Gas exchange in fish, insects, plants and humans

In a nutshell

Gas exchange occurs in all organisms and involves various organs and structures in the body. This summary explores gas exchange in insects, plants and humans as well as the specific adaptations of each.

Adaptations

Gas exchange surfaces are adapted to carry out their function.

Adaptation	Description
Large surface area	Gas exchange surfaces have a large surface area which increases the rate of diffusion.
Thin	Gas exchange surfaces are often one cell thick therefore the diffusion distance is very short. This also increases the rate of diffusion.
Steep concentration gradient	The gas that is exchanged is quickly transported away from the exchange surfaces. This helps to maintain a steep concentration gradient and increases the rate of diffusion.

Gas exchange in fish

The method of gas exchange in fish is known as the counter-current system. Fish have various adaptations to facilitate this exchange.

Biology; Exchange and transport; KS5 Year 12; Gas exchange in fish, insects, plants and humans

1.	Water enters the fish through its mouth.
2.	The water passes back out of the fish through its gills.
3.	Each gill has branches known as gill filaments (or primary lamellae). These increase the surface area for gas exchange.
4.	The gill filaments are covered with gill plates (or secondary lamellae). These provide a further increase in the surface area for gas exchange.
5.	The gill is then supported by a gill arch.
6.	Gill plates are formed of thin cells and they have a good capillary network. This increases the rate of diffusion.
7.	Fish have a counter-current system. This means that the water will flow in one direction and blood will flow in the other direction. The benefit of this is that it increases the concentration gradient. This is because the concentration of oxygen is higher than in the blood.

Gas exchange in insects

Unlike fish, insects use tracheae for gas exchange.

Note: Tracheae is the plural of trachea.

Biology; Exchange and transport; KS5 Year 12; Gas exchange in fish, insects, plants and humans — 1. Thorax, 2. Abdomen, 3. Tracheae and tracheoles, 4. Spiracle, 5. Trachea, 6. Carbon dioxide diffusing into the tracheoles, 7. Tracheole is close to the cells to reduce the diffusion distance, 8. Oxygen diffuses into the respiring cells.

A.	Air enters the tracheae through pores on the insect's body called spiracles.
B.	Oxygen will travel down its concentration gradient towards the cells.
C.	Carbon dioxide travels down its concentration gradient towards the spiracles so it can be released from the insect and into the air.
D.	Tracheae branch into tracheoles which increases the surface area for gas exchange. These have thin and permeable walls and contain fluid for oxygen to dissolve into.
E.	Oxygen will diffuse out of the fluid and into the insect's body cells.
F.	Insects use rhythmic abdominal movements to change their body volume and move air in and out of the spiracles.

Gas exchange in plants

Mesophyll cells in the leaves contain pores called stomata. These are able to open, allowing gases to be exchanged and close to control water loss. The opening and closing of the stomata are controlled by guard cells.

Sufficient water	When a plant has sufficient water, guard cells become turgid and cause stomata to open. This allows gas exchange to occur and water to be lost.
Insufficient water	When a plant has insufficient water guard cells become flaccid and stomata close, preventing any further water loss.

Xerophytes

Xerophytic plants are adapted to survive in environments with little water. Some of their adaptations are listed below.

Adaptation	Explanation
Fewer stomata	Fewer stomata results in less water loss during gas exchange.
Stomata located in pits and curled leaves	Stomata in pits can trap moist air. Stomata in curled leaves are less susceptible to the wind. This decreases the rate of evaporation and lowers the concentration gradient between the air and the leaf.
Hairs on the epidermis	Hairs can trap moisture in the air around the stomata. This decreases the water loss.
A waxy cuticle	The waxy cuticle makes the leaf waterproof and reduces the rate that water is lost.

Gas exchange in humans

In humans, the lungs are the primary organ involved in gas exchange. They allow oxygen to be taken in and carbon dioxide to be released.

Biology; Exchange and transport; KS5 Year 12; Gas exchange in fish, insects, plants and humans

1.	Nose	Air is breathed in through the nose.
2.	Mouth	Air is also breathed in through the mouth.
3 and 4.	Trachea	The trachea is the main airway that splits into bronchi and enters the lung.
5.	Bronchi	The bronchi are the two airway divisions that form from the trachea. The bronchi are supported by cartilage.
6.	Lungs	The lungs are a pair of respiratory organs that are vital for ventilation.
7.	Bronchioles	Bronchi divide into smaller passages called bronchioles. They are primarily made up of muscles which allows them to constrict and dilate to control the volume of air entering and leaving the alveoli.
8.	Diaphragm	The diaphragm is a muscle that controls the volume of air in the thorax. Contraction of the diaphragm increases the volume of air in the thorax.
9.	Alveoli	The bronchioles lead to the alveoli which are small air-sacs where gas exchange occurs. The alveolar walls have a lot of elastic fibres which allows the alveoli to stretch (to fill with air) and then recoil (to expel air). *Note:* Alveolus is the singular term for alveoli.

The following components are not pictured in the diagram but they are also important for ventilation in humans.

Ribcage	The ribcage supports and protects the lungs. The ribs can be moved by the muscles in between them. These are called the intercostal muscles.
Internal intercostal muscles	The contraction of the internal intercostal muscles leads to expiration.
External intercostal muscles	The contraction of the external intercostal muscles leads to inspiration.

Ventilation

Ventilation is the process by which air is inspired (breathed in) and expired (breathed out). The intercostal muscles, diaphragm and rib cage also work together to control ventilation in humans.

Biology; Exchange and transport; KS5 Year 12; Gas exchange in fish, insects, plants and humans — A:The process of inhaling, B: the process of exhaling, 1. Top of the diaphragm, 2. Skeletal muscle of the diaphragm.

Inspiration (A)	Expiration (B)
The external intercostal muscles contract.	The internal intercostal muscles contract.
The ribcage is moved upwards and outwards which increases the volume of the thorax.	The ribcage is moved downwards and inwards which decreases the volume of the thorax.
The diaphragm muscles contract which causes the diaphragm to move downwards and flatten. This also increases the volume of the thorax.	The diaphragm muscles relax which causes the diaphragm to move upwards and curve. This also decreases the volume of the thorax.
The increase in thoracic capacity causes a decrease in lung pressure below atmospheric pressure.	The decrease in thoracic capacity causes an increase in lung pressure above atmospheric pressure.
Air flows from the trachea to the lungs down the pressure gradient.	Air is pushed out of the lungs down the pressure gradient.

Alveoli

The table below explains how gases are exchanged in the alveoli.

Biology; Exchange and transport; KS5 Year 12; Gas exchange in fish, insects, plants and humans — 1. Air from the trachea, bronchi and bronchioles, 2. Alveolar space, 3. Deoxygenated blood, 4. Capillary, 5. Red blood cell

A.	Oxygen from the air travels down the trachea, bronchi and bronchioles into the alveoli.
B.	From the alveoli, the oxygen diffuses across the alveolar epithelium.
C.	Oxygen then diffuses through the capillary endothelium and into the capillary.
D.	Oxygenated blood is then transported to the heart.
E.	Carbon dioxide diffuses from the capillaries into the alveoli. This is then breathed out.