The human circulatory system is a double system, meaning there are two separate systems of blood flow: pulmonary circulation and systemic circulation. The adult human heart consists of two separated pumps, the right side (right atrium and ventricle,) which pumps deoxygenated blood into the pulmonary circulation, and the left side (left atrium and ventricle), which pumps oxygenated blood into the systemic circulation. Great vessels are the major vessels that carry blood into the heart and away from the heart to and from the pulmonary or systemic circuit.
The Venae Cavae
The superior vena cava and inferior vena cava are collectively called the venae cavae. They are the veins that return deoxygenated blood from the body into the heart, emptying into the right atrium. The venae cavae are not separated from the right atrium by valves. The venae cavae, along with the aorta, form the systemic circuit (Figure 1).
The superior vena cava is a large, short vein that carries deoxygenated blood from the upper half of the body to the right atrium. The left and right brachiocephalic veins carry blood from the upper limbs, head, and neck, then converge with the azygous vein carrying blood from the thoracic area to form the superior vena cava. The superior vena cava begins above the heart.
The inferior vena cava is the largest vein in the body and carries deoxygenated blood from the lower half of the body into the heart. The left and right common iliac veins carry converge to form the inferior vena cava. The inferior vena cava begins posterior to the abdominal cavity and travels to the heart next to the abdominal aorta.
It is through the pulmonary circuit (Figure 2) that blood travels from the heart to the lungs for oxygenation, then back to the heart for delivery to the systemic circulation. The pulmonary arteries carry deoxygenated blood from the right ventricle into the lungs for oxygenation. These are the only arteries that carry deoxygenated blood. The main pulmonary artery begins at the base of the right ventricle as a short, wide vessel. It then branches into the left and right pulmonary arteries to which deliver deoxygenated blood to the respective lungs.
The pulmonary veins carry oxygenated blood from the lungs to the left atrium of the heart. These veins are unusual in that they carry oxygenated blood, since most veins carry deoxygenated blood. Four pulmonary veins enter the left atrium. The right pulmonary veins pass behind the right atrium and superior vena cava while the left pass in front of the descending thoracic aorta.
The aorta is the largest of the arteries in the systemic circuit. Blood is pumped from the left ventricle through the aorta into systemic circulation. The aorta is an elastic artery and is able to expand and contract in response to blood pressure and volume. When the left ventricle contracts to force blood through the aortic valve into the aorta, the aorta expands. This expansion, or stretching of the aorta, provides potential energy to help maintain blood pressure during diastole, when the aorta passively contracts. Mean arterial blood pressure is highest in the aorta and diminishes through circulation. The difference in pressure between the aorta and right atrium accounts for blood flow in the circulation. The aortic arch contains baroreceptors (pressure sensors) and chemoreceptors (chemical sensors) that relay information concerning blood pressure, blood pH, and carbon dioxide levels to the medulla oblongata of the brain. This information is processed by the brain and the autonomic nervous system mediates the homeostatic responses. The aorta is often described in five segments and extends from the left ventricle down to the abdomen, where it bifurcates into two smaller arteries, known as the common iliac arteries, for systemic distribution. The five segments are:
- The ascending aorta, describing the section between the heart and the arch of aorta
- The arch of aorta, describing the peak of the aorta
- The descending aorta, describing the section from the arch of aorta to the point where it divides into the common iliac arteries
- The thoracic aorta, describing the part of the descending aorta above the diaphragm
- The abdominal aorta, describing the part of the descending aorta below the diaphragm