Kidneys/Homeostasis

Important definitions • Excretion – removal of metabolic waste made by the body • Homeostasis – maintenance of a constant environment • Negative feedback – occurs when a change in a system produces a second change which reverses the first • Osmoregulation – control of the water potential of the body’s fluid by regulation of the water content of the body • Secondary active transport – coupling of diffusion (sodium ions) down an electrochemical gradient providing energy for the active transport (glucose) up a concentration gradient • Transamination – enzyme catalysed reaction that transfers an amino group to an alpha-keto acid, making it an amino acid

Homeostasis and the excretory system • Homeostasis = maintenance of a constant environment • Maintaining tissue fluids, supplying nutrients and removing fluids • Maintaining these at an optimum protects cells from changes in external environment • Excretory system = structure and function of kidneys and related organs

Kidneys • Remove nitrogenous, metabolic waste from the body • Osmoregulation (regulates the balance between water and dissolved solutes) • 3 main processes – ultrafiltration, selective reabsorption and urine concentration

Nephrons • Each kidney has around 1 million nephrons • Mini filtering device which remove metabolic waste products and regulate water content • Consist of a glomerulus (in cortex) and a tubule (in medulla) • Each tubule consists of proximal and distal tubules connected by the loop of Henle and ends at the collecting duct

Ultrafiltration

Selective reabsorption Facilitated diffusion Co-transport Active transport Diffusion Osmosis Occurs in cells of the proximal convoluted tubule Glucose + Na+ Glucose Na+ Na+ Amino acids + Na+ Amino acids Cl- Cl- H2O H2O Glucose Glucose Na+ Amino acids Cl- Glomerular filtrate Blood

Selective reabsorption - continued • Process by which products are reabsorbed back into the blood as filtrate flows through the nephron • Glucose and amino acids enter by co-transport with sodium ions, glucose leaves by facilitated diffusion (via a carrier) and secondary active transport (diffusion down an electrochemical gradient which provides energy for active transport) • Amino acids leave by facilitated diffusion • Sodium ions leave by active transport via a sodium potassium pumps • Chlorine ions enters and leaves by facilitated diffusion • Water enters, moves through and leaves via osmosis • Everything else diffuses across the cell

Adaptations for reabsorption • Microvilli – provides a large surface area for reabsorption • Numerous mitochondria – provides ATP for active transport of glucose and Na+ • Osmosis – water is reabsorbed passively (doesn’t use energy) following the transport of salt

Glucose threshold and water absorption • If glucose concentration is too high, there may not be enough transport molecules to absorb it all • All of the transport proteins are occupied • Glucose will pass through the loop of Henle and will be lost • Some solutes still remain in the filtrate and so not all of the water will be absorbed due to the water potential

Reabsorption in the loop of Henle 1. Na+ and Cl- are actively transported out of the ascending limb 2. This raises the concentration of Na+ and Cl- in tissue fluid 3. The rise in concentration results in water leaving the descending limb via osmosis 4. Loss of water, raises the concentration of Na+ and Cl- in the descending limb 5. Na+ and Cl- diffuse from this high concentration to a lower one in the lower part of the loop 6. The water is carried away by blood in the capillaries (vasa recta)

Reabsorption in the distal convoluted tubule and collecting duct • Absorb varying volumes of water according to the needs of the body • Operate the fine control of the body’s water content • First part is like the ascending limb of the loop of Henle • Second part is like the collecting duct • In both, Na+ ions are actively pumped from fluid inside the tubule into tissue fluid, the ions then pass into the blood • K+ ions are actively pumped into the tubule • The rate at which the ions are pumped in or out can be varied and helps to regulate the ion concentration in the blood

Osmoregulation • Under control of negative feedback • Restores water potential if blood is more diluted or concentrated • Water potential may fall due to sweating, intake of large amounts of salt or a reduction in water intake • Water potential may rise due to less sweating, no salt intake and an increase in water intake • Change in water potential is detected by osmoreceptors in the hypothalamus • ADH (anti diuretic hormone) is carried in secretory glands in axons to the posterior pituitary gland

ADH • Travels in the bloodstream from the pituitary gland to the kidneys (in various amounts) • Increases/decreases the permeability of the distal convoluted tubule and the collecting duct to water • ADH binds to receptors on distal convoluted tubule and collecting duct membranes • Adenyl cyclase catalyses the production of cyclic AMP (2nd messenger) • Vesicles containing aquaporins move to the cell membrane from the cytoplasm • Aquaporins fuse with and are incorporated into the cell membrane • Water molecules move through the pores into the cell, down a water potential gradient • More/less water is reabsorbed into the medulla • More/less water is absorbed into the vasa recta • Water potential returns to normal • Small/large amount of urine is produced

ADH sequence (water potential too low) Decreased water potential in blood Osmoregulators lose water Dehydration Increase in secretion of ADH More ADH carried in the blood Collecting duct becomes more permeable More water reabsorbed into blood/more urine produced Water potential increases until normal

ADH sequence (Water potential too high) Increased water potential in blood Osmoregulators gain water Too much hydration Decrease in secretion of ADH Less ADH carried in the blood Collecting duct becomes less permeable Less water reabsorbed into blood/less urine produced Water potential decreases until normal

Kidney failure treatments • Medication to control blood potassium and calcium levels • Lower protein diet – more protein in the diet, more urea made which couldn’t be filtered out if the kidneys failed, more protein in the blood makes it more acidic • Medication to reduce blood pressure • Dialysis – blood cleaned artificially by a machine that is outside of the body • Continuous ambulatory peritoneal dialysis – form of dialysis that allows the patient to continue with normal activities whilst dialysis operates • Kidney transplant – may or may not be rejected so immunosuppressants may also be prescribed

Osmoregulation in different animals Animal Main excretory product Toxicity of excretory product Solubility Amoeba Ammonia Diffuses out via a large surface area and gills High High Freshwater fish Ammonia Diffuses out across the gills and is quickly diluted to non toxic levels High High Insects and birds Urea Costs energy to produce but little water needed for excretion so conserves water Low Low Mammals Urea Costs energy for production Medium Tissues can tolerate high concentrations for short time Medium