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Lecture

March 12th_PhysiologyII.docx

8 Pages
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Department
Kinesiology & Health Science
Course Code
KINE 2011
Professor
Gillian Wu

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Summary-Factors regulating Venous Return Venous return – major determinant in cardiac output based on the fact that we need blood in order to fill ventricles with blood, in order to enlarge the volume of ventricle during diastole so you maintain stroke volume. Factors that affect capacity of blood to return back to heart after it goes through the arterial system. Some are Volume related (how much blood you have within the system and if you enlarge the blood vol, SHORTERM: it would have more reabsorption than filtration that would increase in fluid coming into the venous system which would increase venous return. Kidney plays important role in fluid balance. The thing that kidneys do also have major influence OVER A LONGER TERM. Direct link between the amount of blood with the venous return. 2. Maintaining pressure gradient also influences venous return. Lower limbs, the skeletal muscles pump to compress the veins and squeeze blood towards the heart, that works in conjection with the valves in the vein which prenvent backflow of the blood. So when you squeeze the veins within the muscle, the blood only has one direction which is towards the heart. The sympathetic nervous system constrict the veins which is called VENOCONSTRICTION. Veins are capacittant vessels they like to stretch passively out and hold large volume of blood, You need to able to minimize that vol of blood, the sympathetic nerves help with constricting by tightening the walls of the veins so they do less passive stretching, they stay at smaller diameter which causes increase in pressure in the veins, and helps the blood move back to the heart. Influences that affect what’s going on the heart. Influence Respiration: change in vol of thoracic cavity due to inspiration which creates short term negative pressure which helps stimulate more flow to the chest cavity. Small component ---More veins in the body compared to arteries- Cardiac output= since we need to have blood supply to various tissues and organs in the body to maintain oxygen and also care about cardiac output because it is one of the two major regulators of MAP. Venous retun is a determinant of cardiac out ; cardiac out put is a determinant of MAP 2 Slide MAP is a #1 homeostatic regulator in the body. Body can regulate MAP, because there is a very sensitive feedback system which allows it to detect MAP, and to influence the parameters that make up MAP. We use baroreceptors (sensory nerves found) Baroreceptors found in 2 locations both on arterial side, and very close promixity to the heart. 1.Baroreceptors have arch of aorta called Aortic bodies. 2. Baroreceptors found at the carotid sinus called Carotid bodies *both are similar locations for chemoreceptors (control for respiration)* Both locations are used for detection of gasses, the baroreceptor stretch reflex. They are different nerves . Baro receptors are specialized to determine DEGREE OF STRECH IN THE WALL OR AORTA or CAROTOID. They have specilaized nerve endings that are integrated in the wall of the blood vessels, so they are able to detect any change in the stretch of the wall. They are not sensing the blood itself but whats happening in the wall of blood (smooth muscle cell layers). Barorecpeotrs activated by strech When the wall of the blood vessel was strectehd . When there is more pressure inside the blood vessel forcing outwards, trying to expand to wall ofblood vessel. These baroreceptors will become depolarized and fire action potentials. They send these action potential up through the axons then they will synapse in the brain at the cardiovascular control center. This cardiovascular control center is located within the brain stem within medulla oblongata, very close proximity to the respiratory control center. Input from the baroreceptor is one of the major inputs that comes to the cardiovascular center (NOT THE ONLY ONE there are OTHERS) Cardiovascular Center receives all the inputs that are coming to it from various places and decideds what to do with those signals,. In order to make a change , and have an effect, one of two ways: It can either do (i)SNS (sympathetic nerves) (ii)PNS (Parasympathetic nerves) Activation of the cardiovascular center by the baroreceptors causes higher activation of the PNS and at the same time, it reduces the activation of the SNS. The cardiovascular control center uses both outputs at the same time, but is always controlling them in the opposite way that’s good since they have opposite effects. Pathway from the baroreceptor to the cardiovascular control center is called afferent pathway and negative feed back loop. Cardiovascular center is the integrating center SNS and PNS are the outputs rd 3 Slide Examples of Baroreceptor Responses Middle is NORMAL blood pressure. Left is lower blood pressure, right is higher blood pressure. Under normal MAP, the wall of the aorta is subject to an outwards pressure, and that causes a certain level of activation of baroreceptors. This is an important thing . Under resting conditions, what you consider, to be normal blood pressure, the baroreceptors are being activated by the stretch. Each line= action potential Frequency of action potential determines how the nerves are sending the information. The code it by frequency. This frequency of action potentials that is caused (elicited) by MAP, results in some level of parasympathetic activation. And some level of sympathetic. Under resting conditions, MAP is normal, there is some PNS and SNS activation. You are in the middle ground. Level of PNS and SNS is going to help determine the heart rate, the contractility of the heart, and also some other vascular effects. If aortic pressure drops, that means there will be less blood within the aorta pushing outwards, a less of stretching influence, The baroreceptors are getting
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