Question

Explain Henry's law, and describe its importance to gas exchange in the lungs.

Answer 1

Henry's law states that the quantity (concentration) of a gas dissolved in a liquid (solubility) is directly proportional to the partial pressure of the gas and the solubility coefficient, at a constant temperature.

According to Henry's law, S(g)=K × P(g)

where, S(g) = the quantity of gas dissolved

K = solubility coefficient

P(g) = partial pressure of the gas.

Henry's law helps in explaining, the diffusion of gases between the alveoli and pulmonary capillaries as well as between systemic capillaries and tissues/organs.

During external respiration, there is diffusion of oxygen and carbon dioxide between the alveoli and pulmonary capillaries. During internal respiration, there is diffusion of oxygen and carbon dioxide between the systemic capillaries and tissues.

During external respiration, air enters the lungs from the atmosphere due to pressure difference between the thoracic cavity and external atmosphere. Thus, the oxygen rich air is present within the alveoli of the lungs. The deoxygenated blood is brought into the lungs by the pulmonary trunk from the right ventricle of the heart. Thus, the pulmonary capillaries have deoxygenated blood. Deoxygenated blood in the lungs has higher carbon dioxide concentration and lower oxygen concentration as the tissues utilize the oxygen dissolved in the blood and releases carbon dioxide as a by-product which is brought back to the heart by the venous system. This deoxygenated blood is pumped into the lungs for oxygenation.

The partial pressure of oxygen in the pulmonary capillaries is only around 40 mm of Hg while the partial pressure of oxygen in the alveoli of the lungs is around 105 mm of Hg. Thus, there is a pressure gradient between the alveoli and pulmonary capillaries which causes the diffusion of oxygen from alveoli into the pulmonary capillaries i.e. from area of higher concentration/partial pressure to area of lower concentration/partial pressure. Thus, the blood is oxygenated and transported to the heart through two pairs of pulmonary veins from where this oxygenated blood is pumped into the systemic circulation for the entire body.

At the same time, the partial pressure of carbon dioxide in the deoxygenated blood of pulmonary capillaries is around 45 mm of Hg while the partial pressure of carbon dioxide in the alveoli is around 40 mm of Hg. Due to this pressure difference, the carbon dioxide diffuses from pulmonary capillaries into the alveoli i.e. from an area of higher concentration (pulmonary capillaries) to an area of lower concentration (alveoli). From the alveoli, this carbon dioxide is exhaled out of the body during expiration.

Thus, Henry's law, helps in exchange of gases between the alveoli of the lungs and pulmonary capillaries which helps in respiration.