Kidney Lecture 1:Structure and Blood Filtration

Regulation of Water, Salts, Acid-Base balance Removal of metabolic waste Removal of foreign chemicals Gluconeogenesis (formation of glucose) Production of hormones / enzymes Erythropoietin (controls red blood cell production) Renin (controls blood pressure and sodium balance) Activates Vitamin D (regulates calcium)

Each kidney contains about 1 million nephrons Each nephron feeds into collecting ducts These feed into the ureter Kidney branched tubular organ resembling a tree

Glomerular Filtration Tubular Secretion Tubular Reabsorption

surround blood vessels within the glomerulus Foot-like process create a sieve-like structure Allows free passage of small molecules from blood to kidney tubules

Fluid filtered through glomeruli by pressure of flow through blood vessels Glomerular Filtration Rate (GFR) - Volume of fluid filtered from glomeruli to bowman’s space per unit time GFR regulated by adjusting blood pressure either side of glomerulus One way kidneys maintain water and salt balance

Constricting afferent vessels Dilating efferent vessels Reduces water and salt loss through excretion

Constricting efferent vessels Dilating afferent vessels Increases water and salt excretion

Glomerulus, Proximal Convoluted Tubule, Loop of Henle, Distal Convoluted Tubule, Collecting duct

Filtration of water and small molecules from blood to nephron through glomerulus

Dilation and constriction of afferent and efferent blood vessels around the glomerulus

Tubule – Cell : Na/X (e.g Glucose cotransport), Na/H counter transport Na+ moved from tubules into cells through two mechanisms: Co-transport with other molecules. E.g. Glucose Counter-transport of Hydrogen ions (Na+ in , H+ out) Cell – Body : Na/K ATP Pump Na+ moved from cells to interstitial space and then blood via active transport Na+/K+ ATPase pump, requires ATP K+ returns to interstitial fluid via ion channels Down concentration gradient

Tubule – Cell : Na – K – Cl cotransporter (NKCC) Na+ moved from tubules into cells through cotransport with K+ and Cl- Na-K-Cl cotransporter (NKCC) K+ moves back into tubule via ion channels Down concentration gradient Cell – Body : Na/K ATP Pump Na+/K+ ATPase moves Na+ out to interstitial space and K+ into cells K+ returns to interstitial fluid via ion channels Down concentration gradient

Tubule – Cell : Na Channel Tube – Cell : Na+ moves into cell and K+ moves out through ion channel Down concentration gradient Cell – Body : Na/K ATP Pump Cell – Body : Na+/K+ ATPase moves Na+ out to interstitial space and K+ into cells

Aldosterone : Expression of extra ion channels / pumps in collecting duct Aldosterone : steroid hormone Regulates Na+ reabsorption and blood pressure Acts in collecting duct tubule cells Increases expression of ion channels and Na+/K+ ATPase

High salt content in interstitial fluid drives water diffusion from loop Blood flow counter to tubule flow maintains gradients Only descending loop permeant to water Ascending loop reabsorbs salts – creates hypertonic solution in interstitial fluid around loop This causes water to diffuse out of descending loop Blood flow around loop of Henle is counter to flow through loop of Henle. Freshly filtered blood (low in salt) first encounters ascending loop of Henle Encourages salt absorption This salt-rich blood then circulates around descending loop Water from descending loop moves into blood Maintains concentration gradients

Aquaporins (water channels) allow diffusion of water out of collecting ducts Collecting duct tubule cells contain lots of water channels Aquaporins Interstitial space full of salt from Na+ reabsorption Allows free flow of water out of tubule, through cell, and into interstitial space

Vasopressin : stimulation of aquaporin delivery to cell membrane Faster response than aldosterone Vasopressin: hormone that controls water reabsorption Acts on receptors on collecting duct cells This causes aquaporin-rich vesicles inside cell to fuse with cell membrane Increases number of channels water can use to cross into body