Complete the passage by writing a suitable biological term in each blank space.

In the mammalian double circulatory system, blood is forced out of the heart under high pressure into thick-walled vessels known as ...................................................... . As these vessels branch into smaller arterioles, they eventually lead to network beds of microscopic ...................................................... where metabolic exchange occurs; these have walls only one cell thick to minimise diffusion distance.

Blood is collected from tissues and returned to the heart via ...................................................... . Because the pressure in these vessels is low, they possess ...................................................... to ensure unidirectional flow.

The chamber that receives oxygenated blood from the lungs is the left atrium, which then passes it to the highly muscular ...................................................... ventricle. This chamber pumps blood into the body's main systemic artery, the ...................................................... .

Conversely, deoxygenated blood from the right side of the heart is directed towards the respiratory organs via the ...................................................... artery. Inside the red blood cells, oxygen binds reversibly to the respiratory pigment ...................................................... to form oxyhaemoglobin.