Excretion

Kabilan Mahesan

Define Excretion

• Excretion is any process that removes the body of toxic substances, metabolic waste products and excess ions and water.

• The main excretory organs are the liver, kidneys, skin and lungs

Describe the functions of the Kidneys

1. Excretion: the kidneys rid the body of excess urea which is toxic to the body

2. Osmoregulation: the kidneys control water and salt levels in the blood by the hormone ADH which acts on the kidneys preventing dehydration

Label a Diagram of the Urinary System

Renal Vein

Renal Artery

Label a Diagram of a kidney

Nephron

Label a Diagram of a Nephron

Explain how a Nephron Filters

1. Ultrafiltration: An intense pressure is created in the glomerulus when blood travels from a wide artery into a narrow vein. This pressure causes the blood plasma to be filtered out of the blood. The filtrate is collected in the Bowman’s Capsule. It contains WUGI- water, urea, glucose and ions.

Explain how a Nephron Filter (Part II)

2. Selective Reabsorption: Useful molecules like glucose and amino acids are reabsorbed into the blood in the first coiled tube of the nephron. Also, 50% of the urea is also reabsorbed due to there being a concentration gradient between the blood plasma in the nephron and the blood in the capillaries, so urea will diffuse out. Note that glucose and amino acids do not diffuse but are instead actively transported, so that all of the useful glucose and amino acids are reabsorbed, not just 50% of them. Some ions are also reabsorbed.

Osmoregulation

The processes explained previously were the excretory functions of the kidneys. However, they also perform another function, which is osmoregulationOsmoregulation is the way in which the kidney controls water and salt levels in the body.

Describe the process of Osmoregulation in the Kidneys

• ADH is a vital hormone released by the pituitary gland, and osmoregulation depends on it.

• ADH stands for anti-diuretic hormone. Basically, this means it anti-peeing (diuretic means peeing)

• So, ADH stops you from needing to go to the toilet• It works by making the collecting duct more

permeable to water( in high concentrations), and vice-versa.

More on ADH

• Higher levels of ADH means more water is reabsorbed in the collecting duct, leading to a small amount of concentrated urine.

• Lower levels of ADH means less water is reabsorbed in the collecting duct, leading to a large amount of dilute urine.

ADH and Negative Feedback (a bit of homeostasis)

Water content too Kidney reabsorbs more waterhigh- blood plasma less concentrated

Pituitary Gland releases more ADH.

Thirst centre in hypothalamus Optimum water Thirst centre inless stimulated level in blood hypothalamus less stimulatedPituitary Gland releases less ADH Water content too Kidney reabsorbs low- blood plasma

less water more concentrated

Outline the Functions of the LiverThe liver is also an excretory organ. Its functions are:1. Manufacture of bile, which is important for the digestion of fats.2. Storage of excess glucose and glycogen.3. Interconversion of glucose and glycogen, which keeps the glucose

concentration constant for the working tissues of the body.4. Interconversion of amino acids- the liver can convert some amino

acids into others that the body might require in a process called transamination.

5. Excretion of excess amino acids- the amino part of the amino acid is removed in a process called deamination and excreted in the urine as urea.

6. Removal of old red blood cells from the circulation and storage of the iron they contained.

Diagram of the Liver

Deamination occurs here

Describe the process of Deamination

This is the structure of an amino acid:

R H2N C COOH

H

R varies depending on each different types of amino acid (there are 22 different types).Deamination is when the NH2 part of the structure is cut off.

Define Deamination

• Deamination is done by enzymes in the liver. The nitrogen part (NH2) of the amino acids is removed. It is combined with CO2 to form ammonia, which is then broken down to urea. The rest of the amino acid can be respired for energy or stored as fat.

• Urea dissolves in blood plasma and travels to the kidneys where it is excreted as urine.

Describe and explain how a dialysis machine filters the blood of wastes

Sometimes, a person’s kidney may stop working properly. Urea and other toxic substances will build up, which could lead to death. To treat this, a dialysis machine can be used as an artificial kidney. Blood is taken from a vein and passed through a roller pump to push blood through the tubes of the dialysis box (dialyser). The tubes are partially permeable and allow waste products like urea, toxins, excess water and ions to diffuse out of the blood down a concentration gradient into dialysis fluid. The dialysis fluid has a similar composition to blood plasma. The fluid is changed regularly to keep the concentration gradient steep, so urea and other wastes will diffuse. Note that the dialysis fluid can be cleaned and recycled. Before the blood goes back into the body, it passes through a bubble trap to remove air bubbles. Note that useful substances like glucose and amino acids cannot diffuse through the partially permeable tubes because their molecules are too large to fit.

Kidney Transplants

Kidney transplants are basically when a damaged or dead kidney is replaced by another person’s kidney.

Kidney transplants need a donor, and the donor’s kidney must be compatible with the recipient’s body. Family members are the most likely to be compatible (especially brothers and sisters).

Patients must also use immuno-suppressant drugs.

List the advantages of dialysis and kidney transplants.

Dialysis Kidney Transplant

There is an almost 100% chance that it will treat the patient; there is no risk of incompatibility, as there is with kidney transplants.

It is a permanent solution if the operation is successful.

It is cheaper than kidney transplants at first.

It is cheaper than dialysis in the long run.

It is much easier to find a dialysis machine quickly (for example, if the patient needs it immediately or else they will die).

The patient can have a normal diet.

Outline the disadvantages of dialysis and kidney transplants

Dialysis Kidney Transplant

The patient has to spend two to 3 nights a week connected to the machine at 8 hours per session- the treatment disrupts the patient’s life.

It is difficult to find a donor, and even more so to find one whose kidney is compatible.

The patient must eat a special diet that is low in protein so that less urea is made. They can’t take alcohol or drugs.

The body often rejects the donated organ.

It is not a cure and only gives temporary results.

To prevent rejection, the patient is given immuno-suppressant drugs. These make your immune system weaker, so you are more susceptible to diseases e.g. pneumonia, tuberculosis.