Chapter 12 chapter notes
Chapter 12 chapter notes BISC306010
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Date Created: 09/23/15
Chapter 12Pressure Flow and Resistance 12072014 Hemodynamics the relationship between blood pressure blood ow and resistance to blood ow 0 Blood ow F is always from a region of higher pressure to one of lower pressure 0 Pressure exerted by any uid is called hydrostatic pressure but is referred to as pressure which denotes the force exerted by the blood This force is generated in the blood by the contraction of the heart and its magnitude caries throughout the system Units are Lmin and for pressure difference the units are mmHg 0 To determine the ow rate you need the pressure difference and the resistance to R to ow which is how dif cult it is for blood to ow between two points at any given pressure difference F change in pressure resistance Flow rate is directly proportional to the pressure difference between two points and inversely proportional to the resistance Viscosity a function of friction between molecules of a owing uid therefore the greater the friction the greater the viscosity n Increases as hematocrit volume of red blood cells in blood increases Length and radius of the tube are also determinants of resistance Resistance is directly proportional to both the uid viscosity and the vessels length and inversely proportional to the fourth power of the vessels radius The ultimate function of he circulatory system is to ensure adequate blood ow through the capillaries of various organs Arterioles in individual organs are responsible for determining the relative blood ows to those organs at any given mean arterial pressure Arterioles all together are the major factor in determining mean arterial pressure itself Dilating tube creates LOWER resistance greatest ow and narrowing tube HIGHERS resistance Forgan MAP Venous Pressure Resistance of Organ 0 Remember that MAP is the same throughout the body and the difference depend solely on the relative resistance of their respective arterioles 0 They contain smooth muscle which can either relax and cause the vessel radius to increase vasodilation or contract and decrease the radius vasoconstriction o Arteriolar smooth muscle posses a large degree of spontaneous activity independent of any neural hormaonal or paracrine input which is called intrinsic tone or basal tone The baseline level can be increased or decreased by NTs which is induced by changes in the cytosolic Ca 0 Local controls mechanisms independent of nerves or hormones by which organs and tissues alter their own arteriolar resistances thereby self regulating their blood ows Changes caused by autocrine and paracrine agents Active Hyperemia organs and tissues manifest an increased blood ow when their metabolic activity is increased El El Most highly developed in skeletal muscle cardiac muscle and glands Direct result of arteriolar dilation in the more active organ or tissue caused by local chemical changes in the extracellular uid Most obvious change is the decrease in local concentration of oxygen which is used in the production of ATP by oxidative phosphorylation Yet there is an increase in 0 Carbon diozide Hydrogen ions decrease in pH Adenosine K ions bradykinin o Nitric oxide which is released by endothelial cells NO NERVES OR HORMONES INVOLVED Flow Autoregulation Tissue or organ experiences a change in its bloody supply resulting from a change in blood pressure the change in resistance is in the direction of the maintaining blood ow and is nearly constant despite the pressure change El When a decrease in arterial pressure reduces blood ow to an organ the supply of oxygen to the organ diminishes and the local extracellular oxygen concentration decreases and the CO H ions and metabolites increase same a in active hyperemia 0 Therefore the local metabolic changes during decreased bloody supply at constant metabolic activity are similar to that during increased metabolic activity 0 This is because there in both situations there is an imbalance between blood supply and level of cellular metabolic activity I Also happens when arterial pressue increases but opposite occurs oxygen levels are increased and arterioles constrict n Myogenic responses also occur by the changes in Ca movement into the smooth muscle cells where smooth muscle responds directly contract when there is increased arterial pressure and opposite when decreased pressure Reactive Hyperemia when an organ or tissue has had its blood supply completely occluded there is a transient increase in its blood ow if the ow is reestablished n Extreme form of autoregulation a When there is no blood ow the arterioles in the affected area dilate and nally when the occlusion ceases to occur blood ow increases greatly through the dilated arteries Response to Injury causes eicosanoids and other substance to be released locally and cause smooth muscle to relax and cause dilation o Extrinsic Controls Re ex controls Most arterioles are richly innervated by postganglionic sympathetic neurons which primarily release norepinephrine that bind to alpha adrenergic repcetors U Release of norepinephrine causes vasoconstriction Control of sympathetic neurons can