Excretion

The importance of excretion

Carbon Dioxide and urea are the two excretory products produced in the largest quantities.
Carbon Dioxide:
- Produced continuously by almost every cell in the body through aerobic respiration.
- The waste carbon dioxide is transported from the respiring cells to the lungs in the bloodstream. It then diffuses from the blood into the alveoli of the lungs and is excreted in the air that we breathe out.
Urea:
- Formed from excess amino acids, in the liver.
- It is transported from the liver to the kidneys, in solution of blood plasma. The kidneys remove urea from the blood and excrete it, dissolved in water as urine.

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The liver is made up of many lobules.
Between the lobules are branches of the hepatic artery and branches of the hepatic portal vein. Blood flows from here, through the lobules, and into the branch of the hepatic vein.
Lobules:
- In the centre of each lobule is a branch of the hepatic vein.
- Each lobule is made up of many liver cells arranged in rows that radiate out from the centre of the lobule.
- The channels that carry blood between these rows of cells are called sinusoids.
- Other channels cary bile, which is produced by some of the hepatocytes, these chanels are called bile canaliculi.
- The bile flows from the centre of the lobule towards the outside, in the opposite direction of the blood, where it enters a branch of the bile duct.
- The rows of hepatocytes are never more than two cells thick, so that each individual cell is in close contact with the blood in the sinusoids.
Sinusoids:
- They are lined with large, phagocytic macrophages.
- These macrophages capture and destroy bacteria that have entered the liver in the hepatic portal vein, in blood that has come from the small intestine; these are called Kupffer cells.
- The Kupffer cells are very efficient and a bacterium will be taken inside the cell by phagocytosis in 0.01 seconds

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Deamination is the removal of the amino group from an amino acid, forming ammonia.
The rest of the amino acid forms a keto acid, which can then either be respired to release energy or converted to fat to be stored.
In the liver, the ammonia is combined with carbon dioxide to form urea.
Although urea is still toxic, it is much less soluble and much less dangerous than ammonia.
The liver releases urea into the blood where it dissolves in the plasma and is transported all over the body, once it reaches the kidney it is removed from the blood.

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Alcoholic drinks contain ethanol, which is very toxic, the liver helps to prevent problems caused by this and so breaks it down.
Ethanol dehydrogenase catalyses the breakdown of ethanol to ethanal.
Aldehyde dehydrogenase then catalyses the breakdown of ethanal to ethanoate, which then can enter the Krebs cycle in mitochondria and be metabolised to form ATP.

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If large quantities of alcohol are consumed on a regular basis, then the tissues within the liver can be damaged.
The breakdown of ethanol produces reduced NAD, which means that the NAD is not available to oxidise the fatty acids in the liver cells.
The fatty acids accumulate and are converted to fats, which are deposited in the liver.

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The liver breaks down many hormones that are produced within the body.
The liver breaks down thyroxin, oestrogen and testosterone.
Liver damage can therefore lead to the accumulation of hormones in the body, disrupting processes that are affected by these hormones.

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Ureters are long, white tubes that run from the kidney to the bladder, carrying urine.
Each kidney is suppied with blood through a renal artery, which branches off from the aorta.
A renal vein returns the blood to the vena cava.
The kidney is made up of the cortex (a deep red tissue), the medulla (a slightly lighter tissue) and the pelvis (a white tissue), which lies on the centre of one edge of the kidney.

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Each nephron begins with a Bowman's capsule. This is a cup-shaped structure that is located in the cortex of the kidney.
A tube runs from the Bowman's capsule towards the centre of the kidney, first forming a twisted section called the proximal convoluted tubule, and then a long loop in the medulla called the loop of Henle.
The tubule then turns back up to the cortex and forms another twisted section called the distal convoluted tubule.
It then joins the collecting duct, where it travels down through the medulla and into the pelvis of the kidney where is joins the ureter.
Each Bowman's capsule is supplied with blood by a branch of the renal artery called an afferent arteriole.
This arteriole then splits into a net of capillaries in the 'cup' of the Bowman's capsule, called the glomerulus.
The capilleries in the glomerulus then rejoin to form an efferent arteriole.
This arteriole then goes on to form a network of capillaries that run alongside the nephron, and then link with other capillaries to enter a branch of the renal vein.

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