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Oxygenated blood leaves the lungs through pulmonary veins, which return it to the left part of the heart, completing the pulmonary cycle. [3] [6] This blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. [3] [6] From the left ventricle, the blood passes through the aortic valve to the aorta.
The color of human blood ranges from bright red when oxygenated to a darker red when deoxygenated. [2] It owes its color to hemoglobin, to which oxygen binds. Deoxygenated blood is darker due to the difference in shape of the red blood cell when oxygen binds to haemoglobin in the blood cell (oxygenated) versus does not bind to it (deoxygenated).
The right atrium receives deoxygenated blood through coronary sinus and two large veins called venae cavae. The inferior vena cava (or caudal vena cava in some animals) travels up alongside the abdominal aorta with blood from the lower part of the body. It is the largest vein in the human body. [4]
The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body, and returns deoxygenated blood back to the right heart via large veins known as the venae cavae. The systemic circulation can also be defined as two parts – a macrocirculation and a microcirculation.
The blood circulation system via venous heart is close-looped. A venous heart is single circuit and the heart pumps blood all over the body with only a single loop. Deoxygenated blood circulates to the atrium from the body, and afterwards the deoxygenated blood moves to the ventricles from the atrium.
Blood flow diagram of the human heart. Blue components indicate de-oxygenated blood pathways and red components indicate oxygenated pathways. The heart functions as a pump and acts as a double pump in the cardiovascular system to provide a continuous circulation of blood throughout the body.
The systemic circulation then transports oxygen to the body and returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the lungs. [8] The right heart collects deoxygenated blood from two large veins, the superior and inferior venae cavae. Blood collects in the right and left atrium continuously. [8]
The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves. There are two atria in the human heart – the left atrium receives blood from the pulmonary circulation, and the right atrium receives blood from the venae cavae of the systemic circulation.