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Concentration gradient

The difference in concentration between two areas

Respiratory system

The system of organs including the nose, mouth, trachea, bronchi, bronchioles and alveoli through which air is taken into the body

Epithelial cells

Cells that cover the internal and external surfaces of organs


A micro-organism that causes disease.


The spreading out of the particles of a gas or any substance in solution down a concentration gradient


Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus


The tiny air sacs which make up the structure of the lungs. The site of gaseous exchange.


A group of cells in an organism that are specialised to work together to carry out a particular function.


A structure with a particular function which is made up of different tissues.

The breathing system

Aerobic respiration and the need for a breathing system

The glucose used in cellular respiration comes from the food you eat which is broken down in your digestive system. It passes into the blood by diffusion and is carried around your body to the cells. The oxygen needed by the cells comes into the body in the air moved into the breathing system. It passes into the blood by diffusion and is carried around the body by the circulatory system. The oxygen moves into the cells from the blood down a concentration gradient.

The carbon dioxide produced during cellular respiration is poisonous so it must not be allowed to build up in the cells. It passes into the blood stream by diffusion down a concentration gradient and is carried back to the lungs in the blood. There the carbon dioxide passes into the air by diffusion and is removed from the body in the air which is breathed out.

In a small, single celled organism such as an Amoeba, simple diffusion is enough to supply the needs of the cell. Useful substances such as oxygen can diffuse in from the surroundings and waste products can diffuse out fast enough for the organism to survive successfully.

However as organisms get bigger the surface area to volume ratio gets smaller. As a result simple diffusion is no longer enough. The surface area is not large enough for sufficient diffusion to take place, particularly in active organisms which need a lot of oxygen and make a lot of carbon dioxide. A specialised breathing system linked to a transport system becomes necessary.

The link between size and surface area:volume ratio

The link between size and surface area:volume ratio

The breathing system

A specialised breathing system makes sure that oxygen is brought into the body and carbon dioxide is removed. The breathing system is also known as the respiratory system but this can cause confusion. Respiration takes place in the cells and breathing takes place in the airways and the lungs - they are not the same thing.

A simple model of the human breathing system

A simple model of the human breathing system

The structures of the breathing system are closely related to their function:

  • Nose: contains nasal passages with a large surface area, rich blood supply, hairs and a lining that secretes mucus. The hairs and mucus filter out much of the pollen and dust we breathe in, the moist surfaces increase the humidity of the air and the rich blood supply warms it so the air we take in is already warm, moist and clean before it reaches the delicate tissue of our lungs.
  • Mouth: air can move in or out through the mouth but it does not warm and clean incoming air in the same way as the nose.
  • Larynx: the voice box – if we pass the air we breathe out over the vocal cords it makes the sounds we use to speak.
  • Trachea: the biggest airway which is held open by incomplete rings of cartilage. It is lined with cells carrying cilia which beat to move mucus towards the throat where it is swallowed. The mucus traps bacteria and dirt from the air which are carried away from the lungs by the cilia with the mucus.
  • Bronchus (plural bronchi): The trachea splits into these two tubes which carry air to the lungs. Like the trachea, they are supported by cartilage rings and are lined with cilia which move mucus, dirt and pathogens away from the lungs.
Ciliated epithelial cells move mucus, dirt and pathogens out of the breathing system, protecting the lungs from damage and disease

Ciliated epithelial cells move mucus, dirt and pathogens out of the breathing system, protecting the lungs from damage and disease

  • Bronchioles: smaller tubes with no cartilage support and no cilia. They branch repeatedly, becoming smaller and smaller until they lead to the alveoli.
  • Alveolus (plural alveoli): tiny air sacs with a large surface area where gaseous exchange takes place.
  • Lung: the organ where gaseous exchange takes place is made up of millions of alveoli.
  • Pleural membranes: membranes which slide over each other and help the lungs to inflate easily.
  • Diaphragm: a muscular and fibrous sheet which divides the thorax (chest cavity) from the abdomen. It is very important in breathing movements.
  • Ribs: bones which form a protective cage around the heart and lungs.
  • Intercostal muscles: these contract and relax to move the rib cage in breathing movements.