103 terms

Chapter 20

The circulatory system: blood vessels and circulation
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Arteries
Type of blood vessel, efferent vessel of the cardiovascular system, they carry blood away from the heart, sometimes called resistance vessels of the cardiovascular system because they have relatively strong, resilient tissue structure that resists high blood pressure, each beat of the heart creates a surge of pressure in the arteries as blood is ejected into them, they are built to withstand these surges, being more muscular than veins they retain their round shape even when empty, and they appear relatively circular in tissue sections, they are divided into three categories by size: conducting arteries, distributing arteries, and resistance arteries, have sense organs of certain major arteries above the heart in their walls, monitor BP and blood chemistry, trans info to brainstem that serves to regulate the heartbeat, vasomotion and respiration, three kinds: carotid sinuses, carotid bodies, and aortic bodies
Veins
Type of blood vessel, afferent vessels that carry blood back to the heart, capacitance vessels of the cardiovascular system, thin walled and flaccid and expand easily to accommodate an increased volume of blood, have greater capacity for blood containment than arteries do, at rest about 54% of the blood is found in the systemic veins as compared with only 11% in systemic arteries, being distant from ventricles of heart veins can be thin walled because of relatively low BP, also blood flow in veins is steady not pulsating with heartbeat like in arteries, they do not require thick pressure-resistant walls, they collapse when empty and thus have relatively flattened, irregular shapes in histological sections, in arterial system it branches smaller and smaller in venous system they merge to form larger and larger ones as they approach the heart, smaller veins are tributaries, from smallest to largest include: postcapillary venules, muscular venules, medium veins, venous sinuses, and large veins
Capillaries
Type of blood vessel, microscopic, thin-walled vessels that connect the smallest arteries to the smallest veins, capillaries and some venules pass nutrients, wastes, and hormones between the blood and the tissue fluids through the vessel walls, all rest of cardiovascular system serves to supply exchange that occurs here, capillaries greatly outnumber venules they are the more important of the two, they are sometimes called the exchange vessels of the cardiovascular system, composed of only an endothelium and basal lamina, they are thinner at the proximal end where they receive arterial blood and widen at the distal end where they empty into a small vein, they often branch along the way, RBC often have to stretch into elongated shapes to squeeze through these, scarcely any cell in the body is more than 60-80 um (6 cells widths) away from the nearest capillary, few exceptions: capillaries are scarce in tendons and ligaments found only occasionally in cartilage, and absent from epithelia and the cornea and lens of the eye, three types distinguished by the ease with which they allow substances to pass through their walls and by structural differences that account for their greater or lesser permeability: continuous capillaries, fenestrated capillaries, sinusoids
Tunics
Walls of arteries and veins are composed of three layers called these
Tunica interna (tunica intima)
Lines the inside of the vessel and is exposed to blood, consist of simple squamous epithelium called endothelium overlying a basement membrane and a sparse layer of loose connective tissue
Endothelium
Makes up tunica interna, simple squamous epithelium overlying a basement membrane and a sparse layer of loose connective tissue, acts as selectively permeable barrier to materials entering or leaving the bloodstream, it secrets chemicals that stim dilation or constriction of the vessel and it normally repels blood cells and platelets so that they flow freely without sticking to the vessel wall, when it is damaged platelets may adhere to it and form a blood clot and when the tissue around a vessel is inflamed the cells produce cell-adhesion molecules that induce leukocytes to adhere to the surface, this causes leukocytes to congregate in tissues where their defensive actions are needed
Cell-adhesion molecules
Produced by endothelial cells, induce leukocytes to adhere to the surface, when tissues around a vessel is inflamed
Tunica media
The middle layer of blood vessels, usually the thickest, consists of smooth muscle, collagen and in some cases, elastic tissue, the relative amounts of smooth muscle and elastic tissue vary greatly from one vessel to another and form a basis for classifying vessels, it strengthens the vessels and prevents blood pressure from rupturing them, it provides vasomotion
Vasomotion
Provided by tunica media, changes in diameter of blood vessels, quick and powerful way of altering blood pressure and flow, affected by: local, neural, and hormonal mechanisms
Tunica externa (tunica adventitia)
The outer layer of blood vessels, consist of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs, it anchors the vessel and provides passage from small nerves, lymphatic vessels and smaller blood vessels, small vessels (vasa vasorum) supply blood to at least the outer half of the wall of a larger vessels, tissues of the inner half of the wall are thought to be nourished by diffusion from blood in the lumen
Vasa vasorum
Small vessels supply blood to at least the outer half of the wall of a larger vessels, in tunica externa,
Conducting (elastic or large) arteries
