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Excretion is the removal of metabolic waste from the body.

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Importance of removing metabolic waste

Carbon dioxide must be removed as it causes respiratory acidosis; breathing difficulties, headaches, drowsiness, restlessness etc caused by carbon dioxide dissolving in the blood plasma and combining with water to produce carbonic acid, which dissociates to release hydrogen ions. This lowers the pH.

Nitrogenous waste must be removed because the amino group is highly toxic, but proteins and amino acids are very high in energy, so it would be wasteful to excrete them. In the ornithine cycle, the amine group is removed to form ammonia, which forms urea, water and a keto acid when added to oxygen and carbon dioxide. The keto acid can be used in respiration and the urea (which is less toxic) is transported to the kidneys for excretion. 

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Histology of the liver

The hepatic arteries supply the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy.
The hepatic vein takes deoxygenated blood away from the liver - which rejoins the vena cava and normal circulation will proceed.
Bile duct is where the substance bile is secreted, which is carried to the gall bladder where it is stored until it is required in the small intestines.
The hepatic portal vein brings blood from the small intestine, the blood is rich in the products of digetsion, and this means that any harmful substances ingested will be broken down quickly by the liver cells (hepatocytes).

The liver is made up of lobules, which consists of cells called hepatocytes that are arranged in rows.
Each lobule has a central vein in the middle that connects to the hepatic vein. Each lobule has branches of the hepatic artery, hepatic portal vein and bile duct.
Hepatic artery and hepatic vein are connected to the central vein by blood filled spaces called sinusoids.
The blood flows past every hepatocyte via the sinusoid, this ensures that the toxins are broken down quickly. Also the blood provides the liver cells with oxygen. The central vein from all the lobules join up to form the hepatic vein.

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Formation of urea

Amino acid + oxygen --> keto acid + ammonia

Ornithine cycle:
Ammonia + carbon dioxide --> urea + water

Ammonia + carbon dioxide + ornithine --> citrulline + water

Citrulline + ammonia --> water + argenine

Argenine + water --> urea + ornithine

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Role of the liver in detoxification

Catalase can convert 5 million molecules of hydrogen peroxide into harmless substances in a minute. Alcohol contains a lot of chemical potential energy which can be used in respiration. 
Ethanol dehydrogenase catalyses the detoxification of alcohol in hepatocytes.

Ethanol --> Ethanal --> Ethanoic acid --> Acetyl CoA

Ethanal and ethanoic acid are dehydrogenated, and the hydrogen reduces NAD. If too many NADs are busy detoxifying alcohol, there will be too few NAD to break down fatty acids for use in respiration, so the fatty acids are converted back to lipids, which are stored in hepatocytes, making the liver enlarged - fatty liver.

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Histology of the kidney

Supplied with blood from the renal artery and is drained by the renal vein. The kidney is surrounded by a touch capsule, the outer region is the cortex and the inner is the medulla. 
The central region is the pelvis, which leads into the ureter.

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Structure of a nephron

The nephron starts in the cortex, where the capillaries form a knot called the glomerulus, surrounded by the Bowman's capsule. Fluid from the blood is pushed into the capsule by ultrafiltration. The fluid leaves the capsule and flows through the nephron, starting with the proximal convoluted tubule where the composition of the fluid is altered by selective reabsorption, and then into the medulla for the loop of Henle, which is a hairpin countercurrent multiplier. Substances are reabsorbed back into the tissue fluid and capillaries surrounding the nephron tubule. The fluid then passes into the distal convulted tubule, and then into the collecting duct as urine. 

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Production of urine

Blood flows into the glomerulus via the afferent arteriole which has a larger lumen than that of the efferent arteriole, which the blood leaves through, creating high pressure in the glomerular cavity.
Blood enters the glomerulus and must pass through 3 distinct layers in order to enter the Bowman's capsule: 

  • Endothelium of capillaries - contains pores (fenestrations) from which blood passes through as well as the substances dissolved in it
  • Basement membrane - fine mesh of collagen fibres and glycoproteins that do not allow molecules with an RMM larger than 69,000 to pass through
  • Epithelium of Bowman's capsule is formed from podocytes, specialised cells which contain finger like projections that fluid from the glomerulus can pass through into the Bowman's capsule

Selective reabsorption: (Proximal convoluted tubule)
Selective reabsorption of glucose and amino acids, occurs by co-transport, water follows by osmosis, so concentration of ions, nitrogenous waste, urea, remaining substances, increases in the filtrate.

