The liver and the kidneys

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Histology of the liver

Oxygenated blood from the heart. blood travels from the aorta via the hepatic artery into the liver. This supplies the oxygen that is essential for aerobic respiration. The liver cells are active as they carry out many metabolic processes which require energy int he form of ATP.

deoxygenated blood from the digestive system This enters the liver via the hepatic portal vein. This is rich in the products of digestion.

Blood leaves the liver via the hepatic vein. This rejoins the vena cava and the blood returns to normal circulation.

the bile duct contains bile secreted by the liver and carries it to the gall bladder where it is stored until required to aid the digestion of fats in the small intestines.

The liver is divided into lobes which are further divided into cylindrical lobules. The blood from the hepatic artery and hepatic portal vein enter the lobules. the blood from the two vessels are mixed and passed along a chamber called a sinusoid. The sinusoid is lined by liver cells and empties into the interlobular vessel, a branch of the hepatic vein. These branches join together to form the hepatic vein which drains blood from the liver.

As the blood flows along the sinusoid it is in very close contact with the liver cells. They are able to removes molecules from the blood and pass molecules into the blood.

One of the functions of the liver cells is to manufacture bile which is released into the bile canaliculi. These join together to form the bile duct, which transports the bile to the gall bladder.

Liver cells have a simple cuboidal shape with many microvilli on their surface. They many metabolic functions include protein synthesis , transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification.

Kupffer cells are specialised macrophages. They move about within the sinusoids and are involved in the breakdown and recycling red blood cells. One of the products of haemoglobin breakdown is bilirubin which is excreted as part of bile and in faeces.

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Functions of the liver

Deamination

This produces ammonia which is very soluble and highly toxic. it also produces an organic compound called a keto acid, which can enter respiration directly to release its energy.

amino acid + oxygen > keto acid + ammonia

The ornithine cycle

The ammonia is converted to a less toxic form so it is combined with carbon dioxide to produce urea. Urea is less soluble and less toxic than ammonia which can be passed back into the blood and transported around the body to the kidney. in the kidneys the urea is filtered out of the blood and concentrated in the urine. Urine can be stored in the baler until it is released from the body.

ammonia + carbon dioxide > urea + water

Detoxification of alcohol

it is broken down in the hepatocytes but the action of the enzyme ethanol dehydrogenase. The resulting compound is ethanal. This is dehydrogenated further by the enzyme ethanal dehydrogenase. The final compound produced is acetate which is combined with coenzyme A to form acetyl coenzyme A, which enters the process of respiration. The hydrogen atoms released in this process are combined with another coenzyme called NAD to re form reduced NAD.

NAD is also required to oxidise and break down fatty acids for use in respiration. If the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids. These fatty acids are then converted to lipids and are stored in the hepatocytes causing the liver to become enlarged. This can lead to alcohol related hepatitis or cirrhosis.

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Histology of kidneys

They are positioned on each side of the spine just below the lowest rib. Each kidney is supplied with blood from he renal artery and drained by the renal vein. The role of the kidney is to remove waste products from the blood and produce urine. the urine passes out of the kidney down the ureter to the bladder where it can be stored before release.

It has three regions:

the outer region is called the cortex

the inner region is called the medulla

the centre is the pelvis which leads to the ureter.

Nephron is the functional unit of the kidney. It starts in the cortex. In the cortex the capillaries form a knot called the glomerulus surrounded by a cup shaped structure called the Bowman's capsule. Fluid from the blood is pushed into the Bowman's capsule by the process of ultrafiltration. As the fluid moves along the nephron its composition is altered which is achieved by selective reabsorption. The final product in the collecting duct is urine. This passes into the pelvis and down the ureter to the bladder.

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Ultrafiltration

Blood flows into the glomerulus from the afferent arteriole which is wider than the efferent arteriole which carried blood away from the glomerulus. the difference in diameters ensures that the blood in the capillaries of the glomerulus is under increased pressure. The pressure in the glomerulus is higher than in the bowman's capsule which tends to push fluid from the blood not the Bowman's capsule that surrounds the glomerulus.

There are 3 layers between the blood in the capillary and Bowman's capsule:

the endothelium of the capillaries has narrow gaps between its cells that blood plasma and dissolved substances can pass through.

the basement membrane consists of a fine mesh of collagen fibres and glycoproteins . These acts as a filter to prevent the passage of molecules with a relative molecular mass of greater than 69,000. This means that most proteins are held in the capillaries of the glomerulus.

The epithelial cells of Bowman's capsule called podocytes have a very specialised shape. Podocytes have many fingerlike projections called major processes. these ensure that there are gaps between the cells. fluid from the blood in the glomerulus can pass between these cells into the lumen of the Bowman's capsule.

What is filtered out of the blood?

water

amino acids

glucose

urea

inorganic ions

What is left in the capillary?

The blood cells and proteins. The presence of the protein ensures a low water potential so that some of the fluid is retained in the blood.

Selective reabsorption - the cells lining the proximal convoluted tubule are specialised to achieve this reabsorption

the cells surface membrane in contact with the tubule fluid is highly faded to form microvilli. these increases the surface area for reabsorption.

the membrane contains special cotransporter proteins that transports amino acids or glucose in association with sodium ions. This is facilitated diffusion.

the opposite membrane of the cell close tot he tissue fluid and blood capillaries is also folded to increase its surface area. This membrane contains sodium/potassium pumps that pump sodium ions out of the cell and potassium ions into the cell .

the cell cytoplasm has many mitochondria. This indicates that an active process is involved required a lot of ATP.

