Excretion as an example of homeostatic control

• Carbon dioxide from respiration
• Nitrogen containing products, such as urea (i.e. nitrogenous waste)
• Other compounds, such as the bile pigments found in faeces.

Every living cell in the body produces carbon dioxide as a result of respiration.

It is passed from the cells of respiring tissues into the bloodstream, where it is transported to the lungs (most as hydrogencarbonate ions).

In the lungs, the CO2 diffuses into the alveoli to be excreted as you breathe out. 

It has many metabolic roles and some of the substances produced will be passed into the bile for excretion with the faeces, for example the pigment bilirubin.

The liver is also involved in converting excess amino acids to urea.

Amino acids are broken down by the process of deamination.

The nitrogen-containing part of the molecules is then combined with carbon dioxide to make urea.

The urea is passed into the blood stream to be transported to the kidneys.

Urea is transported in solution- dissolved in the plasma.

In the kidneys the urea is removed from the blood to become a part of the urine.

Urine is stored in the bladder before being excreted from the body via the urethra.

The skin is also involved in excretion, but excretion is not the primary function of the skin.

Sweat contains a range of substances including salts, urea, water, uric acid and ammonia.

Urea, uric acid and ammonia are all excretory products.

The loss of water and salts may be an important part of homeostasis- maintaining the body temperature and the water potential of the blood. 

Allowing the products of metabolism to build up could be fatal.

Some metabolic products are toxic.

They interfere will cell processes by altering the pH, so that normal metabolism is prevented.

Other metabolic products may act as inhibitors and reduce the activity of essential enzymes.

Most carbon dioxide is transported into the blood as hydrogen carbonate ions. When they are formed, H+ ions are also formed. 

CO2 + H2O ---> H2CO3 (carbonic acid)

H2CO3 ---> H+ CO3-

This happens in the RBCs and blood plasma under the influence of carbonic anhydrase. The H+ ions affect the pH of the cytoplasm in the RBCs.

The hydrogen ions interact with bonds within haemoglobin, changing its 3D shape.

This reduces the affinity of haemoglobin for oxygen, affecting oxygen transport.

The H+ ions can then combine with haemoglobin, forming haemoglobinic acid.

The CO2 that is not converted to hydrogencarbonate ions can combine directly with the haemoglobin, producing carbaminohaemoglobin.

Both carbaminohaemoglobin and haemoglobinic acid are unable to combine with oxygen as normal-reducing oxygen transport further. 

In the blood plasma, excess H+ ions can reduce the pH of the plasma.

Maintaining the pH of the blood plasma is essential, because changes could alter the structure of the many proteins in the blood that help to transport a wide range of substances around the body.

Proteins in the blood act as buffers to resist the change in pH. 

If the change in pH is small then the extra H+ ions are detected by the respiratory centre in the medulla oblongata of the brain.

This causes an increase in the breathing rate to help remove the excess CO2.

However, if the blood pH drops below 7.35 it may cause headaches, drowsiness, restlessness, tremor and confusion. There may also be a rapid HR and changes in blood pressure. This is respiratory acidosis. It can be caused by diseases or conditions that affect the lungs themselves, such as emphysema, chronic bronchitis, asthma or severe pneumonia. Blockage of the airway due to swelling, a foreign object, or vomit can also induce acute respiratory acidosis.

The body cannot store excess amino acids. However, amino acids contain almost as much energy as carbohydrates. Therefore, it would be wasteful simply to excrete excess amino acids.

Instead they are transported to the liver and the potentially toxic amino group is removed (deamination).

The amino group initially forms the very soluble and highly toxic compound, ammonia.

This is converted to a less soluble and less toxic compound called urea, which can be transported to the kidneys for excretion.

The remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.

Deamination: Amino acid + Oxygen --> Keto acid + ammonia

Formation of urea: Ammonia + Carbon dioxide --> urea + water

                               2NH3       +  CO2                 --> (NH2)2CO + H2O

Hepatocytes carry out hundreds of metabolic processes, so the liver has an important role in homeostasis.

It is therefore essential that the liver has a good supply of blood.

The internal structure of the liver ensures that as much blood as possible flows past as many liver cells as possible.

This enables the liver cells to remove excess or unwanted substances from the blood and return substances to the blood to ensure concentrations are maintained.