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Structure of cells in proximal convoluted tubule

Microvilli - increase the surface area for reabsorption
Co-transporter proteins - contained in the cell surface membrane that is in contact with the tubule fluid. Transports glucose or amino acids.
Na/K pumps - contained in the cell surface membrane opposite to the fluid tubule. Actively transports Na+ and K+ ions against their concentration gradient.
Many mitochondria - provides energy needed to drive the selective reabsorption process, produce a lot of ATP.

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Control of water content of the blood.

In the loop of Henle, salts are transferred from the ascending limb to the descending limb. This means that the tissue fluid in the medulla has a very negative water potential, as so water is lost by osmosis, particularly in the collecting duct.

1. The water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain.
2. When the water potential is very low, they shrink, and stimulate neurosecretory cells in the hypothalamus.
3. These produce and release anti diuretic hormone which flows down the axon to the posterior pituitary gland where it is stored until needed.
4. When the neurosecretory cells are stimulated they send action potentials down their axons and cause the release of ADH.
5. It enters the capillaries running through the posterior pituitary gland. It is transported around the body and acts of the cells of the collecting ducts.
6. When it binds to the receptors, it causes a chain of enzyme catalysed reactions, the end result of which is the insertion of vesicles containing water-permeable channels (aquaporins) in the membranes of the cells, so they are more permeable to water.
7. More water is reabsorbed, by osmosis, into the blood, less urine, with a lower water potential, is excreted. Less ADH is released when the water potenial of the blood rises again.
8. The ADH is slowly broken down and the collecting ducts recieve less stimulus. 

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Kidney failure

Problems: Unable to remove excess water and waste products from the body, e.g. urea and excess salts. Inability to regulate urea and salt levels, death.

Dialysis: Waste, excess fluids and salts are removed from the body by passing the blood over a dialysis membrane. This allows the exchange os substances between the blood and the dialysis fluid, which has the same concentration of substances as blood plasma. Substances diffuse from both sides to create the correct concentration of substances.
Haemodialysis: Blood taken from an artery or vein and is passed through a machine that contains an artificial dialysis membrane before returning via a vein. Heparin (anticoagulant) is used to avoid clotting of blood in the machine. Thrice weekly trips to hospital lasint several hours.
Peritoneal dialysis: The body's own abdominal membrane is used as a filter.

Transplant - Advantages: No dialysis, less limited diet, better physical feeling, better quality of life, no longer 'chronically ill'.
Disavantages - Need immunosuppressants for life of kidney, major surgery, risk of infection, need frequent checks in case of rejection, side effects of medication.

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Testing with urine samples

A human embryo secretes human chorionic gonadotrophin (hCG) as soon as it is implanted on the uterine lining. The hormone is small so can pass from blood into filtrate and can be detected in the mother's urine after as few as 6 days. Pregnancy tests contain monoclonal antibodies which are tagged with a blue bead and bind only to hCG. The hCG-antibody complex moves along the strip until it binds to a band of immobilised antibodies specific to the antibody/hCG complex. The blue beads line up and form a blue line. The unbound antibody specific to hCG binds to another band of antibodies specific to it. One blue line shows the test is working so two lines means pregnancy.
Anabolic steroids:
Urine samples are tested using gas chromatography - the sample is vaporised in the presence of a gaseous solvent. It is passed down a long tube lined with an absorbing agent. Each substance dissolves differently in the gas and stays there for a unique, specific time - the retention time. Eventually, the substances leaves the gas and is absorbed by the lining. It is then analysed to make a chromatogram. Standard samples of drugs and urine samples are run so durgs can be identified and quanified in the chromatogram. Testing is done because of fairness, health risks, down not reflect the athletes natural talent, 'outstanding' performances may be distrusted, there is pressure to keep up with rival competitor, those using drugs can train for longer, recover from injury quicker, build up muscle mass.

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