Explain how the human circulatory system is adapted to supply oxygen to the tissues and remove waste products from tissues.

The humans have a double circulatory system that allows blood to be distributed to the body at a higher blood pressure compared to the lungs. The heart is made of specialized cardiac muscle cells that have protein filaments that allow contraction through shortening. High blood pressures in arteries means that blood is able to reach all parts of the body. A lower pressure to the lungs mean that capillaries in the pulmonary system are not damaged. Capillaries have a large surface area as well as a short diffusion distance, which allows a higher rate of diffusion of oxygen into the blood from the alveoli and CO2 into the alveoli from the blood. Oxygen is bound to haemoglobin in RBC's. Waste products, such as CO2, are transported in the blood plasma. Blood returns to the heart from the body via veins. These veins have valves that prevent backflow.