Christopher J. Berry, MD

• Chief Fellow
• Division of Cardiovascular Medicine
• University of Iowa Hospitals and Clinics
• Iowa City, Iowa

The neuronal channel for Na+ entry is a voltage-gated Na+ channel dust allergy symptoms uk generic zyrtec 10 mg online, but the muscle channel for Na+ entry is the acetylcholine-gated monovalent cation channel allergy shots tallahassee buy zyrtec overnight. Contraction occurs when thin and thick filaments slide past each other as myosin binds to actin allergy testing macon ga cheap zyrtec 5 mg without prescription, swivels allergy treatment los angeles discount zyrtec 10 mg line, and pulls actin toward the center of the sarcomere allergy relief 6 month old cheapest zyrtec. Fatigue-a reversible state in which a muscle can no longer generate or sustain the expected force allergy testing yellowknife cheap zyrtec american express. May involve changes in ion concentrations, depletion of nutrients, or excitation-contraction coupling. The body uses different types of motor units and recruits different numbers of motor units. Small movements use motor units with fewer muscle fibers; gross movements use motor units with more fibers. Fast-twitch glycolytic fibers-largest, rely primarily on anaerobic glycolysis, least fatigue-resistant. Slow-twitch-develop tension more slowly, maintain tension longer, the most fatigue-resistant, depend primarily on oxidative phosphorylation, more mitochondria, greater vascularity, large amounts of myoglobin, smallest in diameter. Pacemaker potentials-repetitive depolarizations to threshold in some smooth muscle and cardiac muscle. Assuming these athletes are lean, differences in weight are correlated with muscle strength, so heavier athletes should have stronger muscles. More important factors are the relative endurance and strength required for a given sport. Any given muscle will have a combination of three fiber types, with the exact ratios depending upon genetics and specific type of athletic training. Leg muscles-fasttwitch glycolytic fibers, to generate strength, and fast-twitch oxidative, for endurance. The arm and shoulder muscles-fast-twitch glycolytic, because shooting requires fast and precise contraction. Leg muscles-fast-twitch oxidative, for moving across the ice, and fast-twitch glycolytic, for powering jumps. Sensor (sensory receptor), input signal (sensory afferent neuron), integrating center (central nervous system), output signal (autonomic or somatic motor neuron), targets (muscles, glands, some adipose tissue). Upon hyperpolarization, the membrane potential becomes more negative and moves farther from threshold. When you pick up a weight, alpha and gamma neurons, spindle afferents, and Golgi tendon organ afferents are all active. A crossed extensor reflex is a postural reflex initiated by withdrawal from a painful stimulus; the extensor muscles contract, but the corresponding flexors are inhibited. Emotional reflexes: blushing, heart rate, gastrointestinal function Two neuron-neuron synapses in the spinal cord and the autonomic ganglion, and one neuron-target synapse. Golgi tendon organ, the muscle spindle, and joint capsule mechanoreceptors tone increase. Voluntary movements, such as playing the piano, and rhythmic movements, such as walking, must involve the brain. Reflex movements are involuntary; the initiation, modulation, and termination of rhythmic movements are voluntary. Heart, blood vessels, respiratory muscles, smooth muscle, and glands are some of the target organs involved. Tetanus toxin triggers prolonged contractions in skeletal muscles, or spastic paralysis. Botulinum toxin blocks secretion of acetylcholine from somatic motor neurons, so skeletal muscles cannot contract, which is flaccid paralysis. The bottom tube has the greater flow because it has the larger pressure gradient (50 mm Hg versus 40 mm Hg for the top tube). Tube C has the highest flow because it has the largest radius of the four tubes (less resistance) and the shorter length (less resistance). If the canals are identical in size and therefore in cross-sectional area A, the canal with the higher velocity of flow v has the higher flow rate Q. Connective tissue is not excitable and is therefore unable to conduct action potentials. It is possible to conclude that myocardial cells require extracellular Ca2+ for contraction but skeletal muscle cells do not. If all Ca2+ channels in the muscle cell membrane are blocked, there will be no contraction. If only some are blocked, the force of contraction will be smaller than the force created with all channels open. Na+ influx causes neuronal depolarization, and K+ efflux causes neuronal repolarization. The refractory period represents the time required for the Na+ channel gates to reset (activation gate closes, inactivation gate opens). Alpha-gamma coactivation allows muscle spindles to continue functioning when the muscle contracts. When the muscle contracts, the ends of the spindles also contract to maintain stretch on the central portion of