cause vasodilation n The primary function of sympathetic neurons to blood vessels are concerned with re exes that serve wholebody needs not just local With few exceptions there is NO important parasympathetic innervation of arterioles Noncholingeric nonadrenegic neurons do not release acetylcholine or norepinephrine they release other vasodilator substances nitric oxide I Also mediate erections drugs that treat erectile dysfunction enhance nitric oxide for this reason Epinephrine just like norepinephrine can bind to alpha adrenergic receptors and cause vasoconstriction but also can bind to beta receptors and cause vasodilation n In skeletal muscles this is most important because unlike other areas there is a large number of beta receptors that epinephrine can bind to Angiotensin II constricts arterioles many drugs are used to prevent its formation to help with high blood pressure Vasopressin also cases arteriolar constriction and is released into the blood by the posterior pituitary gland in response to decreased blood pressure Atrial natriuretic peptide hormone secreted by the cardiac atria vasodilator that regulates NA balance and blood volume Neural Mil quot91quot Filmmal G nltmls 7 l v i 39L lle l lllr ll l At any given moment approximately 5 of the total circulating blood is owing through the capillaries Capilarries permeate every tissue of the body except the cornea 25000 miles of capillaries bood velocity is dependent not on proximity to the heart but rather on total crosssectional area of the vessel type Angiogenesis the act of capillaries developing Anatomy of Cap Network 0 No smooth muscle in capillaries Vasodilation of the arterioles supplying the capillaries causes increased capillary ow constriction decreases ow 0 In some organs metarterioles not arterioles supply the capillaries More active tissue keeps the precapillary sphincter open which causes the caps to receive more blood Diffusion besides the brain diffusion is the number one means by which net movement of nutrients oxygen and metabolic end products occur in c apillary walls Lipidsoluble substances ie oxygen and CO easily diffuse through the plasma membranes of the cap endothelial cells 0 Ions and other polar molecules are poorly soluble in lipid and must use water lled channels Variations in the size of the water lled channels account for great differences in the leakiness of caps in different organs n Caps are tight in the brain and have no intercellular clefts n Liver caps have large intercellular clefts and large fused vesicle channel Transcapillary diffusion gradients occur as a result of cellular utilization of the substance 0 Glucose is continuously transported from interstitial uid into the muscle cell by carriermediated transport mechanisms and oxygen moves in the same direction by diffusion 0 Carbon dioxide is simulataneously produced by muscle cells and diffuses into the interstitial ud o If a tissue increases its metabolic rate it must obtain more nutrients from the blood nad must eliminate ore metabolic end products Active hyperemia increased diffusion gradients between plasma and tissue increased cellular utilization of oxygen and nutrients lowers their tissue concentrations Bulk ow from protein free plasma focuses on the distribution of the extracellular uid volume which includes the plasma 3 L and interstitial uid 11 L o In the presence of hydrostatic pressure across it the capillary wall behaves like a porous lter permitting proteinfree plasma to interstitial uid through the water lled channels referred to as ultra ltration or ltration o Magnitiude of bulk ow is determined partially by the difference between the cap blood pressure usually higher and the interstitial uid hydrostatic pressure 0 Osmosis causes not all of the plasma to lter our into the interstitial space 0 Crystalloids such as Na Cl K have no affect on the water concentration because they easily pass through the cap walls 0 Colloids or plasma proteins are unable to move through cap pores and have low concentrations in the interstitial uid 0 Main points The difference between cap blood hydrostatic pressure and interstitial uid hydrostatic pressure favors lration out of the cap The water concentration difference between plasma and interstitial uid which results from differences in protein concentration favors absorption of interstitial uid into the cap 0 Starling forces Pc capillary hydrostatic pressure which favors uid movement out of the cap Pif interstitial hydrostatic pressure which favors uid momement into the cap Pic plasma protein concentration which favors uid movment into the cap Piif interstitial uid protein concentration which favors uid moement out of the capillary Together they equal the net ltration pressure Lymphatic System is a netowkr of small organs lymph nodes and tubes lymphatic vessels or simply lymphatics through which lymph a uid derivd from interstitial uid ows Not technically part of the circulatory system but provides route for the movement of interstitial uid to the circulatory system Lymphatic capillaries are completely distinct from blood vessel capillaries 0 They are similar to the blood vessel ones but they also have large water lled channels that are permeable