Blood is a bodily fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.

The blood cells present in the blood are mainly red blood cells (also called erythrocytes) and white blood cells (or leukocytes). Red blood cells give the blood its characteristic color.

These cells contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is mostly transported extracellularly as bicarbonate ions are transported in plasma.

White blood cells are the cells of the immune system that are involved in protecting the body against both infectious diseases and foreign invaders. These cells are able to move flexibly through the blood vessels, and then change shape dramatically to enter tissues.

Blood plays a vital role in the body's function, and it is composed of many different types of cells and proteins that work together to keep us healthy.

Red blood cells carry oxygen from the lungs to the rest of the body, and white blood cells help fight infection.

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