Renal System 2 Notes

Renal System Physiology The ionic composition of ECF and the distribution of body water between different fluid compartments Kidney Functions 1. Homeostasis - filtering the blood to remove waste products of metabolism and toxic substances but retains the essential nutrients, which is followed by reabsorption and secretion of water and electrolytes to maintain a normal blood pressure. Urine is produced in the process 2. Hormone secretion - independent of the endocrine system, the kidneys secrete erythropoietin to stimulate RBC production, rennin as part of the blood pressure control and the active form of vitamin D ? Maintains water balance by regulating water intake and water output, ensuring the osmolarity of ECF ~ 285mOsm/L ? Maintains salt balance by regulating the ECF [Na+] ~ 142mmol/L (+3%) and [Cl-] ~ 102 mmol/L (+6%) ? Regulates ECF pH ~ 7.4 (+0.5%) by the rate at which H+ and HCO3- are excreted ? ECF [HCO3-] ~ 25mmol/L (+8%) ? Nitrogenous waste products are excreted only via the kidney - urea, ammonia, creatinine, uric acid Nephron Structure The Renal Corpuscle Found in the cortex, consists of the glomerulus, a knot of capillaries, which are surrounded by the Bowman's capsule into which fluid is forced out of the plasma and into the Bowman's Space to form glomerular filtrate Proximal Convoluted Tubule The PCT cells have fine microvilli on the luminal side, with tight junctions between cells. The basal side on the cell forms folds to increase the surface area for the exchange process. The cells contain lots of mitochondria to provide the energy for this exchange process Loop of Henle The nephron descends into the cortex towards the medulla via the loop of Henle. The thin descending and ascending limb have sparse, narrow cells, with very few Page 1 infolding and mitochondria. The thick ascending limb contains thick cells, packed with mitochondria and cellular intrusion, reflecting their active transport functions Distal Convoluted Tubule Found in the renal cortex, 10-15% of ions and fluid are recovered here Tubules connect from different nephrons to form collecting ducts MESANGEAL CELLS MACULA DENSA JUXTAGLOMER ULAR CELLS (RENIN) Juxta-Glomerular Apparatus The juxta-glomerular cells found in the wall of the afferent arteriole secrete renin into the blood stream in response to a fall in Na+ levels Mesangeal cells and macula densa cells are sensitive to the delivery of Clions from the PDT. A fall results in afferent arteriolar dilation, restoring PERITUBULAR macula densa blood flow Renal Vasculature CAPILLARIES The kidneys together receive about 20% of the cardiac output - 1.2L/min Renal artery ? inter-lobar vessels ? arcuate arteries ? interlobar arteries ? glomerular capillaries VASA RECTA GLOMERULI PROXIMAL TUBULES LOOPS OF HENLE VASA RECTA SUB-CAPSULAR Outer renal cortex Short Just dipping into renal medulla Short JUXTA-MEDULLARY Deep in renal cortex near cortico-medullary boundary Long Extending into renal pelvis Long Glomerular Filtration Barrier Acts as though perforated by (virtual) pores of about 8-10nm diameter Molecular weight <10,000 = freely filtered Molecular weight >100,000 = excluded The rate of filtration is relative to shape: long and thin > spherical And charge: positive > neutral > negative Eg. Albumin ~ 69,000 & negatively charged = excluded Free Hb ~ 68,000 & not charged = leaks into tubule Driving pressure for filtration at afferent end of glomerular capillaries: GFR = (45-10) - (25-0) mmHg = 10 mmHg Driving pressure for filtration at efferent end of glomerular capillaries: GFR = (44-10) - (34-0) mmHg ? GC increases Page 2 

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