B12 Homeostasis in action

Question 1

Why does the human body need to maintain a temperature around 37°C

Answer 1

Because it is the temperature at which enzymes work best

Question 2

Where is body temperature monitored and controlled in the body?

Answer 2

Body temperature is monitored and controlled by the thermoregulatory centre in the hypotalamus in the brain

Question 3

How is the body temperature controlled by the body?

Answer 3

• The thermoregulatory centre contains receptors sensitive to the temperature of the blood
• The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre giving info about the skin temperature. These receptors are very sensitive and they can detect changes in temperature as small as 0.5 degrees

Question 4

What happens when the temperature is too high?

Answer 4

If the body temperature is too high, blood vessels dilate (vasodilation) and so more blood flows through the capillaries. Your skin flushes so you transfer more energy through radiation from your skin to the surroundings, cooling you down and warming the air around you.

Also, sweat is produced from the sweat glands in the skin. This cools your body down as water evaporates from your skin, transferring energy to the environment. In humid weather when the water in sweat does not evaporate, it is harder to keep cool.

Both these mechanisms cause a transfer of energy from the skin to the environment, cooling the body down

Question 5

What happens when the temperature is too low?

Answer 5

If the body temperature is too low, blood vessels constrict (vasoconstriction), to reduce blood flow through the capillaries. This reduces the energy transferred by radiation to the surroundings.
Also sweating stops or is reduced. Less water from sweat evaporates , so less energy is transferred to the environment.
Your skeletal muscles contract and relax rapidly causing you to shiver. This needs lots of respiration, an exothermic process. The energy transferred from these reactions raises your body temperature until shivering stops.

Question 6

Exam tip on Body Temperature

Answer 6

NEVER SAY that capillaries dilate or constrict. They cannot do this as they have no muscle layer. It is the blood vessels supplying the capillaries that dilate or constrict.

Question 7

Where does the waste in the body come from?

Answer 7

They are the products of chemical reactions that take place in the cells

Question 8

What are the two main poisonous waste products of the body?

Answer 8

• Carbon Dioxide
• Urea

Question 9

Why does CO2 need to be removed by the body?

Answer 9

It must be removed because dissolved CO2 produces an acidic solution that would affect how enzymes work.

Question 10

How is CO2 removed from the body?

Answer 10

1. CO2 diffuses out of the cells into the blood down a concentration gradient
2. It then diffuses from the blood into the air in the alveoli
3. This air containing CO2 is then removed from your body when you exhale.

Question 11

What is a side effect of exhalation?

Answer 11

You lose water when the moist air from inside your lungs is forced out of the body.

Question 12

What is urea?

Answer 12

It is the nitrogenous waste produced by the breakdown of excess amino acids in your liver.

Question 13

Why is urea poisonous?

Answer 13

It contains nitrogen and passes from the liver to the blood and its build up can cause extensive damage to cells

Question 14

How is urea expelled from the body?

Answer 14

It is filtered out of the blood through the kidneys and it is then passed out of the body in the urine produced by the kidneys along with excess water and salt.

Question 15

When is urea produced?

Answer 15

Urea is produced when you eat more proteins than you need or when your tissues are worn out.

Question 16

What is deamination?

Answer 16

Deamination is the process of breaking down excess protein and it predominantly occurs in the liver.
Enzymes in the liver split up amino acid molecules, with the part containing carbon turned into glycogen and the other part containing nitrogen (the amino part) turned into ammonia (this is why we say the amino acid has been deaminated).
Ammonia is toxic to cells and so it is immediately converted to urea which can be transported around the body via the blood safely for excretion by the kidneys. The rest of the amino acid molecule can then be used in respiration or to make other molecules

Question 17

Why does the body need to excrete the excess amino- acids?

Answer 17

The digestion of proteins from the diet results in excess amino acids which need to be excreted safely, as they cannot be stored by the body in the same way that excess glucose can.

Question 18

Why it is important that the body cells do not lose or gain too much water?

Answer 18

If body cells lose or gain too much water by osmosis they do not function efficiently:
- Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting)
- Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death

Question 19

How does the body loses water?

Answer 19

Water is lost from the body in the following ways:
- Water leaves the body via the lungs during exhalation (breathing out)
- Water, ions and urea are lost from the skin in sweat.
However, the lungs and skin have no control over how much water, ion or urea is lost via exhalation or sweating
Controlled loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine

Question 20

What are the functions of the kidneys?

