The Kidney Structure

Within each Bowman's Capsule there is a knot of capillaries known as the glomerulus. The afferent arteriole is wider than the efferent artieriole. This creates pressure in the glomerulus so ultrafiltration is also known as filtration under pressure. 

Ultrafiltration-filtration under pressure that separates small soluble molecules from the blood plasma. Small molecules include: glucose, urea, water, salts. Blood entering the glomerulus is separated from the capsular space of the Bowman's Capsule by 2 layers of cells and a basement membranes.

Selective Reabsorption- molecules that are small but useful e.g. all glucose, all amino acids, most hormones, most water, most salts, are removed from the nephron and returned to the blood. Epithelial cells are modified: Microvilli increase the surface area available for reabsorption. Basal Channels remove molecules from the filtrate and return them to the blood. Carrier Proteins for facillitated diffusion and active transport of molecules out of the filtrate. Lots of Mitochondria provide ATP for active transport. Good blood supply means that molecules can be easily removed. 

Selective Reabsorption for glucose (and amino acids)- Glucose (amino acids) is actively transported out of the cell and into the basal channel (against concentration gradient). High concentration of glucose (amino acids) which will then diffuse into the blood, down the concentration gradient. Due to active transport, high concentration of glucose (amino acids) in basal channels-diffuses into the vasa recta. 

The cells that make up the wall of the Proximal Convoluted Tubule have adaptations 

• Microvilli provide a ;arger surface area for reabsorption of substances
• There are lots of mitochondria to provide ATP for the active transport of substances like glucose and amino acids.

Selective Reabsorption for glucose (and amino acids)- Glucose (amino acids) is actively transported out of the cell and into the basal channel (against concentration gradient). High concentration of glucose (amino acids) which will then diffuse into the blood, down the concentration gradient. Due to active transport, high concentration of glucose (amino acids) in basal channels-diffuses into the vasa recta. 

Selective Reabsorption of water- By osmosis down its water potential gradient. As solutes are pumped out of the out of the filtrate, the water potential increase in the filtrate so a gradient is created. This means that water follows the solutes into the blood. At the end of the proximal convoluted tubule, the volume of the filtrate will be lower, but the concentration will be the same as both water and salts are reabsorbed. The filtrate and the blood are similar concentrations-isotonic

THE DESCENDING LIMB IS PERMEABLE TO WATER BUT IMPERMEABLE TO SALTS

THE ASCENDING LIMB IS PERMEABLE TO SALTS BUT IMPERMEABLE TO WATER

Sodium and Chloride ions are actively transported out of the filtrate and into the medulla tissue. This creates a lower water potential outside of the ascending limb of the Loop of Henle. However, because the ascending limb is relatively impermeable to water, water moves out of the loop of Henle through the descending limb, which is relatively permeable. It leaves the filtrate in the descending limb by osmosis (as the water potential is lower in the medulla region than the filtrate) and is carried away by the blood in the vasa recta.

As water moves out of the descending limb, the filtrate becomes more and more concentrated (same mass of salts in a smaller volume of water). It reaches its maximum concentration at the bottom of the loop. 

As filtrate flows up the ascending limb, it becomes more and more dilute (lower mass of salts in the same volume of water)

The Loop of Henle is a (hair-pin) counter-current multiplier

Osmoregulation-the homeostatic control of body water

Operates on the principle of negative feedback 

• receptors that detect changes from the norm are found in the hypothalamus (at the base of the brain)
• the posterior lobe of the pituitary gland acts as the co-ordinator
• the collecting duct of the kidney is the effector

The walls of the Collecting Duct are permeable to water. The walls can be made more permeable by the secretion of ADH (Anti-diuretic hormone). If released, more water is reabsorbed and the urine has a hypertonic concentration compared to general body fluids (urine has a lower water potential) 

Aquatic animals-produce ammonia that is released straight into the water. It diffuses across the gills and dissolves in the water. 

Birds and insects-produce uric acid. Light and non toxic and almost insoluble in water. Takes alot of energy to produce but little water needed for excretion; important for conserving water. Allows organsims to live in dry evironments 

Mammals-produce and excrete urea. Less toxic than ammonia so tissues can tolerate higher concentrations for short periods of time. Length of the Loop of Henle differs depending on environment. Mammals that live in water rich environments have a long loop of henle. Mammals that live in very dry, desert environments have a short loop of henle. Mammals that live in temperate environments have a medium length loop of henle.

Desert animals-survive with little or no water. They live on metabolic water-produced during reactions like respiration and oxidation of food reserves.