Edexcel Biology Gcse - Homeostasis in Humans Part 4

ADH release decreases

ADH release decreases

More water is reabsorbed by nephrons.

Less water is reabsorbed by nephrons.

Urine is more concentrated

Urine is more dilute

A negative feedback control system responds when conditions change from the ideal or set point and returns conditions to this set point. There is a continuous cycle of events in negative feedback.

The kidneys are responsible for the removal of waste products from the blood.

Damage from accidents or disease can lead to a build-up of poisonous wastes in the body.

Treatment is available for kidney failure and can be by organ transplant or by using kidney dialysis.

In this procedure, patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore or maintain the water and ion balance of the blood.

Patients with kidney failure can be kept alive by using kidney dialysis until a transplant becomes available.

Unfiltered blood that is high in urea is taken from a blood vessel in the arm, mixed with blood thinners or an anti-coagulant to prevent clotting, and pumped into the dialysis machine. Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane the blood flows in the opposite direction to dialysis fluid, allowing a concentration gradient to exist and exchange of substances to occur.

As the dialysis fluid has no urea in it, there is a large concentration gradient - meaning that urea moves across the partially permeable membrane, from the blood to the dialysis fluid, by diffusion. This is very important as it is essential that urea is removed from the patients' blood.

As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists. This is very important as the patients' need to retain glucose for respiration.

As the dialysis fluid contains an ion concentration similar to the ideal blood plasma concentration, movement of ions across the membrane only occurs where there is an imbalance.

if the patient's blood is too low in ions, they will diffuse from the dialysis fluid into the blood, restoring the ideal level in the blood

if the patient's blood is too high in ions, the excess ions will diffuse from the blood to the dialysis fluid

a glucose concentration similar to a normal level in the blood

a concentration of mineral ions similar to that found in normal blood plasma

no urea

greatly reduced levels of urea – it is 'cleaned blood'

no overall change in blood glucose levels

the correct water and ion balance maintained or restored (with only excess ions removed)

Kidney dialysis requires highly specialised and expensive machinery. The patient must be connected to this machinery 2-3 times a week for periods (on average) of between 4-6 hours at a time. This is time consuming and restrictive, as this mainly happens in hospital.

As the filtration only works when they are connected, kidney patients must monitor their diet carefully in between dialysis sessions. They need to avoid eating foods with a high salt content or a high protein content as excess amino acids are broken down into urea. This again can be difficult to control and monitor, but will help maintain the health of the patient.

Finally, dialysis will only work for a limited amount of time before a transplant is needed, and sadly many patients will die before a suitable one is found.

Kidney transplantation is an alternative method for treating kidney failure and can save a patient's life. This procedure involves implanting a kidney from an organ donor into the patient’s body to replace the damaged kidney. This is better than using a restrictive dialysis machine, as the recipient can lead a normal life afterwards.

As with all cells, the donor kidney cells will have protein antigens on their surface. Antigens are unique to each of us (with the exception of identical twins), and allow our body to identify our own cells from those of potential pathogens.

Differences in the antigens of the donor kidney cells and those of the patient receiving the transplant would mean that the patient's immune system would quickly form antibodies against the kidney cell antigens, and would ultimately destroy the kidney. This is known as organ rejection. This is potentially very harmful for the patient.

Tissue typing

Immuno-suppressant drugs

Even with these two precautions, most donor kidneys will only survive for an average period of 8-9 years before the patient will require a further transplant or a return to dialysis.

a kidney is given to patients who have antigens that are very similar to the antigens of the donor kidney. This can lead to long waits for transplants while compatible donors become available - during which time patients must undergo dialysis, and in some cases they will die before a match is found.

these drugs must be taken by transplant patients for the rest of their lives. They suppress the immune system, greatly reducing the immune response against the donor kidney. The negative effect of this is that it also suppresses the immune response against pathogens which enter the body, increasing the risk of infections.

Patients can lead a more normal life without having to watch what they eat and drink

Cheaper for the NHS

available to all kidney patients

| - No need for immune-suppressant drugs

Must take immune-suppressant drugs which increase the risk of infection

Shortage of organ donors

Kidney only lasts 8-9 years on average

Any operation carries risks

Patient must limit their salt and protein intake between dialysis sessions

Expensive for the NHS

Regular dialysis sessions - impacts on the patient's lifestyle

Control of body temperature

The hypothalamus contains the thermoregulatory centre and controls temperature.

As sweat evaporates it takes heat energy away. We sweat when we are hot.

Urea is removed from the blood by the kidneys.

Urine is the liquid which contains the urea.

Glucose and water

Glomerulus and Bowman's capsule

The collecting duct reabsorbs water.

Decreased water content of the blood.

Collecting duct.

The glomerulus filters the blood.