Excretion

• Excretion is the process by which the body removes metabolic waste products and toxic materials.

• Metabolic processes consist of anabolic processes and catabolic processes.

• Anabolic processes are ‘building-up’ processes where larger molecules are synthesised from smaller molecules. Examples include: (a) Synthesis of proteins from amino acids (b) Synthesis of glycogen from glucose (c) Photosynthesis with oxygen as waste material

• Catabolic processes are ‘breaking-down’ processes where larger molecules are broken down to form smaller molecules. Examples include: (a) Cellular respiration with carbon dioxide and water as by-products (b) Deamination of amino acids in the liver with urea as a by-product (c) Breakdown of haemoglobin in the liver with bile pigments as by-products

• Waste products have to be removed because they can be harmful if they accumulate in the body.

• The waste products of metabolism are excreted by the following organs:

Human Urinary System:

• The human urinary system consists of : (a) The kidneys, which are two bean-shaped organs located in the abdominal cavity. (b) The ureters, which are narrow tubes that emerge from a depression in the concave surface of the kidney called a hilum. The ureters connect to the urinary bladder. (c) The urinary bladder is an elastic and muscular organ that collects and stores urine excreted by the kidneys. The sphincter muscle at the base of the bladder controls the flow of urine into the urethra. It is controlled by nervous impulses from the brain. (d) The urethra is a duct that connects the urinary bladder to the outside of the body. Urine passes through this tube to the outside.

Structure of Kidney:

Urine formation

• Excess mineral salts, nitrogenous wastes and excess water are excreted through the kidneys through ultrafiltration and selective reabsorption of useful materials.

• Ultrafiltration occurs in the glomerulus. Blood enters the glomerulus through an afferent arteriole from the renal artery. Blood pressure forces water, urea, salts and other small solutes (e.g. glucose, amino acids and vitamins) into the lumen of the Bowman’s capsule. Blood cells and large molecules remain in the capillaries.

• The high blood pressure (high hydrostatic pressure) driving the ultrafiltration in the glomerulus is due to the afferent arteriole having a larger diameter than the efferent arteriole.

• The endothelium of the glomerular capillaries and the basement membrane of the Bowman’s capsule that wraps around the capillaries are partially permeable membranes, thus only small soluble substances are able to pass through.

• The glomerular filtrate passes from the lumen of the Bowman’s capsule into the proximal convoluted tubule.

• Within this tubule, most of the mineral salts and all of the glucose and amino acids are absorbed through active transport or diffusion. Water is reabsorbed by osmosis.

• Reabsorption of water continues in the loop of Henlé.

• Water and salts are reabsorbed in the distal convoluted tubule.

• Water is reabsorbed from the collecting duct.

• Excess salts, nitrogenous waste products, excess water and processed drugs and poisons from the liver enter the renal pelvis as urine.

Kidneys as osmoregulators

• Osmoregulation is the control of water and mineral salts in the blood.

• The water potential of blood has to be maintained for proper functioning of the body.

• Excessive gain in water due to drinking or excessive loss due to diarrhoea or sweating will result in a change in the water potential of blood.

• Excess water could also cause water to move into cells from tissue fluid by osmosis. This causes the cells to swell and burst.

• Too little water would cause water to move out of the cells into tissue fluid causing dehydration.

• Excess water could also lead to an increase in blood pressure due to an increase in volume. This could lead to stroke.

• The amount of water in blood is controlled by a hormone called antidiuretic hormone (ADH).

• ADH is produced in the hypothalamus of the brain and is stored and released from the pituitary gland.

• The hypothalamus contains osmoreceptor cells that can monitor the water potential of blood.

• When blood water potential decreases beyond a certain amount, the pituitary gland is stimulated to secrete more ADH into the blood.

• ADH works on the distal convoluted tubules and the collecting ducts in the kidneys.

• It makes the epithelium more permeable to water.

• This causes more water to be reabsorbed, producing a smaller volume of more concentrated urine.

• The water potential of blood then returns to regular levels.

• When the water potential of blood increases beyond normal levels, the osmoreceptor cells in the hypothalamus stimulate the pituitary gland to release less ADH.

• The epithelium of the kidney tubules and collecting ducts become less permeable to water.

• Less water is reabsorbed resulting in a larger volume of dilute urine.

• The water potential of blood returns to normal levels.

Dialysis (the Kidney Machine) :

• The kidneys function to remove waste products, excess water and excess mineral salts.

• A dialysis machine would have to perform the functions of a kidney.

• In dialysis, blood is passed over a dialysis membrane of a large surface area which is permeable to small molecules but does not allow proteins to pass through.

• On the other side of the dialysis membrane is the dialysis fluid, which contains the same concentration of essential substances as the blood plasma, with the exception of metabolic wastes.

• Substances move from the blood to the dialysis fluid and vice versa through diffusion down a concentration gradient.

• As blood flows through the tubules immersed in dialysis fluid, metabolic waste diffuses out of the tubing into the fluid.

• Fresh dialysis fluid is continually supplied during dialysis in order to maintain a low concentration of urea in the fluid as compared to that in blood plasma.

• The direction of blood flow is opposite to the direction of flow of the dialysis fluid in order to increase the length of exchange surface with the necessary concentration gradients. This is known as countercurrent flow.