Transport System in Mammals

A double, closed circulatory system

Mammals have a double circulatory system — blood passes through the heart twice on each complete circuit — and a closed system, meaning blood always stays inside blood vessels. The two circuits are:

Blood vessels

• Arteries — carry blood away from the heart; thick, muscular walls to withstand high pressure.
• Veins — carry blood back to the heart; thinner walls, with valves to prevent backflow (since pressure is lower).
• Capillaries — tiny vessels, one cell thick, where the exchange of oxygen, nutrients, and waste with body tissues happens.

The cardiac cycle

Each heartbeat is one cardiac cycle — the heart's chambers contract and relax in a coordinated sequence to pump blood in the right direction. Your heart rate (pulse) is the number of these cycles per minute, controlled by internal mechanisms that respond to the body's needs (e.g. rising during exercise).

The lymphatic system

Running alongside the blood system is the lymphatic system, which drains excess fluid from body tissues and returns it to the bloodstream, and also plays a role in the immune system.

Heart and circulatory disease

Problems with the heart and circulatory system include high and low blood pressure, and heart attacks. Common treatments include stents, bypass surgery, pacemakers, valve replacements, and (in severe cases) heart transplants.