The next heartbeat starts a new cycle of systemic circulation. Oxygen-rich blood travels through the pulmonary veins and the left atrium into the left ventricle. When we breathe out, carbon dioxide leaves our body. ![]() This cycle ensures that blood is continuously. Cava (blood from the upper body) or the Inferior Vena. This is where carbon dioxide is released from the blood into the air inside the pulmonary vesicles, and fresh oxygen enters the bloodstream. The cardiac cycle is a sequence contraction (systole) and relaxation (diastole) of the atria and ventricles. Deoxygenated blood returns to the heart through either the Superior Vena. The capillaries form a fine network around the pulmonary vesicles (grape-like air sacs at the end of the airways). Blood travels from the heart into the subclavian artery which continues. It gives off several small branches before continuing on as the axillary artery. It begins near the heart and travels under the clavicle bone toward the shoulder. This is where pulmonary circulation begins: The right ventricle pumps low-oxygen blood into the pulmonary artery, which branches off into smaller and smaller arteries and capillaries. The subclavian artery is the large vessel that begins the blood supply to the upper extremity. The blood, which is now low in oxygen, is collected in veins and travels to the right atrium and into the right ventricle. This process of contracting and relaxing controls blood flow through your heart and to the rest of your body. It also sends signals that tell different parts of your heart to relax and contract (squeeze). Your heart (cardiac) conduction system sends the signal to start a heartbeat. There the blood drops off oxygen, nutrients and other important substances and picks up carbon dioxide and waste products. Muscle cells control your heart’s contractions. The blood travels from the main artery to larger and smaller arteries and into the capillary network. In each cardiac cycle, the heart contracts ( systole), pushing out the blood and pumping it through the body this is followed by a relaxation phase ( diastole). In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery (aorta). ![]() The following phase is called the ejection period, which is when both ventricles pump the blood into the large arteries. Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. Blood circulation starts when the heart relaxes between two heartbeats: The blood flows from both atria (the upper two chambers of the heart) into the ventricles (the lower two chambers), which then expand.
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