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Transport in Mammals: Question 6

Syllabus 8.1

Structured AS 5 marks

In mammals, blood passes through the heart twice during each complete circuit around the body: once through the pulmonary circulation (heart to lungs and back to the heart) and once through the systemic circulation (heart to the rest of the body and back to the heart), rather than travelling through the lungs and then on to the rest of the body in one unbroken loop.

(a) State the term used to describe this arrangement of the mammalian circulatory system. [1]

(b) Fish have a single circulation: blood is pumped from the heart through the gills and then flows on, without returning to the heart, directly to the rest of the body before eventually returning to the heart. Explain, in terms of blood pressure, why the mammalian double circulation is able to deliver blood to body tissues faster than a single circulation of this kind. [4]

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Worked solution

Part (a): Naming the arrangement

Blood in a mammal passes through the heart twice for every complete circuit of the body: once around the pulmonary circulation (heart → lungs → heart) and once around the systemic circulation (heart → rest of the body → heart). This arrangement, in which two separate circuits share the heart as a common pump, is called a double circulation (double circulatory system).

Part (b): Why a double circulation delivers blood to tissues faster

Blood pressure falls considerably whenever blood flows through a capillary bed, because the huge number of very narrow capillaries offers a high resistance to flow. This is true both of the capillaries in a fish’s gills and of the capillaries in a mammal’s lungs.

  • In a fish (single circulation): blood leaves the heart, passes through the gill capillaries (where pressure drops sharply), and then flows on, without returning to the heart, directly to the capillaries supplying the rest of the body. Since there is no further boost to pressure between the gills and the rest of the body, blood arrives at the body tissues only slowly, at the same low pressure it had leaving the gills.
  • In a mammal (double circulation): blood also loses much of its pressure passing through the lung capillaries, but it then returns to the heart (into the left atrium and left ventricle) before being sent on to the rest of the body. Contraction of the thick, muscular wall of the left ventricle re-pressurises this blood, so it leaves the heart for the systemic circulation at a much higher pressure than it had on leaving the lungs.

Because mammalian blood is re-pressurised by a second, separate pumping action before it is sent to the body tissues, it flows to those tissues considerably faster than blood in a single circulation like a fish’s could. This faster delivery of oxygenated blood supports the higher metabolic rate typical of mammals.

Final answers

  • (a) Double circulation (double circulatory system): a pulmonary circuit (heart–lungs–heart) and a systemic circuit (heart–body–heart), with blood returning to the heart between the two.
  • (b) Passing through capillaries causes a large drop in blood pressure; in a single circulation this low pressure alone must drive blood around the rest of the body, so flow is slow. In the mammalian double circulation, blood returns to the heart after the lungs and is re-pressurised by the left ventricle before being sent to the body, so it reaches tissues at higher pressure and flows faster.