Excretion

• Excretion- the removal of waste products of metabolism and substances over requirements.

  Urea- a waste product produced in the liver, from the breakdown of excess amino acids.

  Urine- a solution of urea and other waste materials in water, produced by the kidneys.

  Ureter- one of a pair or tubes that carries urine from the kidneys to the bladder.

  Urethra- the tube that carries urine from the bladder to the outside.

  Deamination- the removal of the nitrogen-containing part of amino acids to form urea.
• The kidneys remove urea and excess water and ions from the blood and produce urine which flows down the ureters and into the bladder. When the sphincter muscle at the entrance to the urethra relaxes, the urine flows out of the body through the urethra.
• Production of urea-

  protein in food is taken into the alimentary canal.

  protein is broken down into amino acids during digestion.

  amino acids absorbed into blood and taken to liver by the hepatic portal vein.

  useful amino acids are made into proteins in liver or released into blood to be assimilated by body cells.

  not useful amino acids are deaminated to carbs and nitrogen-containing compound.

  carbs are stored in liver and nitrogen-containing compound is converted into urea.

  urea is excreted and useful amino acids are taken away by the blood.
• The kidney is made of two parts, the cortex and the medulla.

  Cortex- the tissue making up the outer layer of the kidney.

  Medulla- the tissue making up the inner layers of the kidney.
• Blood flows into the kidney from the renal artery which divides to form many tiny, coiled capillaries, called glomeruli (singular-glomerulus). As blood flows through the glomerulus, it's filtered. Small molecules pass through the filter, but large molecules and blood cells can't. The filtrate includes water, urea, glucose, and ions. These move into the nephrons.
• Each nephron begins in the cortex, loops down into the medulla, back to the cortex, and goes down through the medulla again. The nephrons join up with the ureter.

  Nephron- one of the thousands of microscopic tubes inside a kidney, where urine is made.

  Glomeruli- a little 'network' of blood capillaries, where the blood if filtered in a kidney.
• Filtration- small molecules, such as water, glucose, ions, and urea, are filtered out of the blood into the nephron.

  Reabsorption- any useful substances, such as water and glucose, are taken back into the blood.
• The urea is a toxic waste product and some of the water and ions may be in excess requirements. These substances continue through the nephron. Some substances are not waste. All of the glucose, most of the water, and some of the ions are taken back into the blood. The capillaries come close to the nephrons to make it easy for these substances to move back into the blood.
• Volume of urine produced depends on how much excess water is present in the person's body. If a person has a lot of excess liquid, they will excrete large volumes of dilute urine. If they haven't drank much, or have lost lots of water by sweating, they will produce small volumes of concentrated urine. Dilute urine is lighter and paler in color while concentrated urine is darker in color. Dark urine indicates a person hasn't drank enough fluids.