Blood and lymphatic circulation


Blood circulation is the system of vessels (veins and arteries) and the heart in anatomical and physiological terms. Blood circulation is different in foetuses and adults, which means they are two separate fields of study. Lymphatic circulation drains the lymph into the general blood circulation.

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Blood circulation in foetuses

© Diffomédia/Royal Canin

The first vessels start to form in the embryo when it is no longer able to feed itself by cell-to-cell diffusion. The development of the internal organs demands the supply of vital nutrients directly to the cells involved. The development of the heart is more complex: it is formed from the surface cells of the embryo. It starts out as extra-embryonic before becoming intra-embryonic, gradually taking its final place in the thorax. It is initially straight (a vestige of evolution), acquiring its familiar shape by curving and rotating.

The foetus’s lungs are non-functional. Oxygen is provided through the umbilical veins and carbon dioxide is removed through the umbilical arteries.

The blood circulation is shunted through the intracardiac orifices and a canal – the ductus arteriosus – connecting the aorta (the main artery leaving the left heart) to the pulmonary artery (which leaves the right heart). The chambers do not form in the heart until the end of gestation and the first few hours after birth. The ductus arteriosus, on the other hand, does not close until after birth when the lungs start to function.

Abnormalities include defective development of the heart chambers, retention of the ductus arteriosus or faulty positioning of the heart.

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Blood volume: 80-90 ml blood per kg of body weight
• Red corpuscles: 5.5-8.5 x 106 per mm3 (= 5,500,000-8,500,000)
• White corpuscles: 6,000-17,000 per mm3
• Haemoglobin: 12-18 g per 100 ml
• Platelets: 200,000-500,000 per mm3
• Haematocrit: 37-55 % (volume of red corpuscles by blood volume)
• Calcium: 95-120 mg/l
• Glucose: 0.7-1.1 g/l
• Total lipids: 5.5-14.5 g/l
• Cholesterol: 0.5-2.7 g/l
• Phosphorus: 40-80 mg/l
• Bleeding time (ear): 2-3 minutes
• Clotting time: 6-7.5 minutes

Blood circulation in adults

© Diffomédia/Royal Canin

The heart acts as a pump, but it is the blood vessels which actually convey the blood to the organs. Anatomically speaking, the vessels conveying blood (either oxygenated or deoxygenated) from the heart are called arteries and vessels conveying blood to the heart are called veins. Veins contain tiny valves, which ensures that the blood pressure, although low, is sufficient for circulation. This is why a cut artery spouts blood in fits and starts, whereas a cut vein bleeds at a constant rate.

The aorta, a large artery that carries oxygenated blood, leaves the left heart in the direction of the front of the dog. It curves abruptly, forming the arch of the aorta and moves towards the rear. Just before it curves, the brachiocephalic trunk branches off (to vascularise the head and forelimbs), as does the subclavian artery (towards the thorax).

The aorta then enters the abdominal region to vascularise all the organs and hind limbs through smaller arteries. When it reaches a muscle or an organ the artery branches off into a bundle of arterioles, which mean that oxygen and carbon dioxide can be exchanged. The blood is then carried by venules converging toward a small-diameter vein. These small veins join with the cranial vena cava in the anterior portion of the body or with the caudal vena cava in the posterior portion of the body. These two veins return the blood to the right heart, which pumps it to the lungs through the pulmonary artery. In the lungs, the blood releases its carbon dioxide, before returning, oxygen-rich, to the heart through the pulmonary veins to complete the cycle, and a new cycle begins all over again.

Lymphatic circulation

Duhayer_Royal Canin

Lymphatic circulation drains the lymph into the general blood circulation. Lymph vessels, which also contain valves, gradually converge into two main trunks: a thoracic duct and a right lymphatic duct. The vessels are very difficult to see, but it is very easy to identify the lymph nodes, which filter the lymph in a given part of the body. There are a great number of lymph nodes, some of them superficial, others so deep (in the large cavities) they can only be seen by X-ray or ultrasound.

They can be enlarged, especially if the drainage area is inflamed, which is why they need to be palpated by the veterinarian during a clinical examination. They are also the preferred routes of cancer cells moving from one organ to another, so lymph nodes may be removed along with a tumour to limit the spread of the disease.

© Diffomédia/Royal Canin
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