Biggest arteries includes: the aorta, common carotid and subclavian arteries, pulmonary trunk, and common iliac arteries, they have a layer of elastic tissue called the internal elastic lamina at the border between the interna and media, but microscopically it is incomplete and difficult to distinguish from the elastic tissue of the tunica media, the tunica media consists of 40-70 layers of elastic sheets, perforated, alternating with thin layers of smooth muscle, collagen, and elastic fibers, this elastic tissue dominates, there is an external elastic lamina at the border between the media and externa but it too is difficult to distinguish from the elastic sheets of the tunica media, the tunica extern is less than half as thick as the tunica media and quite sparse in the largest arteries, it is well supplied with vasa vasorum, these arteries expand during ventricular systole to receive blood and recoil during diastole, this expansion takes some pressure off the blood so that smaller arteries downstream are subjected to less systolic stress, their recoil between heartbeats prevents the blood pressure from dropping too low while the heart is relaxing and refilling, these effects lessen the fluctuation in blood pressure that would otherwise occur, arteries stiffened by atherosclerosis cannot expand and recoil as freely, therefore the downstream vessels are subjected to greater stress and are more likely to develop aneurysms and rupture
Internal elastic lamina
Found in arteries, layer of elastic tissue at the border between the interna and media
External elastic lamina
In arteries, at the border between the media and extern, made of elastic tissue
Distributing (muscular or medium) arteries
Smaller branches that distribute blood to specific organs, arteries, branch from conducting arteries, most arteries that have specific anatomical names are in these first two size classes, includes: the brachial, femoral, renal, and splenic arteries, they typically have up to 40 layers of smooth muscle constituting about ¾ of the wall thickness, this smooth muscle is more conspicuous than the elastic tissue, both the internal and external elastic laminae are thick and often conspicuous
Resistance (small) arteries
Type of artery, usually too variable in number and location to be given individual names, exhibit up to 25 layers of smooth muscle and relatively little elastic tissue, compared to large arteries, they have a thicker tunica media in proportion to the lumen, the smallest of these arteries about 40-200 um in diameter and with only one to three layers of smooth muscle, are called arterioles, which have very little tunica externa
Arterioles
Smallest of resistance arteries with only one to three layers of smooth muscle, have very little tunica externa
Metarterioles
Short vessels that link arterioles and capillaries, instead of a continuous tunica media, they have individual muscle cells spaced a short distance apart each forming a precapillary sphincter that encircles the entrance to one capillary, constriction of these sphincters reduces or shuts off blood flow through their respective capillaries and diverts blood to tissues or organs elsewhere, become thoroughfare channels beyond the origins of the capillaries leading directly to a venule, capillaries empty into the distal end of this or directly into the venule,
Precapillary sphincter
Formed by muscle cells in metarterioles, encircles the entrance to one capillary, constriction of these reduces or shuts off blood flow through their respective capillaries and diverts blood to tissues or organs elsewhere, when these are open the capillaries are well perfused with blood and engage in exchanges with the tissue fluid, when they are closed, blood bypasses the capillaries flows through the thoroughfare channel to a venule and engages in relatively little fluid exchange, there is not enough blood in the body to fill the entire vascular system at once, so about ¾ of the body's capillaries are shut down at any given time (ex. Skeletal muscle very little at rest and others like skin and intestines high, then reversed with exercise)
Aneurysm
A weak point in an artery or in the heart wall, forms a thin-walled bulging sac that pulsates with each beat of the heart and may eventually rupture, in a dissecting aneurysm blood accumulates between the tunicas of an artery and separates them usually because of degeneration of the tunica media, the most common sites of aneurysms are the abdominal aorta, renal arteries, and the arterial circle at the base of the brain, even without hemorrhaging, aneurysms can cause pain or death by putting pressure on brain tissue, nerves, adjacent veins, pulmonary air passages, or the esophagus, other consequences include neurological disorders, difficulty in breathing or swallowing, chronic cough, or congestion of the tissues with blood, aneurysms sometimes result from congenital weakness of the blood vessels and sometimes from trauma or bacterial infections such as syphilis, the most common cause however is the combination of atherosclerosis and hypertension
Dissecting aneurysm
When blood accumulates between the tunics of an artery and separates them, usually because of degeneration of the tunica media
Carotid sinuses
An artery sense organ, baroreceptors (pressure sensors) that respond to changes in blood pressure, ascending the neck on each side is a common carotid artery which branches near the angle of the mandible, forming the internal carotid artery to the brain and external carotid artery to the face, these are located in the wall of the internal carotid artery just above the branch point, it has a relatively thin tunica media and an abundance of glossopharyngeal nerve fibers in the tunica externa, a rise in blood pressure easily stretches the thin media and stim these nerve fibers, the glossopharyngeal nerve that trans signals to the vasomotor and cardiac centers of brainstem, and the brainstem responds by lowering the heart rate and dilating the blood vessels, thereby lowering the blood pressure
Carotid bodies
An artery sense organ, licated near the branch of the common carotid arteries, these are oval receptors about 3x5 mm in size, innervated by sensory fibers of the vagus and glossopharyngeal nerves, they are chemoreceptors that monitor changes in blood composition, they primarily trans signals to the brainstem respiratory centers, which adjust breathing to stabilize the blood pH and its CO2 and O2 levels
