Blood has four main components: plasma, red blood cells, white blood cells, and platelets. Together, they make up about 8% of your total body weight. Plasma accounts for roughly 55% of blood volume, red blood cells make up about 44%, and white blood cells plus platelets share the remaining 1%. Each component has a distinct job, from delivering oxygen to fighting infections to sealing wounds.
Plasma: The Liquid That Carries Everything
Plasma is the pale yellow fluid that all the other blood components float in. It’s 91% to 92% water, with the remaining 8% to 9% made up of dissolved solids: proteins, electrolytes, nutrients, hormones, and waste products. Think of it as a delivery system. Plasma carries glucose and amino acids to cells that need fuel, shuttles hormones from the glands that produce them to the organs that respond to them, and hauls metabolic waste to the liver, kidneys, and intestines for disposal.
The proteins dissolved in plasma serve specific purposes. Albumin, the most abundant plasma protein, helps regulate fluid balance by keeping the right amount of water inside blood vessels. Globulins include antibodies that target infections. Fibrinogen is the protein that converts into fibrin strands during clotting, essentially forming the mesh that holds a blood clot together. Plasma also contains electrolytes like sodium, potassium, calcium, and bicarbonate, which help maintain the slightly alkaline pH your blood needs to function.
Red Blood Cells: Oxygen Delivery
Red blood cells are the most numerous cells in your blood. A healthy adult male carries 4.6 to 6.2 million per microliter; for women, the range is 4.2 to 5.4 million. Each one is shaped like a flattened disc with an indentation in the center, similar to a doughnut that isn’t quite punched through. That shape maximizes surface area, giving the cell more room to exchange gases.
The key molecule inside every red blood cell is hemoglobin, a protein that binds to oxygen in the lungs and releases it when the cell reaches tissues that need it. Hemoglobin also picks up carbon dioxide, a waste product of metabolism, and carries it back to the lungs so you can exhale it. This is what gives red blood cells their color: oxygen-rich hemoglobin is bright red, while hemoglobin carrying carbon dioxide is darker.
Red blood cells survive in circulation for about 115 days before the body breaks them down and recycles their components. Your bone marrow continuously produces new ones to replace them, generating millions every second to keep the supply steady.
White Blood Cells: Immune Defense
White blood cells are far less numerous than red blood cells. A normal count falls between 4,500 and 11,000 per microliter. Despite their smaller numbers, they run the entire immune system. There are five main types, each with a different specialty.
- Neutrophils are the first responders. They kill bacteria, fungi, and foreign debris, and they make up the largest share of white blood cells.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, T cells coordinate the immune response or directly attack infected cells, and natural killer cells destroy virus-infected cells and some tumor cells.
- Monocytes clean up damaged or dead cells and help fight chronic infections. They can also mature into specialized cells that patrol specific tissues.
- Eosinophils target parasites and cancer cells and play a role in allergic reactions.
- Basophils trigger allergic responses like coughing, sneezing, and a runny nose by releasing chemicals that drive inflammation.
Platelets: Stopping the Bleeding
Platelets are tiny cell fragments, much smaller than red or white blood cells. A healthy count ranges from 150,000 to 400,000 per microliter. Their job is to stop bleeding whenever a blood vessel is damaged, through a process that unfolds in stages.
First, the injured vessel constricts to slow blood flow. Then platelets stick to the exposed tissue at the wound site. Once attached, they change shape from smooth discs into spiky, multi-armed plugs that dramatically increase their surface area. They also release chemical signals that recruit more platelets to the area. These additional platelets link together using fibrinogen, a protein from plasma, forming bridges between them. The result is a soft, temporary platelet plug.
That initial plug is fragile. To make it durable, the body activates a cascade of clotting proteins that convert fibrinogen into fibrin, a tough, thread-like protein. Fibrin strands weave through the platelet plug like reinforcing mesh, turning it into a stable clot that holds until the vessel heals underneath.
Where Blood Cells Are Made
All blood cells originate from a single type of precursor called a hematopoietic stem cell, found in the spongy tissue inside your bones known as bone marrow. These stem cells can develop into any type of blood cell depending on the signals they receive. From infancy through adulthood, bone marrow is the primary production site for red blood cells, most white blood cells, and platelets. Certain white blood cells called lymphocytes also develop in the thymus, a small gland behind the breastbone.
Before birth, the process looks different. During the first weeks of pregnancy, blood cells form in the yolk sac surrounding the embryo. By months two and three, the liver and spleen take over red blood cell and platelet production, while the liver, spleen, and thymus generate white blood cells. By month five, bone marrow becomes the dominant production site, a role it keeps for life. If disease damages the bone marrow later on, the body can revert to producing blood cells in the liver or spleen as a backup.
What Blood Transports Beyond Oxygen
Oxygen delivery gets the most attention, but blood moves a long list of other substances. Plasma carries glucose and amino acids from the digestive system to cells throughout the body, providing the raw materials for energy and tissue repair. It transports hormones from endocrine glands to their target organs. It distributes vitamins and electrolytes. And it collects metabolic waste, carbon dioxide from cellular activity, nitrogen-containing compounds from protein breakdown, and other byproducts, routing them to the lungs, kidneys, liver, or intestines for removal. Every component of blood contributes to this transport network, with plasma serving as the solvent, red blood cells handling gas exchange, and white blood cells and platelets circulating on standby until they’re needed.