One Minute Movies: Molecular Action at the Nephron Joy Killough / Westwood High School / Austin,TX

Transcription

1 One Minute Movies: Molecular Action at the Nephron Joy Killough / Westwood High School / Austin,TX To prepare your nephron model: ( A nephron is a tubule and the glomerulus. There are about a million of these per kidney!) 1. Tape or glue the four nephron pages together. 2. Cut out the square and rectangular pieces surrounding the juxtamedullary nephron. a. Place the water channel and sodium potassium pump models near the collecting duct. b. Place renin near the arterioles. c. Set aldosterone to the side (off the model beside the arterioles works well). d. Place remaining model pieces in the afferent arteriole. Modeling: 1. Glomerulus/Bowman s Capsule: Filtration- About 20% of the plasma volume is forced out. Consider your model pieces as a representation of this fraction. This liquid is called filtrate. Move model pieces from the afferent arteriole to the Bowman s capsule and then to the proximal tubules according to these guidelines: a. Blood pressure forces out water, salts, ions (bicarbonate, potassium, hydrogen), glucose, amino acids, vitamins and urea and other small molecules. b. Retained are blood cells, platelets, plasma proteins, and other large molecules. 2. Proximal Tubule: Reabsorption- Most components of the filtrate are reabsorbed here. They move out of the filtrate, into the cells of the tubule, out of those cells and into the interstitial fluid, into the capillary cells, then finally into the plasma of the bloodstream. The blood ph is adjusted. A sodium gradient is created and drives the movement of water. Many other molecules are carried by active transport carriers. Move your model pieces according to the guidelines below: a. Moving out actively are: Na +, amino acids and glucose JK 2010 b. Moving out passively are: water, potassium, bicarbonate, and Cl-. 4

2 3. Loop of Henle: Creating a high osmolarity in the medulla- Filtrate moves down the descending Loop of Henle. The interstitial fluid and filtrate are isotonic in the cortex. In the medulla the interstitial fluid is hypertonic to the filtrate. The deep Loop of Henle is found only in mammals and some birds (and only in some nephrons), and the action here creates these hypertonic conditions and allows hyperosmotic urine to be made. a. Move water according to the principles of osmosis. Note restriction to salt in the descending limb due to lack of channels. You should be moving water to the interstitial fluid then the capillaries in the descending Loop of Henle. b. Your model pieces are now in the ascending loop. The tubule here is permeable to salt (ion channels are available), at first passively then actively. This active transport uses a lot of ATP but makes the interstitial fluid osmolarity high. This section of the loop is also permeable to urea which comes from the collecting duct. NaCl and urea are the main source of the high osmolarity in the kidney that allows for creation of a highly concentrated urine at the collecting duct. There is no permeability to water here. c. The countercurrent flow of blood relative to filtrate in the vasa recta keep the solutes from be whisked away. The blood picks up solutes as it flows down, beside the ascending loop, and loses solutes as it flows up, beside the descending loop, leaving the concentration in the medulla as it was. 4. Distal Tubule: Tubular Secretion- Transporters in the walls of the peritubular capillaries move some ions and molecules into the interstitial fluid. Nephron transporters move them across and ultimately into the filtrate. Move model pieces according to the guidelines below. Again, ph is adjusted by secretion of H+ and reabsorption of HCO3- a. Secrete H +, K +, and urea from the peritubular capillary into the distal tubule. b. Secrete toxins and drugs. 5. Collecting Duct- Final adjustments, hormonal control. a. Permeable to urea in the inner medulla. This increases the osmolarity in the medulla even further. Joy Killough

3 Hormonal Control 6. ADH-Antidiuretic Hormone ( AKA vasopressin) works at the collecting duct and the distal tubule. Model ADH release and its effect by moving water channel (aquaporin) models to the collecting duct. The aquaporins are part of a vesicle inside the cell. ADH moves them to the cell membrane. a. Add aquaporins. b. Move more water through these channels and back into the interstitial fluid then blood. 7. RAAS -If the amount of extracellular fluid goes down (blood pressure drop), cells in the juxtaglomerular apparatus secrete renin. Renin converts Angiotensinogen to Angiotensin I which is then converted to Angiotensin II. Angiotensin II causes vasoconstriction. Angiotensis II also causes the release of aldosterone from the adrenal gland above the kidney. Aldosterone causes more NaK pumps to be produced and inserted in the cell membranes of cells making up the collecting duct. More water moves into the interstitial fluid following sodium so volume and therefore pressure goes up. a. Secrete renin from tissue near the affererent arteriole (JGA) to blood b. Move aldosterone to blood (production by adrenal gland not modeled) c. Move aldosterone to the collecting duct. d. Add NaK pumps. e. Move sodium out. Water follows. 8. Atrial natriuretic protein- secreted from the tissue in the heart when heart volume causes the atria to stretch. This hormone causes more sodium to be excreted by increases the rate of filtration at the glomerulus. (Not modeled) Acid-Base Balance- 9. Bicarbonate and H + can be secreted and reabsorbed to adjust blood ph. If the blood is too acidic bicarbonate moves into the peritubular capillaries. H + ions may also be secreted into the tubules combining with ammonia ions 6

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