Aortic bodies
An artery sense organ, these are one to three chemoreceptors located in the aortic arch near the arteries to the head and arms, they are structurally similar to the carotid bodies and have the same function
Continuous capillaries
Type of capillaries, occur in most tissues, such as skeletal msucles, their endothelial cells held together by tight junctions form a continuous tube, a thin protein-carbohydrate layers the basal lamina surrounds the endothelium and separates it from the adjacent connective tissues, the endothelial cells are separated by narrow intercellular clefts about 4 um wide, small solutes such as glucose can pass through these clefts but most plasma protein other large molecules and platelets and blood cells are held back, the continuous capillaries of the brain lack intercellular clefts and have more complete tight junctions that form the BBB, some cont capillaries exhibit cells called pericytes that lie external to endothelium, pericytes have elongated tendrils that wrap around the capillary, they contain the same contractile proteins as muscle, and it is thought that they contract and regulate blood flow through the capillaries, they also can differentiate into endothelial and smooth muscle cells and thus contribute to vessel growth and repair
Basal lamina
Found in continuous capillaries, surround the endothelium and separates it from the adjacent connective tissues
Intercellular clefts
Found in continuous capillaries, the endothelial cells are separated by this, small solutes such as glucose can pass through these clefts but most plasma protein other large molecules and platelets and blood cells are held back, lacked in continuous capillaries of brain
Pericytes
Found in continuous capillaries, have these cells that lie external to the endothelium, they have elongated tendrils that wrap around the capillary, they contain the same contractile proteins as muscle and it is thought that they contract and regulate blood flow through the capillaries, they also can differentiate into endothelial and smooth muscle cells and thus contribute to vessel growth and repair
Fenestrated capillaries
Type of capillaries, have endothelial cells riddled with holes called filtration pores, pores often spanned by a glycoprotein membrane that is much thinner than the cells plasma membrane, they allow for the rapid passage of small molecules, but still retain most proteins and larger particles in the bloodstream, these are important in organs that engage in rapid absorption or filtration, the kidneys, endocrine glands, small intestine, and choroid plexuses of the brain
Filtration pores (fenestrations)
Holes in fenestrated capillaries, spanned by glycoprotein membrane that is much thinner than the cell's plasma membrane, allow for rapid passage of small molecules, but still retain most proteins and larger particles in the bloodstream
Sinusoids (discontinuous capillaries)
Types of capillaries, irregular blood-filled spaces in the liver, bone marrow, spleen, and some other organs, they are twisted, tortuous passageways, typically 30-40 um wide, that conform to the shape of the surrounding tissue, the endothelial cells are separated by wide gaps with no basal lamina, and the cells also frequently have especially large fenestrations through them, even proteins and blood cells can pass through these pores, this is how albumin, clotting factors, and other proteins synthesized by the liver enter the blood, and how newly formed blood cells enter the circulation from the bone marrow and lymphatic organs, some sinusoids contain macrophages or other specialized cells,
Capillary beds
Capillaries are organized into these networks, usually 10-100 capillaries supplied by a single metarteriole, beyond the origins of the capillaries the metarteriole cont. as a thoroughfare channel leading directly to a ventule, capillaries empty into the distal end of the thoroughfare channel or directly into the venule
Thoroughfaree channel
Beyond the origins of the capillaries the metarteriole cont. as this leading directly to a venule
Tributaries
Smaller veins
Postcapillary venules
Smallest of the veins, receive blood from capillaries directly or by way of the distal ends of the thoroughfare channels, they have a tunica interna with only a few fibroblasts around it and no muscle, they are often surrounded by pericytes, are even more porous than capillaries, therefore venules also exchange fluid with the surrounding tissues, most leukocytes emigrate from the bloodstream through the venule walls, 15-20 um
Muscular venules
Types of veins that receive blood from the postcapillary venules, they have a tunica media of one or two layers of smooth muscle, and a thin tunica externa, 1 mm
Medium veins
Veins that range up to 10 mm in diameter, most veins with names are in this category (radial and ulnar and saphenous veins of leg), have a tunica interna with an endothelium, basement membrane, loose connective tissue, and sometimes a thin internal elastic lamina, the tunica media is much thinner than it is in medium arteries, exhibits bundles of smooth muscle, but not a continuous muscular layer as seen in arteries, the muscle is interrupted by regions of collagenous, reticular and elastic tissue, the tunica externa is relatively thick, many esp. in limbs exhibit infolding of the tunica interna that meet in the middle of the lumen forming venous valves directed toward the heart, the pressure in the veins not high enough to push all blood upward against gravity, so depend on massaging action of skeletal muscles and the ability of these valves to keep the blood from dropping down again when the muscles relax, when muscles surrounding a vein contract they force blood through these valves, the population of venous blood by muscular massaging aided by venous valves is a mechanism of blood flow called the skeletal muscle pump,
Venous valves
