The part of the circulatory system that transports blood through the lungs, where it is oxygenated, is called pulmonary circulation. Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. The veins return waste-laden blood to the heart, entering the right atrium through two large veins called vena cavae. The right atrium fills with the waste-laden blood and then contracts, pushing the blood through a one-way valve into the right ventricle. The right ventricle
fills and then contracts, pushing the blood into the pulmonary artery, which leads to the lungs. The pulmonary artery divides into two branches after leaving the heart, with each branch going to a lung. The exchange of carbon dioxide and oxygen takes place in the lungs. The pulmonary artery branches subdivide into arterioles that finally subdivide into many capillary beds in the lungs. These capillary beds are closely associated with alveoli (the smallest air passages of the lungs). A minimum amount of tissue separates the blood in the capillary beds from the alveoli. This allows carbon dioxide (the gaseous waste of animal metabolism) to be exchanged for oxygen from the air. Oxygenated arterial blood is bright red in color, and unoxygenated venous blood is dark or brownish red. The fresh, oxygen-rich blood enters the pulmonary veins and then returns to the heart, re-entering
through the left atrium. The oxygen-rich blood then passes through a one-way valve into the left ventricle where it will exit the heart through the aorta. The left ventricle’s contraction forces the blood into the aorta and the blood begins its journey throughout the body. The pressure required for pulmonary circulation is much less than that required for systemic, or body circulation. Therefore, the muscle mass developed in the right ventricle is much less than that of the left ventricle. The heart and the vessels that move oxygenated arterial blood to all parts of the body and return the unoxygenated venous blood to the heart are referred to as the systemic circulatory system. The blood vessels (arteries, veins, and capillaries) are responsible for the delivery of oxygen and nutrients to the
tissue. Oxygen-rich blood enters the blood vessels through the heart’s main artery called the aorta. The forceful contraction of the heart’s left ventricle forces the blood into the aorta, which then branches into
many smaller arteries that run throughout the body. The inside layer of an artery is very smooth, allowing the blood to flow quickly. The outside layer of an artery is very strong, allowing the blood to flow forcefully. The oxygen-rich blood enters the capillaries where the oxygen and nutrients are released. The waste products are collected, and the waste-laden blood flows into the veins to return to the heart where pulmonary circulation will allow the exchange of gases in the lungs. During systemic circulation, blood passes through the kidneys. This phase of systemic circulation is known as renal circulation. During this phase, the kidneys filter much of the waste from the blood. Blood also passes through the small intestine during systemic circulation. This phase is known as portal circulation. During this phase, the blood from the small intestine collects in the hepatic portal vein, which passes through the liver. The liver filters sugars from the blood and stores them for later use.
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