Pulmonary Circulation

What is pulmonary circulation ?

Pulmonary circulation is the system of blood vessels and associated tissues forming the part of the cardiovascular system that carries oxygen-poor ("deoxygenated") blood away from the heart, to the lungs, and returns oxygen-rich ("oxygenated") blood from the lungs back to the heart.

One way to describe pulmonary circulation is using a diagram - it helps to explain pulmonary circulation with the aid of a diagram.


Diagram of pulmonary circulation

Mammals have double circulatory systems in which blood passes through the heart twice during each complete circuit of the body. This can be described in terms of two 'circuits' through the heart:

Above: Very simple diagram of pulmonary circulation in humans

Describe Pulmonary Circulation in words ...

This diagram and pulmonary circulation can be summarized as follows:

Deoxygenated Blood
(more accurately 'oxygen-poor' blood)

  • Deoxygenated blood leaves the right ventricle (RV) of the heart via the pulmonary artery.
  • The pulmonary artery divides into two main branches, the right pulmonary artery and the left pulmonary artery, which carry deoxygenated blood to the right and left lungs respectively. In both cases deoxygenated blood is taken to the alveolar capillaries of the lungs via networks of arteries and arterioles in each lung.
  • While blood is flowing through the alveolar capillaries gaseous exchange (see also features of exchange surfaces) takes place. During this process, carbon dioxide leaves the blood and oxygen enters the blood. Therefore when the blood moves on from the alveolar capillaries it is oxygenated blood - more *accurately described as 'oxygen-rich blood' (see below re. "oxygenated vs oxygen-rich").

Oxygenated Blood (more accurately 'oxygen-rich' blood)

  • The newly oxygenated blood in the alveolar capillaries flows on into venules leading to pulmonary veins.
  • Pulmonary veins from each lung return oxygen-rich blood to the left atrium (LA) of the heart.
  • From the left atrium of the heart, blood passes through the bicuspid valve into the left ventricle (LV) of the heart, then on to the systemic circulation part of the cardiovascular system.

* 'Oxygenated' and 'deoxygenated' vs 'oxygen-rich' and 'oxygen-poor'

The terms 'oxygenated' and 'deoxygenated' are widely used and understood. However, the terms 'oxygen-poor' and 'oxygen-rich' are sometimes preferred. This is because the latter terms better reflect the continuum between blood that has very little oxygen, i.e. is 'oxygen-poor', and blood that contains a lot of oxygen, i.e. 'oxygen-rich'. Conversely, the term 'deoxygenated' might seem to suggest that 'deoxygenated blood' contains no oxygen at all, which is not the case - it contains some, but very little oxygen compared with 'oxygenated' blood.

More about the structure and functions of the heart and vascular system is included on other pages.

See also systemic circulation and heart problems.

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