excretion in humans

• • metabolic reactions produce waste products, eg. CO2 from respiration and ammonia from deamination of excess amino acids
  • some of these waste products are toxic and can lead to damage of cells if not removed
  • metabolic reactions can be catabolic or anabolic
  • catabolic reactions break complex substances into simple ones
  • anabolic reactions build up complex substances from simple ones
• • excretion is the removal of the toxic substances, metabolic waste materials, and substances in excess of the bodies requirement.
• humans have 3 main excretory substances:

  1) carbon dioxide is produced by all cells during respiration and excreted by the lungs

  2) bile pigments are produced by the breakdown of haemoglobin in the liver and excreted in the faeces

  3) urea is produced by deamination of excess amino acids in the liver and excreted by the kidneys in urine or by sweat glands in skin 
• removal of faeces from the body is not a metabolic process since faeces consists mainly of fibres that cannot be digested and bacteria, thus it is not considered a metabolic waste product.
• conversion of excess proteins to urea for excretion

  • protein consumed is decomposed to amino acids by digestive enzymes in the digestive system
  • absorption of amino acids takes place in the blood capillaries of the villi
  • blood capillaries linked to the hepatic portal vein carry absorbed amino acids to the liver
  • in the presence of oxygen, excess amino acids are chemically converted to ammonia
  • ammonia is toxic, so it is converted to urea for excretion and acid for converting into carbohydrate or fats for storage
  • urea dissolves in blood plasma and is transported to the kidneys for excretion
• • other nitrogenous waste substances are uric acids produced by nucleotides and creatinine produced from creatinine phosphate in the muscle
• functions of the liver (pt1)

  • converts excess amino acids into urea and carbohydrates in a process called deamination 
  • regulates blood glucose concentration with the aid of hormones insulin and glucagon
  • breaks down hormones and harmful substances such as alcohol and toxins produced by intestinal bacteria
  • stores carbohydrates as the polysaccharide glycogen
  • makes bile to be stored in gall bladder for emulsifying consumed fats
• functions of the liver (pt2)

  • breaks down old red blood cells, stores iron, excretes the remainder as bile
  • stores fat soluble vitamins A and D
  • stores excess blood
  • synthesis of cholesterol needed to make and repair cell membranes
  • synthesis of proteins in plasma needed for blood clotting
  • produces heat during metabolic processes to maintain body temperature 
• excretion system and kidney regulate fluid balance (osmoregulation) and remove metabolic wastes from the body (excretion).

  osmoregulation in the body:

  • adequately distributes nutrients that are osmotically active
  • remove wastes, especially nitrogenous wastes/ammonia
  • maintain osmotic balance of cells and tissues with both internal environment and external environment
• • kidney

  > filters blood and removes excess salts, water, and nitrogenous waste as urine

  • ureter

  > transports urine from the kidney to be stored at the bladder via peristalsis using its muscular walls

  • urinary bladder

  > muscular elastic organ that stores urine

  > when organ wall is stretched, nervous system is signalled which results in urination

  • sphincter muscle

  > circular muscle that automatically controls the excretion of urine, when the muscle is relaxed urination happens

  • urethra

  > tube that carries the urine from the bladder

  > in males serves as the passage for sperm
• kidneys

  • sophisticated filter used to remove unwanted substances from blood to be temporarily stored in the bladder
  • comprised of the cortex and medulla
  • cortex is the outer dark red region covered and protected by fibrous capsule
  • medulla is the inner pale red region which contains renal pyramids 
  • within renal pyramids are numerous nephrons. nephrons are tiny kidney tubules in which urine is formed
• what are the 2 regions of the kidney?

  outer dark red cortex and inner pale red medulla
• humans have 3 main excretory substances:

  1) carbon dioxide is produced by all cells during respiration and excreted by the lungs

  2) bile pigments are produced by the breakdown of haemoglobin in the liver and excreted in the faeces

  3) urea is produced by deamination of excess amino acids in the liver and excreted by the kidneys in urine or by sweat glands in skin