Arteries carry blood away from the heart; veins return it. These definitions depend on direction, not oxygen content. Thick elastic arterial walls withstand higher pressure. Veins often have valves that reduce backflow. Capillaries have very thin walls and form extensive networks, supporting exchange with tissues. Mammals have separate pulmonary and systemic circuits.
Some fluid leaves blood capillaries to bathe cells as tissue fluid. The lymphatic system returns excess tissue fluid to the circulation and contributes to immune defence. It also transports absorbed intestinal lipids. Cardiac output is heart rate multiplied by stroke volume. Oxygen delivery depends on blood flow and oxygen content together; a faster heart rate alone does not specify the amount of oxygen delivered.
Example 1
Heart rate is 70 beats/min and stroke volume is 80mL/beat. Calculate output.
70×80=5600mL/min=5.6L/min.
Example 2
The pulmonary artery carries blood low in oxygen. Why is it still an artery?
It carries blood away from the heart towards the lungs. Direction defines an artery.
Watch out:Arteries carry blood away from the heart. Pulmonary arteries carry relatively oxygen-poor blood to the lungs.
Circulation and lymph practice | Year 11 Biology | Ascent Arcade