Formed from infolding of the tunica interna that meet in the middle of the lumen, directed toward the heart, these help to keep the blood from dropping down agina when the muscles relax, failure of these leads to varicose veins, such valves are absent from very small and large veins, veins of abdominal and thoracic cavities and veins of brain
Venous sinuses
Veins with especially thin walls, large lumens, and no smooth muscle, ex. Coronary sinus of heart and the dural sinuses of the brain, unlike other veins they are not capable of vasomotion
Large veins
Veins that have diameter greater than 10 mm, they have some smooth muscle in all three tunicas, they have a relatively thin tunica media with only a moderate amount of smooth muscle, the tunica externa is the thickest layer and contains longitudinal bundles of smooth muscle, large veins include the venae cavae, pulmonary veins, internal jugular veins, and renal veins
Skeletal muscle pump
System with muscles massaging and venous valves that keep blood moving toward heart
Varicose veins
In people who stand for long periods, such as barbers and cashiers, blood tends to pool in the lower limbs and stretch the veins, this is esp. true of superficial veins, which are not surrounded by supportive tissue, stretching pulls the cusps of the venous valves farther apart until the valves become incapable of sealing the vessel and preventing the backflow of blood, as the veins become further distended, their walls grow weak and they develop into varicose veins with irregular dilations and twisted pathways, obesity and pregnancy also promote development of varicose veins by putting pressure on large veins of the pelvic region and obstructing drainage from the limbs, varicose veins sometimes develop because of hereditary weakness of the valves, with less drainage of blood, tissues of the leg and foot may become edematous and painful, hemorrhoids are varicose veins of the anal canal
Simplest circulatory route
Heart -> arteries -> capillaries -> veins -> heart, blood usually passes through only one network of capillaries from the time it leaves the heart until the time it returns, but there are exceptions, like portal systems and anastomoses
Portal system
Blood flows through two consecutive capillary networks before returning to the heart, occur in kidneys connecting the hypothalamus and the anterior pituitary, and connecting the intestines to the liver
Anastomosis
A point where two blood vessels merge
Arteriovenous anastomosis (stunt)
Blood flows from an artery directly into a vein and bypasses the capillaries, they occur in fingers, palms, toes, and ears where they reduce heat loss in cold weather by allowing warm blood to bypass these exposed surfaces, unfortunately this makes these poorly perfused areas more susceptible to frostbit
Venous anastomoses
Most common anastomoses, in which one vein empties directly into another, provide several alternative routes of drainage from an organ so blockage of a vein is rarely as life-threatening as blockage of an artery
Arterial anastomoses
An anastomoses in which two arteries merge, provide collateral (alternative) routes of blood supply to a tissue, they are also common around joints where movement may temporarily compress an artery and obstruct one pathway,
Perfusion
Expresses blood supply, flow per given volume or mass of tissue (mL/min/g),
Blood pressure (BP)
The force that the blood exerts against a vessel wall, can be measure within a blood vessel or the heart by inserting a catheter or needle connected to an external manometer, but of greatest interest is the systemic arterial BP at a point close to the heart measure with a sphygmomanometer connected to an inflatable cuff wrapped around the arm, the brachial artery passing through this region is sufficiently close to the heart that the BP recorded here is approximates the maximum arterial BP found anywhere in the body, two pressures are recorded: systolic pressure and diastolic pressure, typically 120/75 mm Hg, arterial BP is written as a ratio of systolic over diastolic pressure, BP is determined by: CO, blood volume, and resistance to flow
Systolic pressure
The peak arterial BP attained during ventricular contraction
Diastolic pressure
The minimum arterial BP occurring during the ventricular relaxation between heartbeats
Hypertension
High BP, commonly considered to be chronic resting blood pressure higher than 140/90, transient high BP resulting from emotion or exercise is not the same, among other effects, it can weaken the small arteries and cause aueurysms and it promotes the development of atherosclerosis, body tries to reduce with the ability of the arteries to stretch and recoil during the cardiac cycle, healthy conducting arteries expand with each systole and absorb some of the force of the ejected blood, then when the heart is in diastole their elastic recoil exerts pressure on the blood and prevents the BP from dropping to zero, this combo of expansion and recoil maintains a steady flow of blood downstream in the capillaries throughout the cardiac cycle thus the elastic arteries smooth out the pressure fluctuation and reduce stress on the smaller arteries, as we get older our arteries become less distensible and absorb less systolic force so BP rises with age, atherosclerosis also stiffens the arteries and raises the BP
Hypotension
Chronic low resting BP, it may be a consequence of blood loss, dehydration, anemia, or other factors and is normal in people approaching death,
Pulsatile
Blood flow in arteries is this, in the aorta blood rushes forward at 120 m/s during systole and has an average speed 40 cm/s over the cardiac cycle, when measured farther away from the heart, systolic and diastolic pressures are lower and there is less difference between them, in capillaries and veins the blood flows at a steady speed without pulsation because the pressure surges have been damped out by the distance traveled and the elasticity of the arteries, this is why an injured vein exhibits relatively slow, steady bleeding, whereas blood jets intermittently from a severed artery, in the inferior vena cava near the heart, however venous flow fluctuates with the respiratory cycle and there is some fluctuation in the jugular veins of the neck
