Urinary system part # 2 Flashcards
Glomerular filtration -
Glomerular filtrate - fluid that enters the capsular space.filtration fraction - the % of blood plasma that ends up in the capsular space (16-20%) This amounts to 150 liters being filtered for females on a daily basis and 180 liters for males. 99% of this is water is reabsorbed so that only 1-2 liters are excreted per day. Filtration membrane (aka endothelial - capsular membrane) : endothelial cells of the glomerulus (capillary endothelium). Basement membrae - the fused basal lamina of the capillary and the podocytes. Cells of the podocytes - foot processes form filtration slits. Factors that enhance filtration: 1. large surface area of the glomerular capillaries (ball of capillaries) 2. filtration membrane is thin and leaky. 3.blood pressure in the glomerular capillaries is high due to the small size of the efferent arteriole. Net filtration pressure: 3 pressures: 1 favor afiltration and 2 oppose. Glomerular (blood) hydrostatic pressure (GHP) - pressure due to the blood in the glomerular capillaries (55mmHg, favors filtration). Blood colloid osmotic pressure (BCOP) - pressure due to the proteins and solutes in the capillaries (draw or pulls fluid into the capillary, 30mmHg, opposes filtration. Capsular hydrostatic pressure (CHP) - pressure due to fluids already in the capsule as a result of filtration. (15mmHg), opposes filtration. NFP (net filtration pressure) = pressure favoring filtration - 2 pressures opposing filtration. =GHP - BCOP - CHP --- (NFP=55-30-15=10mmHg). Glomerular Filtration Rate (GFR) - males - average 125/ml/min Females - averages 105 ml/min No net filtration if GHP is less than /=45. GFR is tightly regulated to stay constant even if blood pressure is changing.
3 mechanisms: autoregulation, hormonal and neural -
Renal autoregulation of GFR - Occurs completely within the kidney (at a local level) 2 mechanisms: myogenic and tubuloglomerular feedback
Myogenic - afferent arterioles sense the amount of streach they are experiencing. Less streach (lower GFR and BP) results in relaxation of smooth muscle within the arteriole. This mechanism increases GFR because the afferent arteriole dilates. With an increase in BP, GFR would also increase but constriction of the afferent arteriole maintains a constant GFR because blood flow is reduced. (increased streach results in vasoconstriction of the afferent arteriole, still myogenic mechanism).
Tubuloglomerular feedback - when BP increases, the macula densa cells sense an increase in flow and an increase in sodium and chloride concentrations. The maculadensa cells secrete vasoconstrictors (ATP and others) that act on the afferent arteriole, maintaining GFR. Vasodialation is a result of low osmolarity of the filtrate. This is because the vasoconstrictors from the macula densa cells are not secreted. Vasodialation would increase the flow rate.
Hormonal regulation : Angiotensin II - produced by the renin - angiotensis - aldosterone mechanism when systolic BP is below 80mm Hg, it causes vasoconstriction of the systemic arterioles, decreasing GFR (as well as helping to restore blood volume since urine output decreases). Atrial natriuretic peptide ANP - stretch in the srtia is indicative of increased blood volume and results in secretion of ANP. Release of ANP results in less renin release resulting in vasodialation and increased GFR, promotes Na+ and water loss and also a decrease in BP.
Neural regulation: at rest very little sympathetic stimulation - renal blood vesseles are dialated, and GFR is maximized. Extreme sympathetic stimulation - (e.g. exercise or hemorrhaging) release of norepinephrine results in vasoconstriction of the affrent arterioles dramatically decreases the GFR. Systemic arterioles also constrict to increase BP.
Tubular reabsorption review -
This is the process that reclaims nutrients that is filtered out. Cells in the tubules have two surfaces: Apical (luminal) - closest to the lumen. Basolateral membrane - furthest from the lumen, closest to the interstitial fluid. Luminal and basolateral membrane have diffrent types of transport proteins. Third barrier to cross for reabsorption - the endothelial membrane of the peritubular capillaries (or vaso recta). Reabsorption routes: Paracellular reabsorption - transport in between the cells and into the interstitial space. Transcellular reabsorption - transport across/through a cell, from the apical membrane to the basolateral membrane and into the interstitial space and into the capillary.
Transport mechanisms: Primary active transport - ATP is directly used to fuel transport, e.g. sodium pump. Secondary active tramsport - the concentration gradient of an ion is the driving force which must be coupled with the transport of a second ion via symporter ir an antiporter. The ATP is not directly used in the transport. It is used to move the second ion back to where it came from (against the concentration gradient).
Water is reabsorbed by osmosis. Obligatory water reabsorption - “obliged to follow” water follows salt, about 90% of water is reabsorbed this way. Facultative water reabsorption - the remaining water is reabsorbed depending on the need. The hypothalamus monitors osmolarity via osmolarity receptors, ADH is released if needed. Transport maximum - symporters and antiporters have a maximum number of molecules that they can move through in a minute. e.g glycosuria - glucose in the urine if the transport maximum for glucose is exceeded. Nonreabsorbed substances : reasons - no carriers for transport, too large in size and / or not lipid soluble. These substances include : notrogenous waste products : urea , creatitine, and uric acid. Note: some urea is reabsorbed but not all.
Reabsorption in the PCT - proximal convoluted tubule
Most reabsorption takes place in the PTC, most of which is dictated by sodium. Normally, all glucose, all lactic acid, all amino acids, most of the sodium, most of the water, most of the bicarbonate and about half of the chloride and half of the potassium are reabsorbed here. Sodium symporters in the apical membrane move glucose, lactate, amino acids and water soluble vitamins. Sodium anti-porters reabsorb sodium and secrete H+. For every one H+secreted, one bicarbonate ion is reabsorbed into the blood. Carbon dioxide diffuses from the blood or the lumen and combines with water to form carbonic acid catalyzed by carbonic anhydrase.
Tubular secretion in general -
Functions to: rid the body of certain drugs and waste products such as urea and uric acid. remove excess potassium. contro pH
Secretion in the PTC - Ammonia can combine with hydrogen ion to become ammonium ion. Ammonium ion can be anti-ported instead of H+.
Reabsorption in the descending limb of the nephron loop - sodium, potassium,bicarbonate and chloride. Descending limb - %of water is reabsorbed in the descending limb, none in ascending limb (due to the glycoprotein present)
Reabsorption in the thick ascending limb of the nephron loop - Na+ - K+ -Cl- symporters move these substances from the lumen of the thick ascending limb. From there, Na + moves into the blood by active transport, Cl- diffuses into the blood via channels in the basolateral membrane and K+ leaks back into the lumen. K+ leaking back results in the interstitial space being more negative ( the 2Cl-)so cations (Ca2+, Na+, K+, and Mg2+) are attracted to move into the interstitial space and eventually into the blood stream. Remember: the ascending limb is impermeable to water.
Reabsorption in the DCT -
25% of water and 10 % of sodium remain, flow rate is reduced. Na+ -Cl- symporters in the DCT allow for the reabsorption of sodium and chloride. Not much water is reabsorbed because these cells are not very permeable to water.
Reabsorption and Secreation in the Late DCT and Collecting Duct - At this point in the nephron 90 - 95% of the water and solutes have been reabsorbed. Principal cells and intercalated cells fine - tune the urine. Principal cells become more permeable to water and urea in the presence of ADH which fine tunes the final water reabsorption. Principal cells secrete more potassium in the presence of aldosterone (aldosterone secreation is stimulated by high K+). Aldosterone also increases Na+ reabsorption and therefore water. Intercalated cells have H+ ATPases that secrete H+.