the spindle. Parts of the brain include the brain stem, cerebellum, basal ganglia, thalamus, cerebral cortex (visual cortex, association areas, motor cortex). If cardiac Na+ channels are completely blocked with lidocaine, the cell will not depolarize and therefore will not contract. The Ca2+ channels in autorhythmic cells are not the same as those in contractile cells. Autorhythmic Ca2+ channels open rapidly when the membrane potential reaches about - 50 mV and close when it reaches about + 20 mV. The Ca2+ channels in contractile cells are slower and do not open until the membrane has depolarized fully. If tetrodotoxin is applied, nothing will happen because there are no voltage-gated Na+ channels in the cell. Cutting the vagus nerve increased heart rate, so parasympathetic fibers in the nerve must slow heart rate. It also slows down the speed at which those action potentials are conducted, allowing atrial contraction to end before ventricular contraction begins. The fastest pacemaker sets the heart rate, so the heart rate increases to 120 beats/min. Atrial pressure increases because pressure on the mitral valve pushes the valve back into the atrium, decreasing atrial volume. Atrial pressure decreases during the initial part of ventricular systole as the atrium relaxes. Atrial pressure begins to decrease at point D, when the mitral valve opens and blood flows down into the ventricles. Ventricular pressure shoots up when the ventricles contract on a fixed volume of blood. After 10 beats, the pulmonary circulation will have gained 10 mL of blood and the systemic circulation will have lost 10 mL. Phase 2 (the plateau) of the contractile cell action potential has no equivalent in the autorhythmic cell action potential. The heart rate is either 75 beats/min or 80 beats/min, depending on how you calculate it. If you use the data from one R peak to the next, the time interval between the two peaks is 0. There are 4 beats in the 3 sec after the first R wave, so 4 beats/3 sec * 60 sec/min = 80 bpm. In 4, there are no recognizable waves at all, indicating that the depolarizations are not following the normal conduction path. Cardiac muscle has strong cell-to-cell junctions, gap junctions for electrical conduction, and the modification of some muscle cells into autorhythmic cells. The long refractory period prevents a new action potential until the heart muscle has relaxed. Heart rate, heart rhythm (regular or irregular), conduction velocity, and the electrical condition of heart tissue. Calcium channel blockers slow heart rate by blocking Ca2+ entry and decrease force of contraction by decreasing Ca2+@induced Ca2+ release. Beta blockers decrease effect of norepinephrine and epinephrine, preventing increased heart rate and force of contraction. Thus, less blood is being pumped out of the ventricle each time the heart contracts. A ventricular pacemaker is implanted so that the ventricles have an electrical signal telling them to contract at an appropriate rate. Rapid atrial depolarization rate is dangerous because if the rate is too fast, only some action potentials will initiate contractions due to the refractory period of muscle. Veins from the brain do not require valves because blood flow is aided by gravity. The carotid wave would arrive slightly ahead of the wrist wave because the distance from heart to carotid artery is shorter. Epinephrine binding to myocardial b1@receptors increases heart rate and force of contraction. Integrating center: cerebral cortex, with descending pathways through the limbic system. Divergent pathways go to the cardiovascular control center, which increases sympathetic output to heart and arterioles. A second descending spinal pathway goes to the adrenal medulla, which releases epinephrine. Epinephrine on b2@receptors of liver, heart, and skeletal muscle arterioles causes vasodilation of those arterioles. As a result, hydrostatic pressure will have a greater effect in the filtrationabsorption balance, and filtration will increase. Using osmotic pressure rather than osmolarity allows a direct comparison between absorption pressure and filtration pressure, both of which are expressed in mm Hg. If the left ventricle fails, blood backs up into the left atrium and pulmonary veins, and then into lung capillaries. Capillary absorption is reduced while filtration remains constant, resulting in edema and ascites. Sympathetic input causes vasoconstriction but epinephrine causes vasodilation in selected arterioles. Preventing Ca2+ entry decreases ability of cardiac and smooth muscles to contract. Neurons and other cells are unaffected because they have types of calcium channels not affected by the drugs. Lymphatic capillaries have contractile fibers to help fluid flow; systemic capillaries depend on systemic blood pressure for flow. Left ventricular failure causes blood to pool in the lungs, increasing pulmonary capillary hydrostatic pressure. This may cause pulmonary edema and shortness of breath when oxygen has trouble diffusing