B3- Homeostatis

Question 1

Why do waste products need to be excreted?

Answer 1

Waste products are constantly being produced by the body and must therefore be excreted. If they are not, they will increase in concentration and may interfere with chemical reactions or damage cells.

Question 2

Why is carbon dioxide produced and how is it removed?

Answer 2

It is the product of aerobic respiration and is removed through the lungs when we breathe out.

Question 3

Why is urea produced and how is it removed?

Answer 3

It is produced in the liver when excess amino acids are broken down. The kidneys remove it from the blood and make urine - which is temporarily stored in the bladder

Question 4

How do our bodies take in water?

Answer 4

Through food and drink, We even get some water when we respire by burning glucose to release energy.

Question 5

How does our body lose water?

Answer 5

We lose water in sweat, faeces, urine and when we breathe out.

Question 6

What is homeostasis?

Answer 6

The maintenance of a constant internal environment inside a living organism.

Question 7

Why is it important that the water and mineral content in our body remains at the correct level?

Answer 7

If the water and ion content was to change, this would cause too much water to move into or out of cells - leading to them becoming damaged.

Question 8

How is water balance maintained?

Answer 8

The kidneys maintain our water balance by producing urine of different concentrations.
When the water level of our blood plasma is low, more water is reabsorbed back into the blood and the urine becomes more concentrated. When the water level of our blood plasma is high, less water is reabsorbed back into the blood and our urine is more dilute.

Question 9

What are the factors that can affect the level of water in the blood plasma?

Answer 9

External temperature - when it is hot, we sweat more and lose water, which makes the blood plasma more concentrated.
Amount of exercise - if we exercise, we get hot and increase our sweating, so we lose more water and the blood plasma becomes more concentrated.
Fluid intake - the more we drink, the more we dilute the blood plasma. The kidneys respond by producing more dilute urine to get rid of the excess water.
Salt intake - salt makes the plasma more concentrated. This makes us thirsty, and we drink more water until the excess salt has been excreted by the kidneys.

Question 10

What is the role of the kidneys?

Answer 10

Each kidney contains over one million microscopic filtering units called nephrons. Each nephron is made of a tubule and is responsible for ‘cleaning’ the blood by removing urea and excess water and mineral ions

Question 11

What are the three stages in the process of cleaning the blood?

Answer 11

Filtration, selective re absorption, the formation of urine.

Question 12

What happens during filtration?

Answer 12

As blood passes through the capillary at the start of the nephron, small molecules are filtered out and pass into the nephron tubule. These small molecules include glucose, urea, ions and water. However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.

Question 13

What happens during selective reabsorption?

Answer 13

Having filtered out small molecules from the blood - many of which are essential to the body - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.The reabsorption of water takes place by osmosis. The reabsorption of glucose and mineral ions - from the nephron to the blood capillary - takes place by active transport.

Question 14

What are the molecules that are reabsorbed during selective reabsorption?

Answer 14

all of the glucose which was originally filtered out
as much water as the body needs to maintain a constant water level in the blood plasma
as many ions as the body needs to maintain a constant balance of water and mineral ions in the blood plasma

Question 15

How are the cells which make up the wall of the nehpron adapted?

Answer 15

The cells which make up the wall of the nephron are adapted by having a folded membrane (providing a large surface area) and a large number of mitochondria (to supply the energy for active transport).

Question 16

What happens during the formation of urine?

Answer 16

The molecules which are not selectively reabsorbed (the urea and excess water and ions) continue along the nephron tubule as urine . This eventually passes down to the bladder.
In carrying out these processes, the kidney is able to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low.

Question 17

What are the consequences of kidney failure?

Answer 17

Kidney failure has serious consequences as it means that the water and ion balance cannot be regulated, and the levels of toxic urea build up in the body. This would ultimately be fatal if not treated.

Question 18

How does dialysis work?

Answer 18

‘Dirty’ blood (high in urea) is taken from a blood vessel in the arm, mixed with blood thinners to prevent clotting, and pumped into the machine. Inside the machine - separated by a partially permeable membrane the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists.

Question 19

What does dialysis fluid contain?

Answer 19

a glucose concentration similar to a normal level in the blood
a concentration of ions similar to that found in normal blood plasma
no urea

Question 20

How does dialysis fluid clean the blood?

Answer 20

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.
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.
And, 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.

Question 21

How does dialysis fix an imbalance in a person’s ions?

Answer 21

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.

Question 22

What is the overall affect on the blood leaving the dialysis machine?

Answer 22

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)

Question 23

What are the disadvantages of dialysis?

Answer 23

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.
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.

Question 24

What does kidney implantation involve?

Answer 24

his procedure involves implanting a kidney from an organ donor into the patient’s body to replace the damaged kidney.

Question 25

What is organ rejection?

Answer 25

The donor kidney cells will have protein antigens on their surface. Antigens 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.

Question 26

What two ways can prevent organ rejection?

Answer 26

Tissue Typing and immuno-suppressant drugs

Question 27

What is tissue typing?

Answer 27

only giving the kidney to patients who have antigens that are very similar to the antigens of the donor kidney. This can lead to long waits for a transplant for many kidney patients while compatible donors become available - during which time patients must undergo dialysis.

Question 28

What are immuno-suppressant drugs?

Answer 28

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 getting infections.

Question 29

What are the advantages of kidney transplant?

Answer 29

Patients can lead a more normal life without having to watch what they eat and drink
Cheaper for the NHS overall

Question 30

What are the disadvantages of kidney transplant?

Answer 30

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

Question 31

What are the advantages of kidney dialysis?

