Circulation in Vertebrates


All vertebrate animals have a circulatory system, consisting of a central pump (the heart) and a system of tubes (the vascular and lymphatic systems) through which a fluid (blood and lymph) is continuously circulated.

Keywords: blood; capillary blood flow; cardiac cycle; electrocardiogram; haemodynamics; heart rate; lymph; venous system

Figure 1.

Schematic representation of the mammalian circulation and the heart.

(a) The basic design of the mammalian circulatory and lymphatic systems. Four functional characteristics distinguish the vertebrate from the invertebrate cardiovascular system. They are:

1. A single, ventrally located myogenic heart. The chambered heart has muscle cells capable of generating their own regular contraction (myogenic) even though a specialized pacemaker region of the heart starts the heartbeat.

2. A closed network of blood vessels. The branching network provides ordered distribution and collection of blood, often starting with a single artery directing blood cephalad from the heart and ending with one or more veins returning blood to the heart.

3. Passive cardiac, arterial and venous valves. These provide directionality to blood flow, preventing reflux of blood within the circulation.

4.  Muscular blood vessels capable of changing their diameter (vasomotion). Vasomotion controls the distribution of blood flow, blood pressure and blood volume.

(b) A schematic cutaway view of the rear of the mammalian heart. The major anatomical features of this four‐chambered heart are indicated. In addition, the location of sinoatrial node (the pacemaker) and the conducting tissues that spread excitation to the ventricle via the atrioventricular node are indicated. (Figures adapted from Randall, Burggren and French, 1997, with permission from W.H. Freeman and Company.)

Figure 2.

Features of the cardiac cycle in different vertebrates. Representative examples of the changes in blood pressure, ventricular volume and electrical activity (the electrocardiogram) associated with the cardiac cycle are illustrated for (a) the mammalian heart, (b) the trout heart, and (c) the shark heart. (Adapted with permission from Farrell, 1991.)

Figure 3.

Schematic diagrams of the circulation patterns in the main vertebrate groups. (a) The circulation pattern for birds and mammals. This is a fully divided, double circulation with the left and right sides of the heart each having an atrium (A) and a ventricle (V). (b) The single circulation of fish. (c) The circulation of an air‐breathing fish. Oxygenated blood from the air‐breathing organ mixes with deoxygenated blood from the systemic circulation. (d) The partially divided circulation of a lungfish. Two atria assist in better separation of the blood returning from the lungs from that returning from the tissues, as does the spiral ridge in the single outflow vessel from the ventricle. (e) The partially divided circulation of a urodele amphibian. Although there is only a single outflow vessel from the ventricle, the spiral ridge contained in the cordis arteriosus (see Figure ) effectively separates the blood going to the lungs and tissues. An anuran amphibian has an additional vessel from the pulmonary artery to the skin circulation. (f) The partially divided circulation of a non‐crocodilian reptile (snakes, lizards, turtles and tortoises). There are two outflow arteries from the single, but highly partitioned ventricle. (g) The fully divided circulation of a crocodilian reptile. Unlike the mammalian circulation, there are two outflow arteries from the right ventricle and a mechanism to close the pulmonary circulation at the base of the pulmonary artery. The colour blue refers to deoxygenated blood and the colour red refers to oxygenated blood. In some circulations blood can be partially oxygenated because of mixing of deoxygenated systemic blood and oxygenated respiratory blood, and the relative degree of this mixing is indicated by the proportion of red to blue colouring. A, atrium; V, ventricle.

Figure 5.

Highly schematized diagrams of dorsal views of the heart and main arterial plan in: (a) an amphibian; (b) a non‐crocodilian reptile; and (c) a crocodile. The evolution from a single ventricle with a single aortic opening that has a spiral ridge, through a single ventricle with two aortic openings, to a fully divided heart is evident. The predominant patterns for the flow of pulmonary venous blood and systemic venous blood through the heart chambers is indicated by the solid and broken arrows, respectively. The general plan for embryonic aortic arch development is indicated by broken lines and corresponding aortic arches have the same horizontal alignment. LA, left atrium; RA, right atrium; LV, left ventricle; RV, right ventricle; PA, pulmonary artery; V, ventricle; CA, cordis arteriosus. (Adapted with permission from Farrell, 1997, in Burggren and Keller, 1997.)

