Circulatory Systems

• The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood.

Key components

• The heart acts as the central pump, providing the necessary pressure to push blood through the network.
  • The heart and valves ensure a one-way flow of blood around the circulatory system.
• Blood vessels (arteries, veins, capillaries) form a continuous closed loop.
• Valves are seen in the heart and veins. Their crucial function is to prevent the backflow of blood, ensuring a strictly one-way flow.

Single circulation of a fish

• In a fish, blood passes through the heart only once for each complete circuit of the body.

Diagram

Pathway

1. Deoxygenated blood from the body enters the heart.
2. Heart pumps it to the gills (where it picks up oxygen).
3. Oxygenated blood travels directly from the gills to the rest of the body organs.
4. Deoxygenated blood returns to the heart.

Anatomy

• Fish have a single two-chambered heart consisting of one atrium and one ventricle.

Double circulation of a mammal

Diagram

Pathway

• In mammals, blood passes through the heart twice for each complete circuit of the body. It is split into two distinct loops.

1. Pulmonary circuit, heart to lungs to heart. The right side of the heart pumps dexoygenated blood to the lungs to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left side of the heart.
   • When the heart pumps blood to the lungs, there is low pressure required, resulting in little resistance. The reasons why are because:
     • the distance from heart to lungs is short
     • the low pressure is also important to protect the delicate lung tissue, and the capillaries are only one cell thick for efficient diffusion, which can be ruptured if the heart pumped at high pressures.
2. Systemic circuit, heart to body to heart. The left side of the heart pumps this oxygenated blood at high pressure to the rest of the body organs. Deoxygenated blood then returns to the right side of the heart.

Advantages of double circulation

• Oxygenated and deoxygenated blood are kept separate.
• Blood is pumped to lungs at low pressure so they are not damaged.
• Blood is pumped to the rest of the body at high pressure to give fast supply of oxygen and nutrients to respiring tissues.