GCSE Biology: Breathing and gas exchange

Justina:The average human will take between 17,000 and 30,000 breaths per day, depending on how active you are.

Georgina: Wow! That is a lot.Breathing, or ventilation, to use the more scientific term, is the physical movement of air between the outside environment and the lungs.

Justina: This process of ventilation allows air to pass into and out of the body, enabling gas exchange and respiration.

Georgina: Let’s take a closer look at how ventilation and gas exchange works inside the body.First up, is ventilation.This occurs in the thorax.When we inhale, air is taken in through the nasal cavity or mouth, before moving down the trachea, through the bronchi and into the lungs.

Justina: And then, when we exhale, air moves from the lungs, back through the bronchi, up through the trachea and out through the nasal cavity or mouth.The process of ventilation occurs because air moves from a higher pressure area to a lower pressure area, down a pressure gradient.

Georgina: When you inhale, the intercostal muscles contract and pull the ribcage up and out.The diaphragm contracts at the same time, pulling downwards.

Justina: The volume of the thorax increases and the air pressure inside the lungs decreases, which means air is drawn into the lungs, down the pressure gradient.

Georgina: And when we exhale, the opposite happens.The intercostal muscles relax, pulling the ribcage down and inwards and the diaphragm relaxes, creating an upwards dome shape.The volume of the thorax decreases, increasing the pressure inside the lungs and the air is pushed out.

Justina: Interesting fact; our left lung is slightly smaller than the right lung as it has to share space in the body with the heart.The trachea is really fascinating too.The trachea walls are lined with goblet cells and cilia.The goblet cells produce mucus that traps dirt, dust, pollen and bacteria, and the cilia are small hairs that beat and push the mucus back up the trachea so it can be swallowed and destroyed by hydrochloric acid in the stomach.

Georgina: This cleans the air before it flows through the two bronchi and into the lungs.

Justina: Now, let’s take a look at gas exchange that happens inside the lungs and beyond.

Georgina: One of the most important gases we breathe in is oxygen.Oxygen is used along with glucose, throughout the body, for aerobic respiration; a process that releases energy for us to use.

Justina: Oxygen molecules are inhaled with a mixture of other gases: down the trachea, through the bronchi to the bronchioles, and into the alveoli, which are tiny air sacs in the lungs.There are hundreds of millions of alveoli in the lungs, which if spread out next to each other, would cover the surface of a tennis court.

Georgina: Oxygen molecules diffuse out of the alveoli through its thin walls, and into the red blood cells in the surrounding capillaries.

Justina: The alveoli are just one cell thick, which reduces the diffusion distance.The spherical shape of the alveoli increases the surface area to volume ratio, increasing the rate of gas exchange to and from the blood, within the capillaries.

Georgina: The many capillaries provide a rich supply of blood, transporting deoxygenated blood into the alveoli and the newly oxygenated blood away from the alveoli.

Justina: The opposite happens with carbon dioxide.It travels through the capillaries to the alveoli walls, where it diffuses into the alveoli and then it is removed as air is exhaled.

Georgina: The diffusion process of oxygen in and carbon dioxide out works due to a steep concentration gradient.There is a far higher concentration of oxygen in the alveoli than in the arriving deoxygenated blood, which ensures the oxygen molecules diffuse into the blood at a rapid rate.

Justina: I’m out of breath just thinking about all the work going on with every breath we take.

Georgina: It’s amazing really.And it’s oxygen that’s absorbed through the lungs and into the blood that enables aerobic respiration to happen in cells throughout the body.

Justina: I’m feeling energised just thinking about it.