Describe what happens when blood flows through the lung?

Answer 1

When blood flows through the lungs (Which is called the lesser or pulmonary circulation), it has got very low systolic blood pressure. (Systolic blood pressure in the pulmonary aorta is only about 20 mmol of Mercury as opposed to 120 mmolin the systemic aorta). When the blood reaches the pulmonary capillaries it drops to about 15 mmol of mercury. The Oncotic pressure in the capillaries is about 20 mmol of mercury, so fluid does not leak out of the capillaries, but just flows through them (In contrast to systemic circulation, where it leaks out in large quantity, into the interstitial space).

In the pulmonary circulation, the blood contains large quantities of Carbon dioxide, which needs to be exchanged with Oxygen present in the lungs (About 20% in air). It only has a few milliseconds to achieve this. Capillaries are made up of single flat cells only and Lung alveoli are also made up of single flat cells only, which provides a large surface area for the exchange of gases. The pulmonary capillaries give up Carbon dioxide, and Oxygen is taken up because of the large difference in concentration gradient between them. (Oxygen travels from an area of high concentration in the lungs, to an area of low concentration in the pulmonary capillaries). Carbon dioxide is given up and Oxygen is taken up by red blood cells, with the help of Hemoglobin molecules in them (Hemoglobin molecules can hold 4ml of Oxygen per litre, which is 70 times more oxygen than normal plasma can hold).

All this exchange of gases occurs across a very large surface area (100 square meters - approximately the size of a Tennis ground), So exchange of gasses occurs due to the DIFFERENCE in concentration gradient and the reverse occurs in the Systemic circulation, where Oxygen leaves the capillaries to enter the body tissues and carbon dioxide enters the capillaries.