THE KIDNEY Structure and function of the kidney

Function of the kidney The kidney has two main roles. One of the kidney’s roles is to maintain a stable concentration of water in the blood stream. This is known as osmoregulation. The other is to remove waste from the body. This is known as excretion. Note that the lungs and skin are also organs of excretion. The lungs excrete water and carbon dioxide and the skin excretes water and inorganic salts.

Osmoregulation Osmoregulation is the regulation or maintenance of water concentration (and hence also the concentration of salt solutes in water) in the body in order to maintain homeostasis. Water is regulated mostly by the kidney - hormones play a part also. Why is osmoregulation so important…

Osmoregulation All living things are made up of between 70 to 90% water and it is a vital compound for many reasons: It is a solvent that transports and distributes many substances (such as oxygen, nutrients, wastes) between cells in the body. It dissolves substances so that they can take place in chemical reactions. In living things, all chemical reactions need to take place in water. It participates itself in chemical reactions, either being used or produced. It maintains the concentrations of various substances in the blood so that levels are at an optimal range. It helps maintain body temperature as it can absorb and transfer heat readily. Water can also act as a lubricant between surfaces that may rub over each other, such as in the joints between bones. Keeps respiratory surfaces moist to allow gas exchange to take place more easily.

Excretion Excretion is the removal of metabolic wastes from the body of an organism. An organism’s excretory organs are concerned with removing metabolic wastes. Metabolic wastes are the products of chemical reactions taking place in the body and many of these products are poisonous. The body must therefore find a way of removing them. The kidney is an essential part of excretion in mammals because diffusion and osmosis are not adequate alone (as they would be in microscopic organisms with a large SA/V ratio) As diffusion is too slow and is non selective it would be an ineffective filter to remove waste from the human blood. Osmosis alone would mean that only water could enter and leave the blood and solid wastes would remain in the body.

What is excreted? Amino acids (from protein) cannot be stored in the body and are broken down – deanimated – in the liver into various forms of nitrogenous wastes. The various forms of nitrogenous wastes excreted are: \Urea (Humans) \Uric acid (Reptiles, birds & insects) \Ammonia (F ish & invertebrates) Deamination removes the amino group (the part containing nitrogen) from urea. Nitrogenous wastes are the main products excreted by the kidney. Other wastes include any drugs in the blood such as caffeine or alcohol.

Location of the urinary system heart diaphragm adrenal gland aorta vena cava kidney ureter bladder urethra

Mammalian kidney We have two kidneys, one located each side of abdomen The renal artery leads to the kidney The renal vein leads away from kidney A ureter joins each kidney to the bladder The urethra carries urine out of the body from the bladder

The kidney looks simple from the outside BUT….. on the inside its complexity becomes apparent!

Gross Structure of a Kidney Cortex: outer layer, dark red Medulla: middle area, paler red Pelvis: “inner” area, white

Pelvis Cortex Medulla Ureter Renal vein Waves of peristalsis sends urine to the bladder Nephron (about 1 million in each kidney) Glomerulus Bowman’s capsule Kidney tubule The Loop of Henle moves into and out of the Medulla. The rest of the nephron is situated in the Cortex. Loop of Henle Collecting duct Renal artery

The nephron The functional unit of the Kidney Performs the role of ‘cleaning the blood’ It does this by filtering and reabsorbing Its structure is suited to its function

Location of nephron The 3 processes that form urine are: Filtration which occurs in the cortex Reabsorption which occurs in the cortex and medulla, and.., Secretion of urea and water which occurs in the kidney pelvis RenalBlood from the renal artery Glomeruli Collecting duct Cortex Urine moves out of the kidney to the bladder for collection (before urination) Medulla

Filter function Glomerulus A dense capillary network Blood comes in from the arteriole under pressure Small molecules are forced through the wall of the capillary into the Bowman’s Capsule Large molecules and cells remain in circulation as they are too large to cross into the Bowman’s Capsule Arteriole leading to glomerulus

Blood enters glomerulus under pressure through wide arteriole Blood leaves the glomerulus via the narrow arteriole Glomerular filtrate collects and enters kidney tubule Outer wall of Bowman’s Capsule Small filtered particles (filtrate) have crossed into the Bowman’s Capsule Large particles are left in circulation Filtration is passive & selective only by size Glomerulus

Remaining in circulation after filtration The large components of blood that are not forced through the glomerular wall include: Red blood cells White blood cells Large blood proteins such as hormones

Filtration - The Bowman’s capsule  water  urea  glucose  amino acids  sodium  chloride  potassium  carbonate ions  uric acid  creatinine  other substance in small quantities i.e. asprin  The Bowman’s Capsule surrounds the glomerulus  It collects the glomerular filtrate containing: ie. Plasma contents without the protein molecules

Tubular reabsorption After filtration, the substances are actively reabsorbed from the: proximal convoluted tubule the loop of Henle (where concentration of solution occurs – the longer the Loop the more concentrated the urine) the distal convoluted tubule And onto the collecting ducts Blood vessels surround the Loop of Henle to reabsorb useful material.

Tubular reabsorption As the filtrate passes through the tubule, active reabsorption (requires energy) of useful materials occurs: Most salts All glucose All amino acids Most water…….. takes place The materials that are not reabsorbed - such as urea - stay in the kidney tubule and leave the nephron via the collecting duct.

H+, ammonia, drugs (i.e. asprin) = active Water = passive NaCl = active Na+, HCO 3 - = active K+, H+, ammonia, any drugs = active Water = passive NaCl, glucose = active K+, Water = passive Secretion

Active transport Active transport moves substances up/against the concentration gradient – for example from an area of low concentration to an area of high concentration. This is the opposite to diffusion and osmosis that move down/with a concentration gradient – from an area of high concentration to an area of low concentration. Active transport requires the cellular energy of ATP breaking down to ADP + P to take place. This animation shows the sodium – potassium pump that occurs in neurons. It is similar in action to the process of active uptake in the nephron.

Proximal Convoluted Tubule: Reabsorption of ions Complete active reabsorption of glucose occurs here 90% of Sodium is actively reabsorded (pumped out) here An equal number of chloride ions(Cl-) follow sodium ions (Na+) H+ ions pumped in in order to help maintain the pH of the blood

Proximal Convoluted Tubule: Water Reabsorbtion The proximal convoluted tubule is permeable to water 85-90% of the water in the filtrate is passively reabsorbed here Water moves out of this tubule by osmosis as it follows the concentration gradient established by movement of ions from the tubule into the renal capillaries

Kidney Tubule cells These cells are suited to the role of ACTIVE UPTAKE microvili large number of mitochondria special transport molecules in membrane

Urine formation As urine passes down collecting duct, more water is reabsorbed into extracellular fluid Concentrated urine accumulates in kidney pelvis

Composition of Urine mL of urine daily, composed of: water95-97 % solids 3-5 % urea30.0 g creatinine 1.2 g ammonia 0.5 g uric acid 1.0 g ions(salts)25.0g Solids composed of

A quick overview……... Filtration occurs in the cortex from the glomerulus to the Bowman’s Capsule. Reabsorption occurs in the cortex and medulla from the distal and proximal kidney tubules and the Loop of Henle. Collection and Secretion of urea and water occurs in the kidney pelvis.

The End