into the body. Cells (endothelium) in the intact wall detect changes in oxygen and communicate these changes to the smooth muscle. For a 50-kg individual with a resting pulse of 70 bpm, she will pump her weight in blood in about 10 minutes. One-way valves in the veins, skeletal muscle pump, and low pressure in the thorax during breathing 8. Korotkoff sounds occur when cuff pressure is lower than systolic pressure but higher than diastolic pressure. Sympathetic neurons (a@receptors) vasoconstrict, and epinephrine on b2@receptors in certain organs vasodilates. Causes include lower capillary colloid osmotic pressure due to decreased plasma proteins or blockage of the lymphatic vessels by a tumor or other pathology. It goes back up at C because less total energy is needed for velocity, so the potential energy is greater. Characteristics: biconcave disk shape, no nucleus, and red color due to hemoglobin. The two pathways unite at the common pathway to initiate the formation of thrombin. Proteins and vitamins promote hemoglobin synthesis and the production of new blood cell components. This illustrates mass balance: If input exceeds output, restore body load by increasing output. The five types of leukocytes are lymphocytes, monocytes/macrophages, basophils/mast cells, neutrophils, and eosinophils. Erythrocytes and platelets lack nuclei, which would make them unable to carry out protein synthesis. Liver degeneration reduces the total plasma protein concentration, which reduces the osmotic pressure in the capillaries. This decrease in osmotic pressure increases net capillary filtration and edema results. Low atmospheric oxygen at high altitude S low arterial oxygen S sensed by kidney cells S secrete erythropoietin S acts on bone marrow S increased production of red blood cells 8. Hematocrit-percent total blood volume occupied by packed (centrifuged) red cells. External respiration is exchange and transport of gases between the atmosphere and cells. The upper respiratory tract includes the mouth, nasal cavity, pharynx, and larynx.

Another female barrier contraceptive is the contraceptive sponge allergy friendly cats generic 5 mg zyrtec mastercard, which contains a spermicidal chemical allergy medicine nose spray cheap zyrtec 5 mg with mastercard. The male barrier contraceptive is the condom allergy shots reactions swelling order zyrtec paypal, a closed sheath that fits closely over the penis to catch ejaculated semen allergy testing images 10 mg zyrtec fast delivery. Condoms lost popularity when oral contraceptives came into widespread use in the 1960s and 1970s allergy symptoms 6 year molars buy 5 mg zyrtec overnight delivery, but in recent years allergy shots vs acupuncture buy 5 mg zyrtec mastercard, they have regained favor because they combine pregnancy protection with protection from many sexually transmitted diseases. It covers the cervix and completely lines the vagina, providing more protection from sexually transmitted diseases. Implantation Prevention Some contraceptive methods do not prevent fertilization but do keep a fertilized egg from establishing itself in the endometrium. Hormonal Treatments Techniques for decreasing gamete production depend on altering the hormonal milieu of the body. In centuries past, women would eat or drink various plant concoctions for contraception. Some of these substances actually worked because the plants contained estrogen-like compounds. Modern pharmacology has improved on this method, and now women can choose between oral contraceptive pills, injections lasting months, or a vaginal contraceptive ring. The oral contraceptives, also known as birth control pills, first became available in 1960. They rely on various combinations of estrogen and progesterone that inhibit gonadotropin secretion from the pituitary. In addition, progesterones in the contraceptive pills thicken the cervical mucus and help prevent sperm penetration. These hormonal methods of contraception are highly effective when used correctly but also carry some risks, including an increased incidence of blood clots and strokes, especially in women who smoke. Development of a male hormonal contraceptive has been slow because of undesirable side effects. Contraceptives that block testosterone secretion or action are also likely to decrease the male libido or even cause impotence. Both side effects are unacceptable to men who would be most interested in using the contraceptive. Some early male oral contraceptives irreversibly suppressed sperm production, which was also unacceptable. A number of clinical trials are currently looking at both hormonal and nonhormonal methods for decreasing male fertility. Contraceptive vaccines are based on antibodies against various components of the male and female reproductive systems, such as antisperm or antiovum antibodies. However, clinical trials of human vaccines have been disappointing and vaccines may not be a practical contraceptive for humans. Infertility Is the Inability to Conceive While some couples