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The principal functions of the kidneys are: The Mammalian Kidney The principal functions of the kidneys are: Excretion – removal of toxic nitrogenous waste products of metabolism; these include urea and creatine (a waste product of muscle metabolism) Homeostasis – maintenance of optimal water potential of body fluids by regulating the water content, ion composition and pH of the body fluids; their role in the regulation of water content is described as osmoregulation

Vena cava Aorta l Renal vein Renal artery

The kidney is composed of three distinct regions Kidney Section Cortex Pelvis Medulla The kidney is composed of three distinct regions

Draw diagrams to show the following structures: Renal cortex Medulla Renal pyramid Renal pelvis Renal artery Renal vein Ureter Bladder Urethra

Kidney dissection

Dissecting a Kidney: Safety Scalpels are sharp! Wear gloves and goggles at all times. You are NOT a butcher – take care, small cuts only. Label your kidney sections Pins are sharp! Do not dispose of kidney down the sinks, use the bin! Wash desk tops down with disinfectant when you have finished.

The Kidney Tubule; Nephron glomerulus afferent arteriole efferent arteriole Bowman’s (renal) capsule distal convoluted tubule proximal convoluted tubule branch from renal artery branch to renal vein collecting duct loop of Henlé vasa recta There are approximately one million kidney tubules in each human kidney

The nephron This is the functional unit of the kidney It consists of a cup-shaped Bowman’s capsule connected to a tube which is divided into 3 regions Proximal convoluted tubule Loop of Henle Distal convoluted tubule The ends of a number of nephrons join to form a collecting duct, which transfers fluid to the pelvis

The nephron ctd Arteriole blood enters each capsule through an afferent arteriole (arrives) This branches to form a dense capillary network, the glomerulus Blood leaves the capsule through an efferent arteriole (exits) This branches to form a network of capillaries, the vasa recta, around the nephron

Afferent vessel Efferent vessel outer layer Bowman’s capsule inner layer cavity containing the glomerulus

kidney animation

The filter The plasma in the glomerulus is separated from the filtrate in the Bowman’s capsule by 3 layers: The endothelium of the capillaries consisting of a single layer of squamous cells with pores between them. The inner wall of the Bowman’s capsule consists of podocytes; these cells have foot-like processes which surround the capillaries and gaps called filtration slits. Between these is the basement membrane, which is found on the outside of the capillary endothelium and is the effective filter.

The basement membrane is composed of glycoprotein and collagen fibres, forming a mesh; only molecules with a molecular mass less than 68 000 can pass through. The filtrate will contain inorganic ions; glucose; amino acids; urea and other toxic substances dissolved in water. Blood cells and most plasma proteins are too large to pass through.

filtration Efferent vessel Afferent vessel Basement membrane Bowman’s capsule glomerulus podocytes filtration Endothelial cells with pores Glomerular filtrate Glomerular capillaries containing glomerular plasma

glomerulus capillary podocyte Use hair clasp to demo

Ultrafiltration The driving force behind ultrafiltration is the high pressure of the blood entering the glomerulus. P is high because: The renal arteries are wide and relatively close to the heart The afferent arteriole is wider than the efferent arteriole, creating a bottleneck The high hydrostatic pressure created forces fluid from the glomerular plasma into the capsule, forming glomerular filtrate which enters the cavity of the Bowman’s capsule.

• The plasma proteins remaining in the glomerulus cause the water potential to decrease. The filtrate entering the Bowman’s capsule causes the water potential here to increase. • y filtrate now > glomerular plasma This sets up an osmotic gradient from the filtrate in the nephron into the glomerular capillaries, called back pressure which opposes the blood (hydrostatic) pressure and resists further filtration.

FILTRATE ULTRAFILTRATION blood in glomerulus capillary capillary endothelium basement membrane Podocytes on inner wall of Bowman’s capsule lumen of Bowman’s capsule FILTRATE Bowman’s capsule endothelium

The Kidney Tubule (Nephron) Each kidney tubule is responsible for urine formation and this involves TWO basic processes: Ultrafiltration of blood plasma in the glomerulus is filtered into the Bowman’s capsule, all substances below a certain size are filtered, both useful and toxic Selective tubular reabsorption of useful substances from the filtered materials occurs as the filtrate passes along the nephron. The fluid is only called urine when it leaves the collecting ducts

Selective Reabsorption As filtrate moves through the proximal convoluted tubule, 80% of the water is reabsorbed by osmosis into the adjacent blood capillaries of the vasa recta. All glucose and amino acids are reabsorbed by active transport. Small proteins, which may have been filtered are reabsorbed by pinocytosis.

