Altitude Tutorials

The Lungs at High Altitude

Normally, the lungs take up oxygen from the blood as shown in the diagram to the left. Venous blood doesn’t contain much oxygen, because it is returning from the body tissues, where oxygen has been used up. It is pumped through the lungs by the heart, and as it passes through the lungs it is exposed to fresh air in tiny air spaces called alveoli. Oxygen dissolves in the blood and binds to haemoglobin. Haemoglobin changes colour when it binds to oxygen. When it isn’t carrying oxygen it is dark purple, and when it is carrying oxygen, it is red.

Shunt

This diagram shows what happens if there is a blockage to an air space in the lungs. The simplest example is a peanut stuck in one of the air passages in the lungs, but the same process happens in pneumonia, or pulmonary edema. Blood still flows past the air spaces, but because there is no fresh air getting to the blood, it can’t take up any more oxygen. That means that a lot of de-oxygenated blood makes its way straight past the lungs. When it mixes with the blood from all the other parts of the lungs, the total amount of oxygen is less. This causes hypoxia – a shortage of oxygen getting to where it is needed.

Physiological Shunt

Even in completely healthy people, there is always some blood that makes its way past the lungs without encountering any fresh air. This blood passes through the bronchial circulation, and the thebesian veins in the heart. This is called physiological shunt. You can calculate the effect of different amounts of shunt by downloading our oxygen delivery model in microsoft excel (but beware – this is a very large file).

How do the lungs cope with shunt?

We have evolved a clever mechanism to reduce the effect of shunt. When the blood passing through an area of lung isn’t picking up enough oxygen, the blood vessels carrying that blood tighten, so that less deoxygenated blood can get through the lungs. This is called hypoxic pulmonary vasoconstriction. The diagram to the right shows how this means that less deoxygenated blood gets through, so there is more oxygen in the mixture of blood leaving the lungs.

At high altitude, there is less oxygen in the air that you breathe. (Click for an explanation of why there is less oxygen at high altitude.) This means that all of the blood from all areas of the lungs, is relatively short on oxygen or hypoxic.

Unfortunately, the lungs still respond to the shortage of oxygen in the same way: by tightening the blood vessels. But because all areas of the lung are lacking in oxygen, all of the blood vessels in the lungs constrict.

The heart still pumps the same amount of blood through the lungs, but because all of the blood vessels are tightly constricted, the pressure needed to force blood through them is much greater. In fact the pressures get so high that some of the tiniest blood vessels break open, and this is thought to be part of the cause of HAPE (high altitude pulmonary edema).

by J. Kenneth Baillie
Last updated June 2007

SUMMARY
Blood is oxygenated as it passes through the lungs Blood vessels in the lungs tighten when there is a shortage of oxygen At high altitude, this causes high blood pressure in the lungs High lung blood pressre may cause HAPE

"...the pressures [in the lung blood vessels] get so high that some of the tiniest vessels break open, and this is thought to be part of the cause of HAPE"

Have you ever had HAPE? Click to register with the International HAPE Database.