Double Circulation

Simple definition of double circulation:

Double circulation (double circulatory systems) are circulation systems in which blood flows through the heart twice.

In the case of double circulation, pulmonary circulation - i.e. blood flow between the heart and lungs, is separate from systemic circulation - i.e. movement of blood from the heart through the rest of the body (excluding the lungs), then back to the heart.

Reminders about the types of circulation in animals:


Two types of animal circulation systems are:

  • Open Circulatory Systems
    (Circulatory fluid: hemolymph, also called haemolymph)
  • Closed Circulatory Systems
    (Circulatory fluid: blood - which is contained within the structure of the heart and blood vessels through which the blood is conveyed around the body )


Two types of closed circulation systems are:

  • Single Circulatory Systems
    e.g. in fish
  • Double Circulatory Systems
    e.g. in mammals

What are the differences between single and double circulatory systems ?

The differences between single circulatory systems and double circulatory systems include:

  1. During one complete cycle of flow through the whole body (blood system), blood in a double circulatory system passes through the heart twice.
    Another way to say this is the blood passes through the heart twice during each cycle of the circulatory process.

  2. In a double circulatory system there are two circuits for blood passing through the heart:
    • Pulmonary Circulation
      Deoxygenated blood is pumped from the heart to the lungs, oxygenated blood returns to the heart from the lungs.
    • Systemic Circulation
      Oxygenated blood is pumped from the heart around the body (including all the organs). That blood returns to the heart deoxygenated (more accurately 'oxygen poor') because much of the oxygen it contained when it left the heart has been supplied to tissues in the body.
  1. In animals that have double circulation (blood circulation) systems, the animal's heart has more than two chambers, e.g.

    Fish

    Single Circulation

    Two-chambered heart

    Frogs, Lizards

    Double Circulation

    Three-chambered heart

    Mammals, Birds

    Double Circulation

    Four-chambered heart

  1. The pressure of blood flowing through a double circulation system can be higher than that flowing through a single circulatory system because in the case of double circulation the blood is pressurized twice per cycle around the whole blood system. (However, the blood pressure in a double circulatory system isn't twice that in a single circulatory system - it is not that simple because blood pressure in different parts of the heart varies according to the cardiac cycle and blood pressure around the body also varies e.g. according to the type and location of the blood vessel, and with the general state of health of the person or animal.)
    The blood is pumped around the body after it has returned to the heart from the lungs. This is beneficial because blood pressure and flow rate is reduced as it passes through the lungs so if the blood wasn't returned to the heart and pumped onwards it would continue to the tissues of the body at much lower pressure and flow rate than it does in double circulation systems.

  2. The separation of oxygenated (more accurately 'oxygen-rich') and deoxygenated (more accurately 'oxygen-poor') blood is possible in the cases of double circulation systems that include a 'double pump', i.e. a four chamber, heart.
    See for example the diagram of a human heart at the top of this page, which shows the separation of oxygenated and deoxygenated blood within a human heart. Specifically, it illustrates:
    • Oxygen-poor blood, which is often called deoxygenated blood - entering the right atrium of the heart via the inferior vena cava and the superior vena cava then leaving the right ventricle of the heart via the pulmonary artery shown in blue with blue arrows.
    • Oxygen-rich blood, which is often called oxygenated blood or 'reoxygenated blood' - entering the left atrium of the heart via the pulmonary veins then leaving the left ventricle of the heart via the aorta shown in pale red with red arrows.

Advantages of double circulation over single circulation systems

Single circulation systems are adequate for many species e.g. different types of fish.
Double circulation is advantageous for mammals because it increases the pressure and hence the flow rate of blood supplied to the tissues of the body via the systemic circulation.

How ? Double circulation systems deliver greater blood flow rate to tissues around the body because the heart pumps the (oxygenated or 'oxygen-rich') blood returned to it from the lungs. This is useful because when blood passes through the lungs its pressure is reduced. Therefore if it were not returned to the heart then pumped out again (at higher pressure than the pressure at which it reached the heart from the lungs), it would pass onwards around the body much more slowly.

Why does this matter ?

  • Some mammals are very large animals. Sufficient blood pressure to send blood from the heart then eventually back to the heart is needed. The necessary blood pressure is higher in larger animals in which the blood must be pumped from the heart with enough force to send the blood considerable distances around the body before it arrives back at the heart.
  • Animals that have double circulation systems need blood to be supplied to their tissues quickly due to their relatively high metabolic rates i.e. rates of energy used in the chemical reactions within their bodies (compared with e.g. fish). This necessitates sufficient blood flow because the transport functions of blood include supplying the tissues with the chemical substances needed to continue activities including the essential chemical reactions.
  • Animals that have double circulation systems also need to maintain relatively high body temperatures - which requires sufficient blood flow due to the blood's role in maintaining body temperature as necessary for homeostasis.

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