Blood has four main components: red blood cells, white blood cells, platelets, and plasma. Three of these are cellular (or cell-like) elements, while plasma is the liquid that carries everything else. Together, they make up roughly 5 liters of blood in the average adult, with plasma accounting for about 55% of that volume, red blood cells about 44%, and white blood cells plus platelets less than 1%.
Red Blood Cells
Red blood cells are by far the most abundant cells in your blood. A healthy adult man carries between 4.35 and 5.65 trillion per liter; for women, the range is 3.92 to 5.13 trillion per liter. Their single job is oxygen delivery. Each red blood cell is packed with hemoglobin, a protein built around four iron atoms. Every iron atom can grab one oxygen molecule, so a single hemoglobin protein carries four oxygen molecules at a time from your lungs to your tissues.
What makes this system efficient is that hemoglobin doesn’t just passively hold oxygen. It releases more of it in areas where your body is actively burning energy and producing carbon dioxide, so working muscles and busy organs get a bigger share automatically. Red blood cells live about 120 days before the spleen and liver break them down, and your bone marrow constantly produces new ones to replace them.
When your red blood cell count or hemoglobin drops too low, the result is anemia. Common signs include fatigue, shortness of breath, pale skin, and feeling cold. An unusually high red blood cell count (polycythemia) thickens the blood and can increase the risk of clotting.
White Blood Cells
White blood cells are the immune system’s workforce. They’re far less numerous than red blood cells, with a normal count of 3.4 to 9.6 billion per liter, but they come in five distinct types, each with a specialized role.
Neutrophils
Neutrophils make up 50% to 70% of all white blood cells and act as the body’s first responders. When bacteria enter a wound or infection site, neutrophils arrive within minutes and engulf the invaders directly. They’re the main reason pus forms: it’s largely a mass of neutrophils that have done their job and died.
Lymphocytes
Lymphocytes account for 20% to 40% of white blood cells and drive the adaptive immune system, the part that learns and remembers specific threats. This group includes B cells (which produce antibodies), T cells (which destroy infected or abnormal cells), and natural killer cells. Vaccines work by training lymphocytes to recognize a pathogen before you ever encounter it naturally.
Monocytes
Monocytes represent 2% to 8% of white blood cells. They circulate briefly in the blood, then move into tissues and transform into macrophages, large cells that swallow bacteria, dead cells, and debris. Macrophages also present fragments of what they’ve consumed to lymphocytes, helping the adaptive immune system identify new threats.
Eosinophils and Basophils
These two types exist in small numbers but play important roles. Eosinophils specialize in fighting parasitic infections and help regulate allergic inflammation by breaking down histamine and other inflammatory signals. Basophils function similarly to mast cells, releasing chemicals that trigger allergic reactions and inflammation. Together, these cells are often elevated in people with allergies, asthma, or parasitic infections.
A white blood cell count that’s too low (leukopenia) leaves you vulnerable to infections. A count that’s too high (leukocytosis) typically signals that your body is fighting something, though persistently elevated counts can point to more serious conditions like leukemia.
Platelets
Platelets are the smallest blood components, about 2 micrometers across, and they aren’t true cells. They’re fragments that break off from large cells in the bone marrow. A normal count ranges from roughly 135 to 371 billion per liter, depending on sex, and each platelet survives 7 to 10 days in circulation.
Their job is stopping bleeding. When a blood vessel is damaged, the exposed tissue underneath acts like a signal flare. Platelets stick to the damaged area, activate, and sprout arm-like extensions that help them clump together. As they activate, they release chemical signals, including a compound that narrows nearby blood vessels and another (ADP) that recruits even more platelets to the scene. The result is a temporary platelet plug that seals the wound within seconds.
That plug is just the first step. Platelets also provide the surface on which clotting proteins assemble to build a stronger, fibrin-based clot that holds everything together while healing takes place. Too few platelets (thrombocytopenia) leads to easy bruising and prolonged bleeding. Too many (thrombocytosis) raises the risk of dangerous blood clots forming where they shouldn’t.
Plasma
Plasma is the pale yellow liquid that makes up about 55% of blood volume. It’s 91% to 92% water, with the remaining 8% to 9% made up of dissolved proteins, salts, sugars, fats, and hormones. Three proteins do most of the heavy lifting: albumin keeps fluid from leaking out of blood vessels by maintaining pressure inside them, globulins include antibodies and transport proteins, and fibrinogen is the raw material platelets and clotting factors convert into fibrin to seal wounds.
Plasma also serves as the body’s main delivery and disposal system. It carries nutrients from your digestive tract to your cells, shuttles hormones from glands to target organs, and transports waste products to the kidneys and liver for removal. Without plasma, the cellular components of blood would have no way to travel.
Where Blood Cells Are Made
All blood cells originate in the bone marrow, the spongy tissue inside your larger bones. A single type of cell, the hematopoietic stem cell, gives rise to every red blood cell, white blood cell, and platelet your body produces. These stem cells can both copy themselves (to maintain the supply) and mature into specialized cells depending on what the body needs. Chemical signals tell the marrow to ramp up production of a specific cell type: more red blood cells when oxygen levels drop, more white blood cells during infection, more platelets after blood loss.
This production system is remarkably prolific. Your bone marrow generates hundreds of billions of new blood cells every day to replace those that wear out, get used up fighting infections, or are consumed during clotting.