B8: Gas exchange and respiration

B8.1: Gas exchange

Organs of gas exchange

Lungs: Bring oxygen into our bodies and force carbon dioxide out
Diaphragm: Contracts to become flat, and increasing the area of the chest cavity to pull air into the lungs. It then returns to its original shape to force air out of the lungs
Ribs: Arrangement of bones protecting internal organs like the lungs
Intercostal muscles: Muscles located within the ribcage. The external intercostal muscles contract to decrease air pressure and suck air inside the lungs
Larynx: An organ connected to the top of the trachea. Air passes through the larynx to the lungs. It can also make sounds through vibrations
Trachea: A windpipe connect the nose and the mouth to the lungs
Bronchi (singular: bronchus): Large branches of the trachea. There is one bronchus connected to the left lung and one connected to the right
Bronchioles: Smaller branches off the bronchi that connect to the alveoli
Alveoli: Air sacs where gas exchange takes place

The surfaces for gas exchange are specialised to performs its functions, and allow quicker diffusion of gases. They all have...

Differences between inhaled and exhaled air

The air we inhale is around 20-21% oxygen, and the air we exhale is around 16%. This is because the blood returning to the lungs will have a lower concentration of oxygen than the air in the alveoli (as oxygen diffused into cells for respiration), meaning that oxygen will diffuse to the blood in the lungs
The air we inhale is around 0.04% CO2, and the air we exhale is around 4%. This is because the blood blood returning the the lungs will have a higher concentration of carbon dioxide than the air in the alveoli (as carbon dioxide is produced from respiration in the body's cells), meaning that carbon dioxide will diffuse into the alveoli
The air we inhale has lower water vapour than the air we exhale. This is because the moist lining of the alveoli causes water to evaporate into vapour from the warmth of the body, causing the air we exhale to have more water vapour

Using limewater to test for CO₂

Air is inhaled from the tube on the left
Limewater is normally clear, but becomes cloudy when carbon dioxide is added
As the air exhaled will bubble carbon dioxide, the tube on the right will become clouded, while the tube on the left will remain clear
This shows that there is more carbon dioxide in the air exhaled than the air inhaled

Effects of physical activity on the rate and depth of breathing

Physical activity increases the depth and rate of breathing
The amount of breaths and chest expansion will increase after exercises
This happens because muscles need more oxygen to aerobically respire to provide the amount of energy needed
When there is not enough oxygen to reach the energy demand, cells respire anaerobically without oxygen, and produces lactic acid
Lactic acid denatures enzymes with its low pH and needs to be removed by combining with oxygen. Therefore after exercise, oxygen needs to be distributed to the rest of the body to remove lactic acid
When respiration increases, more carbon dioxide is released
Carbon dioxide is acidic, and could affect the activity of enzymes. Therefore, it needs to be removed
Receptor cells transmit messages to the muscles and the lungs to contract faster and stronger to lower the carbon dioxide concentration
Therefore, the depth and rate of breathing increases

Goblet cells, mucus, and ciliated cells

The passage to the lungs are lined with ciliated cells, which have hair like projectiles with sweep mucus towards the nose and the throat
The mucus traps harmful particles, bacteria, and dust to make sure these particles do not get to and damage the lungs
The mucus is made by goblet cells

Smoking

Smoking has several harms on the breathing system and can cause several diseases. Some of these are...

Smoking damages the alveoli, causing gas exchange to be slower
Therefore, those who suffer from the disease have less oxygen in their blood. In consequence, they are often tired, have more trouble breathing, and more

Tobacco contains many substances that can cause cancer (eg. tar)
Therefore, it poses threat of lung cancer (as well as cancer of the mouth, oesophagus, and throat)

Carbon monoxide combines with haemoglobin, the protein in red blood cells carrying oxygen
The ability to carry oxygen of the haemoglobin is reduced, and less oxygen could be transported, causing coronary heart disease

Carbon monoxide: Carbon monoxide combines with haemoglobin and reduces its ability to carry oxygen
Nicotine: Narrows blood vessels which causes higher blood pressure and more difficulty in transporting blood
Tar: Tar is a carcinogen, and could cause cancers in the lung, mouth, oesophagus, and throat

B8.2: Respiration

Respiration is used to breakdown nutrient molecules to release the energy stored in the bonds of the molecules
This energy is used for different functions in the body such as...

There are two types of respiration: aerobic respiration and anaerobic respiration

Aerobic Respiration

The chemical reactions in cells that use oxygen to break down nutrient molecules for energy
It completely breaks down glucose and releases quite a good amount of energy
The balanced equation for aerobic respiration is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
The word equation for aerobic respiration is Glucose + Oxygen → Carbon dioxide + Water

Anaerobic Respiration

The chemical reactions in cells that do not use oxygen to break down nutrient molecules for energy
It partially breaks down glucose and does not release as much energy as aerobic respiration
It produces different products in different usages

Anaerobic respiration in muscles

In muscles, anaerobic respiration is used when there is vigorous physical activity. As only a limited amount of oxygen can be delivered, anaerobic respiration is used to release the remainder of energy needed
When anaerobic respiration occurs in the muscles, glucose is broken down and produces lactic acid
There will be more lactic acid in muscle cells, which lowers the pH
A lower pH could threaten to denature the enzymes in the cells, and therefore lactic acid needs to be removed
The cells expel lactic acid into the blood where it carries the lactic acid to the liver to be oxidised, and turns into carbon dioxide and water
This is why we breathe heavily and have a high pulse rate after exercising (as we need to transport lactic acids from the muscles to the liver and continue delivering oxygen to oxidise lactic acid)

Anaerobic respiration in yeast

The process of anaerobic respiration in yeast is used in bread making
The balanced equation for anaerobic respiration in yeast is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
When the glucose in the sugar is added in the dough to make the bread, the glucose goes through anaerobic respiration
The carbon dioxide produced makes the bread rise, while the alcohol evaporates when the bread is baked