The Kidneys and Homeostasis Homeostasis is the ability to control the internal environment to enable organisms to be independent of the external environment Homeostasis is the ability to control the internal environment to enable organisms to be independent of the external environment Cells are bathed in tissue fluid in which conditions are maintained at a constant level (e.g. temperature, pH, salt content) Cells are bathed in tissue fluid in which conditions are maintained at a constant level (e.g. temperature, pH, salt content) The internal environment is maintained by physiological mechanisms which are controlled by feedback mechanisms The internal environment is maintained by physiological mechanisms which are controlled by feedback mechanisms

Control Mechanisms Control Mechanisms Self-regulating by means of feedback response: Self-regulating by means of feedback response: 1.Reference point: set level of operation 2.Detector: signals deviation from reference point 3.Controller: co-ordinates info from detectors, sends out instructions 4.Effector: brings about necessary change 5.Feedback Loop: informs detector of any change in system as a result of action by effector

Feedback is usually NEGATIVE Feedback is usually NEGATIVE E.g. temperature control E.g. temperature control –Temp detectors in skin relay info on changes in external temp. to hypothalamus in brain (controller). Message sent to skin and blood vessels (effectors) to start corrective responses. As temp adjusts then negative feedback to hypothalamus to stop corrective measures Rare occasions when feedback is POSITIVE Rare occasions when feedback is POSITIVE –E.g Oxytocin during labour stimulates uterine contractions. A positive feedback as more is released as pressure on cervix continues

The kidneys have several homeostatic functions; primarily regulating the composition of body fluids by OSMOREGULATION The kidneys have several homeostatic functions; primarily regulating the composition of body fluids by OSMOREGULATION They control the water and salt content of the body and the pH of the blood They control the water and salt content of the body and the pH of the blood

They excrete nitrogenous products in solution through the ureter into the bladder where it is stored. They excrete nitrogenous products in solution through the ureter into the bladder where it is stored. The exit of the bladder is controlled by a voluntarily controlled sphincter. The exit of the bladder is controlled by a voluntarily controlled sphincter. Urea is formed in the liver from deamination of excess amino acids (removal of the NH 2 ), this is combined with CO 2 to form urea in a sequence of reactions known as the ornithine cycle. Urea is formed in the liver from deamination of excess amino acids (removal of the NH 2 ), this is combined with CO 2 to form urea in a sequence of reactions known as the ornithine cycle.

Each kidney is supplied with oxygenated blood by the RENAL ARTERY, and deoxygenated blood returns via the RENAL VEIN. Each kidney is supplied with oxygenated blood by the RENAL ARTERY, and deoxygenated blood returns via the RENAL VEIN. Each kidney contains around 1.2 million NEPHRONS; each of which have blood vessels closely associated with it Each kidney contains around 1.2 million NEPHRONS; each of which have blood vessels closely associated with it The AFFERENT ARTERIOLE brings blood into each renal capsule, dividing to form a network of capillaries called the GLOMERULUS The AFFERENT ARTERIOLE brings blood into each renal capsule, dividing to form a network of capillaries called the GLOMERULUS Blood leaves the capillaries in the EFFERENT ARTERIOLE Blood leaves the capillaries in the EFFERENT ARTERIOLE

The Nephron The functional unit of the kidney The functional unit of the kidney Carry out the process of osmoregulation and excretion Carry out the process of osmoregulation and excretion Made up of the following parts Made up of the following parts –Bowman’s Capsule –Glomerulus –Proximal convoluted tubule –Loop of Henle –Distal convoluted tubule –Collecting duct

The Bowmans Capsule The ultrafiltration unit The ultrafiltration unit It is a single layer of flattened epithelial cells which contains the glomerulus (a tightly coiled network of capillaries) It is a single layer of flattened epithelial cells which contains the glomerulus (a tightly coiled network of capillaries) The inner layer of the Capsule has “slit pores” which enable ultrafiltration The inner layer of the Capsule has “slit pores” which enable ultrafiltration It filters large particles (stay in blood) from small ones (pass into nephron) It filters large particles (stay in blood) from small ones (pass into nephron)

Afferent arteriole has larger diameter than the efferent arteriole which leads to a high pressure in the glomerulus capillaries Afferent arteriole has larger diameter than the efferent arteriole which leads to a high pressure in the glomerulus capillaries This pressure forces substances through pores in the capillary walls This pressure forces substances through pores in the capillary walls Glomerular filtrate contains glucose, amino acids, vitamins, hormones, urea, ions and water Glomerular filtrate contains glucose, amino acids, vitamins, hormones, urea, ions and water All cells, platelets and proteins remain in the blood All cells, platelets and proteins remain in the blood Most of the filtrate produced will be reabsorbed as it passes through the renal tubules Most of the filtrate produced will be reabsorbed as it passes through the renal tubules

