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5.1.2 Excretion and the Kidneys (Ultrafiltration (3 layers (endothelium of…
5.1.2 Excretion and the Kidneys
Ultrafiltration
filtration of the blood at a molecular level under pressure
blood is pushed from glomerulus into Bowman's capsule
3 layers
endothelium of cavity: narrow gaps between cells; cells contain pores called fenestrations; allow blood plasma to pass out of capillary
basement membrane: fine mesh of collagen fibres and glycoproteins; filter to prevent passage of molecules with Mr > 69000; most proteins and rbc's held in glomerulus
epithelial cells of Bowman's capsule: podocytes contain many projections (major processors) and hold cells away from endothelium of cavity; ensure gap between cells so fluid can pass between cells into lumen of Bowman's capsule
blood enters glomerulus via afferent arteriole (wider so less pressure) and leaves glomerulus via efferent arteriole (narrower so more pressure); difference in diameter means blood in capillaries is at higher pressure than blood in Bowman's capsule, so is pushed out
left with only red blood cells and proteins in capillary as they are too large to pass through filters; low water potential of blood to help with reabsorption later on
filter: water, amino acids, glucose, urea, inorganic mineral ions etc
Production of urine
if consume more water; less ADH produced; collecting duct walls less permeable, less water reabsorbed into blood; more urine produced; more dilute urine
if consume less water, more ADH produces, collecting duct walls more permeable, more water reabsorbed into blood, less urine produces, more concentrated urine
Control water potential of blood
osmoreceptors in hypothalamus detect change in water potential
posterior pituitary gland; where ADH is stored in vesicles
ADH (antidiuretic hormone): controls permeability of the collecting duct walls; makes walls more permeable
ADH binds to receptors on collecting ducts; enzyme controlled reactions; aquaporins fuse with plasma membrane; walls more permeable to water
if water potential too low, osmoreceptors lose water by osmosis and shrink; stimulate neurosecretory cells in the hypothalamus to produce and release ADH
Kidney failure
renal dialysis
haemodialysis
partially permeable membrane
heparin is added to avoid clotting
counter-current; blood runs in opposite direction to dialysis fluid
several hours, a few days per week
only in hospital
blood from artery or vein is passed into a machine
peritoneal dialysis:
permanent tube is implanted into body
dialysis solution is poured into tube and removed after several hours
can be carried out at home
dialysis membrane is the body's own peritoneum (abdominal membrane)
mechanism to artificially regulate the concentration of solutes in the blood
dialysis fluid contains correct concentrations of mineral ions, urea, water etc in blood
kidney transplants
advantages: no more time consuming dialysis; physically fitter; can travel; improve quality of life
disadvantages: immunosuppressant drugs for rest of life; have side effects; major surgery; kidneys don't last forever
GFR: glomerular filtration rate; measure of how much fluid passes into the nephron each minute
normal GFR is 90-120 cm³ min⁻¹
if below 15cm³ min⁻¹ then kidney failure
protein in urine indicates kidney damage; protein is usually too large to fit through basement membrae
increased level of electrolytes in blood as kidney is not controlling it anymore
Selective reabsorption:
involves active transport and cotransport
proximal convoluted tubule: plasma membrane: microvilli, contains cotransport proteins , many mitochondria for active transport
fluid altered by reabsorption of all sugars, mineral ions and some water
Loop of Henle
Descending limb of Loop of Henle: water potential of blood decreased by addition of mineral ions and removal of water
Ascending limb of Loop of Henle: water potential increased as mineral ions removed by active transport, and diffusion at the base; top of ascending limb is impermeable to water
decending limb descends into the medulla; ascending limb ascends into the cortex
hairpin countercurrent multiplier system: increase efficieny of transfer of mineral ions from ascending limb to descending limb in order to create water potential gradient in medulla
Collecting duct
Collecting duct: water potential is decreased by removal of water to produce urine
fluid passes from top to medulla, out of loop of henle, into short distal convoluted tubule to adjust concentration of mineral ions, and into the collecting duct
Urine analysis
pregnancy testing
1) urinate on test stick
2) hCG binds to mobile antibodies attached to blue bead
3) mobile antibodies-complex moves down test stick
4) if hCG present then hCG binds to fixed antibodies so a blue line is shown; if no hCG then no binding and no blue line
5) mobile antibodies with no hCG attached bind to another row of fixed antibodies so a blue line is shown
monoclonal antibodies: antibodies made from one type of cell; specific to one complementary molecule
hCG (human chorionic gonadotrophin) is a hormone found only in urine of pregnant women
testing for anabolic steroids:
anabolic steroids increase protein synthesis in cells; increase muscle mass; banned in sports
unfair advantage in sports
half life of ~16 hours and remain in blood for many days
dangerous side effects
use gas chromatography to test urine sample in laboratory
test for glucose in diabetes diagnosis
test drivers for alcohol levels
test for recreational drugs
Sammer Sheikh