WJEC Unit 3.7 Homeostasis & the Kidney PDF

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Summary

Negative feedback, Positive feedback, Kidney Anatomy, Ultrafiltration, Glomerular Filtration Rate (GFR), Selective reabsorption by the Proximal Convoluted Tubule, The glucose threshold, Reabsorption of water by the loop of Henle, Variation in loop length, Osmoregulation, Dialysis, Kidney Transplant...

Description

Homeostasis & the Kidney ● ● ●

Homeostasis is the maintenance of a constant internal environment by negative feedback. Core body temperature, pH and water potential may change due to changes in our activity or external environment, but they fluctuate around a set point1. The body is kept in dynamic equilibrium; constant changes occur, but corrective mechanisms bring the internal environmental conditions back towards a set point.

Negative feedback ● ● ●

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A receptor detects a deviation from the set point in the internal environment and sends instructions to a co-ordinator. The co-ordinator communicates with one or more effectors which make corrective responses. The factor returns to normal (the set point), this is monitored by the receptor and information is fed back to the effectors, which stop making the correction. Negative feedback restores conditions to their optimum levels counteracts the change + ensures minimal fluctuation around the set point. Glucose regulation of the blood via hormone insulin which is produced in the beta cells of the pancreas. Thermoregulation - maintains at its core internal temperature. (at resting; 37oC / 98.6oF) Water potential is maintained by Antidiuretic Hormone (ADH) which alters water reabsorption in the kidney.

Positive feedback

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the role of +ve feedback is to enhance the size of a given stimulus - amplifies the change the rate of a process is increased as the product concentration increases

Oxytocin is a hormone that stimulates the contraction of the uterus at the end of pregnancy and this aids in the delivery of the baby through the birth canal. Blood Clotting - when skin is cut or capillaries are damaged, the first stage of clot formation is when platelets in the blood adhere to the surface of the open cut.

Egestion – is the removal of undigested semi-solid waste (faeces) via the anus in mammals. Excretion is the removal of wastes produced by the body due to metabolism.

1 Homeostasis prevents wild fluctuations, beyond the optimal range, allowing cells and metabolism to function efficiently.

Ammonia ● ● ● ● ● ● ●

Highly toxic. Very water soluble. Needs large volumes of water to dilute it to non-toxic concentrations. Freshwater animals pass - large volumes of water through their gills. Most ammonia is excreted through the gills. Some is removed by the kidneys.

Urea Excess amino acids are deaminated in the liver; the amino group is removed and converted into ammonia (highly toxic) and then to urea (less toxic). Urea is removed by the kidneys. ● Excreted by mammals. ● Less toxic than ammonia. ● Water soluble. ● Does not need to be diluted as much as ● Ammonia. ● Energy is used to convert excess amino ● acids and nucleic acids to urea.

Uric Acid ● Insects. ● ● ● ● ● ● ● ●

Excreted by reptiles, birds and Low toxicity. Very low solubility in water (almost insoluble). Very little water is needed (advantage to animals with poor access to water). A lot of energy is used to produce Uric acid.

Kidney Anatomy Functions: Excretion – the removal of nitrogenous waste from the body Osmoregulation – the control of water potential of the body’s fluids ● ● ● ● ● ● ● ●

Renal vein (blood returns to the general circulation) Renal artery (blood enters the kidney) Medulla (reabsorption of water occurs here) Cortex (ultrafiltration and selective reabsorption occurs in this region) Pelvis (empties urine into the ureter) Urethra (carries urine out of the body) Ureter (transports urine to the bladder) Bladder (stores urine)

Nephrons which collectively provide a large surface area for exchange of

materials. Large SA:V ratio allows maximal diffusion between Filtrate & blood

Ultrafiltration ● ●

Ultrafiltration is filtration under high blood pressure. Bowman’s capsule and the capillary knot of the glomerulus are responsible for ultrafiltration.

Blood enters the glomerulus via the afferent arteriole, which wider diameter than the efferent arteriole, and leaves via the efferent arteriole. The narrowing of the capillaries in the capillary knot of the glomerulus generates a high hydrostatic pressure, providing the driving force for ultrafiltration. Small molecules and ions are forced through ● the fenestrations of the endothelial cells (capillary walls) ● then the selective molecular filter of the glomerular basement membrane, which only allows small molecules to pass through; blood cells, platelets and large proteins, such as antibodies, are too large to be filtered. ● and finally through the filtration slits of the pedicels (podocyte extensions) into the Bowman’s space The glomerular filtrate contains: • Water • Glucose • Salts • Urea • Amino acids

Glomerular Filtration Rate (GFR) The rate at which fluid passes from the blood in the capillary knot (glomerulus) into the Bowman’s space The volume of glomerular filtrate formed per minute by the kidneys.

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Pure water has the highest water potential (0 kPa) Solutes lower water potential. Hydrostatic Pressure increases water potential.

