Gas Exchange & Breathing

Breathing brings oxygen into the body and removes carbon dioxide produced by cells. Blood carries these gases between the lungs and body tissues.

Air can enter at the nose or mouth. Nasal passages warm and moisten it. Hairs and mucus catch many dust particles and germs, although this filtering does not stop everything. After passing through the throat, air reaches the windpipe, called the trachea. Cartilage rings support this tube so it stays open.

In the upper chest, the trachea divides into a bronchus for each lung. Each bronchus has smaller branches called bronchioles. Their branching shape resembles a tree. Near the ends are millions of alveoli, the tiny sacs used for gas exchange.

At these sacs, blood receives oxygen from inhaled air and releases carbon dioxide into the air. Exhaling carries that carbon dioxide out.

Many small air sacs give the lungs a large exchange surface. A commonly used estimate is about 300 million alveoli across both lungs, providing roughly 75 square metres of surface altogether. These estimates vary with lung size and inflation. Packing the surface into tiny pockets lets a large area fit within the chest. Many gas particles can cross this surface at once.

An alveolar wall is about a single cell thick. Capillaries, which are tiny blood vessels, lie close around each sac. These thin barriers keep the diffusion distance short.

Diffusion comes from random particle motion. Each gas has a net movement down its own gradient. For gases, the relevant gradient is partial pressure: the share of pressure contributed by that gas. Oxygen moves into arriving blood because its partial pressure is higher in alveolar air. Carbon dioxide has the opposite gradient, so it enters the sacs and is exhaled.

Moving air through the airways is a separate process. Chest expansion lowers lung air pressure below outside pressure, drawing air inward. As the chest becomes smaller during relaxed breathing out, lung pressure rises above outside pressure and air leaves.

Below the voice box, air travels through a branching network that reaches deep into the lungs. Its main parts are the trachea, bronchi, bronchioles and alveoli. The trachea connects the throat region to the bronchi; it is not itself part of either lung.

Each of the two main bronchi enters a lung. Repeated branching creates the narrower bronchioles. This network is called the bronchial tree because its shape resembles tree branches.

Millions of alveoli form small clusters near the ends of the airways, rather like clusters of grapes. These sacs provide the exchange surface. Inhaled oxygen enters nearby blood, while carbon dioxide passes into the sacs from the blood. Breathing out removes this waste gas.

Trillions of cells make up your body. They need nutrients and a way to remove waste, and most use oxygen. The respiratory and circulatory systems work together to meet these needs.

Breathing supplies air through the nose or mouth. The nose helps warm and filter it. Air continues through the throat to the trachea, then follows a bronchus into each lung. Smaller bronchioles lead toward the alveoli, where oxygen enters blood.

The heart keeps blood moving through vessels. Blood arriving at the lungs carries carbon dioxide from body tissues. At the alveoli it releases that gas and gains oxygen. The oxygen-rich blood returns to the heart, which pumps it onward to the tissues.

Air movement and blood flow have different roles. Without blood flow, oxygen entering the lungs would not reach distant tissues. Without breathing, the lungs would not keep receiving fresh oxygen or removing carbon dioxide. Circulation also delivers nutrients and carries waste from cells.

The respiratory system is the body system that swaps gases between your body and the outside air. Air first passes the pharynx, which is the tube at the back of your throat, and then the larynx, your voice box. From there it enters the trachea, the windpipe, a tube that connects the larynx to the lungs.

Inside the chest, the trachea splits into two branches called bronchi, one for each lung. The bronchi divide into smaller airways called bronchioles. At the very ends of the bronchioles are the alveoli, tiny air sacs where gas exchange happens. Oxygen moves from the air in the alveoli into your blood, and carbon dioxide moves the other way, from your blood into the air, so you can breathe it out.

Below your lungs sits a flat muscle called the diaphragm. When the diaphragm and the muscles between your ribs pull to make your chest bigger, the air pressure inside your lungs drops and air flows in. When those muscles relax and your chest gets smaller, the pressure rises and air flows out. That squeeze and release is the whole mechanism of breathing.