THE ARTERIES AND VEINS: THE CAPILLARIE & THE VEINS
The capillary walls are very thin, allowing small molecules to pass through them. The role of the capillaries depends on the tissues in which they are located. Pulmonary capillaries release carbon dioxide from the blood into air sacs (alveoli) and at the same time absorb oxygen from the air sacs into the blood. The capillaries in the kidneys send waste products into tubules where urine is formed. In the walls of the intestines, nutrients are absorbed from broken-down food. Capillaries in the muscles exchange oxygen and nutrients for waste products and carbon dioxide.
The pressure in your veins is very low, and their walls are thinner than the walls of the arteries. Your veins have valves that keep the blood flowing only toward the heart. Veins are capable of contracting and expanding in response to the needs of the body. They can act as a reservoir because their walls are 6 to 10 times as expandable as those of the arteries. When you need blood drawn for most diagnostic tests, it is taken from a vein.
Cardio & Blood