Angiotensin II in Peripheral Capillary Beds • Causes brief, powerful vasoconstriction – of arterioles and precapillary sphincters • Elevating arterial pressures throughout body Angiotensin II in the CNS • Stimulate thirst • Triggers release of antidiuretic hormone (ADH) – stimulates reabsorption of water in distal portion of DCT and collecting system • Increases sympathetic motor tone: – mobilizing the venous reserve – increasing cardiac output – stimulating peripheral vasoconstriction Natriuretic Peptides • Atrial natriuretic peptide (ANP) is released by atria • Brain natriuretic peptide (BNP) is released by ventricles • Are released by the heart: – in response to stretching walls – due to increased blood volume or blood pressure • Trigger dilation of afferent arterioles and constriction of efferent arterioles • Elevates glomerular pressures and increases GFR • Decrease tubular reabsorption of sodium ions: – increasing urine production
Exam 5 – decreasing blood volume and blood pressure Autonomic Regulation of GFR • Mostly consists of sympathetic postganglionic fibers • Sympathetic activation: – constricts afferent arterioles – decreases GFR – slows filtrate production • Changes in blood flow to kidneys due to sympathetic stimulation: – may be opposed by autoregulation at local level Increased Blood Volume • Automatically increases GFR: – to promote fluid loss • Hormonal factors further increase GFR: – accelerating fluid loss Reabsorption and Secretion • Reabsorption : – recovers useful materials from filtrate • Secretion : – ejects waste products, toxins, and other undesirable solutes • Occur in every segment of nephron: – except renal corpuscle • 1% of daily filtrate is excreted as urine • 400 ml minimum – obligatory water loss • Water moves by osmosis – must have concentration gradient 5 Functions of the PCT 1. Reabsorption of organic nutrients 2. Active reabsorption of ions 3. Reabsorption of water 4. Passive reabsorption of ions 5. Secretion
Exam 5 PCT • PCT cells normally reabsorb 60–70% of filtrate produced in renal corpuscle • Reabsorbed materials enter peritubular fluid: – and diffuse into peritubular capillaries Sodium Ion Reabsorption • Is important in every PCT process • Ions enter tubular cells by: – diffusion through leak channels – sodium-linked cotransport of glucose – counter-transport for hydrogen ions – Na + / K + pump The Loop of Henle • Reabsorbs about 1/2 of water, and 2/3 of sodium and chloride ions remaining in tubular fluid by the process of countercurrent multiplication • Is exchange that occurs between 2 parallel segments of loop of Henle: – the thin, descending limb – the thick, ascending limb – Are very close together, separated only by peritubular fluid – Have very different permeability characteristics The Thin Descending Limb • Is permeable to water • Osmosis to vasa recta – not in tissue • Is relatively impermeable to solutes • Thus tubular fluid reaches high osmolality The Thick Ascending Limb •
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