Blood volume
Helps to determine BP, it is regulated mainly by the kidneys, the kidneys have a greater influence than any other organ on blood pressure (except heart)
Resistance to flow
Helps to determine BP, which results from the friction of the blood against the walls of vessels, it in turn hinges on three variables: blood viscosity, vessel length, and vessel radius, consider systemic circuit, flow is fastest in the aorta because it is a large vessel close to the pressure source, from aorta to capillaries velocity diminishes because: the blood has traveled a greater distance so friction has slowed it down, the arterioles and capillaries have smaller radii and therefore put up more resistance, even though the radii of individual vessels become smaller as we progress farther from the heart the number of vessels and their total cross-sectional area becomes greater and greater, so a given volume of aortic blood is distributed over a greater total area in capillaries which collectively form a wider path in the bloodstream, blood slows down as it enters pathway with a greater total width or volume, velocity rises again from capillaries to vena cava, one reason is veins are larger than capillaries, so they create less resistance, also a large amount of blood is being forced into a progressively smaller channel, blood in veins never regains the velocity it has in large arteries, because veins are farther from heart and pressure is much lower,
Peripheral resistance
The opposition to flow that the blood encounters in vessels away from the heart, a moving fluid has no pressure unless it encounters at least some resistance, thus pressure and resistance are not independent variables in blood flow, pressure is affected by resistance and flow is affect by both, arterioles are the most significant point of control
Blood viscosity
Affects blood resistance, most significant variable in this is the RBC count and albumin concentration, a deficiency of RBC (anemia) or albumin (hypoproteinemia) reduces viscosity and speeds up blood flow, viscosity increases and flow declines in such conditions as polycythemia and dehydration, this is quite stable in a healthy person
Vessel length
Affects blood resistance, the farther a liquid travels through a tube the more cumulative friction it encounter thus pressure and flow decline with distance, if you were to measure mean arterial pressure in a reclining person you would obtain a higher value in the arm than in the ankle, this would not be true in a standing person because of the influence of gravity, a strong pulse in the dorsal pedal artery of the foot is a good sign of adequate CO, if perfusion is good at that distance from the heart, it is likely to be good elsewhere in the systemic circulation, this is quite stable in the short term
Vessel radius
Only significant way of controlling peripheral resistance from moment to moment, vasomotion which is the adjusting the radius of the blood vessels, this includes vasoconstriction and vasodilation, this has affects due to the friction of the moving blood against the vessel walls, blood normally exhibits smooth silent laminar flow where is flows faster in center with less friction, when a blood vessel dilates a greater portion of the blood is in the middle of the stream and the average flow may be quite swift, when the vessel constricts, more of the blood is closer to the wall and the average flow is slower, the radius of a vessel greatly affects blood velocity, blood flow is proportional to vessel radius and to the fourth power or radius, a mere 3-fold increase in radius increases a 81-fold increase in velocity, blood vessel are capable of substantial changes in radius,
Vasoconstriction
Affect vessel radius, the narrowing of a vessel, aspect of vasomotion, this occurs when the smooth muscle of the tunica media contracts
Vasodilation
Affect vessel radius, the widening of a vessel, aspect of vasomotion, this occurs not by any muscular effort to widen a vessel but rather by muscular passivity, relaxation of the smooth muscle allowing BP to expand the vessel
Laminar flow
Blood normally exhibits this, it flows in 'layers' faster near the center of a vessel where it encounters less friction and slower near the wall where it drags against the vessel
Arterioles
are the most significant point of control because: they are on the proximal sides of the capillary beds, so they are best positioned to regulate flow into the capillaries, they greatly outnumber any other class of arteries and thus provide the most numerous control points, and they are more muscular in proportion to their diameters than any other class of blood vessels and are highly capable of vasomotion, arterioles alone account for about half of the total peripheral resistance of the circulatory system, but larger arteries and veins are also capable of considerable vasomotion and control of peripheral resistance
Vasomotor center
Neural control of vasomotion, remote control by the CNS and ANS, in the medulla it exerts SNS control over blood vessels throughout the body, (precapillary phincters have no innervation however and respond only to local and hormonal stim), SNS fibers stim most blood vessels to constrict, but they dilate the vessels of skeletal and cardiac muscle in order to meet the metabolic demands of exercise, this center is an integrating center for three autonomic reflexes: baroreflexes, chemoreflexes, and medullary ischemic reflex
Barorefelx
