4.5.3.3 Maintaining Water and Nitrogen Balance in the Body (biology only)

Question 1

How should the body keep the levels of water in the blood?

Answer 1

important that body keeps level of water in blood as constant as possible

Question 2

What is the effect on cells of osmotic changes in body fluids?

Answer 2

• if blood becomes too dilute then water moves into cells by osmosis
• if blood becomes too concentrated water move by osmosis out of cells into blood

Question 3

What happens to body cells if they gain or lose too much water and what does the body have in place to prevent this?

Answer 3

• if body cells gain or lose too much water by osmosis then they don’t work efficiently
• so body has a system in place to maintain its water balance - part of homeostasis

Question 4

How does the body take in water?

Answer 4

body takes in water through food and drink

Question 5

How does the body lose water?

Answer 5

• water leaves body via lungs during exhalation
• water, ions (e.g. sodium ions) and urea are lost from the skin in sweat
  
  • no control over water, ion or urea loss by the lungs or skin - as sweating is part of body’s temperature control system
• excess water, ions and urea are removed by the kidneys in the urine

Question 6

How does the body control water loss through urine?

Answer 6

• if blood is too dilute, the kidneys remove the excess water and produce greater volume of urine
• urea and excess ions also removed in urine

Question 7

How many kidneys do humans have?

Answer 7

2 kidneys

Question 8

How does blood enter and leave the kidney and what does the kidney remove?

Answer 8

• blood enters kidneys through an artery and this blood contains the waste product urea
• the kidney removes this urea as well as excess ions and excess water
• these leave the kidney as urine and is stored in the bladder
• blood leaves the kidney through a vein and the blood then contains no urea

Question 9

How do the kidneys adjust the levels of molecules in the blood?

Answer 9

• blood passes through capillaries
  
  • small molecules are filtered out of the blood into tiny tubules - includes urea, ions and water as well as the sugar, glucose
  • protein and blood cells not filtered out of blood due to larger molecule size
  • these pass into a tube - all of the glucose, some of the ions and some of the water is reabsorbed back into the blood (selective reabsorption)
  • glucose and ions get reabsorbed via active transport, which requires cells to have and use energy using aerobic respiration which requires oxygen
• urea, excess ions and excess glucose are released as urine
• kidneys remove all of the waste product urea out of the blood and adjust the levels of ions and water

Question 10

How do the concentration of urea, ions and glucose change in the blood after it passes through the kidney?

Answer 10

• concentration of ions has decreased after it has passed through the kidneys - kidneys filter ions out of the blood
  
  • then reabsorb certain amount of ions back into the blood depending on the body’s needs
• concentration of urea in the blood falls to virtually 0 as it passes through the kidneys
  
  • kidneys filter out all of the urea and do not reabsorb it
• concentration of glucose in blood remains same as kidney filters out the glucose and then the blood reabsorbs it
• all of urea has been removed from the blood and the concentrations or water and ions have been adjusted - kidneys carrying out homeostasis
• urea has higher concentration in urine as other substances have been removed (reabsorbed - glucose, most of ions and water)
  
  • water gets reabsorbed - due to ADH

Question 11

How do the kidneys produce urine?

Answer 11

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

Question 12

What are proteins?

Answer 12

proteins are long chains of amino acids

Question 13

How does the kidney deal with excess amino acids?

Answer 13

• get a lot of protein in our diet
• digestion of protein leads to excess amino acids in the blood which need to be excreted safely
• often eat more protein than the body needs so the excess amino acids are deaminated in the liver to form the chemical ammonia
• ammonia is a toxic chemical so it is immediately converted to urea by the liver for safe excretion
• urea can then be safely excreted by the kidneys

Question 14

What hormone controls the water level in the body and how?

Answer 14

hormone ADH cause kidney tubules to become more permeable

Question 15

What happens if the blood becomes too concentrated?

Answer 15

• if blood becomes too concentrated (level of water in blood falls) then the pituitary gland releases the hormone ADH into the bloodstream
• ADH travels to the kidneys and causes the kidney tubules to become more permeable to water - so more water can pass out of the kidney tubules → more water reabsorbed back into the blood from the kidney tubules
  
  • because of this less urine is produced and the amount of water in the blood rises back to its normal level
  • controlled by negative feedback
• makes concentrated + low volume urine
• as levels of water in blood returns to normal, pituitary gland stops releasing ADH
• negative feedback cycle

Question 16

What happens if blood becomes too dilute?

Answer 16

• if blood becomes too dilute pituitary gland stops releasing ADH
• kidneys tubules stop becoming more permeable to water and so reabsorb less water into the blood
• makes dilute + high volume urine
• more urine produced and the conc of water in blood returns back to normal

Question 17

What happens if your kidneys fail?

Answer 17

• You have two kidneys, but you can manage with just one
• But if both kidneys fail, urea (waste) rises, and the body is poisoned

Question 18

What are the treatments for kidney failure?