Figure 6.

The major factors that influence cardiac output and arterial blood pressure.

Figure 4.

Representative arterial (branchial, systemic and pulmonary) blood pressures in selected vertebrates. There are clear evolutionary trends towards (a) low arterial blood pressures in the respiratory circulation, and (b) higher arterial pressures in the systemic circulation. These evolutionary shifts are tied to the evolution of a divided, double circulation and the associated increase in metabolic rate. The higher systemic blood pressures permit a faster blood flow and a higher filtration pressure for filtering wastes at the kidney. The lower respiratory blood pressures are associated with thinner diffusion barriers at the lungs that facilitate rapid gaseous diffusion without excess fluid filtration. (Adapted with permission from Farrell, 1991.)

Figure 7.

The intrinsic mechanical properties of the heart. (a) The Frank–Starling mechanism is illustrated by a ventricular function curve. Muscle fibres are stretched with an increased end‐diastolic volume (EDV) as a result of increased cardiac filling pressure. As a result, SVH can increase with increases in venous filling pressure. Factors that alter cardiac contractility, such as adrenergic stimulation and vagal inhibition, shift the ventricular function curve. (b) In a similar manner as above, increased arterial pressure can increase both EDV and end‐systolic volume (ESV) with little effect on SVH (homeometric regulation). (Adapted with permission from Farrell, 1991.)

Figure 8.

The structure of blood vessels. (a) Schematic cutaways of the major vessel types to illustrate the various tissues that comprise the vessel wall. (b) A schematic of a capillary bed to illustrate the innervation and vascular smooth muscle arrangement in arterioles and venules. (Figures adapted from Randall, Burggren and French, 1997, with permission from W.H. Freeman and Company.)

Figure 9.

The distribution of blood pressure, vascular resistance, flow velocity and blood volume through the various parts of the mammalian circulation. (Adapted with permission from Farrell, 1991.)


Further Reading

Berne RM and Levy MN (eds) (1998) Physiology, 4th edn. St Louis, MO: CV Mosby.

Bourne GH (ed.) (1980) Hearts and Heart‐like Organs. New York: Academic Press.

Burggren WW and Keller B (eds) (1997) Development of the Cardiovascular System: Molecules to Organisms. Cambridge: Cambridge University Press.

Burggren WW, Farrell AP and Lillywhite H (1997) Vertebrate cardiovascular physiology. In: Dantzler WH (ed.) The Handbook of Physiology, pp. 215–308. New York: American Physiological Society.

Farrell AP (1991) Circulation of body fluids. In: Prosser CL (ed.) Comparative Animal Physiology, pp. 509–558. New York: John Wiley and Sons.

Farrell AP, Gamperl AK and Francis ETB (1998) Comparative aspects of heart morphology. In: Gans C and Gaunt AS (eds) Biology of the Reptilia, vol. 19 (Morphology G). Ithaca, NY: Society for the Study of Amphibians and Reptiles. (Contributions in Herpetology, vol. 14, pp. 375–424.)

Fishman AP (ed.) (1990) The Pulmonary Circulation, Normal and Abnormal. Philadelphia, PA: University of Pennsylvania Press.

Graham JB (1997) Air‐Breathing Fishes – Evolution, Diversity, and Adaptation. San Diego, CA: Academic Press.

Hoar WS, Randall DJ and Farrell AP (eds) (1992) The Cardiovascular System. Fish Physiology, vols 12A and 12B. San Diego: Academic Press.

Jones DR and Johansen K (1972) The blood vascular system of birds. In: Avian Biology, vol. 2, pp. 157–285. New York: Academic Press.

Randall DJ, Burggren WW, Farrell AP and Haswell MS (1981) The Evolution of Air Breathing in Vertebrates. Cambridge: Cambridge University Press.

Randall DJ, Burggren W and French K (1997) Eckert Animal Physiology Mechanisms and Adaptations, 4th edn. New York: Freeman WH.

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Farrell, AP(Apr 2001) Circulation in Vertebrates. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001829]