The Liver and Kidney Excretion

What is excretion?

All the chemical reactions that happen in cells make up the metabolism. Metabolism prduces waste products- substances that aren't needed by the cells, such as carbon dioxide and nitrogenous waste. Many of these products are toxic, so if they are allowed to build up in the body they would cause damage. Excretion is the removal of the waste products of metabolism from the body. 

Example

Carbon dioxide is a waste product of respiration. Too much in the blood is toxic, so it is removed from the body by the lungs or gills. The lungs and gills act as excretory organs. 

The Liver

One function of the liver is to break down metabolic easte products and other substances that can be harmful, like drugs and alcohol. 

• The hepatic artery supplies that liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration.
• The hepatic vein takes deoxygenated bloodt away from the liver.
• The hepatic portal vein brings blood from the duodenim and ileum, so it is rich in the products of digestion.
• This means any ingested harmful substances are filtered out and broken down straight away. 
• The bile duct takes bile to the gall bladder to be stored. 

Liver Histology

The liver is made up of liver lobules- cylindical structures made up of cells called hepatocytes that are aranged in rows radiating out from the centre. Each lobule has a central vein in the middle that connects to the hepatic vein. Many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule.

The hepatic artery and the hepatic portal vein are connected to the central vein by capillaries called sinusoids. Blood runs through the sinusoids, past the hepatocytes that remove harmful substances and oxygen from the blood. The harmful substances are broken down by the hepatocytes into less harmful substances that then re-enter the blood. The blood runs to the central vein, and the central veins from all the lobules connect to form the hepatic vein. Cells called Kupffer cells are also attached to the walls of the sinusoids. They remove bacteria and break down old red blood cells. The bile duct is connected to the central vein by tubes called canaliculi.

Deamination and the ornithine cycle

One of the liver's most important roles is getting rid of excess amino acids produces by eating and digesting protein. Amino acids contain nitrogen in their amino groups. Nitrogenous substances can't usually be stored by the body. This means excess amino acids can be damaging to the body, so they must be used by the body or be broken down and excreted.

1) The nitrogen-containing amino groups are removed from any excess amino acids, forming ammonia and organic acids- this is called deamination. 

2) The organic acids can be respired to give ATP or be converted to carbohydrate and stored as glycogen. 

3) Ammonia is too toxic for excrete directly, so it is combined with carbon dioxide in the ornithine cycle to create urea and water. 

4) The urea is released from the liver into the blood. The kidney then filters the blood and removes the urea as urine, which is excreted from the body. 

Detoxification 

The liver also breaks down other harmful substances, like alcohol, drugs, and unwanted hormones. They are broken down into less harmful compounds that can then be excreted from the body- this process is called detoxification. Some of the harmful products broken down by the liver by:

Alcohol
   Alcohol is a toxic substance that can damage cells. It is broken down by the liver into ethanal, which is then broken down into a less harmful substance called acetic acid. Excess alcohol over a long period can lead to cirrhosis of the liver- this is when the cells of the liver die and scar tissue  blocks blood flow. 

Paracetamol
   Paracetamol is a common painkiller that is broken down by the liver. Excess paracetamol in the blood can lead to liver and kidney failure.

Insulin
   Insulin is a hormone that controls blood glucose concentration. Insulin is also broken down by the liver as excess insulin can cause problems with blood sugar levels. 

Excretion of waste products

Blood enters the kidney through the renal artery and then passes through capillaries in the cortex of the kidneys. As the blood passes through the capillaries, substances are filtered out of the blood and into long tubules that surround the capillaries. This process is called ultrafiltration.

Useful substances are reabsorbed back into the blood from the tubules in the medulla and cortex- this is called selective reabsorption.

The remaining unwanted substances pass along the tubules, then along the ureter to the bladder, where they are expelled as urine. The filtered blood passes out of the kidneys through the renal vein. 

The nephrons

The long tubules along with the bundle of capillaries where the blood is filtered are called nephrons- there are thousands in each kidney.

Ultrafiltration
   Blood from the renal artery enters smaller arterioles in the cortex. Each arteriol splits into a structire called a glomerulus- a bundle of capillaries looped inside a hollow ball called a renal capsule (Bowman's capsule). The arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole. The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure. The high pressure forces liquid and small molecules in the blood out of the capillary and into the renal capsule.

