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:

  • Pulmonary Circulation - oxygen-poor blood is pumped from the heart to the lungs and oxygenated blood returns to the heart from the lungs
  • Systemic Circulation - oxygen-rich blood is pumped from the heart around the body. That blood returns to the heart deoxygenated (more accurately 'oxygen poor') because much of the oxygen it contained when it left the heart has been supplied to tissues in the body.

Above: Very simple diagram of pulmonary circulation in humans

Describe Pulmonary Circulation in words ...

Notes about human pulmonary circulation:

The above diagram is a much simplified representation of the human pulmonary circuit. It has been simplified for clarity. The following statements add some anatomical details.

  • Re. Entry of blood into the right atrium (RA): As shown, deoxygenated blood returning to the heart from the systemic circulation arrives at the RA via the superior vena cava and the inferior vena cava. Each of these 2 veins has its own opening directly into the RA.
  • Re. Pulmonary veins: FOUR pulmonary veins open into the left atrium (LA) of the heart; TWO pulmonary veins from each of the right and left lungs (although the 2 left veins are often terminated by a common opening into the LA).
  • Re. Heart Structure: The RA is slightly larger than the LA, though its walls are thinner. The RV has an approximately triangular shape. The LV is longer and has a more conical shape than the R V (these differences are not shown in the diagram above).

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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