When the right atrium contracts, it forces the blood through the tricuspid valve into the right ventricle. When the right ventricle contracts, it sends the blood through the pulmonary semilunar valve into the pulmonary circulation. The pulmonary artery carries the blood to the lungs where the arteries branch numerous times connecting with capillary beds around the lungs' alveoli. Here, carbon dioxide is discharged into the air contained in the alveoli, and oxygen is picked up by the hemoglobin contained in the red blood cells of the blood. The pulmonary veins transport the oxygenated blood from the capillaries to the left atrium or auricle of the heart. When this atrium contracts, the blood flows through the mitral or bicuspid valve into the left ventricle. When the left ventricle contracts, the oxygenated blood flows through the aortic semilunar valve into the body's largest artery, the aorta. All the arteries of the systemic circulation branch from the aorta to carry the oxygenated blood to the cells of the body's tissues. The arteries branch numerous times, eventually, branching into the small arterioles which connect to the capillary beds found in within all tissue. In the capillary beds of the systemic circulation not only are oxygen, nutrients, and hormones delivered to cells and wastes collected from cells, but, also, nutrients are picked up in capillary beds found in the intestine and liver, wastes discharged in capillary beds in the kidneys, and hormones
picked up in capillary beds in the endocrine glands. Substances such as nutrients and hormones acquired by the blood in the systemic circulation will be delivered to all