Answer 20

• The kidneys help to control the water content of the body and the concentrations of substances (such as sodium and potassium ions) dissolved in the fluids of the body

Question 21

How do the kidneys work?

Answer 21

The kidney contains highly branched capillary networks that filters the blood. Glucose, urea, water and mineral ions all move out of the blood into the kidneys.
Larger molecules (such as blood cells and proteins) are too big to pass out of the filter and so remain in the blood plasma.
The kidneys then selectively reabsorb substances needed by the body back into the bloodstream (this is an active process called selective reabsorption).
In a healthy kidney, this includes all of the glucose and some ions from the filtrate.
Anything not reabsorbed forms urine, which is then stored in the bladder until it is excreted.
Urea, formed from the deamination of amino acids in the liver, is not selectively reabsorbed.

Question 22

What is meant by selective reabsorption?

Answer 22

It is the process through which the kidneys reabsorb substances needed by the body back into the bloodstream (glucose, some water and mineral ions). They do this thanks to their structure. The kidneys are in fact made up millions of tiny tubules which act as a filter.

Question 23

Exam tip

Answer 23

Exam Tip
Take care to describe clearly where substances are moving from and to in the kidneys (ie. glucose moves from the filtrate into the bloodstream when it is selectively reabsorbed. Using your technical terminology incorrectly here could lose you marks. Also – small substances such as urea are forced out of
the blood during filtration as a result of high-pressure mass flow, they don’t diffuse out of the blood.

Exam Tip
You should be able to translate tables and bar charts of glucose, ions and urea before and after filtration – expect to see this in an exam.

Question 24

Explain body parts in the kidney process

Answer 24

See picture pg 186 biology book

Question 25

How are water levels maintained at a constant level in the body?

Answer 25

Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys.
This is done by a negative feedback system involving the hormone ADH.
ADH is secreted by the pituitary gland in the brain and how much ADH is released depends on how much water the kidneys should reabsorb from the filtrate.
ADH affects the permeability of the tubules to water.
If the water content of the blood is too high, the pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water).
If the water content of the blood is too low and the blood is too concentrated, the pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water).
See diagram pg 187 of Biology book

Question 26

What happens if the kidneys are not functioning?

Answer 26

Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed.

Question 27

What can be the causes of kidney failure?

Answer 27

Infections, accidents, genetic problems are an example

Question 28

What is Dialysis?

Answer 28

Dialysis is the usual treatment for someone with kidney failure.
Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood.
As you carry on with normal life urea and other substances build up again and so the dialysis needs to be carried out at regular internals.

Question 29

How does dialysis work?

Answer 29

Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm.
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 exchange to occur between the two where a concentration gradient exists. The whole process depends on diffusion down a concentration gradient. There is no active transport involved.
See diagram pg 189 Biology book

Question 30

What does the Dialysis fluid contains?

Answer 30

Dialysis fluid contains:
- the same concentration of glucose as a person with healthy kidneys. This ensures there is no net movement of glucose out of the blood.
- normal plasma levels of mineral ions, so any excess ions are removed by the blood by diffusion down a concentration gradient.
- and no urea. This makes a steep concentration gradient from the blood to the fluid. As a result, much of the urea leaves the body.

Question 31

Talk about the diet of people with kidney issues

Answer 31

• Control protein intake carefully between dialysis sessions to keep urea levels low
• Control the amount of salt they eat as the kidneys cannot get rid of the excess mineral ions

Question 32

What are the disadvantages of dialysis?

Answer 32

• Need to follow a very controlled diet between sessions
• Very long sessions connected to the machine
• You have only a very short time when the balance of chemicals is right so towards the time of another session you might feel tired or unwell.
• Over many years the balance of substances in the blood can be come more difficult to control even with dialysis

Question 33

Advantages and disadvantages of kidney transplants.

Answer 33

Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:
- Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney with potential risk of rejection.
- Risk of rejection has to be suppressed by taking immunosupressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to
infections
- Eventually dialysis causes serious damage to the body
- There are not enough donors to cope with demand. Usually donated kidneys come from people who dies from suddenly (stoke, heart attack, accidents). People need to register as donors or a relative can give consent. Sometime a family member donates one of their kidneys.

Benefits:
- The patient has much more freedom as they are not tied to having dialysis several times a week in one place
- Their diets can be much less restrictive than they are when on dialysis
- Use of dialysis machines is very expensive and so this cost is removed
- A kidney transplant is a long term solution whereas dialysis will only work for a limited time