are trying to prevent pregnancy, others are spending thousands of dollars trying to get pregnant. Infertility is the inability of a couple to conceive a child after a year of unprotected intercourse. For years, infertile couples had no choice but adoption if they wanted to have a child, but incredible strides have been made in this field since the 1970s. Male infertility usually results from a low sperm count or an abnormally high number of defective sperm. Female infertility can be mechanical (blocked Fallopian tubes or other structural problems) or hormonal, leading to decreased or absent ovulation. By some estimates, as many as a third of all pregnancies spontaneously terminate-many within the first weeks, before the woman is even aware that she was pregnant. Because of the expense and complicated nature of the procedure, multiple embryos are usually placed in the uterus at one time, which may result in multiple births. In vitro fertilization has allowed some infertile couples to have children, with a 2009 live-birth delivery rate in the United States averaging 37%. Fertilization Requires Capacitation Once an egg is released from the ruptured follicle, it is swept into the Fallopian tube by beating cilia. Meanwhile, sperm deposited in the female reproductive tract must go through their final maturation step, capacitation, which enables the sperm to swim rapidly 26. Normally, capacitation takes place in the female reproductive tract, which presents a problem for in vitro fertilization. Those sperm must be artificially capacitated by placing them in physiological saline supplemented with human serum. Much of what we know about human fertilization has come from infertility research aimed at improving the success rate of in vitro fertilization. Fertilization of an egg by a sperm is the result of a chance encounter, possibly aided by chemical attractants produced by the egg. Apparently they bind to the epithelium of the Fallopian tube while awaiting a chemical signal from a newly ovulated egg. Of the millions of sperm in a single ejaculation, only about 100 reach this point. To get past these barriers, capacitated sperm release powerful enzymes from the acrosome in the sperm head, a process known as the acrosomal reaction. The enzymes dissolve cell junctions and the zona pellucida, allowing the sperm to wiggle their way toward the egg. The fusion of sperm and oocyte membranes triggers a chemical reaction called the cortical reaction that excludes other sperm. In the cortical reaction, membrane-bound cortical granules in the peripheral cytoplasm of the egg release their contents into the space just outside the egg membrane. These chemicals rapidly alter the membrane and surrounding zona pellucida to prevent polyspermy, in which more than one sperm fertilizes an egg. At this point, the 23 chromosomes of the sperm join the 23 chromosomes of the egg, creating a zygote nucleus with a full set of genetic material. Once an egg is fertilized and becomes a zygote, it begins mitosis as it slowly makes its way along the Fallopian tube to the uterus, where it will settle for the remainder of the gestation period gestare, to carry in the womb. The inner cell mass of the blastocyst will develop into the embryo and into three other extraembryonic membranes. These membranes include the amnion, which secretes amniotic fluid in which the developing embryo floats; the allantois, which becomes part of the umbilical cord that links the embryo to the mother; and the yolk sac, which degenerates early in human development. Implantation of the blastocyst into the uterine wall normally takes place about seven days after fertilization. The blastocyst secretes enzymes that allow it to invade the endometrium, like a parasite burrowing into its host. As it does so, endometrial cells grow out around the blastocyst until it is completely engulfed. As the blastocyst continues dividing and becomes an embryo, cells that will become the placenta form fingerlike chorionic villi that penetrate into the vascularized endometrium. The 26 blood of the embryo and that of the mother do not mix, but nutrients, gases, and wastes exchange across the membranes of the villi. Many of these substances move by simple diffusion, but some, such as maternal antibodies, must be transported across the membrane. The placenta continues to grow during pregnancy until, by delivery, it is about 20 cm in diameter (the size of a small dinner plate). The tremendous blood flow to the placenta is one reason sudden, abnormal separation of the placenta from the uterine wall is a medical emergency. After approximately six weeks, mature eggs are harvested surgically and fertilized in vitro. Under the influence of progesterone, smooth muscle of the tube relaxes, and transport proceeds slowly. By the time the developing embryo reaches the uterus, it consists of a hollow ball of about 100 cells called a blastocyst. First polar body Egg Sperm Egg Corona radiata Cells of corona radiata Capacitated sperm First polar body