Blood is the river of life. It feeds oxygen and essential nutrients to living cells and carries awaywaste. It transports the foot soldiers of the immune system, white blood cells, which seek out and destroy invading bacteria and parasites. And it speeds platelets to the site of injury or tissue damage, triggering the body’s miraculous process of self-repair.
Blood looks like a thick, homogenous fluid, but it’s more like a watery current of plasma – a straw-coloured, protein-rich fluid – carrying billions of microscopic solids consisting of red blood cells, white blood cells and cell fragments called platelets. The distribution is far from equal. Over half of blood is plasma, 45 per cent is red blood cells and a tiny fragment, less than one per cent, is composed of white blood cells and platelets.
Red blood cells are so numerous because they perform the most essential function of blood, which is to deliver oxygen to every cell in the body and carry away carbon dioxide. As an adult, all of your red blood cells are produced in red bone marrow, the spongy tissue in the bulbous ends of long bones and at the centre of flat bones like hips and ribs. In the marrow, red blood cells start out as undifferentiated stem cells called haemocytoblasts. If the body detects a minuscule drop in oxygen carrying capacity, a hormone is released from the kidneys that triggers the stem cells to become red blood cells. Because red blood cells only live 120 days, the supply must be continuously replenished; roughly 2 million red blood cells are born every second.
A mature red blood cell has no nucleus. The nucleus is spit out during the final stages of the cell’s two-day development before taking on the shape of a concave, doughnut-like disc. Like all cells, red blood cells are mostly water, but 97 per cent of their solid matter is haemoglobin, a complex protein that carries four atoms of iron. Those iron atoms have the ability to form loose, reversible bonds with both oxygen and carbon dioxide – think of them as weak magnets – making red blood cells such an effective transport system for respiratory gases. The haemoglobin, which turns bright red when oxygenated, is what gives blood its characteristic colour.
To provide oxygen to every living cell, red blood cells must be pumped through the body’s circulatory system. The right side of the heart pumps CO2-heavy blood into the lungs, where it releases its waste gases and picks up oxygen. The left side of the heart then pumps the freshly oxygenated blood out into the body through a system of arteries and capillaries, some as narrow as a single cell. As the red blood cells release their oxygen, they pick up carbon dioxide molecules, then course through the veins back toward the heart, where they are pumped back into the lungs to ‘exhale’ the excess CO2, and collect some more precious O2.
White blood cells are greatly outnumbered by red blood cells, but they are critical to the function of the immune system. Most white blood cells are also produced in red bone marrow, but white blood cells – unlike red blood cells – come in five different varieties, each with its own specialised immune function. The first three varieties, collectively called granulocytes, engulf and digest bacteria and parasites, and play a role in allergic reactions. Lymphocytes, another type of white blood cell, produce anti-bodies that build up our immunity to repeat intruders. And monocytes, the largest of the white blood cells, enter organ tissue and become macrophages, microbes that ingest bad bacteria and help break down dead red blood cells into reusable parts.
Platelets aren’t cells at all, but fragments of much larger stem cells found in bone marrow. In their resting state, they look like smooth oval plates, but when activated to form a clot they take on an irregular form with many protruding arms called pseudopods. This shape helps them stick to blood vessel walls and to each other, forming a physical barrier around wound sites. With the help of proteins and clotting factors found in plasma, platelets weave a mesh of fibrin that stems blood loss and triggers the formation of new collagen and skin cells.
But even these three functions of blood – oxygen supplier, immune system defender and wound healer -only begin to scratch the surface of the critical role of blood in each and every bodily process. When blood circulates through the small intestine, it absorbs sugars from digested food, which are transported to the liver to be stored as energy. When blood passes through the kidneys, it is scrubbed of excess urea and salts, waste that will leave the body as urine. The proteins in plasma transport vitamins, hormones, enzymes, sugar and electrolytes. Pause for a second to listen to your pumping heart and be thankful for the river of life coursing through your veins.
Components of blood
Blood is a mix of solids and liquids, a blend of highly specialised cells and particles suspended in a protein-rich fluid called plasma. Red blood cells dominate the mix, carrying oxygen to living tissue and returning carbon dioxide to the lungs. For every 600 red blood cells there is a single white blood cell, of which there are five different kinds. Cell fragments called platelets use their irregular surface to cling to vessel walls and initiate the clotting process.
Life cycle of red blood cells
Every second, roughly 2 million red blood cells decay and die. The body is keenly sensitive to blood hypoxia – reduced oxygen carrying capacity- and triggers the kidney to release a hormone called erythropoietin. The hormone stimulates the production of more red blood cells in bone marrow. Red blood cells enter the bloodstream and circulate for 120 days before they begin to degenerate and are swallowed up by roving macrophages in the liver, spleen and lymph nodes.
The macrophages extract iron from the haemoglobin in the red blood cells and release it back into the bloodstream, where it binds to a protein that carries it back to the bone marrow, ready to be recycled in fresh red blood cells.