to all interstitial uid constituents including protein Small amounts of interstitial uid continuously enter the lymphatic caps by bulk ow which then goes to lymphatic vessels which form larger and larger vessels Lymph nodes part of the immune system are found primarily the neck armpits groin and around the intestines then the entire network ends in two large lymphatic ducts that drain into the veins near the jugular and subclavian veins in the upper chest Movement from the lymphatic system to the cardiovascular system is important because the amount of uid lter out of all the blood vessel caps exceeds that absorbed by 4 L each day then those 4L are returned to the blood via the lymphatic system Overall they drain excess interstitial uid provide a pathway by which fat absorbed from the GI ract reaches blood and a route by which cancer cells spread from their origin to other parts of the body Re exes that homeostatically regulate arterial pressure originate primarily with arterial recpetors that respond to changes in pressure They are found where the left and right common carotid arteries divide into two smaller arteries that supply the head with blood Baroreceptors pressure receptors Example are carotid sinuses and aortic arch baroreceptor Afferent neurons tracen from the arterial baroreceptors to the brainstem and provide input to the neurons of the cardiovascular control centers Medullary Cardiovascular Center Primary integrating center for the baroreceptor re exes is a diffuse network of highly interconnected neurons Located in the medulla oblongata Receive input from the various baroreceptors which determines the action ptotential frequency from the cardiovascular center along neural pathways that terminate upon the cell bodies and dendrites of the vagus parasympthatic neurons to the heart and the sympathetic neurons to the heart arterioles and veins Arterial barocrepcetors increase their rate of discharge l decrease in sympathetic neuron activity and an increase in parasympathetic neuron activity Decreased arterial pressure results in increased concentration of angiotensin II and vasopressin which constrict arterioles which in turn increase arterial pressure Operation of the Arterial Baroreceptor Re ex lf arterial pressure decreases like during a hemorrhage then the discharge rate of the arterial baroreceptors decrease l fewer action potentials occur which induces o Increases heart rate because of the increased sympathetic activity and decreased parasympathetic activity 0 Increased ventricular contractility because of the increased sympathetic activity to the ventricular myocardium o Arteriolar constriction because of the increased sympathetic activity to the arterioles increase plasma concentration of angiotessin II and vasopressin 0 Increased venous constriction because of increased sympathetic activity to the veins 0 Overall causes an increase in cardiac output and total peripheral resistance Short term regulator of arterial blood pressure so if BP remains high for a few days arterial baroreceptors adapt to this new pressure and decrease their frequency of AP ring Hypotension low blood pressure Signi cant loss of blood volume as in a hemorrhage The most serious consequence of hypotension is reduced blood ow to the brain and cardiac muscle Can also occur due to volume depleteion that does not result form loss of whole blood but through severe sweating or burns or through GI tract as in vomiting or diarrhea or through high urinary losses Can also be caused by decrease in cardiac contractilitity as in during a heart attack Or strong emotion which can cause fainting as well 0 Vasovagal syncope fainting and is usually transient Autotransfusion restores blood volume to virtually levels within 12 to 24 hours after a moderate hemorrhage through the movement of interstitial uid into the vascular system Losses of as much as 30 of total blood volume can be sustained with only slight reductions of mean arterial pressure or cardiac output Absorption of interstitial uid only redistributes the extracellular uid ultimate restoration of blood volume involves the control of uid ingestion and minimizing water loss via the kidneys Shock any situation in which a decrease in blood ow to the organs and tissues damages them Arterial pressure is usually low in shock Hypovolemic shock caused by a decrease in total peroperal resistance secondary to excessive release of vadodilators like allergy and infection Cardiogenic shock due to an extreme decrease in cardiac output from any of a variety of factors as in a heart attack During exercise cardiac output may increase from a resting potential of 5 Lmin to a maximum of 35 Lmin Most of the cardiac output goes to the exercising muscles Also increases in ow to the heart to provide for the increased metabolism and workload as CO increases and to the skin Local metabolic factoris mediate the vasodilation in skeletal and cardiac muscle but in skin vasodilation is mainly achieved through decreasing the ring of sympathetic neurons to the skins o What happens to arterial blood pressure during exercise The MAP is simply the product of CO x total peripheral resistance CO usually increases more than TPR during exercise