remnants Second meiotic division suspended Zona pellucida (c) the first sperm to fuse with the egg fertilizes it. It obtains oxygen and nutrients from the mother through the placenta and umbilical cord. Yolk sac Chorion Amniotic fluid Amnion Umbilical cord Maternal blood bathes the chorionic villi. Unless the developing embryo sends a hormonal signal, the corpus luteum disintegrates, progesterone and estrogen levels drop, and the embryo is flushed from the body along with the surface layers of endometrium during menstruation. The placenta secretes several hormones that prevent menstruation during pregnancy, including human chorionic gonadotropin, human chorionic somatomammotropin, estrogen, and progesterone. By the seventh week of development, however, the placenta has taken over progesterone production, and the corpus luteum is no longer needed. Human chorionic gonadotropin production by the placenta peaks at three months of development, then diminishes. Today, with modern biochemical techniques, women can perform their own pregnancy tests in a few minutes in the privacy of their home. This hormone, structurally related to growth hormone and prolactin, was initially believed to be necessary for breast development during pregnancy and for milk production (lactation). Maternal glucose moves across the membranes of the placenta by facilitated diffusion and enters the fetal circulation. With high circulating levels of these steroid hormones, feedback suppression of the pituitary continues throughout pregnancy, preventing another set of follicles from beginning development. During pregnancy, estrogen contributes to the development of the milk-secreting ducts of the breasts. Progesterone is essential for maintaining the endometrium and also helps suppress uterine contractions. The placenta makes a variety of other hormones, including inhibin and prorenin, but the function of most of them remains unclear. Pregnancy Ends with Labor and Delivery Parturition normally occurs between the 38th and 40th week of gestation. For many years, researchers developed animal models of the signals that initiate parturition, only to discover recently that there are no good nonprimate models that apply to humans. Signals that initiate these contractions could begin with either the mother or the fetus, or they could be a combination of signals from both. In many nonhuman mammals, a decrease in estrogen and progesterone levels marks the beginning of parturition. A decrease in progesterone levels is logical, as progesterone inhibits uterine contractions. In humans, however, levels of these hormones do not decrease until labor is well under way. Another possible labor trigger is oxytocin, the peptide hormone that causes uterine muscle contraction. As a pregnancy nears full term, the number of uterine oxytocin receptors increases. However, studies have shown that oxytocin secretion does not increase until after labor begins. Synthetic oxytocin is often used to induce labor in pregnant women, but it is not always effective. Apparently, the start of labor requires something more than adequate amounts of oxytocin. Another possibility is that the fetus somehow signals that it has completed development. Although we do not know for certain what initiates parturition, we do understand the sequence of events. In the days prior to the onset of active labor, the cervix softens ("ripens"), and ligaments holding the pelvic bones together loosen as enzymes destabilize collagen in the connective tissue. The control of these processes is not clear and may be due to estrogen or the peptide hormone relaxin, which is secreted by ovaries and the placenta. Once the contractions of labor begin, a positive feedback loop consisting of mechanical and hormonal factors is set into motion. Cervical stretch triggers uterine contractions that move in a wave from the top of the uterus down, pushing the fetus farther into the pelvis. The lower portion of the uterus stays relaxed, and the cervix stretches and dilates. The contractions are reinforced by secretion of oxytocin from the posterior pituitary [p. Prostaglandins are very effective at causing uterine muscle contractions at any time. They are the primary cause of menstrual cramps and have been used to induce abortion in early pregnancy. The placenta then detaches from the uterine wall and is expelled a short time later. Uterine contractions clamp the maternal blood vessels and help prevent excessive bleeding, although typically the mother loses about 240 mL of blood. The Mammary Glands Secrete Milk During Lactation A newborn has lost its source of maternal nourishment through the placenta and must rely on an external source of food instead. Primates, who normally have only one or two offspring at a time, have two functional mammary glands. Each lobe branches into lobules, and the lobules terminate in clusters of cells called alveoli or acini. During pregnancy, the glands develop further under the direction of estrogen, growth hormone, and cortisol.

