Excretion

Excretion is the removal of etabolic waste from the body. This means the removal from the body of the unwanted products of cell metabolism.

What products must be excreted?

Many substances need to be excreted. Almost all products that are formed in excess by the chemical processes occurring in the cells must be removed from the body, so that they do not build up and inhibit enzyme activity or become toxic. The main excretory products are:

• CO2 from respiration
• Nitrogen-containing compounds suchas urea

• Other compounds like bile pigments found in faeces.

Lungs
All cells in the body produce CO2 as a result of respiration. CO2 is passed from cells of respiring tissues into the bloodstream, where it is transported (mostly in the form of hydrogencarbonate ions) to the lungs. In the lungs the CO2 diffuses into the alveoli to be excreted as you breathe out.

Liver
Has many metabolic roles, such as the pigment bilrubin being passed into the bile for excretion with faeces. It is also involed in deamination of amino acids and converts them into urea. The nitrogen-containing part is then combined with CO2 to make urea. 

Kidneys
Urea passes into the bloodstream 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 part of urine.

Skin
The skin is also involved in excretion. Sweat contains substances like 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 water potential of the blood.

Allowing the products of metabolism to build up could be fatal. Some metabolic products such as carbon dioxide and ammonia are toxic. They interfere with 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 CO2 is transported in the blood as hydrogencarbonate ions. However, the formation of this also forms hydrogen ions:

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

The carbonic acid dissociates to release hydrogen ions:

H2CO3 ----> H+  + HCO3-

This occurs inside the red blood cells, under the influence of enzyme carbonic anhydrase, but can also occur in the blood plasma.

Metabolic functions

• Controls blood glucose levels, amino acid levels, lipid levels
• Synthesis of bile, plasma proteins, cholesterol
• Synthesis of red blood cells in the foetus
• Storage of vitamins A, D and B12, iron, glycogen
• Detoxification of alcohols and drugs
• Breakdown of hormones
• Destruction of red blood cells

Glycogen storage
The liver is able to store approx 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 (liver cells). This glycogen can be broken down to release glucose into the blood as required.

• Detoxification
  The liver also has to detoxify substances that cause harm. Some of the compounds, such as hydrogen peroxide, are produced in the body. Others, such as alcohol, may be consumed as a part of our diet or may be taken for health or recreational reasons, like medicines or recreational drugs.
• Toxins can be rendered harmless by oxidation, reduction, methylation or by combination with another molecule. Liver cells contain many enzymes that render toxic molecules, less toxic. These include:
  Catalase, which converts hydrogen peroxide into oxygen and water. Catalase has a very high turnover number (the number of catalse molecules that are turned into oxygen and water in one second) of 5 million.
  Cytochrome P450, which is a group of enzymes usd to breakdown drugs like cocaine and various medicinal drugs. Cytochromes are also used in the ETC.

Ethanol is a drug that depresses nerve activity. It also 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 ethanal dehydrogenase. The final product is acetate.. The acetate combines with co-enzyme 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 for 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 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 condition is 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 two processes: deamination followed by the ornithine cycle.

The hepatic arteries supply the liver with oxygenated blood from the heart, so the liver has a good supply ofoxygen 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 digestion, and this means that any harmful substances ingested will be broken down quickly by the liver cells (hepatocytes).The blood flows past every hepatocytes via the sinusoid, this ensures that the harmful stuff are broken down quickly. Also the blood provides the liver cells with oxygen.

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.　Every single lobule has branches of the hepatic artery, hepatic portal vein and bile duct.　

Hepatic artery and hepatic vein are connected to the central vein via capillaries called sinusoid.　The central veins from all the lobules join up to form the hepatic vein.

Microvilli - increase the surface area for re-absorption.　

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+ against their concentration gradient.　

Many Mitochondria - provides the energy in the form of a lot of ATP. The energy is needed to drive the selective re-absorption process.

Selective Reabsorbtion: - (Proximal Convoluted Tubule)
1.Na ions are actively transported out of the wall of the convoluted tubule and enter the surrounding tissue fluid.
2. Na ions are actively pumped out of the cells lining in the convoluted tubule.
3. Sodium ions diffuse into the cell through a cotransport protein, carryingglucose or an amino acid with it at the same time.
4. Water moves into the cell by osmosis.
5. Glucose/amino acids diffuse into the blood.

• 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 salt 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 three sources: food, drink and metabolism (e.g. respiration). Water is lost from the body in urine, sweat, water vapour exhaled air and faeces.
• These gains losses of water must be balanced. The kidneys act 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 fluid, the kidneys will produce a large volume of dilute urine. Alternatively, on a hot day when you have drunk very little, the kidneys will produce smaller volumes of more concentrated urine.

There is a water potential gradientdown the collecting duct to the medullary tissue of the kidney. This causes water to move out of the collecting duct by osmosis. Water reabsorption in the collecting duct like this is controlled by the levels of ADH(antidiuretic hormone) in the blood vessels in contact with the walls of the collecting duct. ADH is a hormone which in larger amounts will cause more water to be absorbed, and so urine is more concentrated and less urine is released. The water potential of the blood is detected by sensory cells called osmoreceptorsin a region of the brain called the hypothalamus, which you will be familiar with from thermoregulation, as well as other functions. Normal osmosis rules apply to these cells, as in when the water potential of the blood is low, water will move out of osmoreceptors by osmosis (because the cells will have a higher water potential, and water always moves down the potential gradient), causing the cells to shrivel – and so more ADH is released (which will ultimately increased the water potential of the blood, as ADH will make the collecting duct walls more permeable to water so more will be reabsorbed).　

When the osmoreceptors shrink due to a lower water potential, they stimulate neurosecretory cellsin the hypothalamus to manufacture and release ADH. When ADH is made, it travels down the axon of the cell it is made in (ADH is produced in the cell body), and is stored in the posterior pituitary glanduntil needed. When the neurosecretory cells have been stimulated, the ADH is released into the bloodstream by the posterior pituitary gland.

Causes and problems of kidney failure

Kidney failure does not arise from one sole cause. The most common factors which can cause kidney failure are

• Diabetes (both type 1 and type 2)
• Hypertension
• Infection
• Heart disease

Intro

When kidneys fail, the body is left unable to remove waste substances and excess water from the blood, which of course includes urea, which is harmful. This will ultimately lead to death, and will not take too long.

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 90-120cm3min-1. A figure below 60cm3min-1 shows that there may be some form of chronic kidney disease. A figure below 15cm3min-1 indicates kidney failure and a need for  immediate medical attention.