13 1 OsmoRegulation BIO361T Fall 2014 1 When an animal eats an abundance of salt why will it retain more water Total osmolarity in the body increases because salt has increased but water has not salt will be absorbed into the plasma through the digestive epithelia Kidneys need to increase salt excretion while maintaining water retention Kidneys will excrete small amounts of concentrated urine to bring salt back down and keep water up 2 When water is retained blood volume increases However the kidney has many mechanisms to maintain filtration rates in spite of changes in blood volume and pressure Fill in the following flow chart to illustrate how the kidney uses myogenic regulation to maintain normal rates of filtration when blood volume increases 3 How does the kidney also use tubuloglomerular feedback to maintain normal rates of filtration in the face of an increase in blood volume 4 How does the juxtaglomerular apparatus prevent both the afferent and efferent arteriole from being vasoconstricted simultaneously When the macula densa release ATP and it is converted into adenosine this constricts the afferent arteriole and inhibits renin release from the juxtaglomerular cells This prevents constriction of the efferent arteriole 5 Although systemic changes in blood pressure and volume do not cause changes in filtration filtration does affect systemic blood pressure and volume For example when blood osmolarity is too high arginine vasopressin AVP is released into the bloodstream How does AVP then decrease blood osmolarity back to normal values Insert aquaporins into collecting ducts increase H2O reabsorption 6 Why is vasopressin vasotocin also called anti diuretic hormone ADH It has the opposite effect of a diuretic which results in the excretion of large volumes of dilute urine Vasopressin increases H2O reabsorption which resultsin the excretion of small volumes of more concentrated urine 7 How is the effect of ADH magnified by the loop of Henle Urine is at its maximal concentration at the tip of the loop due to the countercurrent multiplier between the ascending and descending limbs Thus the osmolarity of the kidney increases deeper into the medulla As urine goes down the collecting duct it goes deeper into the kidney through the medulla and into the pelvis The osmolarity gradient will be higher which will draw more H2O out of the aquaporins inserted by ADH further concentrating the urine as it approaches the renal pelvis 8 When blood pressure drops too low renin is released from the juxtaglomerular apparatus How does renin increase blood pressure By angiotensin II causes vasoconstriction and increases thirst to increase blood volume By aldosterone increases Na and thus H2O reabsorption in distal tubule and collecting duct to increase blood volume 9 Why does aldosterone take several hours to increase Na reabsorption It exerts its effects by altering gene expression The genes have to be transcribed and translated and the proteins have to be transported to and inserted into the membrane Because aldosterone is a steroid hormone it is non polar and can diffuse across the cell membrane just like sex steroid hormones like testosterone or estradiol It translocates to the nucleus where it binds a transcription factor and activates transcription of ion channel genes 10 Predict how each of the following will affect renin release a Increase in blood pressure Inhibits renin release which will prevent H2O reabsorption in the distal tubule and collecting duct and thereby increase H2O excretion Detected by stretch receptors in the atria carotid artery and aorta that also decrease ADH release to further decrease H2O reabsorption in the collecting duct b Increase in plasma K Activates renin release which stimulates aldosterone release which increases Na K ATPase in distal tubule and collecting duct that actively secretes K into filtrate Renin also stimulates release of ADH c Decrease in GFR and filtrate flow Activates renin release which vasoconstricts the efferent arteriole and increases blood volume 11 When blood volume is high atrial natriuretic peptide is released What pathways are inhibited by ANP and how will this decrease blood volume 1 stimulus for ANP release is atrial distension The renin angiotensin aldosterone pathway ANP inhibits the release of renin which inhibits the formation of angiotensin II which decreases thirst and inhibits aldosterone release ANP also directly inhibits the release of aldosterone which will decrease Na and H2O reabsorption and increase K secretion in the distal tubule and collecting duct If Na is excreted instead H2O will follow reducing the volume of fluid in the body ANP also inhibits the release of ADH which will further prevent H2O reabsorption in the collecting duct 12 Fill in the following table showing how each regulatory mechanism responds to changes in the kidney or cardiovascular system Note that these are negative feedback mechanisms of regulation If a mechanism responds to high GFR that means it will lower GFR and vice versa Intrinsic regulatory mechanism High GFR causes Low GFR causes Mesangial cells Mesangial cell contraction Mesangial cell relaxation which which decreases GFR increases GFR Myogenic regulation Afferent arteriole constriction Efferent arteriole dilation Afferent arteriole dilation Efferent arteriole constriction Tubuloglomerular feedback Macula densa releases ATP which constricts afferent arteriole Macula densa releases prostaglandins juxtaglomerular cells release renin which constricts efferent arteriole Extrinsic regulatory mechanism Vasopressin ADH High blood pressure causes Less ADH release less H2O reabsorption more H2O excretion Renin angiotensin aldosterone pathway Less renin release less Na and H2O reabsorption more excretion more K secretion Atrial natriuretic peptide ANP release less renin angiotensin aldosterone ADH release more Na and H2O excretion Low blood pressure causes ADH release more aquaporins in collecting duct more H2O reabsorption higher blood volume Renin release more Na and H2O reabsorption higher blood volume vasoconstriction more H2O intake Less ANP release more renin angiotensin aldosterone ADH release more reabsorption higher blood volume 13 Elasmobranchs also use ANP for osmoregulation when they eat salty foods Based on how ANP works in mammals predict how ANP will affect the rectal gland of an elasmobranch Increase ingested salt more salt diffuses through gut epithelia into blood circulates to
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