Excretion

Ammonia is small, soluble and highly toxic, cannot be stored. Large volumes of water needed to dilute it to safe levels before exretion. Freshwater fish have body fluids with lower water potential than surroundings, so absorb loads of water by osmos through their highly permeable gills, so they can safely exrete ammonia. Some exreted from kidneys, most diffuses from gills as very soluble.

Urea is less toxic so can be stored in the tissues and needs less water to dilute it to safe levels, however is energetically expensive to produce. Adaptation to life on land, and helps prevent dehydration.

Uric acid has very low toxicity and can be stored for long periods of time with very little water, so is removed from the body as white paste. Can accumulate inside bird and reptile eggs without embryo damage. Energetically expensive to produce but allows birds, reptiles and insects to survive in arid environments, and reduces their mass, which is an advantage for flight.

Deamination of amino acids occurs in the liver. Ammonia produced quickly converted to urea in the ornithine cycle. Urea diffuses into blood where it is removed by the kidneys.

The kidneys are located in the abdominal cavity. recieve oxygenated blood from the renal artery. Urine produced in kidney travels via ureter to the bladder. 2 regions: inner medulla, outer cortex

Ultrafiltration is the removal of water and small solutes from the blood and the formation of glomerular filtrate. Occurs between the glomerular capillaries and the Bowmans capulse.

Capillaries are highly permeable as made of a single layer of endothelial cells with fenestrations between them (pores).

Basement membrane surrounding the capillary acts as a molecular filter, as it forces water and small solutes out of the blood into the capsular space, but preventing blood cells and large solutes leaving the capillaries. 

Inner layer of Bowmans capsule made of podocyte cells which have foot like processes that wrap around the capillaries.

Hydrostatic pressue of blood in the glomerulus is very high due to contraction of the left ventricle (renal artery branches from aorta) and the efferent arteriole being narrower than the afferent arteriole, creating a bottleneck effect. This hydrostatic pressure forces fluid (water, small solutes) through the endothelium and basement membrane, down through the spaces between podocytes into the lumen of the Bowman's capsule - ultrafiltrate.

Longest region of the nephron. Made of a single layer of epithelial cells adapted for selective reabsorption of solutes: they possess microvilli to provide larger surface area for reabsorption by carrier proteins, and have many mitochondria for ATP for active transport of solutes.

Renal capillaries are close to the tubule, so short diffusion pathway for reabsorption. All of the glucose and amino acids are reabsorbed, and about 85% of the sodium ions by active transport. About half of the urea passes back into the blood by facilitated diffusion. 

All of this reabsorption, along with the presence of plasma proteins in the capillaries, lowers the water potential of the blood in the capillaries of that about 85% of the water in the filtrate is reabsorbed by osmosis.

The plasma membrane near to the capillaries is highly folded to form lots of intercellular spaces. When solutes, e.g. Na+ are actively transported out of the epithelial cells into the intercellular spaces, it increases the ion concentration in the intercellular spaces so that they can move into the capillaries by faciliated diffusion. They are then carried away in the plasma to the renal vein.

As the concentration of solutes inside the epithelial cells is lower than in the lumen, the solutes move out of the filtrate by facilitated diffusion. So: into epithelial cells by facilitated diffusion, into intercellular spaces by active transport, into capillaries by facilitated diffusion.

The descending limb is highly permeable to water but impermeable to ions.

The ascending limb is highly permeable to ions but impermeable to water.

Capillaries surrounding the loop are permeable to both water and ions.

Function of ascending limb: to create a water potential gradient between the filtrate and medulla tissue fluid, by active transport of Na+ and Cl- ions from filtrate into medulla. This causes water to move out of the filtrate from the descending limb by osmosis (cant happen in ascending limb as impermable to water). The water is reabsorbed into the capillaries and removed by the blood so it doesnt dilute the medulla tissue fluid.

As filtrate moves down the descending limb, it loses water, so has a high ion concentration. Apex of loop has highest ion concentration. Moves up the ascending limb, filtrate loses ions, lowering the ion concentration.

Countercurrent multiplier system: countercurrent = flitrate flows in opposite directions in loop, multiplier= generation of the water potential gradient in the medulla caused by active transport of ions in the ascending limb, but is multiplied by the constant flow of filtrate though the loop/

As the filtrate enters the DCT it has a higher water potential the medulla tissues.

As it enters the collecting duct, the medulla tissues have a lower water potential due to the pumping of solutes from the ascending limb. Therefore, water moves out of the collecting duct by osmosis and is reabsorbed into the blood, producing concentrated urine.

ADH affects the permeability of the DCT and collecting duct walls: this allows mammals to control the volume of water reabsorbed, maintaining the water potential and solute composition of the blood at a relative constant - osmoregulation. Balancing water uptake from diet with water loss from sweating, evaporation from lungs, urine and faeces.