The circulatory system of the body consists of a pump, the heart, and a system of vessels that circulate blood around the body. The vessels that carry blood away from the heart are called arteries, and those that carry blood back to the heart are called veins. Blood carries oxygen and nutrients to the muscles and vital organs, and also carries carbon dioxide and waste products away. The system includes the lymphatics and spleen.


The heart is a cone-shaped, hollow, muscular organ situated in the centre of the thorax and in the chest cavity. It is partly surrounded by a membrane called the pericardium that produces fluid, and acts as a lubricant for the outside of the heart which is constantly moving as it beats.

Figure 1: The Heart

The walls of the heart are called the myocardium and are composed of specialised, involuntary muscle called cardiac muscle. They are known as involuntary muscle because they work automatically and cannot be stopped or started by the animal. The heart is divided down the middle by a layer of cardiac muscle called the septum, and in the normal, healthy animal there is no connection between the two sides; blood cannot move from the left into the right side of the heart. Each side is further divided into two chambers, the upper chamber called the atrium, and the lower chamber called the ventricle, so that at the top of the heart there is the left and right atrium, and at the bottom the left and right ventricle. Between the atrium and ventricle on each side, there is a valve called the atrio-ventricular valve, sometimes simply the A-V valve, and this controls the flow of blood from one chamber to the other. Have a good look at the two diagrams and find the myocardium, the pericardium, the atrium and ventricle on each side, and the A-V valves. These valves are made of flaps of muscle.

The main blood vessels carrying blood away from, and into, the heart are:

  • The pulmonary artery, which carries blood from the right ventricle to the lungs;
  • The aorta which carries blood from the left ventricle to other parts of the body;
  • The pulmonary veins which bring blood from the lungs to the left atrium; and
  • The vena cava, which brings blood from other parts of the body to the right atrium.

Figure 2: The heart showing the chambers and main vessels

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All the vessels leading from the ventricles have what are called semi-lunar valves which prevent the blood running back from the vessel into the heart.

The heart is able to function quite independently of any outside control because the beating of the heart, which is simply the contracting and relaxing of the muscular wall, is controlled by impulses sent out by two centres of specialised cardiac cells. Contraction of the atrium is governed by the sino-atrial node which is situated in the wall of the right atrium. The ventricles are controlled by the atrio- ventricular node also situated in the muscular wall.


The blood vessels form a complicated system which extends throughout the body. The diagram below shows the general system, the darker areas showing vessels carrying oxygenated blood, that is blood

+ oxygen, and the clear areas carrying de-oxygenated blood, or blood + carbon dioxide.

From the heart, large arteries spread out into the body. These branch into smaller and smaller arteries and then divide further to form the very small arterioles. These lead to the capillaries, the smallest blood vessels which form a network inside the organs of the body. The capillaries unite to form venules, the smallest veins, and these join together to form larger veins that eventually drain back

into the heart.

Figure 3: The Blood System of a Mammal

Figure 4: A Capillary Network


These are tubular structures carrying blood away from the heart. They have thick walls composed of five layers of tissue, and these are:

  • Inner Endothelial Coat;
    • Yellow Elastic Fibrous Coat;
    • White Smooth Muscle Coat;
    • Yellow Elastic Fibrous Coat; and
    • Loose Connective Tissue Coat.

Figure 5: An Artery

Arteries require a thick wall because they help to pump the blood around the body; the blood in the arteries is pumped with substantial pressure. The elastic tissue is important in maintaining this pressure, while the smooth muscle layer controls the size of the artery. This, in turn, controls the amount of blood flowing through the vessels and the distribution of that blood throughout the body. Tissues and organs require different amounts of blood at different times. When you are running, your leg muscles require extra blood to supply the energy to increase their activity. After a heavy meal, the digestive system requires extra blood to carry out the digestion. During pregnancy, the foetus, the young inside the mother, requires blood for growth. If blood is lost from the body due to injury, the blood pressure is reduced, and the result is what is called a state of shock.


These are also tubular structures but they are larger than arteries and have much thinner walls. They serve to carry blood from the organs of the body back to the heart. The walls of veins have only a small amount of muscle, and on the inside of the veins are a number of valves scattered along their length at irregular intervals. These valves keep the blood flowing towards the heart and prevent any flow back in the wrong direction, a necessity as the pressure of blood is very low in the veins, and the thin walls cannot pump the blood along.

Figure 6: A Vein


These are very small, thread-like tubes made of endothelial tissue similar to the inner layer of the arterial walls. This very thin wall acts as a semi-permeable membrane which provides a passage for nutrients, water, and oxygen to exit the vessel, and an entry for carbon dioxide and waste products.


This is a network of vessels which transport excess fluid from the tissues and organs of the body to the veins. The vessels are very thin-walled, and have valves similar to those found in veins. Small lymph vessels unite to form larger vessels which, in turn, unite and eventually drain into the vena cava and enter the heart.

The spleen is a dark red-blue organ associated with the circulatory system and situated near the stomach.


The following is the sequence of events that occur during one complete heartbeat. Follow the steps on the simplified diagram of the heart above.

  • Blood enters the right atrium from the big veins, the vena cava. At the same time, blood enters the left atrium from the pulmonary vein which comes from the lungs.
  • When they are full of blood, the left and right atrium contract together, this contraction or pumping action is called the atrial systole.
  • Because of this contraction pressure builds up, the atrio-ventricular valves are forced open and the blood flows into the ventricles which are relaxed.
  • Both atria start to relax, the action known as atrial diastole. At the same time, both ventricles

begin contracting (ventricular systole).

  • Pressure in the ventricles increases causing the A-V valves to snap shut and close off the gap between the atrium and the ventricle.
  • As the pressure continues to rise in the ventricles, the semi-lunar valves open and blood is forced out of both ventricles. The right ventricle pump’s blood into the pulmonary artery, and the left into the aorta.
  • The ventricles begin to relax (ventricular diastole) and the semi-lunar valves snap shut. The whole cycle then repeats itself.

These contractions and relaxations or heartbeats are controlled by the two nodes situated in the wall of the heart. The sino-atrial node controls the actions of both atria, and the atrio ventricular node, those of the ventricles.

Figure 7: A simplified Diagram of the Heart

Source: petmd

Although the heart beats independently on its own, the nervous system does have some control over the rate of heartbeat and the strength of the contractions. When the pressure in the heart and arteries rise, the rate of heartbeats drops through a reflex action. During increased muscular activity, as in running, the amount of blood returned to the heart is increased, a reflex action that increases the rate of the heartbeat.


In general, the smaller the animal the faster the heart beats.

  • Elephant                                          20 beats per minute
    • Horse                                                32 – 44 beats per minute
    • Cow                                                   60 – 70 beats per minute
    • Pig                                                      60 – 80 beats per minute
    • Sheep & Goat                                70 – 80 beats per minute
    • Chicken                                            200 – 400 beats per minute
    • Mouse                                             500 – 1000 beats per minute

It is essential that once blood has been forced into the large arteries, pressure is maintained. This is achieved by:

  • The closure of the semi-lunar valves to prevent the backflow of blood into the heart; and
  • The elasticity of the walls of the arteries. During systole the walls expand to accommodate the extra volume of blood. During diastole the walls recoil in order to maintain the pressure on the blood inside the artery.

Figure 8: Systole (Expansion)

Figure 9: Diastole (Contraction)