Remote neural control of vasomation, autonomic reflex controlled by vasomotor center, an autonomic negative feedback response to changes in BP, the changes are detected by the carotid sinuses, glossopharyngeal nerve fibers from these sinuses transmit signals continually to the brainstem, when the BP rises their signaling rate rises, this input reduces sympathetic tone and it excites the vagal fibers to the heart, thus it reduces the heart rate and CO, dilates the arteries and veins, and reduces the BP, when BP drops below normal on the other hand the opposite reactions occur and BP rises back to normal, these are important in short-term regulation of BP, such as adapting to changes in posture, if you jump out of bed you feel dizzy, this is because gravity draws blood into large veins of abdomen and lower limbs when you stand which reduces venous return to heart and CO to brain, these receptors respond quickly to drop in pressure and restore cerebral perfusion, they are not effective in chronic hypertension, they adjust their set point to the higher BP and maintain dynamic equilibrium at this new level
Chemoreflex
Remote neural control of vasomation, autonomic reflex controlled by vasomotor center, an autonomic response to changes in blood chemistry, esp. pH and concentrations of O2 and CO2, initiated by aortic bodies and carotid bodies, their primary role is to adjust respiration to changes in blood chemistry, but they have a secondary role in stim vasomotion, hypoxemia (blood O2 deficiency), hypercapnia (CO2 excess), and acidosis (low blood pH) stim these receptors and act through the vasomotor center to induce widespread vasoconstriction, this increases overall BP, thus increasing perfusion of lungs and the rate of gas exchange, they also stim breathing so increased ventilation of the lungs matches their increased perfusion, increasing on without the other would be of little use
Capillary exchange
Two-way movement of fluid, most important blood in body, mainly across capillary walls that exchanges occur between the blood and surrounding tissues, chemicals given off by the capillary blood to serve the perivascular tissues include oxygen, glucose, amino acids, lipids, and other organic nutrients, minerals, antibodies, and hormones, chemical take up by them include CO2, ammonia, and other wastes, and many of the same substances as they give off like glucose, and fatty acids released from storage in the liver and adipose tissue, calcium and other minerals release from bone, antibodies secreted by immune cells, and hormones secreted by the endocrine glands, thus many of these chemicals have a tow-way traffic between the blood and connective tissue, leaving the capillaries at one point and entering at another, along with all these solutes there is substantial movement of water in and out of the bloodstream across the capillary walls, the mechanisms of this exchange are difficult to study because hard to measure pressure and flow in such small vessels, those at base of nails can be observed with stereomicroscope, there are three routes: the endothelial cell cytoplasm, intercellular clefts between the endothelial cells, and filtration pores (fenestrations) of the fenestrated capillaries; mechanisms of movement through the walls include: diffusion, transcytosis, filtration, and reabsorption
Diffusion
Mechanisms of capillary exchange, most important one, glucose and oxygen being more concentrated in the systemic blood than in the tissue fluid, diffuse out of the blood, CO2 and other wastes being more concentrated in the tissue fluid, diffuse into the blood, this is only possible if the solute can either permeate the membranes of the endothelial cells or find passages large enough to pass (filtration pores and intercellular clefts), such lipid-soluble substances as steroid hormones, O2, and CO2 diffuse easily through the membranes, substances insoluble in lipids such as glucose and electrolytes must pass through membrane channels, filtration pores, or intercellular clefts, large molecules such as protein are usually held back
Transcytosis
A process in which endothelial cells pick up material on one side of the membrane by pinocytosis or receptor-mediated endocytosis, transport the vesicles across the cell, and discharge the material on the other side by exocytosis, this probably accounts for only small fraction of solute exchange across capillary wall, but fatty acids, albumin, and some hormones such as insulin move across the endothelium by this mechanism, cytologists think that filtration pores of fenestrated capillaries might be only a chain of pinocytotic vesicles that have temporarily fused to form a continuous channel through the cells,
Filtration and reabsorption
Important for capillary fluid exchange, fluid filters out of the arterial end of a capillary and osmotically reneter it at the venous end, this fluid delivers materials to the cells and removes their metabolic wastes, due to shifting balance between hydrostatic and osmotic forces, the only pressure that changes significantly is capillary blood pressure which is responsible for shift from filtration to reabsorption, net reabsorption pressure 7 mm Hg and net filtration pressure of 13 mm Hg it would seem like more fluid would leave capillaries than reenter them but capillaries branch along their length so there are more of them at the venous end than arterial end which compensates for difference, and typically are two the diameter at venous end so more capillary SA to reabsorb than give off, capillaries reabsorb about 85% of fluid they filter and other 15% absorbed and returned to blood through limbic system
Hydrostatic pressure
The physical force exerted by a liquid against a surface such as a capillary wall, blood pressure is an example, typically capillary has this at about 30 mm Hg at arterial end, and -3 mm Hg (negative/slight suction, drawing fluid out of capillary) in interstitial space (but hard to measure), the (+) within capillary and (-) interstitial work in same direction creating a total outward force of 33 mm Hg, these forces are opposed by colloid osmotic pressure
Colloid osmotic pressure (COP)
