Blood is a specialized fluid tissue that circulates through your body, delivering oxygen and nutrients to every cell while carrying away waste. The average adult has about 5 liters of it, with women typically carrying slightly less than men. Despite looking like a simple red liquid, blood is a complex mix of cells, proteins, and dissolved substances that keeps you alive by performing dozens of jobs simultaneously.
What Blood Is Made Of
Blood has two main parts: a liquid component called plasma and a solid component made up of cells and cell fragments. Plasma accounts for about 55% of total blood volume. It’s mostly water (around 90%), but dissolved in that water are proteins, salts, sugars, fats, hormones, and dissolved gases. The remaining 45% consists of red blood cells, white blood cells, and platelets.
If you let a tube of blood sit still or spin it in a centrifuge, it separates into visible layers. Plasma rises to the top as a pale yellow liquid. Red blood cells, the heaviest component, settle to the bottom. A thin, whitish layer of white blood cells and platelets sits between the two. The ratio of red blood cells to total blood volume is called hematocrit. Normal hematocrit runs 40% to 54% in adult men and 36% to 48% in adult women.
Red Blood Cells and Oxygen Delivery
Red blood cells are by far the most abundant cells in your blood. Their sole job is ferrying oxygen from your lungs to your tissues and bringing carbon dioxide back to be exhaled. They do this using hemoglobin, an iron-containing protein packed inside each cell. A single hemoglobin molecule has four iron atoms, and each one can grab one oxygen molecule, so every hemoglobin molecule carries up to four oxygen molecules at a time.
Hemoglobin is also what makes blood red. When it’s loaded with oxygen (in arteries), it’s bright red. When it has released its oxygen and picked up carbon dioxide (in veins), it turns a darker, bluish-red. Normal hemoglobin levels range from 13 to 18 grams per deciliter in adult men and 12 to 16 g/dL in adult women. When hemoglobin drops below those ranges, you’re considered anemic, which is why low iron intake or blood loss can leave you feeling exhausted and short of breath.
White Blood Cells and Immune Defense
White blood cells make up a tiny fraction of your blood by volume, but they’re the backbone of your immune system. A healthy adult has between 4,500 and 11,000 white blood cells per microliter of blood. There are five main types, each with a distinct role.
- Neutrophils are the most common, making up 50% to 70% of all white blood cells. They’re first responders: when bacteria invade or tissue is damaged, neutrophils rush to the site and engulf the invaders.
- Lymphocytes account for 20% to 40% and drive your adaptive immune system, the branch that “learns” to recognize specific threats. This group includes the cells that produce antibodies and the cells that destroy virus-infected or cancerous cells.
- Monocytes circulate in the blood and then migrate into tissues, where they mature into larger cells called macrophages. Macrophages devour bacteria, dead cells, and debris, and they also alert other immune cells to threats.
- Eosinophils specialize in fighting parasitic infections and play a role in allergic reactions and chronic inflammation.
- Basophils are the rarest type. They release histamine and other chemicals during allergic responses, contributing to the swelling, redness, and itching you feel during an allergic reaction.
Platelets and Blood Clotting
Platelets aren’t full cells. They’re small fragments that break off from large cells in your bone marrow, and their primary job is stopping bleeding. When a blood vessel is damaged, the exposed inner surface triggers platelets to stick to the wound site, change shape, and clump together into what’s called a platelet plug. This is the first stage of clotting, known as primary hemostasis.
That initial plug is fragile, though. To make it durable, the body activates a chain reaction called the coagulation cascade. Proteins in the blood interact in a precise sequence, ultimately producing tough strands of a protein called fibrin. Fibrin weaves through the platelet plug like reinforcing mesh, creating a stable clot that seals the wound while the tissue underneath heals. Without enough platelets or clotting proteins, even minor injuries can bleed excessively.
What Blood Does Beyond Carrying Oxygen
Oxygen delivery gets the most attention, but blood performs several other critical functions. It carries nutrients absorbed from your digestive tract (glucose, amino acids, fats, vitamins) to cells throughout the body. It transports hormones from the glands that produce them to the distant organs that respond to them. And it picks up metabolic waste products, shuttling them to the liver, kidneys, and intestines for processing and removal.
Blood also acts as your body’s temperature regulation system. Plasma absorbs and releases heat efficiently. When you’re overheating, blood vessels near the skin’s surface widen, allowing more blood to flow close to the surface and release heat. When you’re cold, those vessels narrow to conserve warmth in your core. This is why your face flushes during exercise and your fingers turn pale in the cold.
On top of all that, blood maintains a tightly controlled chemical balance. Its pH stays between 7.35 and 7.45, averaging 7.40. That’s slightly alkaline. Even small shifts outside this range can disrupt how proteins function and how cells communicate. The body maintains this balance partly through a buffer system: carbon dioxide produced by cells combines with water in the blood to form carbonic acid, which breaks apart into bicarbonate and hydrogen ions. This reaction can run in either direction, absorbing or releasing acid as needed to keep pH stable.
Blood Types
Not everyone’s blood is interchangeable. Red blood cells carry surface markers called antigens, and these determine your blood type. The ABO system classifies blood into four groups: A, B, AB, and O, based on which antigens sit on the surface of your red blood cells. Type A cells carry the A antigen, type B carry the B antigen, type AB carry both, and type O carry neither. Your immune system produces antibodies against whichever antigens your own cells lack, which is why receiving the wrong blood type during a transfusion triggers a dangerous immune reaction.
A second classification layer is the Rh factor. If your red blood cells carry the Rh antigen, you’re Rh-positive; if not, you’re Rh-negative. Combined with the ABO system, this gives eight common blood types (A+, A-, B+, B-, AB+, AB-, O+, O-). Type O is generally the most common blood type worldwide, followed by types A and B. Rh-positive blood is far more common than Rh-negative in most populations.
How Blood Is Produced
Most blood cells are made in your bone marrow, the spongy tissue inside certain bones like the pelvis, sternum, and thighbones. Stem cells in the marrow continuously divide and mature into red blood cells, white blood cells, and the large cells that shed platelets. Red blood cells live about 120 days before being broken down in the spleen and liver, so your body produces millions of new ones every second to maintain a steady supply. White blood cells have much shorter lifespans, from hours to days for neutrophils, though some lymphocytes can persist for years as memory cells. Plasma proteins are produced mainly by the liver.
This constant production is why nutritional deficiencies can affect your blood so directly. Iron is essential for hemoglobin, certain B vitamins are needed for red blood cell formation, and protein supplies the building blocks for plasma and cell production. It’s also why bone marrow disorders can have such widespread effects on health: when the factory slows down or malfunctions, every part of the blood supply is affected.