Answer 18

• dialysis
• kidney transplant

Question 19

How does dialysis work?

Answer 19

• You are attached to a kidney machine via a tube from a vein
• when a patient has a kidney failure, their blood will contain a higher concentration of water ions and urea than it should
• in kidney dialysis the patients blood passes over a semi-permeable membrane, flows into a machine, is cleansed and then returns to your body
• excess salt moves out of the blood along a concentration gradient
• urea moves out of the blood by diffusion along a steep concentration gradient
• dialysis fluid also contain normal concentrations of water and ions so some of the water and some of the ions will diffuse from the blood into the dialysis fluid
• because of this, concentrations of water and ions in the patient’s blood will return to normal
• larger molecules like blood cells and proteins cannot pass through the dialysis membrane
• dialysis membrane - partially membrane
• dialysis fluid - low in salt and ureas; sugar and mineral concentration is the same as blood
• there is no net movement of sugar and other substances needed by the body
• fluid and blood moves in opp directions and dialysis fluid constantly refreshed to maintain high conc gradient

Question 20

What does dialysis do?

Answer 20

• Restores the concentrations of dissolved substances in the blood to normal levels
• It removes the waste product urea and excess water

Question 21

Which substances are retained in kidney dialysis?

Answer 21

• Dialysis fluid contains the same concentration of useful substances as the blood so glucose and useful mineral ions are not lost – as if it has a lower conc then the useful substances will diffuse into the dialysis fluid from the blood from area of high con to area of low conc
• But urea passes out from the blood into the dialysis fluid – dialysis fluid does not contain any urea so diffuses into dialysis fluid from area of high urea conc

Question 22

How long are you attached to a dialysis machine?

Answer 22

you are attached for about 4 hours, 3 times per week

Question 23

Haemodialysis:

Answer 23

• being ‘on dialysis’ in the traditional understanding means being linked up to a machine for several hours while the machine filters your blood
• Most people need three sessions of haemodialysis a week, with each session lasting around four hours
• The dialysis machine is made up of a series of membranes that act as filters and a special liquid called dialysing fluid

Question 24

Haemodialysis diagram:

Answer 24

Question 25

Peritoneal dialysis:

Answer 25

• where a fluid is pumped into your abdomen and drains away into a bag
• waste products are removed from the blood by diffusion as the dialysis fluid sits in the abdominal cavity

Question 26

Peritoneal dialysis diagram:

Answer 26

Question 27

What is a kidney transplant?

Answer 27

• An operation to remove your failed kidney and replace it with a working kidney from a donor
• One or both kidneys may need to be transplanted

Question 28

What is needed for a successful organ match?

Answer 28

• For a successful organ match, the donor and transplant patient ideally should have compatible:
  
  • Blood group
  • Tissue type
  • And a variety of other factors involving the white blood cells that would reduce the chances of rejection

Question 29

Pros of dialysis:

Answer 29

• immediately available - no donor needed
• cleans and filters the blood like a kidney
• don’t need medication
• no change of rejection
• no surgery risks

Question 30

Cons of dialysis:

Answer 30

• dialysis isn’t cheap – about £23,000 per year, for life - more expensive long term
• time consuming and inconvenient- attached for about 4 hours, 3 times per week
• eat controlled diet so don’t produce too much urea
• limits mobility

Question 31

Pros of kidney transplant:

Answer 31

• someone with a successful transplant can lead a completely normal life + have full mobility
• diseased kidney is replaced with healthy kidney from a donor
• changes in concentrations of substances in blood and urea in blood is minimised → so less chance of causing damage to body cells
• not repeatedly puncturing skin/in contact with machine unlike in dialysis → less chance of infection / less or no chance of blood clots / don’t need to take anti-clotting drug
• can eat what you like
• less expensive in long term

Question 32

Cons of kidney transplant:

Answer 32

• taking drugs (immunosuppressants) to stop their body rejecting the new kidney
• expensive - a transplant costs about £42,000 for the operation plus £6,500 a year in drugs and check-ups
• in some cases, donated kidney might be rejected by patient’s immune system
• may be a long waiting list for donor
• surgery risk - risk of infections

Question 33

Why is urea more concentrated in urine than in blood?

Answer 33

• after filtering water is reabsorbed
• urea not reabsorbed

Question 34

What do the bubble traps, anti-coagulants and warm temps do in a dialysis machine?

Answer 34

• bubble traps - get rid of air bubbles
• anticoagulant - stops blood clotting in machines
• warm temp - stops blood clotting

Question 35

What does a carbon dioxide build up lead to?

Answer 35

low pH/acidic conditions = denatures enzymes

Question 36

What does a salt build up lead to?

Answer 36

• lowers water potential in blood
• water moves by osmosis into blood from cells
• raises blood pressure
• leads to heart problems