Excretion

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  • Created by: imanilara
  • Created on: 19-01-16 18:32

The structure of the liver

The liver cells (hepatocytes) carry out many hundreds of metabolic processes + the liver has an important role in homeostasis. Must have a good supply of blood. The internal structure of the liver maximises the blood supply to the liver cells. 

Blood flow to+from the liver

From two sources:

  • Oxygenated blood from the heart; from aorta via the hepatic artery into the liver. This supplies the oxygen that is essential for aerobic respiration. The liver cells are very active as they carry out many metabolic processes. The processes require ATP, so it is important there is a good supply of oxygen. 
  • Deoxygenated blood from the digestive system-enters liver via the hepatic portal vein. This blood is rich in the products of digestion. The concentrations of various compounds will be uncotrolled and the blood may contain toxic compounds that have been absorbed in the intestine.
  • Blood leaves the liver via the hepatic vein and rejoins the vena cava.
  • A fourth vessel connected to the liver is not a blood vessel. It is the bile duct. Bile is a secretion from the liver 
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Excretion

Excretion: the removal of metabolic waste from the body- metabolic waste - potentially harmful and toxic substances made from the reactions inside the body. 

Substances to be removed: CO2 and urea - CO2 removed via bloodstream+ breathed out by lungs, urea removed by deamination. 

CO2 must be removed bc 3 reasons:- 

  • majority of Co2 formed into hydrogencarbonate ions which form H+ ions thru the carbonic anhydrase enzyme- H+ combine w haemoglobin and compete for space w the oxygen. It can reduce O2 transport
  • combine straight w haemoglibin to form carbaminohaemoglobin
  • can cause respiratory acidosis:- CO2 dissolves into blood plasma, forming carbonic acid that dissociates to release H+, this lowers pH causing asthma/pneumonia- resp. acidosis. 
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Excretion

Nitrogenous compounds:-

-amino acids contain the same amount of energy as carbs and so wasteful to remove them-the toxic amino group must be removed (deamination).

-first forms toxic ammonia which is converted to less soluble and less toxic urea which is transported to kidneys for excretion

-keto acid used in resp or converted to a fat for storage

Deamination: amino acid+O2=keto acid+ammonia

Urea formation= ammonia+CO2=urea+water

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The arrangement of cells in the liver

hep artery and hep portal vein enter the liver and split into interlobular vessels- the blood from the two vessels mixed and enter sinusoid chamber. Sinusoid lined w liver cells and empty into interlobular vessel, branch of hepatic vein. 

When blood enters sinusoid it is in close contact w blood- can take away and add things to and from blood. 

Liver cells produce bile which releases into bile canaliculi and they join to make bile duct. 

Liver cells- have microvilli and involved in protein sysnthsis transformation and carb storage and so have dense cytoplasm and a controlled amount of organelles. 

Kupffer cells- macrophages -breakdown of RBC's and are found in sinusoid- bilirubin is product of RBC's. 

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Functions of the liver

 Formation of urea: 

2 processes- deamination+ornithine cycle.

Deamination=amino acid+O2 producing ammonia (highly toxic)+keto acid which can be used in resp.

Ornithine cycle: 

Ammonia too toxic and is converted to urea by adding it to CO2, to form urea and water- urea can pass thru bloodstream to kidney where it it is filtered out. 

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Functions of liver- detoxification

Detoxification- conversion of toxic molecules into less toxic molecules.

Catalase converts hydrogen peroxide into oxygen and water.

Detoxification of alcohol:

-ethanol dehydrogenase breaks down ethanol

-ethanol-ethanal-ethanoic acid-acetyl CoA

-The final compound is ethanoate (acetate) which is combined w CoA to form acetyl CoA-enters the process of resp. The H+ released join w NAD coenzyme to form reduced NAD.

NAD also used break down fatty acids in respiration-if the liver has to detoxify lots of alcohol it has less NAD to break down the fatty acids- they are then converted back into lipids and stored in the hepatocytes causing fatty liver which can lead to alcohol related hepatitis. 

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The kidney

  • Outer region- cortex
  • Middle region- medulla
  • Centre- Pelvis which leads to ureter

Nephron -:

Each nephron starts w the cortex, where the capillaries form a knot- glomerulus. This is surrounded by Bowman's capsule. Fluid is pushed into the B. capsule by ultrafiltration. The nephron is split into: proximal convoluted tubule, loop of henle, distal convoluted tubule, collecting duct. 

As the fluid moves along the nephron it goes thru selective reabsorption- urine passes intto pelvis, ureter then to  bladder.

Composition changes: In p.c.t, 85% of fluid is reabsorbed. In DL of LOH water pot. is decreased by the addition of salts and the removal of water. In the AL water pot. is increased as salts are removed by Active Transport. In the collecting duct the water pot. is decreased again by the removal of water. This means urine has a low water pot. 

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