5.2.1: Excretion

Question 1

What is excretion?

Answer 1

The removal of metabolic waste from the body.

Question 2

What is metabolic waste?

Answer 2

A substance that is produced in excess by the metabolic processes in cells; it may become toxic.

Question 3

Why do nearly all products produced in excess by the chemical processes occurring in the body need to be removed quickly?

Answer 3

So that they do not build up and inhibit enzyme activity or become toxic.

Question 4

What are the main excretory products?

Answer 4

• Carbon dioxide from respiration.
• nitrogen-containing compounds, such as urea (i.e.nitrogenous waste)
• other compounds, such as the bile pigments found in faeces.

Question 5

Name four excretory organs.

Answer 5

• The lungs
• The liver
• The kidneys
• The skin

Question 6

What do the lungs excrete and how?

Answer 6

• Carbon dioxide.
• every cell in the body produces carbon dioxide as a result of respiration.
• The carbon dioxide passes from the cells of respiring tissues into the bloodstream where it is transported to the lungs.
• In the lungs, the carbon dioxide diffuses into the alveoli to be excreted as you breath out.

Question 7

What does the liver excrete and how?

Answer 7

• It has many metabolic roles and some of the substances produces will be passed into the bile for excretion with the faeces, For example, the pigment bilirubin.
• Also involved in converting excess amino acids into urea.

Question 8

How is excess amino acids converted into urea?

Answer 8

-Amino acids are broken down by the process of deamination. The nitrogen-containing part of the molecule is then combined with carbon dioxide to produce urea.

Question 9

What happens to urea once it is excreted from the liver?

Answer 9

Urea is passed into the bloodstream to be transported to the kidneys. urea is transported in solution - dissolved in the plasma.

Question 10

What do the kidneys excrete and how?

Answer 10

In the kidneys, the urea is removed from the blood to become a part of the urine. Urine is stored in the bladder before being excreted from the body via the urethra.

Question 11

What are some substances that sweat contains?

Answer 11

Sweat contains a range of substances including…

• salts
• urea
• water
• uric acid
• ammonia.

Question 12

Which substances in sweat are excretory products?

Answer 12

• Urea
• Uric acid
• Ammonia

Question 13

Why is the loss of water and salts an important part of homoeostasis?

Answer 13

It contributes to maintaining the body temperature and the water potential of the blood.

Question 14

Why could allowing the products of metabolism to build up be fatal?

Answer 14

• Because some metabolic products such as carbon dioxide and ammonia are toxic and they interfere with cell processes by altering the pH, so that the normal metabolism is prevented.
• Other metabolic products may act as inhibitors and reduce the activity of essential enzymes.

Question 15

What is most carbon dioxide transported in the blood as?

give equations

Answer 15

Hydrogencrbonate ions. However, forming hydrogencarbonate ions also forms hydrogen ions.
CO2 + H2O —> H2CO3 (carbonic acid)
The carbonic acid dissociates to release hydrogen ions:
H2CO3 —> H+ + HCO3- (hydrogencarbonate)

Question 16

where does carbonic acid dissociate to release hydrogen ions?

Answer 16

In the red blood cells, under the influence of the enzyme carbonic anhydrase, but can also occur in the blood plasma.

Question 17

How do hydrogen ions reduce the affinity of haemoglobin for oxygen?

Answer 17

-The hydrogen ions affect the pH of the cytoplasm in the red blood cells.
-The hydrogen ions interact with the bonds within haemoglobin, changing its tertiary structure.
This reduces the affinity of haemoglobin for oxygen, affecting oxygen transport.

Question 18

How are haemoglobinic acid and carbaminohaemoglobin formed?

How do they affect oxygen transport?

Answer 18

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid.
• Carbon dioxide that is not converted to hydrogencarbonate ions can combine directly with haemoglobin, producing carbaminohaemoglobin.
• Both are unable to combine with oxygen as normal which reduces oxygen transport further.

Question 19

Why is maintaining the pH of the blood plasma essential?

Answer 19

Because changes could alter the structure of the many proteins in the blood that help to transport a wide range of substances around the body.

Question 20

What acts as a buffer in the blood?

Answer 20

proteins in the blood plasma act as buffers to resist changes in pH.

Question 21

What is the response when the pH change is small?

Answer 21

• The extra hydrogen ions are detected by the respiratory centre in the medulla oblongata of the brain.
• This causes an increase in the breathing rate to help remove the excess carbon dioxide.

Question 22

What happens when the blood pH drops below 7.35?

Answer 22

• Headaches -Drowsiness -restlessness -tremor
• confusion
• There may also be a rapid heart rate and changes in blood pressure.
• These are symptoms of respiratory acidosis.

Question 23

What can cause respiratory acidosis?

Answer 23

• Diseases or conditions that affect the lungs themselves, such as emphysema, chronic bronchitis, asthma or severe pneumonia.
• Blockages of the airway due to swelling, foreign objects or vomit can also induce respiratory acidosis.

Question 24

The body cannot store amino acids, however, it would be a waste to excrete the excess amino acids. Why?

Answer 24

Because amino acids contain almost as much energy as carbohydrates.

Question 25

What happens to excess amino acids?

Deamination

Answer 25

-They are transported to the liver and the potentially toxic amino group is removed (deamination).

amino acid + oxygen —> keto acid + ammonia

Question 26

How is urea formed?

Answer 26

-The highly toxic and very soluble ammonia is converted to the less toxic compound called urea, which can be transported to the kidneys for excretion.

ammonia + carbon dioxide —> urea + water
2NH3 + CO2 —> (NH2)2CO + H2O

Question 27

What happens to the remaining keto acid produced in deamination?

Answer 27

-The remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.