Proximal Convoluted Tubule The longest region of the nephron for Reabsorption The longest region of the nephron for Reabsorption It is formed by a layer of cuboidal epithelial cells which have microvilli projecting into the lumen of the tubule It is formed by a layer of cuboidal epithelial cells which have microvilli projecting into the lumen of the tubule They contain a large number of mitochondria due to their high energy requirement They contain a large number of mitochondria due to their high energy requirement The volume of the filtrate is reduced by % in the proximal tubule The volume of the filtrate is reduced by % in the proximal tubule

Amino acids, glucose and ions diffuse into cells of proximal tubule and are then actively transported into the surrounding where they diffuse into the nearby capillaries Amino acids, glucose and ions diffuse into cells of proximal tubule and are then actively transported into the surrounding where they diffuse into the nearby capillaries The microvilli of the epithelial cells greatly increase the SA for uptake of ions and other solutes The microvilli of the epithelial cells greatly increase the SA for uptake of ions and other solutes Ion uptake Ion uptake –Na + ions are actively pumped out of the tubule, as these leave an electrochemical gradient is formed, meaning negative ions such as Cl - and PO 4 3- follow –The ion concentration so increases in the capillaries so an osmotic potential formed so water moves out of the tubule

Glucose uptake Glucose uptake –This involves a specific type of active transport; Sodium Cotransport –This involves a sodium ion and glucose binding to the same carrier molecule in the surface membrane –Normally all glucose is reabsorbed and non is lost in the urine Urea uptake Urea uptake –Urea is in relatively high concentrations in the remaining tubule fluid so diffuses into the blood

The Loop of Henle Has an descending limb which starts in the cortex and a ascending limb which returns to the cortex Has an descending limb which starts in the cortex and a ascending limb which returns to the cortex This reabsorbs water by osmosis which enables concentrated urine production This reabsorbs water by osmosis which enables concentrated urine production It functions thanks to a Countercurrent exchange mechanism with the surrounding capillaries It functions thanks to a Countercurrent exchange mechanism with the surrounding capillaries The mechanism involves exchange between 2 liquids moving in opposite directions e.g the descending limb and the ascending limb The mechanism involves exchange between 2 liquids moving in opposite directions e.g the descending limb and the ascending limb

The loop of Henle is a countercurrent multiplier due to the fact it actively secretes solutes into the surroundings to produce a very high conc. gradient The loop of Henle is a countercurrent multiplier due to the fact it actively secretes solutes into the surroundings to produce a very high conc. gradient The ascending limb actively secretes Cl - ions out as so Na + ions follow. The ascending limb actively secretes Cl - ions out as so Na + ions follow. The descending limb is impermeable to solutes but permeable to water The descending limb is impermeable to solutes but permeable to water The water moves out of the descending limb due to osmosis into the capillaries and the contents becomes very conc. The water moves out of the descending limb due to osmosis into the capillaries and the contents becomes very conc. As a consequence the fluid entering the ascending limb contains high conc. of ions so more pumped out, so more water out of descending limb and so on.. As a consequence the fluid entering the ascending limb contains high conc. of ions so more pumped out, so more water out of descending limb and so on..

Distal Convoluted Tubule + Collecting Duct Controls the amount of water reabsorbed into the blood; Osmoregulation Controls the amount of water reabsorbed into the blood; Osmoregulation In times of water shortage mammals can produce Hypertonic urine (a higher conc. of solutes than blood) In times of water shortage mammals can produce Hypertonic urine (a higher conc. of solutes than blood) In times of over hydration urine can be very dilute (little water reabsorption in kidney) In times of over hydration urine can be very dilute (little water reabsorption in kidney) The amount of water reabsorbed from the distal tubule and collecting duct depends on their permeability to water, which is controlled by negative feedback involving ADH (antidiuretic hormone) The amount of water reabsorbed from the distal tubule and collecting duct depends on their permeability to water, which is controlled by negative feedback involving ADH (antidiuretic hormone)

ADH is released by the posterior pituitary gland ADH is released by the posterior pituitary gland It acts on cells in the tubule and collecting duct to increase the permeability to water It acts on cells in the tubule and collecting duct to increase the permeability to water The more ADH the more water is reabsorbed, so the more conc. the urine is The more ADH the more water is reabsorbed, so the more conc. the urine is The mechanism is controlled by Osmoreceptors in the hypothalamus. The mechanism is controlled by Osmoreceptors in the hypothalamus. If the conc. of solutes in blood is raised then osmoreceptors increase activity and ADH secretions increase, so more water reabsorbed If the conc. of solutes in blood is raised then osmoreceptors increase activity and ADH secretions increase, so more water reabsorbed If the conc. decreases ADH release is decreased If the conc. decreases ADH release is decreased