The Anatomy Of Dog: The Circulatory System

The Circulation is the body’s transport system, conveying nutrients, water minerals, blood chemicals and defender cells in the blood stream to every part of the body. All these substances are carried in the blood which owes its colour to billions of red oxygen-carrying cells. White cells, involved in the body’s defence, are also present in blood but in far fewer numbers. The fluid component of blood contains proteins, some of which are involved in blood-clotting, as well as glucose, carbon dioxide, inorganic salts, and organic compounds such as vitamins, hormones and fats.
The function of the heart is to propel the blood round the body at sufficient velocity and pressure to ensure that all tissues constantly receive the substances they require for efficient function, an on-going process called general circulation. At the same time the heart must pump blood round the lungs so that the red cells can pick up fresh oxygen and so that carbon dioxide can be eliminated. This is called the pulmonary (lung) circulation.
The heart lies approximately between ribs three and six and a little to the left of the midline. In a slim-chested dog like the Whippet, the heart beat can be seen as a regular movement or flutter of the chest wall where the apex of the heart touches the inside of the chest wall. The heart is similar in construction to that of the human, consisting of two compartments or pumps divided down the middle by a muscular wall, the septum. Each compartment contains two chambers, the atrium, into which blood flows, and the ventricle, which propels blood into the arteries. Each chamber is guarded by a one-way valve, which prevents the backflow of blood. If any of the valves is faulty, blood may leak in the reverse direction; this can be common in older dogs, and a faulty valve causes extra strain on the heart and inefficient re-oxygenation of the blood in the lungs.
The right atrium and ventricle pump blood through the pulmonary circulation, picking up oxygen in the lungs; the oxygen-rich blood is then conveyed back to the left atrium and ventricle from which it is propelled into the aorta and distributed throughout the body by a network of Arteries. The heart beat can be heard as a ‘Iup-dup’, ‘Iup-dup’ sound through the chest wall, corresponding to the closure times of the two pairs of valves.

Arteries and veins

Thick-walled elastic vessels, the arteries, convey blood away from the heart. During every heart beat the walls of the larger arteries expand to receive the new delivery of blood, and then recoil; where an artery crosses a bone, the movement of the arterial wall can be felt as the pulse. In the dog this is best felt about half-way down on the inner surface of the thigh in the midline. Arteries branch and rebranch, becoming smaller until they form a huge network of microscopic vessels or capillaries. These allow nutrients to pass through their walls to the tissues, then join and rejoin to become thin-walled vessels of increasing size, the veins. The main arteries are the aorta, from the heart; the right and left common carotid arteries, to the head; the brachial and femoral arteries, to the forelimbs and hindlimbs respectively; and the pulmonary arteries, to the lungs. Veins convey blood towards the heart; they are wider and with thinner walls than the arteries, lying in certain parts of the body in superficial positions where they can be prone to damage, especially cuts; at the same time they are accessible to the veterinarian for intravenous injection. Blood escapes in spurts from a cut artery, but flows steadily from a cut vein. The volume of blood lost over a period of time can be great unless the hemorrhage is stemmed.

The main veins include the cranial and caudal and vena cava, to the heart; the right and left jugular veins, from the head the cephalic and brachial veins, from the forelimbs the saphenous and femoral veins from the hindlimbs and the right and left pulmonary veins from the lungs.

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