As organisms increase in size and complexity, simple diffusion becomes insufficient to supply cells with oxygen and nutrients or to remove metabolic waste. To overcome this, animals have evolved specialized circulatory systems to transport fluids under pressure. The IGCSE 0654 Extended syllabus requires a comparative understanding of the single circulatory system of fish and the double circulatory system of mammals, focusing on the physiological advantages of the latter.

In a single circulatory system, blood passes through the heart only once for every complete circuit of the body.

Pathway of Flow

1. Deoxygenated blood enters the single atrium and is pumped by the single ventricle.
2. The blood travels to the gills, where gas exchange occurs (uptake of O2, release of CO2).
3. The now oxygenated blood flows directly from the gills to the rest of the body tissues.
4. Blood returns to the heart in a deoxygenated state.

Physical Constraint

The primary limitation of this system is the loss of blood pressure at the gills. As blood passes through the fine capillaries of the gill filaments, the resistance is high, causing the pressure to drop significantly. Consequently, the blood flows to the body tissues at a relatively low pressure, which limits the rate of oxygen delivery and metabolic activity.

Mammals possess a double circulatory system, where blood passes through the heart twice for every complete circuit. This creates two distinct pathways: the pulmonary circuit (heart to lungs) and the systemic circuit (heart to body).

Pathway of Flow

1. Right Side: Deoxygenated blood is pumped to the lungs at a lower pressure (pulmonary circuit).
2. Left Side: Oxygenated blood returns to the heart and is pumped out to the rest of the body at a very high pressure (systemic circuit).

The evolution of double circulation is a key factor in the success of endothermic (warm-blooded) animals. Its primary advantages include:

1. Maintenance of High Blood Pressure

Because blood is "re-pressurized" by the heart after picking up oxygen from the lungs, it can be pumped to the systemic circuit at much higher pressures than in fish. This ensures that blood reaches distant tissues and organs rapidly.

2. Enhanced Delivery Rates

Higher pressure leads to a faster flow rate. This allows for a more rapid delivery of the oxygen and glucose required for cellular respiration, and a more efficient removal of carbon dioxide and urea.

3. Support for High Metabolic Rates

Mammals require vast amounts of energy to maintain a constant internal body temperature and power complex movements. Double circulation provides the high-performance nutrient delivery system necessary to sustain these high metabolic demands.

The transition from single to double circulation represents a major evolutionary leap in physiological efficiency. By separating the gas exchange circuit from the systemic delivery circuit, mammals can maintain the high-pressure bulk flow required to support the energy-intensive lifestyle of a complex multicellular organism.