Oxygenated blood from the heart 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 very active, because they carry out many metabolic processes.

Many of these processes require energy, in the form of ATP, so it is important that the liver has a good supply of oxygen for aerobic respiration.

Deoxygenated blood from the digestive system enters the liver via the hepatic portal vein.

This blood is rich in the products of digestion.

The concentrations of various substances will be controlled as they have just entered the body from the products of digestion in the intestines.

The blood may also contain toxic compounds that have been absorbed from the intestine.

It is important that such substances do not continue have been adjusted.

Blood leaves the liver via the hepatic vein.

The hepatic vein rejoins the vena cava and the blood returns to the body's normal circulation.

A fourth vessel is connected to the liver.

However, it is not a blood vessel- it is the bile duct.

Bile is a secretion from the liver which has functions in digestion and excretion.

The bile duct carries bile from the liver to the gall bladder, where it is stored until required to aid the digestion of fats in the small intestine.

Bile also contains some excretory products such as bile pigments like bilirubin, which will leave the body with the faeces.

The cells, blood vessels and chambers inside the liver are arranged to ensure the greatest possible contact between the blood and the liver cells.

The liver is divided into cyclindrical lobules.

As the hepatic artery and hepatic portal vein enter the liver, they split into smaller and smaller vessels.

These vessels run between and parallel to the lobules- they are known as inter-lobular vessels.

At intervals, branches from the hepatic artery and the hepatic portal vein enter the lobules.

The blood from the 2 blood vessels is mixed and passes along a special chamber called a sinusoid, which is lined with liver cells.

As the blood flows along the sinusoid it is in close contact with the liver cells.

These cells are able to remove substances from the blood and return other substances to the blood. 

Specialised macrophages called Kupffer cells move about within the sinusoids.

Their primary function appears to be to breakdown and recycle old RBCs.

One of the products of haemoglobin breakdown is bilirubin, which is one of the pigments excreted as part of the bile.

Bile is made in the liver cells and released into the bile canaliculi.

The bile canaliculi join together to form the bile duct which transports the bile to the gall bladder.

When the blood reaches the end of the sinusoid, the concentrations of many of its components have been modified and regulated.

At the centre of each lobule is a branch of the hepatic vein known as the intra-lobular vessel, where the sinusoids empty into..

The branches of the hepatic vein, from different lobules, join together to form the hepatic vein, which drains blood from the liver.

Hepatocytes appear to be relatively unspecialised.

They have a simple cuboidal shape with many microvilli on their surface.

However, they have many metabolic functions such as protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts and detoxification.

This means that their cytoplasm must be very dense and is specialised in the number of certain organelles that it contains.

• control of blood glucose, amino acid and lipid levels
• synthesis of bile, plasma proteins and cholesterol
• synthesis of RBCs in the foetus
• storage of vitamins A, D, B12, iron and glycogen
• detoxification of alcohol and drugs
• breakdown of hormones
• destruction of RBCs

The liver stores sugars in the form of glycogen.

It is able to store approximately 100-120g of glycogen, which makes up about 8% of the fresh weight of the liver.

The glycogen forms granules in the cytoplasm of the hepatocytes.

This glycogen can be broken down to release glucose into the blood as required.

One important role of the liver is to detoxify substances that may cause harm. Some of these compounds, such as hydrogen peroxide, are produced in the body. Others, such as alcohol may be consumed as part of our diet or may be take for health or recreational reasons, for example medicines and recreational drugs. Toxins can be rendered harmless by oxidation, reduction, methylation or by a combination with another molecules. Liver cells contain many enzymes that render toxic molecules less toxic. These include:

• Catalase: Which converts H2O2 to O2 and H2O. Catalase has a particularly high turnover number of 5 million.
• Cytochrome P450: A group of enzymes used to breakdown drugs including cocaine and various medicinal drugs. The cytochromes are also used in other metabolic reactions such as electron transport during respiration. Their role in metabolising drugs can interfere with other metabolic roles and cause the unwanted side effects of some medicinal drugs.

Alcohol, or ethanol, is a drug that depresses nerve activity. In addition, alcohol contains chemical potential energy, which can be used for respiration.