Air went from the lungs to the heart allergy medicine inhaler order zyrtec 5 mg overnight delivery, where it was digested and picked up "vital spirits allergy treatment runny nose order zyrtec 5mg without prescription. Anomalies-such as the fact that a cut artery squirted blood rather than air-were ingeniously explained by unseen links between arteries and veins that opened upon injury allergy medicine during ivf cheap zyrtec 10 mg fast delivery. According to this model of the circulatory system allergy testing plano trusted zyrtec 10 mg, the tissues consumed all blood delivered to them allergy symptoms caused by pollen zyrtec 10 mg fast delivery, and the liver had to synthesize new blood continuously allergy zithromax symptoms generic zyrtec 5mg without prescription. Once it became obvious that the liver could not make blood as rapidly as the heart pumped it, Harvey looked for an anatomical route that would allow the blood to recirculate rather than be consumed in the tissues. He showed that valves in the heart and veins created a one-way flow of blood, and that veins carried blood back to the heart, not out to the limbs. He also showed that blood entering the right side of the heart 14 had to go to the lungs before it could go to the left side of the heart. Today, we understand the structure of the cardiovascular system at microscopic and molecular levels that Harvey never dreamed existed. Even now, with our sophisticated technology, we are searching for "spirits" in the blood, although today we call them by names such as hormone and cytokine. In addition, the cardiovascular system plays an important role in cell-to-cell communication and in defending the body against foreign invaders. This article focuses on an overview of the cardiovascular system and on the heart as a pump. Later, you will learn about the properties of the blood vessels and the homeostatic controls that regulate blood flow and blood pressure. As blood moves through the cardiovascular system, a system of valves in the heart and veins ensures that the blood flows in one direction only. Like the turnstiles at an amusement park, the valves keep blood from reversing its direction of flow. Notice in this illustration, as well as in most other diagrams of the heart, that the right side of the heart is on the left side of the page, which means that the heart is labeled as if you were viewing the heart of a person facing you. Each half functions as an independent pump that consists of an atrium atrium, central room; plural atria and a ventricle ventriculus, belly. The atrium receives blood returning to the heart from the blood vessels, and the ventricle pumps blood out into the blood vessels. The right side of the heart receives blood from the tissues and sends it to the lungs for oxygenation. The left side of the heart receives newly oxygenated blood from the lungs and pumps it to tissues throughout the body. This is a convention used to show blood from which the tissues have extracted oxygen. Although this blood is often described as deoxygenated, it is not completely devoid of oxygen. In living people, well-oxygenated blood is bright red, and lowoxygen blood is a darker red. Under some conditions, low-oxygen blood can impart a bluish color to certain areas of the skin, such as around the mouth and under the fingernails. This condition, known as cyanosis kyanos, dark blue, is the reason blue is used in drawings to indicate blood with lower oxygen content. From there it is pumped through the pulmonary arteries pulmo, lung to the lungs, where it is oxygenated. From the lungs, blood travels to the left side of the heart through the pulmonary veins. The blood vessels that go from the right ventricle to the lungs and back to the left atrium are known collectively as the pulmonary circulation. Blood from the lungs enters the heart at the left atrium and passes into the left ventricle. Blood pumped out of the left ventricle enters the large artery known as the aorta. The aorta branches into a series of smaller and smaller arteries that finally lead into networks of capillaries. Materials Entering the Body Oxygen Nutrients and water Lungs Intestinal tract All cells All cells Materials Moved from Cell to Cell Wastes Immune cells, antibodies, clotting proteins Hormones Stored nutrients Some cells Present in blood continuously Endocrine cells Liver and adipose tissue Liver for processing Available to any cell that needs them Target cells All cells Materials Leaving the Body Metabolic wastes Heat Carbon dioxide All cells All cells All cells Kidneys Skin Lungs Oxygen enters the body at the exchange surface of the