Blood is a fluid tissue made up of two main parts: a pale yellow liquid called plasma, which accounts for about 55% of blood’s volume, and cells and cell fragments that make up the remaining 45%. An average adult carries roughly 4.5 to 5.5 liters of blood, depending on body size and sex. Every component plays a distinct role, from carrying oxygen to fighting infections to sealing wounds.
Plasma: The Liquid Foundation
Plasma is mostly water, about 90% by volume. The remaining 10% is a concentrated mix of proteins (8%), inorganic salts like sodium, potassium, calcium, and chloride (0.9%), and organic substances including glucose, amino acids, vitamins, and hormones (1.1%). Despite being a small fraction, these dissolved substances are what make plasma so functionally powerful. It serves as the body’s main transport highway, delivering nutrients to cells and carrying metabolic waste to the kidneys, liver, and lungs for removal.
Blood pH stays in a narrow band between 7.35 and 7.45, slightly alkaline. Even small shifts outside that range can disrupt cell function, so plasma proteins and dissolved salts act as chemical buffers to keep things stable. They account for roughly 7% of the blood’s total buffering capacity.
The Three Major Plasma Proteins
Three protein families dominate plasma, and each does very different work.
Albumin is the most abundant, making up about 60% of all plasma protein. Its primary job is maintaining oncotic pressure, the force that keeps fluid inside your blood vessels rather than leaking into surrounding tissue. Albumin is three times more effective at this than globulin. It also acts as a molecular taxi, binding and transporting fatty acids, calcium, zinc, hormones, and even medications like aspirin and penicillin through the bloodstream.
Globulins make up around 36% of plasma protein. This family includes antibodies (also called immunoglobulins) that recognize and neutralize bacteria, viruses, and other foreign invaders. Other globulins help transport lipids and fat-soluble vitamins.
Fibrinogen accounts for about 4% of plasma protein, but its role is critical. When a blood vessel is damaged, fibrinogen converts into long, sticky threads of fibrin that form the structural mesh of a blood clot. Without it, even a small cut could bleed indefinitely.
Red Blood Cells: Oxygen Carriers
Red blood cells, or erythrocytes, are by far the most numerous cells in blood. A healthy adult male has 4.7 to 6.1 million red blood cells in every microliter of blood. For women, the typical range is 4.2 to 5.4 million per microliter. A single drop of blood contains millions of them.
Their sole purpose is gas exchange. Each red blood cell is packed with hemoglobin, an iron-containing protein that binds oxygen in the lungs and releases it in tissues that need it. Hemoglobin also picks up some carbon dioxide from tissues and carries it back to the lungs to be exhaled. Healthy hemoglobin levels range from 13.2 to 16.6 grams per deciliter in men and 11.6 to 15 grams per deciliter in women. When hemoglobin drops below these ranges, you’re anemic, which is why low red blood cell counts cause fatigue and shortness of breath.
Red blood cells are shaped like flattened discs with a slight indentation on both sides. This shape maximizes their surface area for gas exchange and lets them flex through capillaries narrower than the cells themselves. They lack a nucleus, which frees up internal space for more hemoglobin. The body produces roughly two million new red blood cells every second, primarily in bone marrow, and each cell circulates for about 120 days before being broken down and recycled in the spleen and liver.
White Blood Cells: The Immune Defense
White blood cells, or leukocytes, are far less numerous than red blood cells but essential for survival. They patrol the bloodstream and tissues looking for pathogens, damaged cells, and foreign material. There are five major types, each with a specialized function.
- Neutrophils are the first responders. They arrive quickly at infection sites and engulf bacteria and fungi. They make up the largest share of circulating white blood cells.
- Lymphocytes include B cells, which produce antibodies, and T cells, which directly attack infected or abnormal cells. They are central to both immediate immune responses and long-term immunity.
- Monocytes migrate into tissues where they mature into macrophages, large cells that consume pathogens and dead cells and help activate other parts of the immune system.
- Eosinophils target parasites and play a role in allergic reactions and inflammation.
- Basophils are the rarest type. They release histamine and other chemicals during allergic responses, helping to increase blood flow to affected areas.
Platelets: The Clotting Crew
Platelets, or thrombocytes, aren’t full cells. They’re tiny fragments that break off from large cells in the bone marrow. A healthy platelet count ranges from 150,000 to 400,000 per microliter of blood. When a blood vessel is damaged, platelets rush to the site, stick to the exposed tissue, and clump together. They then work with fibrinogen and other clotting factors to form a plug that seals the wound and stops bleeding. This entire process, called hemostasis, can begin within seconds of an injury.
Platelet counts that drop too low increase the risk of uncontrolled bleeding. Counts that climb too high can raise the risk of abnormal clots forming inside blood vessels, potentially leading to stroke or heart attack.
Hematocrit: The Cell-to-Plasma Ratio
Hematocrit measures the percentage of blood volume occupied by red blood cells. For men, a typical hematocrit falls between 38.3% and 48.6%. For women, the range is 35.5% to 44.9%. Since red blood cells vastly outnumber white blood cells and platelets, hematocrit is essentially a measure of red blood cell concentration. A hematocrit that’s too low suggests anemia, while one that’s too high can indicate dehydration or conditions where the body overproduces red blood cells, thickening the blood and straining the heart.
What Blood Actually Does
The composition of blood maps directly to its functions. Plasma transports nutrients, hormones, and waste. Red blood cells handle oxygen and carbon dioxide exchange. White blood cells defend against infection. Platelets and clotting proteins repair vascular damage. Beyond these, blood also distributes heat from active muscles and organs throughout the body, helping regulate core temperature, and its buffering systems keep pH stable even when metabolism produces large amounts of acid during exercise or illness.
Every component is continuously produced, used, and recycled. Bone marrow generates new blood cells, the liver produces most plasma proteins, and organs like the spleen filter out old or damaged cells. The entire system turns over constantly, keeping the composition of blood remarkably stable from day to day.