How does oxygen and carbon dioxide move between the alveoli and the blood?

The YouTube video below (3:09) illustrates oxygen transport from the lungs to cells throughout the body. 

Cellular Gas Exchange

Oxygen is necessary for cells to be able to generate cellular energy from glucose or fatty acids. After blood is reoxygenated in the lungs, it leaves the left ventricle of the heart and is distributed throughout the body in increasingly branched blood vessels. Arteries branch into arterioles, which, in turn, eventually branch into capillaries. The image below on the left shows an arteriole branching into smaller capillaries which supply nutrients and oxygenated red blood cells (RBCs) to metabolizing cells (shown in stippled brown). As RBCs pass through the capillaries and release oxygen to the cells, they become progressively deoxygenated (purple RBCs). At the same time, carbon dioxide and other wastes are transferred into capillary blood for excretion. Capillary blood then enters venules to be returned to the heart and lungs, where CO2 is exhaled and blood becomes reoxygenated.The image on the right shows a capillary that has been cut in cross-section. The lumen (interior) of the capillary is shown in blue with a single RBC, since capillaries are so fine that RBCs often pass through them one at a time. The endothelial cells lining the capillary are shown in green, and the metabolizing cells receiving oxygen and nutrients from the capillary are shown in stippled brown.

Our lungs are among our largest vital organs. The oxygen you breathe in goes into your lungs and passes into your blood from there. It is then transported to all the cells in your body through your bloodstream. The lungs are located in the chest region, protected by the ribs in the rib cage. Their structure can be compared to that of an upside-down tree: The windpipe branches into two airways called bronchi, which lead to the lungs. Inside the lungs, the airways keep branching into narrower airways until the air sacs are reached.

What is pulmonary circulation?

When you breathe in (inhale), air containing oxygen enters your windpipe, passes through the bronchi and eventually reaches the air sacs. These air sacs, called alveoli, are responsible for gas exchange. They look a bit like grapes at the end of the bronchial branches. Healthy lungs have about 300 million air sacs in them. Each air sac is surrounded by a network of fine blood vessels (capillaries).

The oxygen in inhaled air passes across the thin lining of the air sacs and into the blood vessels. This is known as diffusion. The oxygen in the blood is then carried around the body in the bloodstream, reaching every cell. When oxygen passes into the bloodstream, carbon dioxide leaves it. Carbon dioxide (CO2) is a waste product of cellular metabolism. You get rid of it when you breathe out (exhale). This gas is transported in the opposite direction to oxygen: It passes from the bloodstream – across the lining of the air sacs – into the lungs and out into the open.

Gas exchange in the lungs

What happens when you breathe?

When you breathe in, your chest and lungs expand. When you breathe out, your lungs get smaller again. Both of these movements are caused by the diaphragm and muscles that run between the ribs (intercostal muscles). We breathe without having to think about it.

When at rest, adults breathe 14 to 16 times per minute. About half a liter of air is inhaled during one normal breath. When you are more active, your breathing becomes faster and deeper in order to get more oxygen into your blood.

A person’s general fitness greatly depends on how well their lungs and heart work. Your lung function can be measured using various breathing tests.

The structure of the lungs

In adults, the windpipe (trachea) is about ten centimeters long and branches into two main bronchi known as the right bronchus and the left bronchus. These main bronchi then divide into smaller secondary bronchi (lobar bronchi) – three in the right lung and two in the left lung. There is less room in the left lung because it shares space with the heart.

The secondary bronchi then branch into a number of tertiary bronchi (segmental bronchi). The right lung is made up of ten areas known as bronchopulmonary segments. The left lung is made up of nine of these segments. Each segment is supplied by its own tertiary bronchus and its own branch of the pulmonary (lung) artery. This means that individual segments can be removed if necessary, for instance due to a serious lung disease or injury.

The windpipe and bronchi are lined with mucus-producing cells and millions of tiny hair-like projections called cilia. If you breathe in harmful substances like dust or other particles, the mucus and cilia ensure that they don’t stay in your lungs: Foreign matter gets caught in the mucus, and the cilia constantly move back and forth, carrying the mucus out of your lungs into your throat, where you either swallow it or cough it out. If larger foreign objects enter the windpipe, a cough reflex is triggered.

Sources

• Andreae S. Lexikon der Krankheiten und Untersuchungen. Stuttgart: Thieme; 2008.

• Menche N. (Hg.) Biologie Anatomie Physiologie. München: Urban & Fischer/ Elsevier; 2012.

• Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.

• IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

  Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. We do not offer individual consultations.

  Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

  How does carbon dioxide move from blood to alveoli?

  Once the venous blood returns to the lungs, the carbon dioxide diffuses out of the bloodstream, through the capillaries, and into the alveoli from where it is expelled, during which time oxygen simultaneously binds with hemoglobin to be carried back to the tissues.

  How does oxygen and carbon dioxide move between the blood and the muscles?

  Haemoglobin carries oxygen to be exchanged at the working muscle and carbon dioxide to be exchanged at the lung. At the muscles, the opposite occurs as carbon dioxide enters the blood from the muscle while oxygen enters the muscle from the blood. Capillaries surround the alveoli in the lungs.

  How does oxygen and carbon dioxide move with breathing?

  When you inhale (breathe in), air enters your lungs, and oxygen from that air moves to your blood. At the same time, carbon dioxide, a waste gas, moves from your blood to the lungs and is exhaled (breathed out). This process, called gas exchange, is essential to life.