Opposes hydrostatic pressure, the portion of the osmotic pressure due to protein, blood has about 28 mm Hg, due mainly to albumin, tissue fluid has less than 1/3 the protein concentration of blood plasma at about 8 mm Hg, this difference is called oncotic pressure, typically 20in which tends to draw water into capillary by osmosis opposing hydrostatic pressure
Oncotic pressure
Difference between COP of blood and tissue, typically 28in-8out=20in, therefore tends to draw water into the capillary by osmosis, opposing hydrostatic pressure
Net filtration pressure (NFP)
Opposing forces of oncotic pressure and hydrostatic pressure create this, about 13 mm Hg, 33out(hydrostatic pressure)-20in(oncotic pressure) = 13out, causing .5% of blood plasma to leave capillaries at the arterial end,
Net reabsorption pressure
At the venous end capillary blood pressure is lower (10 mm Hg), 13out-7in = 7in, the prevailing force is inward at venous end because osmotic pressure overrides filtration pressure, causing capillaries to reabsorb fluid at this end,
Solvent drag
Process in which other chemicals dissolved in blood, other than water, cross capillary walls by filtration and reabsorption and movement of water taking them with it
Edema
The accumulation of excess fluid in a tissue, often shows swelling of face, fingers, abdomen, or ankles, but also in internal organs, occurs when fluid filters into tissue faster than it is reabsorbed, three causes: increased capillary filtration, reduced capillary reabsorption, and obstructed lymphatic drainage, it has multiple consequences, congested tissues with fluid causes impaired oxygen delivery and waste removal and the tissues may die, pulmonary ones threat suffocation as fluid replaces air in lungs, and cerebral ones an cause headaches, nausea, and sometimes delirium, seizures, and coma, in severe causes so much fluid may transfer from vessels to tissues that BV and BP may drop low enough to cause circulatory shock
Increased capillary filtration
Causing edema, due to kidney failure leading to water retention and hypertension thus raising capillary BP and filtration rate, histamine dilates arterioles and raises capillary pressure and also makes the capillary wall more permeable, capillaries generally become more permeable in old age increasing elderly risk for edema, capillary blood pressure also rises in cases of poor venous return (flow back to heart), good venous return depends on muscular activity so common in people in wheelchairs, failure of right ventricle tends to cause pressure to back up in systemic veins and capillaries resulting in systemic edema, failure of left ventricle causes pressure to back up in lungs, causing pulmonary edema
Reduced capillary reabsorption
Causing edema, capillary reabsorption depends on oncotic pressure, which is proportional to the concentration of blood albumin, a deficiency in it (hypoproteinemia) produces edema by reducing reabsorption of tissue fluid, albumin is produced by liver so liver diseases such as cirrhosis tend to lead to hypoproteinemia and edema, edema is commonly seen in regions of famine due to protein deficiency, so result from severe burns due to protein loss from body surfaces without skin coverage, also caused by kidney disease that allow protein to escape in urine
Obstructed lymphatic drainage
Causing edema, lymphatic system is a network of one-way vessels that collect fluid from the tissues and return it to the bloodstream, obstruction of these vessels or the surgical removal of lymph nodes can interfere with fluid drainage and lead to the accumulation of tissue fluid distal to the obstruction
Venous return
The flow of blood back to the heart, achieved by five mechanisms: pressure gradient, gravity, skeletal muscle pump, thoracic (respiratory) pump, and cardiac suction, this increases with exercise, the heart beats faster and harder increasing CO and BP, vessels of skeletal muscles, lungs, and coronary circulation dilate to increase flow, increase respiratory rate and depth enhances the action of the thoracic pump, muscle contractions increase venous return by means of skeletal muscle pump, increased venous return increases CO which is important in perfusion of the muscles just when they need it most, when a person is still blood accumulates in the libs because venous pressure is not high enough to override the weight bo the blood and drive it upward, this is called venous pooling
Pressure gradient
Allows for venous return, pressure generated by the heart is most important force in venous flow, even though it is substantially weaker in the veins than in arteries, pressure in the venules ranges from 12-18 mm Hg, and pressure at the point where the venae cavae enter the heart called central venous pressure is about 4.6 mm Hg, this creates this term of about 7-13 mm Hg favoring the flow of blood toward the heart, both this and venous return increases in the event of generalized, widespread vasoconstriction because this reduces the volume of the circulatory system and raises BP and flow
Central venous pressure
The pressure at the point where the venae cavae enter the heart, averages 4.6 mm Hg, contributes to pressure gradient that affects venous return
Gravity
Allows for venous return, when you are sitting or standing, blood from head and neck returns to heart by flowing downhill through the large veins above the heart, large veins of neck are normally collapsed or nearly so, and their venous pressure is close to zero, the dural sinuses of the brain however have more rigid walls and cannot collapse, their pressure is as low as -10 mm Hg, creating a risk of air embolism if they are punctured
The skeletal muscle pump
Allows for venous return, in the limbs the veins are surrounded and massaged by the muscles, contracting muscles squeeze the blood out of the compressed part of a vein, and the valves ensure that their blood can go in only one direction toward the heart
The thoracic (respiratory) pump