Alcohol is broken down in the hepatocytes by 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 ethanoate. This acetate is combined with coenzyme A to form acetyl coenzyme A, which enters the process of aerobic respiration. The hydrogen atoms released from alcohol are combined with another coenzyme, called NAD, to form reduced NAD.

NAD is also required to oxidise and breakdown fatty acids to use in respiration. If the liver has to detoxify too much alcohol, it uses up its stores of NAD and has insufficient left to deal with the fatty acids.

These fatty acids are then converted back to lipids and stored as fat in the hepatocytes, causing the liver to become enlarged. This is a condition known as 'fatty liver', which can lead to alcohol-related hepatitis or to cirrhosis. 

Every day we need 40-60g of protein. However, most people in developed countries eat far more than this.

Excess amino acids cannot be stored, because the amino groups make them toxic. However, the amino acid molecules contain a lot of energy, so it would be wasteful to excrete the whole molecule.

Therefore excess amino acids undergo treatment in the liver to remove and excrete the amino component.

This treatment consists of 2 processes: deamination followed by the ornithine cycle.

The process of deamination removes the amino group and produces ammonia.

Ammonia is very soluble and highly toxic.

Therefore, ammonia must not be allowed to accumulate.

Deamination also produces and organic compound, a keto acid, which can enter respiration directly to release its energy.

Because ammonia is soluble and toxic, it must be converted to a less toxic form very quickly. The ammonia is combined with carbon dioxide to produce urea. This occurs in the ornithine cycle. Ammonia and carbon dioxide combine with the amino acid ornithine to produce citrulline. This is converted to arginine by addition of further ammonia. The arginine is then re-converted to ornithine by the removal of urea.

Urea is both soluble and less toxic than ammonia. It can be passed back into the blood and transported to the kidneys. In the kidneys the urea is filtered out of the blood and concentrated in the urine. Urine can be safely stored in the bladder until it is released from the body.

They are positioned on each side of the spine, just below the lowest rib. Each kidney is supplied with blood from a renal artery and is drained by a renal vein. The role of the kidneys is excretion. The kidneys removed 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 is it released.

In a longitudinal section you can see that the kidney consists of 3 regions surrounded by a tough capsule.

•The outer region is called the cortex

•The inner region is called the medulla

•The centre is the pelvis, which leads into the ureter. 

The bulk of each kidney consists of 1 million tiny tubules called nephrons.

Each nephron starts in the cortex at a cup-shaped structure called the Bowman's capsule.

The remainder of the nephron is a coiled tubule that passes through the cortex, forms a loop down into the medulla and back to the cortex, before joining a collecting duct that passes back down into the medulla.

The renal artery splits to form many afferent arterioles, which each lead to a knot of capillaries called the glomerulus.

Blood from the glomerulus continues into an efferent arteriole which carries the blood to more capillaries surrounding the rest of the tubule.

These capillaries eventually flow together into the renal vein. 

Each glomerulus is surrounded by the Bowman's capsule.

Fluid from the blood is pushed into the Bowman's capsule by the process of ultrafiltration.

The filter is the barrier between the blood in the capillary and the lumen of the Bowman's capsule; this barrier consists of 3 layers. 

There are narrow gaps between the cells of the endothelium of the capillary wall.

The cells of the endothelium also contains pores, called fenestrations.

The gaps allow blood plasma and the substances dissolved in it to pass out of the capillary.

This membrane consists of a fine mesh of collagen fibres and glycoproteins.

This mesh 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 (and all blood cells) are held in the capillaries of the glomerulus.

These cells, called podocytes, have a specialised shape- they have many finger-like projections, called major processes.

On each major process are minor processes or foot processes that hold the cells away from the endothelium of the capillary.

These projections 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.

The Bowman's capsule leads to the rest of the tubule, which has 3 parts:

•proximal convoluted tubule

•loop of Henle

•distal convoluted tubule

The fluid from many nephrons enters the collecting ducts, which pass down through the medulla to the pelvis at the centre of the kidney.

This is the filtering of the blood at the molecular level.

Blood flows into the glomerulus through the afferent arteriole, which is wider than the efferent arteriole that carries blood away from the glomerulus.

The difference is diameter ensure that the blood in the capillaries of the glomerulus maintains a pressure higher than the pressure in the Bowman's capsule.