The liquid and small molecules pass through three layers to get into the renal capsule and enter the nephron tubules. Larger molecules like proteins and blood cells can't pass through and stay in the blood. The filtrate passes along the rest of the nephron and useful substances are reabsorbed. Finally, the filtrate flows through the collecting duct and passes out of the kidney along the ureter. 

Selective reabsorption 

Selective reabsorption of the useful substances takes place as the filtrate flows along the proximal convoluted tuble, through the loop of Henle, and along the distal convoluted tubule. Useful substances leave the tubules of the nephrons and enter the capillaries network that is wrapped around them. 

The epithelium of the wall of the proximal convoluted tubule has microvilli to provide a large surface area for the reabsorption of useful materials from the filtrate into the blood. Useful solutes like glucose, amino acids, vitamins and some salts are reabsorbed along the proximal convoluted tubule by active transport and facilitated diffusion. Some urea is also reabsorbed by diffusion. Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate. water is reabsorbed from the loop of Henel, distal convoluted tubule and the collecting duct. 

Urine
   Urine is usually made up of water and dissolved salts, ures and other substances such as hormones and excess vitamins. Glucose, amino acids and vitamins are actively reabsorbed.  

Regulation of water content 

Water is essential to keep the body functioning, so the amount of water in the blood needs to be kept constant. Mammals excrete urea in solution, which means water is lost during excretion. Water is also lost in sweat. The kidneys regulate the water content of the blood, so the body has just the right amount: 

• If the water content of the blood is too low, more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated, so less water is lost during excretion. 
• If the water content of the blood is too high, less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion. 

Regulation of the water content of the blood takes place in the middle and last parts of the nephron- the loop of henle, the distal convoluted tubule and the collecting duct. the volume of water reabsorbed is controlled by hormones. 

The loop of Henle

The loop of Henle is made up of two parts- the descending limb and the ascending limb. 

Countercurrent multiplier

1) Near the top of the ascending limb, sodium and chloride ions are actively transported out into the medulla. The ascending limb is impermeable to water, so water stays inside the tubule. This creates a low water potential in the medulla.

2) The lower water potential means water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated. Water in the medulla is reabsorbed into the blood through the capillary network. 

3) Near the bottom of the ascending limb, sodium and chloride ions diffuse out into the medulla, further lowering the water potential. 

4) The water potential in the medulla is very low which causes water to move out of the collecting duct by osmosis. the water in the mesulla is reabsorbed into the blood through the capillary network. 

Loop of Henle length in different animals

the longer an animal's loop of Henle, the more water they can reabsorb from the filtrate. When there is a longer ascending limb, more ions are actively pumped out into the medulla, which creates a really low water potential in the medulla. This means more wter moves out of the nephrin and collecting duct into the capillaries, giving very concentrated urine. Animals that live in areas where there is little water usually have long loops to save as much water as possible.

Examples

• The fennec fox, desert kangaroo rat and camel live in hot, dry environments such as deserts. As a result they have evolved long loops of Henle, which enables them to produce small volumes of concentrated urine in order for them to conserve water. 
• In contrast, frogs and toads don't have a loop og Henle at all, so they can't produce concentrated urine. This is because they live in a wet environment, so they don't have to conserve water. 

Antidiuretic hormone (ADH)

The water content, and so water potential, of the blood is monitored by cells called osmoreceptors in the hypothalamus. When the osmoreceptors are stimulated by low water content in the blood, the hypothalamus sends nerve impulses to the posterior pituitary gland to release a hormone called ADH into the blood.

ADH molecules bind to receptors on the plasma membrances of cells in the distal convoluted tubule and the collecting duct. When this happens, protein channels called aquaporins are inserted into the plasma membrane. These channels allow water to pass through via osmosis, making the walls of the distal convoluted tubule and collecting duct more permeable to water. This means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis. A small ampunt of concentrated urine is produced, which means less water is lost from the body.

ADH and dehydration

• The water content of the blood drops, so its water potential drops.
• This is detected by osmoreceptors in the hypothalamus.
• The posterior pititary gland is stimulated to release more ADH into the blood. 
• More ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis.
• A small amount of highly concentrated urine is produced and less water is lost.

