Blood flows through your capillaries in small sacks called Alveoli where it releases Carbon Dioxide and picks up Oxygen.
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 mm of mercury as opposed to 120 mm in the systemic Aorta). When the blood reaches the pulmonary capillaries, it drops to about 15 mm of mercury. The Oncotic pressure in the capillaries is about 20 mm 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, about 11 liters, into the interstitial space.)
In the pulmonary circulation, the blood contains large quantity of Carbon bi oxide. Which is generated in tissue during oxidation of food used by cells. Which needs to be exchanged with Oxygen present in the alveoli of lungs. (About 20% in air.) It has got only 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. So both Oxygen and Carbon bi oxide has to cross very thin layer. But blood and air do not come in contact with each other.Number of alveoli is very large. (It comes to be 167772160.) Which provides a large surface area for the exchange of gases.(About 100 square meters or 1100 square feet.) The blood from pulmonary capillaries give up Carbon bi oxide which enters alveoli from lungs and Oxygen is taken up by blood from pulmonary capillaries from lungs,because of the large difference in concentration of both of them.(Oxygen is much more in air of alveoli and Carbon bi oxide much more in pulmonary cappilaries.) So both Oxygen and Carbon bi oxide travel from high concentration to low concentration. So exchange of gasses occurs due to the DIFFERENCE in concentration and the reverse occurs in the Systemic circulation, where Oxygen leaves the capillaries to enter the body tissues and Carbon bi oxide enters the blood to leave body tissue.(There Oxygen is consumed in Oxidation process to form Carbon bi oxide and Energy is liberated. Which is used for metabolic activities.