This pressure difference tends to push fluid from the blood into the Bowman's capsule that surrounds the glomerulus.

Blood plasma containing dissolved substances is pushed under pressure from the capillary into the lumen of the Bowman's capsule. The blood plasma contains the following substances:

•water

•amino acids

•glucose

•urea

•inorganic mineral ions (Na+, Cl-, K+)

The concentrations of dissolved solutes will depend on the water balance in the organism, and are therefore variable.

Blood cells and proteins are left in the capillary.

The presence of proteins means that the blood has very low (very negative) water potential.

This ensures that some of the fluid is retained in the blood, and this contains some of the water and dissolved substances.

The very low water potential of the blood in the capillaries is important to help reabsorb water at a later stage. 

As the fluid from the Bowman's capsule passes along the nephron tubule, its composition is altered by selective reabsorption- substances are absorbed back into the tissue fluid and blood capillaries surrounding the nephron.

•In the proximal convoluted tubule, the fluid is altered by the reabsorption of all the sugars,most mineral ions and some water. In total, 85% of the fluid is reabsorbed here. The cells of these tubules have a highly folded surface producing a brush border which increases the surface area.

•In the descending limb of the loop of Henle, the water potential of the fluid is decrease by the addition of mineral ions and the removal of water.

•In the ascending limb of the loop of Henle, the water potential is increased as mineral ions are removed by active transport.

•In the collecting duct, the water potential is decreased again by the removal of water. The final product in the collecting duct is urine.

This process ensures that urine has a low water potential. The urine, therefore, has a higher concentration of solutes than is found in the blood and tissue fluid. Urine passes into the pelvis and down the ureter to the bladder. 

The movement of Na+ and glucose into the cell is driven by the concentration gradient created by pumping sodium ions out of the cell. The Na+ move into the cell by facilitated diffusion but they cotransport glucose or amino acids against their concentration gradient, called secondary active transport. The movement of these substances reduces the water potential of the cells so that water is drawn in from the tubule by osmosis. As the substance move through to the blood, the water follows. Larger molecules such as small proteins that may have entered the tubule, can be reabsorbed by endocytosis.

Each minute about 125cm3 of fluid is filtered from the blood and enters the nephrons.

After selective reabsorption in the proximal convoluted tubule, about 45cm3 of fluid is left.

By the time this fluid reaches the bladder, the volume has dropped to about 1.5cm3.

It consists of a descending limb that descends into the medulla and an ascending limb that ascends back out of the cortex.

The arrangement of the loop of Henle allows mineral ions to be transferred from the ascending limb to the descending limb.

The overall effect is to increase the concentration of mineral ions in the tubule fluid, which has a similar effect upon the concentration of mineral ions in the tissue fluid.

This gives the tissue fluid in the medulla a very low (negative) water potential.

As mineral ions enter the descending limb, the concentration of the fluid in the descending limb rises.

This means that its water potential decreases (becomes more negative).

It becomes increasingly more -ve the deeper the tubule descends into the medulla.

As the fluid rises up the ascending limb, mineral ions leave the fluid.

At the base, this movement is by diffusion.

However, higher up the ascending limb, active transport is used to move mineral ions out.

The upper portion of the ascending limb is also impermeable to water.

The effect of the ionic movements, creates a higher water potential in the fluid of the ascending limb and decreases the water potential in the tissue fluid of the medulla.

The water potential of the tissue fluid becomes lower (much more negative) towards the bottom of the loop of Henle.

As fluid passes down the collecting duct, it passes through tissues with an ever-decreasing water potential.

Therefore, there is always a water potential gradient between the fluid in the collecting duct and that in the tissues.

Allowing water to move out of the collecting duct to the tissue fluid by osmosis.

The arrangement of the loop of Henle is known as a hairpin countercurrent multiplier system.

The overall effect of this arrangement is to increase the efficiency of transfer of mineral ions from the ascending limb to the descending limb, in order to create the water gradient.

From the top of the ascending limb the tubule fluid passes along a short distal convoluted tubule, where active transport is used to adjust the concentrations of various mineral ions. From here the fluid flows into the collecting duct.

At this stage the tubule fluid still contains a lot of water- it has a high water potential.