ADH and hydration

• 
  The water content of the blood rises, so its water potential rises.
• This is detected by the osmoreceptors in the hypothalamus.
• The posterior pititary gland releases less ADH into the blood.
• Less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis.
• A large amount of dilute urine is produced and more water is lost. 

What is kidney failure?

Kidney failure is when the kidneys can't carry out their normal function because they can't work properly. Kidney failure can be caused by many things, including kidney infection and high blood pressure.

Kidney infections 

Kidney infections can cause inflammation of the kidneys, which can damage the cells. This interferes with filtering in the renal capsules, or with reabsorption in the other parts of the nephron.

High blood pressure

High blood pressure can damage the glomeruli. The blood in the glomeruli is already under high pressure gets too high. This means larger molecules like proteins can get through the capillary walls and into the urine.

Problems arising from kidney failure

Kidney failure causes lots of problems, for example:

• Waste products that the kidney would normally remove begin to build up in the blood. Too much urea in the blood causes weight loss and vomiting.
• Fluid starts to accumulate in the tissues because the kidneys can't remove excess water from the blood. This causes parts of the body to swell, e.g. the person's legs, face and abdomen can swell up.
• The balance of ions in the body becomes unbalanced. the blood may become too acidic, and an imbalance of calcium and phosphate can kead to brittle bones. Salt build-up may cause more water retention. 
• Long term kidney failure causes anaemia- a lack of haemoglobin in the blood.

If the problems caused by kidney failure can't be controlled, it can lead to death.

Treating kidney failure- when the kidneys are no longer functions correctly.

Renal dialysis

Reanal dialysis is where a machine is used to filter a patients blood. the patient's blood is passed through a dialysis machine- the blood flows on one side of a partially permeable membrane and dialysis fluid flows in the opposite direction in order to maintain a steep concentration gradient between them, to increase the rate of diffusion. During dialysis, waste products and excess water and ions diffuse across the membrane into the dialysis fluid, removing them from the blood. Blood cells and large molecules like proteins are prevented from leaving the blood.

One of the problems with renal dialysis is that the patient can feel increasingly unwell between sessions because waster products and fluid start to build up in their blood. Also, each dialysis sessio takes three to five hours, and patients need two or three sessions a week, usually in hospital. This is quite expensive and it is inconvenient for the patient. But dialysis can keep a person alive until a transplant is available.

Kidney transplant

A kidney transplant is where a new kidney is implanted into a patient's body to replace a damaged kidney. The new kidney has to be from a person with the same blood type and tissue type. They are often donated from a living relative, as people can survive with only one kidney. They can also be donated from people who have recently died.

Transplants have a lot of advantages, For example, it is cheaper to give a person a transplant than keep them on dialysis. Having a kidney transplant is more convenient for a person than having regular dialysis sessions, and patients don't have the problem of feeling unwell between sessions.

However, there are also disadvantages to having a kidney transplant. These inculde the fact the patient will have to undergo a major operation, which is risky. There is also the risk that the immune system may reject the kidney. This means that the patient has to take drugs to suppress it. 

Human chorionic gonadotrophin (hCG) is a hormone that is only found in the urine of pregnant women. 

Testing for pregnancy

• A stick is used with an application area that contains antibodies for hCG bound to a colour bead, blue.
• When urine is applied to the application area any hCG will bind to the antibody on the beads.
• The urine moves up the test *****, carrying the beads with it.
• The test ***** has antibodies to hCG immobilised.
• If there is hCG present the test ***** turns blue because the immobileised antibody binds to any hCG attached to the blue beads.
• If there is no hCG present, the beads will pass through the test area without binding to anything, so it won't turn blue. 

Anabolic steroids

Anabolic steroids are drugs that build up muscle tissue. Testosterone is an example, and there are other common ones such as Nandrolone. Some athletes are banned from taking ananbolic steroids as they can have dangerous side-effects, such as liver damage. Also it can be concidered unfair on other athletes. 

However, there are some athletes who continue to take steroids, as there is an increasing pressure on elite athletes to perform well. Taking steroids can have positive effects on performance, such as increased strength and power.

Testing for steroids

Steroids are removed from the blood in the urine, so athletes regularly have urine tests, by a technique called gas chromatography. The urine sample is vapourised and passed through a column at different speeds. The retention times for each substance is compared to the known retention times for a steroid to pass through. The the times are the same then the sample contains steroids.