Red blood cell, also known as erythrocyte, is a small and biconcave blood component that distinctively gives blood its red colour and transports oxygen from the lungs to different body parts and tissues. The cell lacks a nucleus and is enclosed with a protein membrane. It contains haemoglobin, a protein rich in iron, which combines with oxygen to form oxyhaemoglobin. The red blood cell enters the heart twice, either as a deoxygenated cell which is transported to the pulmonary circuit or as an oxygenated cell for the systemic circulation. Deoxygenated red blood cells are pumped by the right ventricle into the pulmonary artery which later branches to pulmonary capillaries leading to the lungs. Oxygen from the inhaled air passes through the alveoli walls into the blood where each molecule of haemoglobin binds with four oxygen molecules to form oxyhaemoglobin.
The oxygenated red blood cells flow through the pulmonary capillaries from the lungs to form the pulmonary vein that leads to the left atrium. The cells are transported to the left ventricle which is responsible for pumping blood into the aorta, which supplies blood to organs, tissues and cells. For instance, if the blood flows to the foot, the aorta branches to the femoral artery, a large artery in the thigh, then to smaller arteries and capillaries which enable the red blood cells to interact with tissues and cells. At lower oxygen concentrations, the oxyhaemoglobin dissociates to individual molecules of haemoglobin and oxygen. The oxygen molecules leave the capillaries to support the metabolic processes in the cells. The metabolic wastes and carbon dioxide of the cells enter the blood which is lower in oxygen concentration. At times, haemoglobin can combine with carbon dioxide to form carbaminohaemoglobin.
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The deoxygenated red blood cells and the metabolic wastes stream into the blood capillaries which join to form the vena cava. As a result, the vena cava enables the blood to stream back into the right atrium from the systemic circulation. The blood later flows to the right ventricle from the right atrium and is pumped to the pulmonary circuit.