Allows for venous return, this aids in the flow of venous blood from the abdominal to the thoracic cavity, when you inhale, your thoracic cavity expands and its internal pressure drops, while downward movement of the diaphragm raises the pressure in your abdominal cavity, the inferior vena cava, you largest vein, Is a flexible tube passing through both of these cavitites, if abdominal pressure on the IVC rises while thoracic pressure on it drops, then blood is squeezed upward toward the heart, it is not forced back into the lower limbs because of the thoracic pump, central venous pressure fluctuates from 2 mm Hg when you inhale to 6 mm Hg when you exhale, and blood flows faster when you inhale
Cardiac suction
Allows for venous return, during ventricular systole, then tnedinous cords pull the AV valve cusps downward, slightly expanding the atrial space, this creates a slight suction that draws blood into the atria from the venae cavae and pulmonary veins
Venous pooling
Accumulation of blood, occurs when person is still and blood accumulates in limbs because venous pressure is not high enough to override the weight of blood to drive upward, tis is troublesome to people who must stand for prolonged periods, if enough accumulates CO may become low and brain is inadequately perfused leading to dizziness or syncope, prevent by periodically tensing calf and other muscle to keep skeletal muscle pump active, military pilots wear pressure suits that inflate and tighten on lower limbs during maneuvers to cause black out, they tense abdominal muscle to prevent this
Circulatory shock
State in which CO is insufficient to meet the body's metabolic needs, all forms fall into two categories: cardiogenic shock and low venous return shock
Dissecting aneurysm
Splitting of the layers of an arterial wall from each other because of the accumulation of blood between layers, results from either a tear in the tunica interna or rupture of the vasa vasorum
Fat embolism
The presence of fat globules traveling in the bloodstream, globules originate from bone fractures, fatty degeneration of the liver, and other causes and may block cerebral or pulmonary blood vessels
Orthostatic hypotension
A decrease in BP that occurs when one stands, often resulting in blurring of vision, dizziness, and syncope (fainting), results from sluggish or inactive baroreflexes
Syncope
Fainting
Arterial pressure points
Places where major arteries come close to the body surface so they can be palpated, can be used to take pulse, firm pressure can be applied to temporarily reduce arterial bleeding in emergencies, one is the femoral triangle of the upper medial thigh, important for arterial supply, venous drainage, and innervation of the lower limb, bounded by sartorius muscle laterally, inguinal ligament superiorly, and adductor longus muscle medially, the femoral artery, vein, and nerve run close to the surface at this point
Hypertension
Most common cardiovascular disease, affects 30% of Americans over 50, 50% by 74, it's a silent killer with affects 10-20 years before noticed, major cause of heart failure, stroke, and kidney failure, damages heart by increasing afterload making ventricles work harder to expel blood, myocardium enlarges to a point (hypertrophic response), but eventually becomes excessively stretched and less efficient, creates lesion that become focal points of atherosclerosis which then worsens it causing positive feedback, with stress kidney arterioles thicken and their lumens become narrower and renal blood flow decline, in response to resulting drop in BP the kidneys release renin, leading to formation of the vasoconstrictor angiotensin II and the release of aldosterone, a hormone that promotes salt retention, this worsens hypertension, pressure above 120 mm Hg causes kidneys and heart to deteriorate rapidly, vessels in eye hemorrhage, blindness, and death within 2 years
Primary hypertension
About 90% of cases, results from complex web of behavioral, hereditary, and other factors difficult to sort, called essential hypertension because so common with aging, chief culprit is obesity, each pound of extra fat requires additional vessels to serve, this added length increases peripheral resistance and BP, also increases workload on heart with added weight, also sedentary behavior is culprit, aerobic exercise helps reduce this by controlling weight, reducing emotional tension, and stim vasodilation, diet high in cholesterol and sat fat contribute to atherosclerosis, K and Mg reduce BP, salt relationship controversial the kidneys compensate so effectively for excess salt intake that dietary salt has little effect on BP but my help control in older people and people with reduced renal function, nicotine stimulates myocardium to beat faster and harder while stim vasoconstriction and increases afterload against which the myocardium must work, when heart needs extra oxygen nicotine causes coronary vasoconstriction and promotes myocardial ischemia, hypertension runs in families, higher in blacks also strokes, more common in men 18-54, above 65 more common in women, treatments include weight loss, diet, certain drugs, diuretics low BV and BP by promoting urination, ACE inhibitors block the formation of the vasoconstrictor angiotensin II, beta-blockers such as propranolol block the vasoconstrictive action of the SNS, Ca channel blockers like verapamil and nifedipine inhibit inflow of Ca into cardiac and smooth muscle thus inhibiting their contraction, promoting vasodilation, and reducing cardiac workload,
Secondary hypertension
Accounts for 10% of cases, is high blood pressure that is secondary to (results from) other identifiable disorders, these include kidney disease (which may cause renin hypersecretion), atherosclerosis, hyperthyroidism, Cushing syndrome, polycythemia, corrected by treating underlying disease
Tissue necrosis
consequence of edema, oxygen delivery and waste removal impaired
Pulmonary edema
consequence of edema, suffocation threat
Cerebral edema
consequence of edema, headaches, nausea, seizures, and coma
severe edema or circulatory shock
consequence of edema, excess fluid in tissue spaces causes low blood volume and low blood pressure