🫁 Human Respiratory System

The air around us is called the **atmosphere**. It contains a mixture of gases including Nitrogen, Oxygen, Carbon dioxide, water vapour, and dust particles.

Breathing is the process of taking in air (inhaling) and letting out air (exhaling). It allows organisms to survive. Respiration is what happens inside our body cells — they use oxygen to release energy from food. Breathing brings in the oxygen needed for respiration.

Photosynthesis — Plants take in carbon dioxide and give out oxygen during photosynthesis (in the presence of light).

The function of the respiratory system is to enable gaseous exchange between the surroundings and the body. It consists of three main parts:

The lungs are made up of tiny air sacs, which are surrounded by many tiny blood vessels.

The air sacs in the lungs increase the exposed surface area of the lungs, allowing gaseous exchange between the air in the lungs and the bloodstream to take place faster.

Diaphragm — Elastic sheet of muscle located below the lungs. It controls inhalation and exhalation:

• Inhalation — Diaphragm contracts and moves down, pulling air into the lungs.
• Exhalation — Diaphragm relaxes and moves up, pushing air out of the lungs.

In the lungs:

• Oxygen in the air enters the blood.
• The blood then transports oxygen to all parts of the body.
• Carbon dioxide is removed from the lungs when a person exhales.

The breathing rate is the number of breaths taken in one minute.

The rate and depth of our breaths increase when we engage in vigorous activities. These changes enable more oxygen to be taken into the lungs, and more carbon dioxide to be removed from the body.

Mammals breathe using lungs. Some animals like frogs can also breathe through their moist skin. Insects have a different respiratory system — they use spiracles and tracheal tubes.

Fish breathe underwater using gills, which allow gaseous exchange to take place in water.

• Fish take in dissolved oxygen from the water through their mouth, which is absorbed into the bloodstream at the gills.
• Carbon dioxide in the bloodstream passes through the walls of the blood vessels and is removed when water flows out from under the gill covers.

The gill filaments increase the exposed surface area of the gills, allowing gaseous exchange between the gills and the surrounding water to take place faster.

Similarities:

• Both gills and lungs allow oxygen to enter the blood and carbon dioxide to leave the blood.
• Both gills and lungs have a rich supply of blood vessels.
• Both air sacs in human lungs and gill filaments increase exposed surface area, enabling gaseous exchange to take place faster.

Differences:

• Gills take in dissolved oxygen from water, but lungs take in oxygen from the surrounding air.

Plants respire all the time. Gaseous exchange between the plant and surrounding air takes place mainly through tiny openings called stomata, found mostly on the underside of leaves.

The stomata also allows water vapour to escape.

• In the day: The tiny openings become bigger for the plant to take in more carbon dioxide to make more food.
• At night: The tiny openings become smaller to reduce water vapour loss, leaving only a small gap for gaseous exchange.

In a lung model, the bell jar represents the chest cavity, the rubber sheet at the bottom represents the diaphragm, and the balloons represent the lungs.

• Pulling down the rubber sheet (diaphragm) simulates inhalation — air enters the balloons (lungs).
• Pushing up the rubber sheet simulates exhalation — air leaves the balloons.

• Humans and plants take in oxygen from the air, while fish take in dissolved oxygen in water.
• Humans, fish, and plants all carry out respiration by taking in oxygen and giving out carbon dioxide.
• Lungs, gills, and stomata are the organs for gaseous exchange in humans, fish, and plants respectively.
• Plants carry out both respiration and photosynthesis during the day. Carbon dioxide is taken in and oxygen is given out.