lungs. Once all these materials are in the blood, the cardiovascular system distributes them. A steady supply of oxygen for the cells is particularly important because many cells deprived of oxygen become irreparably damaged within a short period of time. For example, hormones secreted by endocrine glands travel in the blood to their targets. Blood also carries nutrients, such as glucose from the liver and fatty acids from adipose tissue, to metabolically active cells. Finally, the defense team of white blood cells and antibodies patrols the circulation to intercept foreign invaders. The cardiovascular system also picks up carbon dioxide and metabolic wastes released by cells and transports them to the lungs and kidneys for excretion. Some waste products are transported to the liver for processing before they are excreted in the urine or feces. Heat also circulates through the blood, moving from the body core to body surface, where it dissipates. Kidneys Pelvis and legs After leaving the capillaries, blood flows into the venous side of the circulation, moving from small veins into larger and larger veins. The blood vessels that carry blood from the left side of the heart to the tissues and back to the right side of the heart are collectively known as the systemic circulation. The first branch represents the coronary arteries, which nourish the heart muscle itself. Blood from these arteries flows into capillaries, then into the coronary veins, which empty directly into the right atrium at the coronary sinus. The abdominal aorta supplies blood to the trunk, the legs, and the internal organs such as liver (hepatic artery), digestive tract, and the kidneys (renal arteries). Both regions receive well-oxygenated blood through their own arteries, but, in addition, blood leaving the digestive tract goes directly to the liver by means of the hepatic portal vein. The liver is an important site for nutrient processing and plays a major role in detoxifying foreign substances. Most nutrients absorbed in the intestine are routed directly to the liver, allowing that organ to process material before it is released into the general circulation. A second portal system occurs in the kidneys, where two capillary beds are connected in series. A third portal system, discussed earlier but not shown here, is the hypothalamic-hypophyseal portal system, which connects the hypothalamus and the anterior pituitary [p. What is the difference between (a) the pulmonary and systemic circulations, (b) an artery and a vein, (c) an atrium and a ventricle In physiology, we are also concerned with how blood flows-in other words, with the mechanisms or forces that create blood flow. For this reason, blood can flow in the cardiovascular system only if one region develops higher pressure than other regions. Blood flows out of the heart (the region of highest pressure) into the closed loop of blood vessels (a region of lower pressure). As blood moves through the system, pressure is lost because of friction between the fluid and the blood vessel walls. The highest pressure in the vessels of the cardiovascular system is found in the aorta and systemic arteries as they receive blood from the left ventricle. The lowest pressure is in the venae cavae, just before they empty into the right atrium. Many of these principles apply broadly to the flow of all types of liquids and gases, including the flow of air in the respiratory system. However, in this chapter we focus on blood flow and its relevance to the function of the heart. For example, a column of fluid in a tube exerts hydrostatic pressure on the floor and sides of the tube. In addition, the pressure exerted by moving fluid has two components: a dynamic, flowing component that represents the kinetic energy of the system, and a lateral component that represents the hydrostatic pressure (potential energy) exerted on the walls of the system. Pressure within our cardiovascular system is usually called hydrostatic pressure even though it is a system in which fluid is in motion. Some textbooks are beginning to replace the term hydrostatic pressure with the term hydraulic pressure. Pressure Changes in Liquids without a Change in Volume If the walls of a fluid-filled container contract, the pressure exerted on the fluid in the container increases. You can demonstrate this principle by filling a balloon with water and squeezing the water balloon in your hand. Water is minimally compressible, and so the pressure you apply to the balloon is transmitted throughout the fluid. As you squeeze, higher pressure in the fluid causes parts of the balloon to bulge. The water