Excretion

Excretion

The liver and excretion
1. Excretion is the removal of the waste products of metabolism from the body
2. The liver is involved in excretion
  1. Hepatic artery and the hepatic portal vein enter the liver, a bile duct exits the liver. Hepatic (central) vein takes blood away from the liver.
  2. Liver is made of liver lobules. Sinusoids connect the hepatic artery and the hepatic portal vein.
3. Excess amino acids
  1. The amino groups are broken down into ammonia and keto acid - (deamination)
    1. Keto acid can be respired to give ATP or converted to carbohydrates and stored as glucose
    2. Ammonia is toxic to excrete directly. It is combined with CO2 in the ornithine cycle to create urea
4. The liver breaks down toxic substances through detoxification.
  1. Alcohol -> ethanal -> acetic acid
  2. NAD is needed to break down ethanol but is also used to break down fatty acids.
The kidneys and excretion
1. Blood enter through renal artery and passes through capillaries in the cortex. Ultrafiltration occurs. Selective reabsorbtion occurs in the medulla.
2. Blood from the renal artery carries blood into the afferent arteriole into the glomerulus in the Bowman's capsule.
  1. Ultrafiltration happens. Blood taken away is the efferent arteriole. High pressure forces liquid and small molecules in the blood out of the capillaries into the renal capsule.
    1. Enters the renal tubes after it goes through the capillary wall, basement membrane and epithelium of the renal capsule.
3. Selective reabsorption happens in the Proximal Convoluted Tubule.
  1. The epithelium of the PCT is filled with microvilli to increase surface area. It has many mitochondria to help with active transport of glucose. It has the sodium ion glucose cotransporter proteins to allow glucose to be reabsorbed.
Controlling water content
1. Water content is too low then more water is reabsorbed. Water content is high less water is reabsorbed.
2. The Loop of Henle is a countercurrent multiplier
  1. At the top of the ascending limb, Na+ and Cl- ions are actively pumped out into the medulla. The limb is impermeable to water so it stays in the tubule. The medulla has a low water potential because of the ions pumped into it.
    1. As there is a lower water potential in the medulla, water in the descending limb diffuses via osmosis down the CG.
  2. The longer the loop of Henle is more water can be reabsorbed. More ions pumped out and a more concentration urine.
3. Hydrated
  1. Water content in blood rises so does water potential. Detected by the osmoreceptors in the hypothalamus. The posterior pituitary gland releases less ADH. The CDT and the collecting duct have fewer aquaporins resulting in a large dilute urine.
4. Dehydrated
  1. Water content in blood drops and so does the water potential. Detected by the osmoreceptors in the hypothalamus. The posterior pituitary gland releases more ADH so more aquaporins are in the collecting duct meaning a small concentrated urine.

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