Nephron and Associated Blood Vessels

1. Nephron and Associated Blood Vessels 20-8

3. Urine Formation • Glomerular Filtration • substances move from blood to glomerular capsule • Tubular Reabsorption (reclaim substances necessary for homeostasis) • substances move from renal tubules into blood of peritubular capillaries • water and organic nutrients • sodium ions first and the rest follow • Tubular Secretion • substances move from blood of peritubular capillaries into renal tubules • drugs and ions (H+, K+, HCO3-, organic acids), excess materials • Elimination = glomerular filtration + tubular secretion – tubular reabsorption 20-14

4. Glomerular Filtration Glomerular filtrate passes through the filtration membrane into the capsule 20-15

5. Net Filtration Pressure (NFP) Pressure within the glomerulus that forces filtrate into capsule Glomerular Hydrostatic Pressure -- glomerular blood pressure --pushes water and solutes ouf of the blood across the filtration membrane --large proteins don’t filter out of blood; maintain osmotic pressure Colloid Osmotic Pressure of Glomerular Blood – osmotic pressure created by proteins that don’t filter into capsule; trying to pull water into glomerulus Capsular Hydrostatic Pressure – fluid in glomerular capsule; trying to push water into glomerulus

6. NFP cont. • Net Filtration Pressure = force favoring filtration – forces opposing filtration • (glomerular capillary ( capsular hydrostatic pressure • hydrostatic pressure) and glomerular capillary • osmotic pressure ) 20-17

8. Glomerular Filtration Rate (GFR) -- volume of filtrate formed each minute by all glomeruli of the kidney Controlled by -- total filtration surface area --filtration membrane permeability --NFP Intrinsic Factors – renal autoregulation -- Myogenic Mechanism >afferent arteriole reacts to systemic (whole body) blood pressure to control blood pressure of glomerulus

9. --Tubuloglomerular Feedback Mechanism .directed by macula densa (measure _____ content of filtrate -- Increased GFR = ???? -- Decreased GFR = ????

10. Extrensic Control --Sympathetic Nervous System >extreme stress or emergency >norepinephrine and epinephrine constrict afferent arteriole >triggers renin-angiotensin mechanism --renin released from juxtaglomerular cells in kidneys --Renin-angiotensin Mechanism > Renin acts as enzyme >Angiotensin II -- vasoconstricts all arterioles raising systemic BP -- controls reabsorption of Na+ (water follows Na+) -- stimulates release of anti-diuretic hormone and activates thirst center -- decreases surface area of glomerular capsule

11. Tubular Reabsorption of Water and Ions 20-21

12. Secretion Removal of substances from the peritubular capillary to the renal tubule Importance – remove substances bound to plasma proteins --eliminate reabsorbed undesirable substances --get rid of extra K+ --control blood pH In distal convoluted tubules, potassium ions or hydrogen ions may be passively secreted in response to active reabsorption of sodium ions 20-24

13. Countercurrent Mechanism of Vasa Recta • fluid in descending limb becomes hypertonic as it loses water by osmosis • fluid in ascending limb becomes hypotonic as solute is reabsorbed 20-26

14. Effect of ADH on Renal Tubules • without ADH, DCT and collecting duct are impermeable to water • with ADH, DCT and collecting duct become permeable to water • with ADH, water is reabsorbed by osmosis into hypertonic medullary interstitial fluid 20-27

15. Elimination of Urine • nephrons • collecting ducts • renal papillae • minor and major calyces • renal pelvis • ureters • urinary bladder • urethra • outside world 20-30

16. Micturition • bladder distends and stretch receptors stimulated • micturition center activated in sacral portion of spinal cord • parasympathetic nerve impulses cause detrusor muscle to contract • need to urinate is sensed • voluntary contraction of external urethral sphincter prevents urination • when decision is made to urinate, external urethral sphincter relaxes, detrusor muscle contracts, and urine is expelled 20-36