4.5.3.3 - Maintaining water and nitrogen balance in the body

How is water lost from the body?

Water leaves the body via the lungs during exhalation.

Water is also lost from the skin in sweat

What else is lost via the skin in sweat?

Water, ions and urea

Can you control the amount of water, ions and urea lost (via sweat) ?

No

How is excess water, ions and urea removed from the body?

Excess water, ions and urea are removed via the kidneys in the urine.

What can occur if the ion / water content of the body is wrong?

If the ion / water content of the body is wrong it can lead to body cells losing or gaining too much water by osmosis - leading to them not functioning efficiently.

What leads to excess amino acids?

The digestion of proteins from the diet results in excess amino acids which need to be excreted safely.

What happens in the liver (to the excess amino acids)

In the liver, amino acids are deaminated to form ammonia

Ammonia is toxic and so it is immediately converted to urea for safe excretion

Then is excreted in urine by the kidneys

Is ammonia toxic?

Yes so it’s converted into urea

How do the kidneys produce urine?

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.

What hormone controls the concentration of urine?

ADH - anti-diuretic hormone

Where is ADH released by?

Released into the blood by the pituitary gland

Describe how ADH is used to control our water concentration.

The brain monitor the water content of the body and instructs to release ADH according to how much is needed

What is the name of the process that controls ADH?

This entire process if controlled by negative feedback ( so whenever water content gets too high or too low a mechanism is triggered to bring the water content back to normal)

What does ADH act on?

ADH is a hormone which affects the permeability of the kidney tubules

What happens when the blood is too concentrated? (too little water)

ADH is released by the pituitary gland

Causes more water to be reabsorbed back into the blood from the kidney tubules - as the kidney tubules are more permeable

Leads to more concentrated urine

What occurs if our kidney’s don’t work properly?

• waste substances build up in the blood

• leads to an individual no longer being able to control the levels of ions and water in their body

• can be fatal

What are the two main treatments for kidney failure?

• Kidney dialysis

• Kidney transplant

Describe how kidney dialysis works.

In a dialysis machine, a persons blood flows between partially permeable membranes which are surrounded by dialysis fluid

The membranes are permeable to things like ions and waste substances - but not large moeclules like proteins

The dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood

Meaning that no useful dissolved ions and glucose will be lost during the dialysis

Only waste substances - like urea - and excess ions and water can diffuse across the barrier

Why does kidney dialysis have to be done regularly?

To keep the concentrations of dissolved substances in the blood at normal levels and to remove any waste substances

Why does the dialysis fluid have to regularly replaced?

to maintain the concentration gradient - not reach an equilibrium where waste substances no longer dissolve out of the blood

What are the disadvantages / limitations of kidney dialysis?

Many patients with kidney failure have to have dialysis 3 times a week, each session lasting 3-4 hours

Inconvenient , lack of flexibility

Dialysis can cause blood clots

Being on a dialysis machine is unpleasant

Expensive for the NHS to run

What is the main advantage of kidney dialysis?

Allows a patient with kidney failure to buy valuable time - until a donor organ is found

Describe kidney transplants.

Only cure available for kidney failure

Healthy kidneys are transplanted from people who have died suddenly (the person has to be on the organ donor register / carry a donor card)

Kidneys can also be transplanted from people who are still alive (yet their is a small risk from the person donating the kidney)

What are the disadvantages of kidney transplants?

Risk that the donor kidney can be rejected by the patients immune system , so the patient has to regularly take immunosuppressants

Invasive surgery - risk of infection

Long waiting lists for kidneys

What are the advantages of kidney transplants?

Transplants are cheaper in the long run compared to dialysis

Much more convent and allow the patient to maintain a flexible lifestyle

Cure the issue at hand