How does reabsorption occur?

the sodium potassium pumps remove sodium ions from the cells lining the proximal convoluted tubule. this reduces the concentration of sodium ions in the cell cytoplasm.

sodium ions are transported into the cell along with glucose or amino acid molecules by facilitated diffusion

as the glucose and amino acid concentrations rise inside the cell, these substances are able to diffuse into the blood and are carried away

reabsorption of salts, glucose and amino acids reduces the water potential in the cells and increases the water potential into the tubule fluid. This means that the water enter the cells and then be reabsorbed into the blood by osmosis.

larger molecules such as small proteins that may have entered the tubule will be reabsorbed by endocytosis.

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Water reabsorption

The role of the loop of hence is to create a very low water potential in the tissue of the medulla. this ensures that even more water can be reabsorbed from the fluid in the collecting duct.

The loop of Henle

It consists of a descending limb that descends into the medulla and an ascending limb that ascends back out into the cortex. The overall effect is to increase the concentration of salts in the tubule fluid so that they diffuse out from there thin walled ascending limbo into the surrounding medulla tissue, giving the tissue fluid in the medulla a very low water potential.

How?

as the fluid descends deeper in the medulla its water potential becomes lower due to loss of water by osmosis to the surrounding fluid and the diffusion of sodium and chloride ions into the tubule from the surrounding tissue fluid.

as the fluid ascends back up towards the cortex its water potential becomes higher because at the base Na and Cl diffuse out of the tubule into the tissue fluid, higher up the tubule Na and Cl ions are actively transported out into the tissue fluid, the wall of the ascending limb is impermeable to water so it cannot leave the tubule, the fluid loses salts but not water as it moves up the ascending limbs.

This arrangement is knows as the hairpin countercurrent multiplier system. the overall effect is to increase the efficiency of salt transfer from he ascending limb to the descending limb causing a build up of salt concentration in the surrounding tissue fluid.

The movement of salts from the ascending limb into the medulla creates a high salt concentration in the tissue fluid of the medulla so that there is a low water potential. The water potential becomes increasingly negative deep into the medulla.

The removal of ions from the ascending limb means that the top of the ascending limb the urine is dilute. water may then be reabsorbed from the urine in the distal convoluted tubule and collecting ducts. The amount of water reabsorbed depends on the needs of the body and so the kidney is also an organ of osmoregulation

The collecting duct

the fluid still has a high water potential. The collecting duct carries the fluid back down though the medulla into the pelvis. The tissue fluid in the medulla has a lower water potential that becomes even lower deeper into the medulla. As the fluid passes down the duct water moves into the surrounding tissue by osmosis from the tubule fluid into the surrounding tissue. It then enters blood capillaries by osmosis and is carried away.

The amount of water depends on the permeability of the walls of the collecting duct.

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Osmoregulation

osmoregulation is the control of water and salt levels in the body.

water is gained from food, drink and metabolism. It is lost in urine, sweat, faeces and water vapour.

the permeability of the collecting duct respond to the level of antidiuretic hormone (ADH) in the blood. Cells in the walls have membrane bound receptors for ADH. The ADH binds to these receptors and causes a chain of enzyme controlled reactions inside the cell. The end result is to insert vesicles containing water permeable channels (aquaporins) into the cell surface membrane. This makes the walls more permeable to water. if there is more ADH, there are more water permeable channels inserted. This allows more water to be reabsorbed by osmosis into the blood. Less urine passes out of the body.

if there is less ADH in the blood then cell surface membrane folds inwards to create new vesicles that remove water permeable channels. This makes the walls less permeable and less water is reabsorbed by osmosis into the blood. More water passes out in the urine.

Adjusting the concentration of ADH in the blood:

the water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain

they respond to the effects of osmosis. when the water potential of the blood is very low the osmoreceptor cells lose water by osmosis. This causes them to shrink and stimulate the neurosecretory cells in the hypothalamus.

the neurosecretory cells are specialised neurones that produce and release ADH. The ADH is manufactured in the cell body of these cells which lies in the hypothalamus. ADH flows down the axon to the terminal bulb in the posterior pituitary gland. It is stored there until needed.

When the neurosecretory cells are stimulated they send action potentials down their axons and causes the release of ADH.

The ADH enters the blood capillaries running through the posterior pituitary gland. It is transported around the body and acts on the cells of the collecting ducts.

once the water potential of the blood rises again, less ADH is released

ADH is slowly broken down, it has a half life of about 20 minutes. The ADH present in the blood is broken down and the collecting ducts will receive less stimulation.

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Kidney failure

kidney failure can be due to:

diabetes mellitus

hypertension

infection

Once the kidneys fail completely, the body is unable to remove excess water and certain waste products from the blood which will rapidly lead to death.

Treatment

Dialysis

this removes wastes, excess fluid and salt from blood by passing the blood over a dialysis membrane. The dialysis membrane is partially permeable which allows exchange of substances between the blood and dialysis fluid. This fluid contains the correct concentrations of salts, urea,water and other substances in the blood plasma. Nay substances in excess in the blood diffuse across the membrane into the dialysis fluid. This must be combined with a carefully controlled diet.

In haemodialysis blood from a vein is passed into a machine that contains an artificial dialysis membrane. Heparin is added to avoid clotting and any bubbles are removed before the blood is returned to the body.

In peritoneal dialysis the filter is the body's own abdominal membrane. A surgeon implants a permanent tube in the abdomen. Dialysis solution is poured though the tube and fills the space between the abdominal wall and the organs. After several hours the sued solution is drained.

Kidney transplant - the surgeon implants the new organ into the lower abdomen and attaches it to the blood supply and the bladder. Many patients fell much bettie immediately which is the best life extending treatment for kidney failure. However, the patient's immune system will recognise the new organ as a foreign object and produce a reaction. Patients are given immunosuppressant drugs to help prevent rejection.