CIE AS Level Biology: Gas Exchange

Structure and function of human gas exchange system

Trachea: The airway that leads from the mouth and nose to the bronchi. The trachea is lined with mucus- secreting Goblet cells and cilia. The cilia sweep microorganisms and dust away from the lungs.

Lungs: Humans have two lungs,both which are a central part of the respiratory system and where gas exchange takes place.

Bronchi: The bronchi lead to bronchioles.

Bronchioles: These are narrow tubes which carry air from the bronchi to the alveoli. As they are narrow, they have no supporting cartilage and so can collapse

Alveoli: The main site of gas exchange in the lungs, These are tiny sacs with many structural adaptations to enable efficient gas exchange, such as their thin walls and large surface area to volume ratio.

Capillary network: An extensive network of capillaries surrounds the alveoli and are an exchange surface between the lungs and the blood. During gas exchange, oxygen diffuses from the alveoli and into the capillaries, while carbon dioxide diffuses the other way and is exhaled.

Role of cartilage in the trachea

Cartilage is a strong and flexible tissue found in various places around the body. One place is in rings along the trachea, called Tracheal rings. These rings help to support the trachea and ensure it stays open, while allowing it to move and flex while we breathe

Ciliated epithelium

Ciliated epithelium is a specialised tissue found along the trachea down to the bronchi. Each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs and the epithelium itself

Goblet cells

Goblet cells can be found scattered throughout the ciliated epithelium in the trachea. They are mucus-producing cells that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs. The mucus is then swept along by the cilia of the ciliated epithelium upwards and is swallowed. The mucus and any microorganisms will then be destroyed by the acid in the stomach

Squamous epithelium

The alveoli have a lining of thin squamous epithelium, that allows for gas exchange. The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases

Smooth muscle

Smooth muscle can be found throughout the walls of the bronchi and bronchioles. It helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed

Capillary network

Each alveolus is surrounded by an extensive network of capillaries. Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from alveoli and into the capillaries to be carried around the body

These capillaries have a diameter of around 3-4µm, which is only wide enough for one red blood cell to travel through at any one time. This ensures that there is sufficient time and opportunity for gas exchange to occur

Protection against pathogens

Cilia beat to move mucus away from the lungs and up towards the mouth. Mucus is produced by goblet cells and acts as a barrier to pathogens from entering the ciliated epithelium. The mucus also traps bacteria and microorganisms, while blood vessels bring macrophages to engulf any pathogens in phagocytosis.

Mucous glands

Mucous glands with the goblet cells and bronchi produce mucus

Describe and explain the cause and effect of cystic fibrosis

Cystic fibrosis is caused by a recessive allele of the CFTR gene. Cystic fibrosis affects the lungs by goblet cells producing thick, sticky and dehydrated mucus. This mucus is much harder for cilia to sweep up the trachea and to the mouth and so mucus accumulates in the airways. As the mucus traps bacteria but the mucus can't be removed, this leads to frequent infections and eventually scar tissue in the lungs. The fact that mucus obstructs the airways means that gas exchange is less efficient as it is harder for oxygen to reach the lungs and harder for carbon dioxide to be exhaled, causing breathing problems.

Gas exchange process

The exchange of oxygen and dioxide occurs between the alveoli and the capillaries in the lungs
Oxygen and carbon dioxide are exchanged in a process of simple diffusion; (passive movement from high to low concentration)
The air in the alveoli contains a high concentration of oxygen
The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration
The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide.
The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled

Features of alveoli

1-Large surface area: The enormous surface area increases the space available for oxygen and carbon dioxide to diffuse.

2-Thin walls: The walls of the alveoli are only one cell thick. This means that gases have a very short diffusion distance and so gas exchange is as quick and efficient as possible.