The collecting duct carries the fluid back down through the medulla to the pelvis.

The tissue fluid in the medulla has a low water potential that becomes even lower deeper into the medulla.

As the tubule fluid passes down the collecting duct, water moves by osmosis from the tubule fluid into the surrounding tissue. It then enters the blood capillaries by osmosis, and is carried away.

The amount of water that is reabsorbed depends on the permeability of the collecting duct walls. Only about 1.5-2.0dm3 of fluid (urine) reaches the pelvis each day. By the time the urine reaches the pelvis, it has a low (very negative) water potential and the concentration of minerals and urea is higher than in blood.

1. Glucose decreases in concentration as it is selectively reabsorbed from the proximal tubule.

2. Na+ ions diffuse into the descending limb of the loop of Henle, causing the concentration to rise. They are then pumped out of the ascending limb, so the concentration falls.

3. The urea concentration rises as water is withdrawn from the tubule. Urea is alsoactively moved into the tubule.

4. Na+ ions are removed from the tubule, but their concentration rises as water is removed from the tubule, and K+ ions increase in concentration as water is removed. K+ ions are also actively transported into the tubule to be removed in urine.

It is the control of water potential in the body. Water potential is the tendency of water to move from one place to another.

Osmoregulation involves controlling levels of both water and salts in the body. The correct water balance between cells and the surrounding fluids must be maintained to prevent water entering cells and causing lysis or leaving cells and causing crenation.

The body gains water from food, drink and metabolism. Water is lost from the body in urine, sweat, water vapour in exhaled air, and faeces. 

These gains and losses of water must be balanced. The kidneys acts as an effector to control the water content of the body and the salt concentration in the body fluids.

• On a cool day or when you have drunk a lot of water, the kidneys will produce a large volume of dilute urine.
• Alternatively, on a hot day or when you have drunk very little, the kidneys will produce smaller volumes of more concentrated urine.

The kidneys alter the volume of urine produced by altering the permeability of the collecting ducts. The walls of the collecting ducts can be made more or less permeable according to the needs of the body:

•If you need to conserve less water, the walls of the collecting ducts become less permeable. This means that less water is reabsorbed and a greater volume of urine will be produced.

•If you need to conserve more water, the collecting duct walls will become more permeable so that more water can be reabsorbed into the blood. A smaller volume of urine is produced. 

The cells in the walls of the collecting duct respond to the level of antidiuretic hormone (ADH) in the blood. These cells 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 of these reactions is to cause vesicles containing water-permeable channels (aquaporins) to fuse with the cell surface membrane, making the walls more permeable to water.

When the level of ADH in the blood rises, more water-permeable channels are inserted. This allows more water to be reabsorbed, by osmosis, into the blood. Less urine is produced and the urine has a lower water potential.

If the level of ADH in the blood falls, then the cell surface membrane folds inwards (invaginates) to create new vesicles that remove water permeable channels from the membrane. This makes the walls less permeable and less water is reabsorbed, by osmosis, into the blood. More water passes on down the collecting duct to form a greater volume of urine which is more dilute (higher water potential).

The hypothalamus  contains specialised cells called osmoreceptors. They are sensory receptors that monitor water potential in the blood. These cells respond to the effects of osmosis.

When the water potential of the blood is low (very negative), the osmoreceptor cells lose water by osmosis and shrink. Resulting in the secretion of neurosecretory cells in the hypothalamus.

The neurosecretory cells are specialised neurones that produce and release ADH. The ADH is manufactured in the cell body, which lies in the hypothalamus. ADH moves down the axon to the terminal bulb in the posterior pituitary gland, where it is stored in vesicles.

When the neurosecretory cells are stimulated by the osmoreceptors, they carry action potentials down their axons and cause the release of ADH by exocytosis.

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 (target cells). 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. Therefore the ADH present in the blood is broken down and the collecting ducts will receive less stimulation.

If the kidneys fail completely they are unable to regulate the levels of water and electrolytes (substances that form charge particles in water) in the body or to remove waste products such as urea from the blood.

This will rapidly lead to death.

Kidney function can be assessed by estimating the glomerular filtration rate (GFR) and by analysing the urine for substances such as proteins. Proteins in the urine indicate the filtration mechanism has been damaged.

