The Excretory System

The excretory system is the name given to the skin, lungs and kidneys which release metabolic wastes to the outside of the body.

The kidneys, skin and the lungs are the principle organs of excretion in the human body. These organs remove the harmful wastes of cellular respiration - CO2 and metabolism of amino acids by the liver - Urea.

The skin moves wastes out of the body by dissolving them in the sweat it pours out on to our skin. It is the wastes within the sweat that will make you smell.

The kidneys remove wastes and excess salts by filtering the blood The kidneys remove wastes and excess salts by filtering the blood. They produce a watery substance called urine. Within the urine can be found: urea, salts, blood cells, hormones, minerals. Kidneys control the amount of water leaving the body, in doing so they regulate your water homeostasis.

Kidney Structure (Figs. 39-8 & 39-9)

The kidney contains millions of nephrons The kidney contains millions of nephrons. These structures that remove waste while resorbing useful molecules.

This fluid with wastes travels into the Bowman's capsule and then into the proximal tubule. Blood travels from renal artery, to capillaries, and then to the glomerulus for filtration. The pressure forces the fluid portion of the blood out. Blood vessels are in close contact with the tubules and this allows for the recovery of valuable water (osmosis) and food molecules (active transport). The fluid decends to the loop of Henle and turns back to the cortex with the distal tubule. The distal tubule drains into the collecting duct and this sends urine to the renal pelvis.

Water, ions and small molecules filter from the blood into the Bowman's capsule. 180 l filtered/day, with only 1 litre excreted as urine (resorbtion). Cells in the tubules and collecting ducts pump ions to the blood capillaries by active transport, water follows as a result of osmosis. This ensures 99% of H2O and salts are recovered and that wastes like urea remain in the tubules for excretion.

Urine travels from the renal pelvis in the kidney out to the bladder via the ureters. Urine is released through the urethra.

Homeostasis of Body Fluids The kidneys enable the tissue fluid of the body to remain at a constant concentration. We lose 1 L from lungs/day, 100mL via feces, 1 L via kidneys, up to 10 000 mL through the skin. This water loss can cause serious problems for our body. Our kidneys must regain this H2O but too much water is not good either.

What our kidneys control by increasing the permeability of the collecting duct to water (retain water). if you drink too much, the distil tubule will become impermeable to water (release H2O) Pumping of ions can have an influence on the concentration of salt in the urine, which in turn is important for blood buffers and blood pH.

A.P. Biology (pg 891) Tubule function in relation to specific ions and other molecules