Excretion
Kidneys clean the blood by removing waste products, including urea, excess salts and excess water — theses leave the body in the urine.
Excretion
The term ‘excretion’ refers to the removal of toxic substances or the waste products generated from respiration. Remember not to confuse ‘excretion’ with ‘egestion’ which is the removal of undigested food from the body as faeces.
Animals produce the following forms of excretory waste:
Urine - produced by the kidneys and stored in the bladder
Sweat - excreted from the body through the skin
Carbon dioxide - exhaled from the lungs
Plants generate excretory waste in the form of carbon dioxide and water vapour from respiration. Both of these diffuse out of the leaf through the stomata.
Kidney structure
The inner part of the kidney is called the medulla and the outer part is the cortex. Blood is carried to the kidney via the renal artery for the kidney to filter the blood and remove waste products. The filtered blood is taken away from the kidneys by the renal vein. The individual structures which filter the blood are called nephrons.
The waste products include urea, excess salts and water which come together to form urine. Urine is transported along a tube called the ureter to the bladder where it is stored. Urine is expelled from the body through another tube called the urethra when we urinate.
Nephron structure
Ultrafiltration: the Bowman’s capsule surrounds a ball of capillaries called the glomerulus where blood is placed under high pressure. The blood vessels leading up to the glomerulus become smaller and smaller, which has the effect of pushing molecules right up against the capillary walls. Small molecules (glucose, urea, water and salts) pass out of the bloodstream and into the kidney nephron, while larger molecules such as blood cells and protein stay inside the capillaries.
Selective reabsorption: in the proximal convoluted tubule, glucose is reabsorbed into the bloodstream by the energy-requiring process active transport. It is important that the blood can claim back all of the glucose to be used for respiration. Further along the nephron, in the collecting duct, as much water as needed is reabsorbed back into the bloodstream. If we happen to be dehydrated, more water will move from the nephron into the bloodstream. This leaves the waste products (urea, excess salts and water) in the nephron tubule which travel as urine through the ureter to be stored in the bladder.
Anti-diuretic hormone (ADH)
ADH controls the water content of our urine by increasing the reabsoprtion of water from the collecting duct. It works by increasing the permeability of the collecting duct wall, making it more porous and allowing more water to pass from the kidney into the bloodstream. It is released by the pituitary gland in the brain when water levels in our blood plasma are becoming low. This type of control is an example of a negative feedback mechanism.
Kidney failure
Kidney failure can be caused by diabetes as large amounts of glucose are are forced into and out of bloodstream, damaging the blood vessels within the kidney. Kidney failure can be treated with dialysis, which involves connecting the patient’s blood supply to a dialysis machine. Their blood is passed through a partially permeable membrane surrounded by dialysis fluid. The urea diffuses out of the blood and into the dialysis fluid, whereas glucose, water and salts move between the blood and dialysis fluid to reach normal concentrations. Dialysis involves multiple hospital visits where the patient is connected to the machine for long periods of time. In addition, the treatment is expensive and the patient still needs to maintain a carefully controlled diet.
Another form of treatment is a kidney transplant, however this requires major surgery and the associated risks. The new kidney may be rejected by the patient’s body so they are given immunosuppressant drugs which suppress the immune system and make the individual susceptible to other diseases.