Answer 31

Available to all kidney patients (no shortage)

No need for immune-suppressant drugs

Question 32

What are the disadvantages of kidney dialysis?

Answer 32

Patient must limit their salt and protein intake between dialysis sessions
Expensive for the NHS
Regular dialysis sessions – impacts on the patient’s lifestyle

Question 33

How does our body maintain a constant temperature?

Answer 33

Temperature receptors in the skin detect changes in the external temperature. Sensory and relay neurones transmit this information as impulses to the thermoregulatory centre of the brain – the area of the brain responsible for monitoring and controlling temperature.
The thermoregulatory centre also has temperature receptors which detect changes in the temperature of the blood flowing through the brain.
In the event of a change in temperature away from 37oC, the thermoregulatory centre sends electrical impulses to effectors (predominantly in the skin) which bring about responses that correct the temperature back to 37oC.

Question 34

Responses to an increase in temperature?

Answer 34

If the body temperature rises, the thermoregulatory centre’s receptors detect this and coordinate responses which lower the temperature back to 37oC.
Sweat glands secrete sweat onto the skin. The evaporation of sweat requires heat energy, which in turn cools the skin down.
Vasodilation occurs – the muscles in the wall of the blood vessels supplying the skin capillaries relax causing the blood vessel to dilate. This increases the flow of blood into the capillaries and allows more blood to flow near the surface of the skin. This in turn increases the amount of heat lost by radiation and results in the skin appearing red and flushed.
Hair muscles relax. Hairs lie flat so heat can escape.

Question 35

Responses to a decrease in temperature?

Answer 35

If the body temperature falls, the thermoregulatory centre’s receptors detect this and coordinate responses which raise the temperature back to 37oC.
To do this, electrical impulses are sent, via relay and motor neurones , to effectors in the skin and muscles. This causes muscles attached to our skeleton to start to shiver. Shivering - the rapid contraction of muscles - requires muscles to increase the rate of respiration. This increase in respiration generates more waste heat to warm the body back up.
Vasoconstriction occurs – the muscles in the wall of the blood vessels supplying the skin capillaries contract causing the blood vessel to constrict. This reduces the flow of blood into the capillaries and allows less blood to flow near the surface of the skin. This in turn decreases the amount of heat lost by radiation and results in the skin appearing pale.
Hair muscles pull hair erect, causing hot air to become trapped.

Question 36

NOTE: about vasoconstriction and vascodilation

Answer 36

Note that the capillaries themselves do not constrict/dilate – it is the blood vessels supplying the capillaries that do this. Nor do the blood vessels move closer to/further from the skin surface. These are two common mistakes made in exams.

Question 37

What does the pancreas do?

Answer 37

The pancreas monitors and controls the concentration of glucose in the blood.

Question 38

What happens if there is too much glucose in the blood?

Answer 38

In response to an increase in blood glucose level above the normal level, the pancreas produces a hormone called insulin which is released into the bloodstream.
Insulin causes glucose to move from the blood into cells, where it is either used for respiration or stored in liver and muscle cells as glycogen. The effect of this is to lower the blood glucose concentration back to normal.

Question 39

What happens if there is too little glucose in the blood?

Answer 39

Then the pancreas does not produce insulin, so less glucose is absorbed by body cells, so the level of blood glucose increases.

Question 40

What causes type 1 diabetes?

Answer 40

when the pancreas does not produce enough insulin. The body is therefore unable to lower blood sugar level when it rises too high.

Question 41

How do type 1 diabetes sufferers control their blood glucose level?

Answer 41

by being careful with their diet (eating foods which will not cause big spikes in their blood sugar level) and by exercising (which can lower blood glucose levels due to increased respiration in the muscles).
However, Type 1 diabetics must also inject insulin to control their blood glucose level. This requires a person to conduct a blood test to provide a reading of their blood glucose level (using a blood glucose meter), from which they can then work out the dose of insulin they are required to inject.

Question 42

What are the two ways to inject insulin?

Answer 42

Traditionally, diabetics have had to inject themselves with multiple injections of insulin throughout the day to try to regulate their blood sugar level.
However, some diabetics now wear an insulin pump. This supplies insulin continuously at low levels and can be programmed to adjust the supply at meal times or times of exercise.

Question 43

What are the advantages of injecting insulin multiple times a day?

Answer 43

Equipment is cheaper

More discrete as needles, insulin and blood glucose monitor are easy to conceal

Question 44

What are the disadvantages of injecting insulin multiple times a day?

Answer 44

Uses more insulin
Does not control blood glucose levels as well, leading to more swings in blood glucose levels (which can lead to health effects).
Requires more careful control of diet and exercise

Question 45

What are the advantages of a blood pump?

Answer 45

Allows the delivery of more precise volumes of insulin – and therefore offers better control of blood glucose level
Reduced risk of long-term effects of diabetes (due to better control
Uses less insulin
More flexibility over diet and exercise
Computer keeps accurate record of insulin usage history

Question 46

What are the disadvantages of a blood pump?

Answer 46

Equipment is more expensive
Pump may be uncomfortable to wear and may present problems for some activities, eg contact sports
Users may have to do more blood glucose tests per day to identify if pump is working effectively
Increased amounts of scar tissue around injection sites

Question 47

When might the level of blood glucose fall beneath normal?

Answer 47

following periods of exercise, or when you have not eaten for a while

Question 48

What happens when there is a fall in the level of blood glucose?

Answer 48

The pancreas detects the fall in the blood glucose level and releases another hormone, glucagon. This causes the cells in the liver to turn some of the stored back into glucose which can then be released into the blood. The blood sugar levels will then rise back to a normal level.