Selective reabsorption by the Proximal Convoluted Tubule Selective reabsorption is the process by which useful substances such as glucose, amino acids and salts are reabsorbed back from the glomerular filtrate into the blood plasma. This takes place in the PCT (in the cortex) by facilitated diffusion and active transport. ● All the glucose & most of the water & mineral ions (e.g.sodium & chloride ions) are reabsorbed in the PCT. The nephron is closely associated with blood capillaries - vasa recta, where reabsorbed substances pass from the PCT. Epithelial cuboidal cells of the PCT: ● ● ●

Microvilli provide an extensive surface to volume area for maximal selective reabsorption. Mitochondria provide the energy for Active Transport of ions Folded basement membrane provides an extended surface area for maximal numbers of Na+/K+ active transport complexes.

Cotransport is the transport of molecules or ions together through the same transport protein Secondary active transport is the coupling of diffusion, down an electrochemical gradient, providing energy for active transport Glucose and amino acids enter the cell from the lumen of the PCT by cotransport with sodium ions. Chloride ions enter by facilitated diffusion and water by osmosis. Once inside the cell they diffuse across the cell cytoplasm towards the opposite cell membrane. Glucose leaves the cell by facilitated diffusion and secondary active transport via a carrier and pump respectively. Na+ are actively transported out of the epithelial cells via a sodium-potassium pump. Amino acids and chloride ions leave by facilitated diffusion and water by osmosis.

The glucose threshold Glucose is essential for respiration and is usually all reabsorbed by the PCT and re-enters the bloodstream via the vasa recta. If the concentration of glucose in the filtrate is too high intrinsic transport proteins may become limiting (there are not enough of them), which means that not all the glucose will be reabsorbed. In cases of excess glucose, glucose will pass through into the Loop of Henle Glucose will remain in the filtrate and pass out of the body in urine, which can be an Indication of Diabetes This may occur if a person secretes too little insulin (diabetes type 1) or liver cells no longer respond to insulin (diabetes type 2 or gestational diabetes in some pregnant women).

Reabsorption of water by the loop of Henle ● ● ● ● ● ● ●

Filtrate leaves the PCT and enters the descending limb of the loop of Henle. The descending limb is permeable to water and water leaves the filtrate and enters the blood by osmosis, down a water potential gradient. At the same time Na+ & Cl- ions diffuse into the descending limb from the medulla. As water leaves the descending limb by osmosis the filtrate becomes more concentrated, reaching maximum concentration at the apex of the loop. The medulla has a low water potential which is maintained by the thick ascending limb of the loop of Henle (impermeable to water) expelling Na+ & Cl- by facilitated diffusion and then active transport into the tissue fluid of the medulla Due to low water potential in the medulla, water moves out by osmosis (reabsorbed) and readily removed by blood in vasa recta Interstitial fluid has low water potential so water is reabsorbed from the filtrate along the whole length of the DCT and collecting duct

The loop of Henle is called a countercurrent multiplier because the filtrate flows in opposite directions and the concentration of solutes in the filtrate increases towards the apex (is multiplied); the longer the loop of Henle the higher the concentration of solutes at the apex.

Variation in loop length Animals with long loops of Henle are adapted to dry environments, such as the desert e.g. camels. Animals with short loops live in freshwater environments e.g. otters. The longer the loop the more ions can be pumped into the medulla. This lowers the water potential of the medulla further allowing more water to be reabsorbed into the bloodstream by osmosis.

Osmoregulation ● ● ● ●

Osmoregulation Is the control of body fluid water potential by negative feedback (homeostasis). Antidiuretic hormone binds to receptors on cells in the collecting ducts of the kidney and promotes reabsorption of water back into the circulation. Affects the DCT and collecting duct In the absence of antidiuretic hormone, the collecting ducts are virtually impermeable to water, and it flows out as urine.

Eg Decrease in Water Potential in blood (i.e. Dehydration) ● ● ● ● ● ● ● ● ● ●

Low water levels in blood/high osmotic potential/low Ψ; Detected by osmoreceptors in hypothalamus Increases amount of ADH secretion from the posterior pituitary gland into the bloodstream to receptor proteins on the walls of the distal convoluted tubule and collecting duct (effectors) This causes increased permeability - aquaporins are added to the cell membranes of the effectors allowing more water to be reabsorbed by osmosis. More water reabsorbed into the blood from the filtrate, because of low Ψ in medulla Small volume of hypertonic urine (lower water potential in urine) Water potential increases in blood, back towards the set point This information is fed back to the hypothalamus and less ADH (or no ADH) is produced. Thirst (triggered by the hypothalamus) will encourage the individual to drink more fluids

Dialysis ● ●

Dialysis is a method of method of replacing kidney function in patients with kidney failure. The blood to be cleaned and the dialysing fluid are separated by a selectively permeable membrane.

Haemodialysis Blood is taken from an artery, and travels through thousands of long, narrow fibres of dialysis tubing, which is selectively permeable. These fibres are surrounded by the dialysis fluid. Small molecules/ions in the blood can diffuse down a conc. gradient into the dialysis fluid.

Counter - current mechanism The blood and the dialysis fluid flow in opposite directions. This ensures that a concentration gradient is maintained between the blood and the dialysis fluid.

Continuous Ambulatory Peritoneal Dialysis (CAPD) The dialysing membrane in this process is the patients own peritoneum (a membrane that lines the abdomen). The dialysis fluid can pass across the peritoneum in the abdomen. The peritoneum has a rich blood supply, therefore exchange of material can take place. The patient is free to walk around hence the term ‘ambulatory’.

Kidney Transplant...