L5 Renal Physiology: Renal Handling of Na, Water & Glucose (Urinary System) PDF

Level Semester3 Module (Renal) Physiology  Renal handling of Na, K water& glucose By: Prof. DR. Nahid Tahoon References: ❖ Ganong, W.F.: Review of medical physiology, 23 th ed. New York, Mc Graw- Hill Co, 2011. P 648,649, 652: 654 & 659. ❖ Gyton, A.A. and Hal...

Level Semester3 Module (Renal) Physiology  Renal handling of Na, K water& glucose By: Prof. DR. Nahid Tahoon References: ❖ Ganong, W.F.: Review of medical physiology, 23 th ed. New York, Mc Graw- Hill Co, 2011. P 648,649, 652: 654 & 659. ❖ Gyton, A.A. and Hall, J.E.: Text book of medical physiology, 12 th ed. Philadelphia, Saunders Co., 2011.P 361:366 Intended Learning Outcomes (ILOs) On completion of this lecture, the student will be able to: Describe of Na+ , K+ , water and glucose handling by different segments of renal tubules. Outline regulation of Na+ , K+ , water and glucose handling by renal tubules.  Na+ handling by renal tubules Na+ is filtered through the glomeruli, but most of filtered amounts are actively reabsorbed along the whole length of renal tubules.  Tubular reabsorptionof sodium (Utilizes 80% of energy utilized by kidneys) Na+ Reabsorption % (99.5% of filtered) Proximal tubule 65% Obligatory Loop of Henle 25% Obligatory Distal and Controlled collecting 9.5% tubules (varied)  Importance of Na+ reabsorption Proximal Play a key role in reabsorption of many Convoluted substances; Tubules Glucose, amino acids (actively reab.), water, urea, & chloride (passively reabsorbed). Loop of Critical for kidneys ability to dilute and Henle concentrate urine by producing medullary hyperosmolarity Distal Important to regulate ECF volume, convoluted (hormonally regulated). tubules  A)At the proximal convulted tubules Early part of PCTs: Na+ is reabsorbed actively by co-transport along with glucose, amino acids, sulfate and organic acids. This active reabsorptive process depends on the action of Na+-K+ ATPase enzyme at the basolateral border of tubular cell.  Peritubular Lumen Tubular cell Interstitial fluid capillary Diffusion Na+ Active transport channel Basolateral Na+– K+ ATPase carrier Lateral space Diffusion Mechanism of sodium reabsorption Late part of PCTs: Na+ reabsorption across luminal border of tubular cell is accompanied by H + secretion through Na+- H+ counter transport mechanism (facilitated diffusion) This process is accompanied by HCO3- reabsorption.  B) At the loop of Henle & early part of distal convoluted tubules The descending limb of the loop of Henle: this bp part is impermeable to Na + due to absence of Na + carrier proteins or channels at the luminal border of its cells. Thin ascending limb of loop of Henle :NaCl reabsorption in thin part is passive by its concentration gradient. Thick ascending limb of loop of Henle & early DCTs: 25 % of the of filtered Na + are reabsorbed in this segment by primary active transport.  Glomerulus Bowman’s capsule Distal tubule Cortex Proximal tubule Collecting tubule Medulla Collecting Long tubule loop of Henle  C)At the late part of distal convoluted tubules &collecting ducts The principle cells can reabsorb Na + actively in exchange with K + through Na + -K + counter transport mechanism at the basolateral border of the tubular cells. Aldosterone hormone is responsible for regulation of Na + reabsorption in DCTs &CDs Regulation of Na reabsorption through two systems 1) Na + 2) Na + Retaining system Loosing system Is the most Atrial Natriuretic important system peptide (ANP) (RAAS)  NaCl / ECF volume / Arterial blood pressure H2O Adrenal conserved Liver Kidney Lungs Kidney cortex Na+ (and CI–) osmotically hold more H2O in ECF Na+ (and CI–) Angiotensin- conserved converting Renin enzyme Na+ reabsorption by kidney tubules Angiotensinogen Angiotensin I Angiotensin II Aldosterone ( CI– Reabsorption, follows passively) Na Retaining Syst Vasopressin Thirst Arteriolar vasoconstriction H2O reabsorption Fluid intake by kidney tubules  Angiotensin II stimulates Aldosterone secretion Vasopressin secretion Vasoconstriction of blood vessels Thirst center to increase water intake