The GFR is a measure of how much fluid passes into the nephrons each minute.

• A normal reading is in the range 90-120cm3 min-1.
• A figure below 60cm3 min-1 indicates there may be some form of chronic kidney disease.
• A figure below 15cm3 min-1 indicates kidney failure and needs immediate medical attention.

• Diabetes mellitus (Type 1 and 2)
• Heart disease
• Hypertension
• Infection

The main treatments for kidney failure are renal dialysis and kidney transplant.

The most common form of treatment for kidney failure.

Waste products, excess fluid and mineral ions are removed from the blood by passing over a partially permeable dialysis membrane that allows the exchange of substances between the blood and dialysis fluid.

The dialysis fluid contains the correct concentrations of mineral ions, urea, water and other substances found in blood plasma.

Any substances in excess in the blood diffuse across the membrane into the dialysis fluid.

Any substances that are too low in concentration diffuse into the blood from the dialysis fluid.

There are 2 types of renal dialysis.

Must be combined with a carefully monitored diet.

Blood from an artery or vein is passed into a machine that contains an artificial dialysis membrane shaped to form many artificial capillaries, which increases the surface area for exchange.

Heparin is added to reduce blood clotting.

The artificial capillaries are surrounded by dialysis fluid, which flows in the opposite direction to the blood (a countercurrent).

This improves the efficiency of exchange.

Any bubbles are removed before the blood is returned to the body via a vein.

Haemodialysis is usually performed at a clinic 2 or 3 times a week for several hours at each session. 

Some patients may learn how to carry it out at home.

The dialysis membrane is the body's own abdominal membrane (peritoneum).

First, a surgeon implants a permanent tube in the abdomen. 

Dialysis solution is poured through a tube and fills the space between the abdominal wall and organs.

After several hours, the use solution is drained from the abdomen.

PD can be carried out at home or work.

Because the patient can walk around while having dialysis, the method is sometimes called ambulatory PD.

The best life-extending treatment for kidney failure. While the patient is under anaesthesia, the surgeon implants the new organ into the lower abdomen and attaches it to the blood supply and bladder. Patients are given immunosuppressant drugs to help prevent their immune system recognising the new organ as a foreign object and rejecting it.

Advantages:

• Freedom from time consuming renal dialysis
• Feeling physically better immediately after
• Improved quality of life- able to travel.
• Improved self image- no longer have a feeling of being chronically ill

Disadvantages:

• Need to take immunosuppressants drugs which have side effects- fluid retention, high blood pressure , susceptibility to infections
• Need for major surgery under GA
• Need regular checks for signs of rejection

Molecules with a relative molecular mass of less than 69,000 can enter the nephron.

Any metabolic product or other substance in the blood can therefore be passed into the urine if it is small enough. If these substances are not reabsorbed further down the nephron, they can be detected in urine.

For example, urine can be tested for:

• glucose in the diagnosis of diabetes
• alcohol to determine blood alcohol levels in drivers
• many recreational drugs
• human chorionic gonadotrophin in pregnancy testing (hCG)
• anabolic steroids 

Once a human embryo is implanted in the uterine lining, it produces a hormone called human chorionic gonadotrophin (hCG).

This a relatively small glycoprotein, with a molecular mass of 36,700, that can be found in urine as early as 6 days after conception.

Pregnancy testing kits use monoclonal antibodies which bind to hCG in urine.

1. Urine poured onto test stick.

2. hCG binds to mobile antibodies attached to a blue bead.

3. Mobile antibodies move down the test stick.

4. If hCG is present, it  binds to fixed antibodies holding beads in place- a blue line forms.

5. Mobile antibodies with no hCG attached bind to another fixed site to show the test is working.

One blue line- Negative test

Two blue lines- Positive test

Anabolic steroids increase protein synthesis within cells, which results in the build up of cell tissue, especially in the muscles.

Non-medical uses for anabolic steroids are controversial because they can give advantage in competitive sports and they have dangerous side effects.

All major sporting bodies ban the use of anabolic steroids.

Anabolic steroids have a half life of about 16 hours and remain in the blood for many days.

They are relatively small molecules and can enter the nephron easily.

Testing for anabolic steroids involves analysing a urine sample in a laboratory using gas chromatography.