Adaptations for Reabsorption Cuboidal epithelial cells line the proximal convoluted tubule. The surface in contact with the filtrate has many microvilli, and the surface next to the capillaries has many infoldings of the cell membrane (basal invaginations). These adaptations increase the surface area for reabsorption. These cells have many mitochondria that supply ATP for active transport.

The movement of solutes out of the proximal convoluted tubule by AT & pinocytosis lowers the solute potential in the cuboidal epithelium and blood capillaries; This sets up an osmotic gradient between the proximal convoluted tubule and the blood capillaries causing water to move into the capillaries by osmosis. This is responsible for the bulk of water reabsorbed in the kidney.

interstitial fluid lumen epithelial cells microvilli intercellular spaces many mitochondria

Microvilli increase SA Mitochondria: Resp Energy AT Cell membrane infolding

urea water glucose sodium urea water sodium filtrate flow reabsorption Microvilli increase SA reabsorption  none  some  all Not filtered Intrinsic carrier protein for AT  some Mitochondria: Resp Energy AT Blood flow

Distal the ionic composition and pH of the blood are adjusted. other toxic substances such as creatine, a waste product of metabolism, are secreted into the filtrate from the blood facultative reabsorption of water, under the influence of the hormone ADH.

The Loop Of Henle This is found deep in the medulla. Water and ions are moved between the filtrate in the loop and tissues in the medulla. This causes the tissues deeper in the medulla to be more concentrated with ions, i.e. the solute potential becomes increasingly negative through the medulla. Creates a salt gradient, and thereby increasingly negative solute potential,in the medulla tissue of the kidney.

This salt gradient is important in the reabsorption of water. The Loop Of Henle This salt gradient is important in the reabsorption of water. This means there is a potential for osmotic extraction from the filtrate in the descending limb of the Loop of Henle, distal convoluted tubule and collecting duct. It results in urine which is hypertonic to the blood.

Increasing concentration The purpose of the Loop of Henlé is to create a salt gradient (increasing concentration of salt) within the tissue of the medulla Increasing concentration of salt The water potential of the descending limb is greater than the medulla therefore water is lost from the loop by osmosis allowing the production of concentrated urine Water is not gained into or lost from the ascending limb as it is impermeable to water

Ultrafiltration of the blood takes place at the glomerulus The Kidney Tubule; Nephron Ultrafiltration of the blood takes place at the glomerulus Water reabsorption occurs from the distal convoluted tubule and collecting duct, allowing for the production of a concentrated urine The glomerular filtrate flows into the proximal convoluted tubule where useful substances are reabsorbed back into the blood The resulting fluid flows into the loop of Henlé where a salt gradient is created deep in the medulla The walls of the distal tubules and collecting ducts are variably permeable to water, depending upon the presence or absence of antidiuretic hormone (ADH)

osmoregulation Osmoreceptors in the hypothalamus are sensitive to the solute potential of the blood Cells in the hypothalamus produce anti-diuretic hormone (ADH) ADH is secreted into, stored and released from the posterior pituitary gland into the bloodstream

A rise in blood concentration (water potential more negative) is detected by the osmoreceptors, Which send impulses to the posterior lobe of the pituitary gland Resulting in release of ADH into the blood.

The water passes back into the blood capillaries of the vasa recta ADH increases the permeability of the distal convoluted tubule and collecting ducts to water When the walls are more permeable, more water is re-absorbed by osmosis, from the filtrate in the distal convoluted tubule the filtrate in the collecting duct The water passes back into the blood capillaries of the vasa recta A reduced volume of hypertonic urine is discharged.

blood solute potential Thirst centre cerebrum blood solute potential e.g. due to exercise, little water intake, high salt intake More ADH produced in the hypothalamus and released from pituitary gland

Less water is reabsorbed producing large volumes of dilute urine Less/no ADH Produced or released blood solute potential e.g. large water intake Without ADH the walls of the distal convoluted tubules and collecting ducts remain relatively impermeable to water. Less water is reabsorbed producing large volumes of dilute urine

ADH and negative feedback No ADH produced Less water reabsorbed Large volumes dilute urine increased blood solute potential decreases blood solute potential normal level blood solute normal level blood solute More ADH produced More water reabsorbed small volumes concentrated urine increases blood solute potential decreased blood solute potential e.g. during exercise

Read page 321 Answer Q 6-10 page 325 Question booklet and answers

in excretion and osmoregulation essay describe and explain the function of the mammalian kidney in excretion and osmoregulation