stretches the elastic aorta as aortic pressure reaches its peak. • In healthy adults, this pressure peak averages 120 mm Hg • Blood moves forward into the arterial bed because the pressure in the aorta is higher than the pressure in the more distal vessels. Diastolic pressure = as the aortic valve closes, it prevents blood from flowing back into the heart and the walls of the aorta recoil, maintaining sufficient pressure to keep the blood flowing forward into the smaller vessels. • Aortic pressure drops to its lowest level—approx. 70 to 80 mm Hg in health adults. o Essentially, the volume and energy of blood stored in the elastic arteries during systole are given back during diastole. Pulse pressure (pulse) = the difference between the systolic and diastolic pressures.