Orignal M6 P2.pdf

Renal effects on blood pressure The kidneys are able to control blood pressure because they are able to alter ECF/plasma composition and volume. Kidneys have several mechanisms whereby they can change blood pressure: 1. Intrinsic Mechanisms: a. Autoregulation: which protects the kidneys...

Renal effects on blood pressure The kidneys are able to control blood pressure because they are able to alter ECF/plasma composition and volume. Kidneys have several mechanisms whereby they can change blood pressure: 1. Intrinsic Mechanisms: a. Autoregulation: which protects the kidneys from the minute-to-minute changes in blood pressure in the body. b. RAA system: Juxtaglomerular cells detect blood pressure in the afferent arteriole and secrete an enzyme called renin to restore bp. Both these mechanisms rely on the juxtaglomerular apparatus (JGA) 2. Extrinsic Mechanisms: Sympathetic NS quickly reduces the glomerular filtration rate (GFR) in order to increase the blood pressure if necessary 7  Justaglomerular apparatus The Juxtaglomerular apparatus consists of: a. Juxtaglomerular cells or granular cells, are modified muscle cells around the afferent arteriole, secrete renin in response to drop in blood pressure or salt concentration. b. Macula densa cells of the distal convoluted tubule detect changes in filtrate composition (salts) and stimulate the release of renin from the juxtaglomerular cells. 8 The Renin-Angiotensin-Aldosterone System Blood pressure is monitored in the afferent arteriole by the juxtaglomerular cells. When they detect a drop in blood pressure, juxtaglomerular cells release renin stimulating a cascade of mechanisms involving angiotensin I and II, ADH (also called vasopressin) and aldosterone. The series of mechanisms involved is called Renin-Angiotensin- Aldosterone system. The interrelated mechanisms are summarised in the following slide. 9 March 17 ISAP: Renal System Part C 10  The effect of aldosterone 1. Aldosterone produced by the adrenal cortex stimulates Na+ reabsorption in the distal convoluted tube and collecting ducts. 2. Aldosterone (steroid hormone) induces DNA to code mRNA to manufacture Na+ / K+ transport proteins. 3. In complete absence of aldosterone a person may excrete 20 g of salt per day. 4. In the presence of aldosterone almost no salt is lost. 11 The RAA system - Summary 12  Regulation of urine volume Fluid homeostasis is maintained by kidneys via regulation of urine output This is achieved by the loops of Henle of juxtamedullary nephrons. They form part of a system called Counter- Current Multiplier Mechanism (CCMM) based upon The anatomical arrangement of the juxtamedullary nephrons and vasa recta. Different permeability of the two limbs. 13 Meaning of Counter-Current Multiplier Mechanism Counter-current, because the filtrate in both limbs of the loop of Henle flows in opposite direction respect to the blood in vasa recta. Multiplier mechanism, because the movement of ions is achieved in small steps which multiply into a big effect. 14  Regulation of urine volume The loop of Henle of the juxtamedullary nephrons sets up a steep osmotic gradient in the kidney: cortex (300 mOsm/L) to medulla (1200 mOsm/L) This is achieved by two mechanisms: 1. Movement of water and ions from filtrate (LoH) into blood (vasa recta) 2. Relative permeabilities of tubular cells in the descending and ascending limbs of the loop of Henle 15  Set up of the osmotic gradient Descending limb permeable to water and impermeable to Na+ and Cl- : Water moves out of the descending tubule by osmosis Water is collected by the vasa recta and removed from the medulla 16  Set up of the osmotic gradient Ascending limb is impermeable to water and permeable to Na+ and Cl- : the "salt pumps" remove Na+ and Cl- so the osmolality of the filtrate drops to 100 mOsm/L Salts are collected by vasa recta Vasa recta also provides nutrients and O2 without affecting the osmolality of the interstitial fluid 17  Reabsorption in the collecting ducts The collecting ducts run parallel to the loops of Henle. Normally the collecting ducts are IMPERMEABLE to water. However, the permeability of the collecting ducts can be changed by ADH. 18  Antidiuretic hormone (ADH) ADH controls whether dilute or concentrated urine is formed Kidneys set up the steep osmotic gradient in the medulla irrespective of the body’s fluid balance Osmoreceptors in the hypothalamus monitor the plasma osmolality ADH secretion is stimulated by a water deficit in the body ADH induces water reabsorption in DCT and CD (normally impermeable to water movement) Once ADH reaches the distal convoluted tubule and CD, it makes the tubule cells become permeable to water at their luminal membrane Normal urine production is 1-1.5 L/day 19  Effect of ADH on urine volume Presence of ADH: In case of dehydration ADH is released The collecting ducts become permeable to water movement Water reabsorbed The output is a small volume of concentrated urine (< 1200 mOsm/L) Approx. 500 ml/day 20  Effect of ADH on urine volume Absence of ADH: The collecting ducts are impermeable to water movement No water reabsorption occurs The output is a large volume of dilute urine (< 100 mOsm/L) 21 Summary of the loop of Henle 22