Aldosterone induces Sodium and water retention  Sodium loosing system Atrial Natriuretic peptide (ANP)  Helps correct NaCl / ECF volume / Helps correct Arterial blood pressure Cardiac Na Losing Syste atria Atrial natriuretic peptide (ANP) Na+ reabsorption Salt-conserving Smooth muscle Sympathetic by kidney tubules renin-angiotensin- of afferent arterioles nervous system aldosterone system Afferent Cardiac Total arteriolar output peripheral vasodilation resistance Na+ excretion GFR Arterial blood in urine pressure Na+ and H2O filtered H2O excretion in urine K + Transport Both types of transport occur Reabsorption and Secretion  K Transport PCT reabsorb 65% (obligatory) Loop of Henle reabsorb 25-30% (obligatory) almost K excreted in urine is due to secretion by DCT & CT secretion (Secretion= 10-15% of filtered load)  Mechanism of K+ Reabsorption: Active, both 1ry and 2nd transport mechanisms Reabsorption in PCT ,Loop 2nd active transport (Co transport with Na &CL) Secretion in DCT & collecting 1ry active transport  Peritubular Lumen Distal tubular cell Interstitial fluid capillary Primary active transp Diffusion K+ channel Active transport Diffusion K Secretion The only part of K transport that is hormonally regulated  Na+/ ECF volume/ arterial pressure ntrol of K Secretion by Renin istal and CollectingAngiotensin I Plasma K+ Angiotensin II Aldosterone Tubular K+ secretion Tubular Na+ reabsorption Urinary K+ excretion Urinary Na+ excretion Water handling by renal tubules Normally, about 99.7% of the filtered water is reabsorbed in the renal tubules By osmosis aii through as follows: ▪65% in the PCTs. ▪15% in the descending limb loop of Henle. ▪5% in the DCTs. ▪10% in the cortical CDs. ▪4.7% in the medullary CDs. Only, 0.3% of the glomerular filtrations excreted producing about 1.5 liters urine daily with osmolarity about 400 mOsm/liter.  Water reabsorption in PCT About 65% of water is reabsorbed in PCT by passive diffusion following Na reabsorption. This process is called obligatory water reabsorption.  Peritubular Lumen Proximal tubular cell Interstitial fluid capillary Osmosis Water channel Hydrostatic pressure Osmosis Water Reabsorption in PCT  Water handling in loop Henle The descending limbs of LH highly permeable to water, 15% of water passively diffuses obligatory outwards down an osmotic gradient into medullary interstitium. The initial thin part of ascending limbs of LH is impermeable to water.  Water reabsorption in DCT Only about 5% of filtered water is reabsorbed 2ry to Na+ in DCT because their walls poorly permeable to H2O under effect of aldosterone H. Water reabsorption is affected by antidiuretic hormone (ADH) leading to more H2O reabsorption to the blood. Water reabsorption from this segment of the kidney is called facultative water reabsorption.  Regulation of water balance In case of decreased body water(dehydration) Corrected by Increased water intake Decreased water loss Through stimulation of thirst Through antidiuretic sensation hormone (ADH) Thirst center is stimulated by ADH causes 1-hyperosmalarity of plasma 1-water retention through its action 2-hypovolaemaia on DCTs &CDs 3-Dryness of mouth and throat 2-Vasoconstriction of the blood vessels.  In case of increased body water (overhydration) Corrected by decreased water intake increased water loss in urine Through inhibition of thirst Through inhibition of sensation antidiuretic hormone (ADH) secretion  Control of vasopressin or ADH secretion ECF osmolarity ECF volume through through Osmoreceptors in Volume receptors in atria hypothalamus (less important) (most important) Other stimuli; angiotensin II, trauma, stress  (4) Glucose reabsorption: This is usually complete in normal condition. It occurs only in the PCTs. Glucose reabsorption at the brush borders occurs by the secondary active transport by co transport with Na. Then, Glucose reabsorption at the basolateral borders of the cells of PCTs to the interstitium occurs by facilitated diffusion. Co transport, both Na+ & the transported substance move together into the cells. It occurs by utilizing a symport carrier. e.g. glucose & amino acids