volume inside the balloon did not change, but the pressure in the fluid increased. In the human heart, contraction of the blood-filled ventricles is similar to squeezing a water balloon: pressure created by the contracting muscle is transferred to the blood. This high-pressure blood then flows out of the ventricle and into the blood vessels, displacing lower-pressure blood already in the vessels. The pressure created in the ventricles is called the driving pressure because it is the force that drives blood through the blood vessels. The Pressure of Fluid in Motion Decreases over Distance Pressure in a fluid is the force exerted by the fluid on its container. In the heart and blood vessels, pressure is commonly measured in millimeters of mercury (mm Hg), where 1 millimeter of mercury is equivalent to the hydrostatic pressure exerted by a 1-mm-high column of mercury on an area of 1 cm2. For this reason, when the heart relaxes and expands, pressure in the fluid-filled chambers falls. Volume changes of the blood vessels and heart are major factors that influence blood pressure in the cardiovascular system. Blood Flows from Higher Pressure to Lower Pressure As stated earlier, blood flow through the cardiovascular system requires a pressure gradient. Flow through the tube is directly proportional to (a) the pressure gradient (P): Flow a P (1) this expression says that flow is inversely proportional to resistance: if resistance increases, flow decreases; and if resistance decreases, flow increases. For fluid flowing through a tube, resistance is influenced by three components: the radius of the tube (r), the length of the tube (L), and the viscosity (thickness) of the fluid (h, the Greek letter eta). This relationship says that the higher the pressure gradient, the greater the fluid flow. However, because there is no pressure gradient between the two ends of the tube, there is no flow through the tube. On the other hand, two identical tubes can have very different absolute pressures but the same flow. The identical bottom tube has a hydrostatic pressure of 40 mm Hg at one end and 15 mm Hg at the other end. This tube has lower absolute pressure all along its length but the same pressure gradient as the top tube: 25 mm Hg. Because the pressure difference in the two tubes is identical, fluid flow through the tubes is the same. Resistance Opposes Flow In an ideal system, a substance in motion would remain in motion. Just as a ball rolled across the ground loses energy to friction, blood flowing through blood vessels encounters friction from the walls of the vessels and from cells within the blood rubbing against one another as they flow. We speak of people being resistant to change or taking the path of least resistance. This concept translates well to the cardiovascular system because blood flow also takes the path of least resistance. An increase in the resistance of a blood vessel results in a decrease in the flow through that vessel. You do not need to suck as hard on a short straw as on a long one (the resistance offered by the straw increases with length). Drinking water through a straw is easier than drinking a thick milkshake (resistance increases with viscosity). And drinking the milkshake through a fat straw is much easier than through a skinny cocktail straw (resistance increases as radius decreases). How significant are tube length, fluid viscosity, and tube radius to blood flow in a normal individual The length of the systemic circulation is determined by the anatomy of the system and is essentially constant. Blood viscosity is determined by the ratio of red blood cells to plasma and by how much protein is in the plasma. Normally, viscosity is constant, and small changes in either length or viscosity have little effect on resistance. This leaves changes in the radius of the blood vessels as the main variable that affects resistance in the systemic circulation. If we assume that the length of the straw and the viscosity of the milkshake do not change, this system is similar to the cardiovascular system-the radius of the tube has the greatest effect on resistance. If we consider only resistance (R) and radius (r) from equation 4, the relationship between resistance and radius can be expressed as R a 1/r 4 (5) If the skinny straw has a radius of 1, its resistance is proportional to 1/14 or 1. Because flow is inversely proportional to resistance, flow increases 16-fold when the radius doubles. As you can see from this example, a small change in the radius of a tube has a large effect on the flow of a fluid through 14.

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