Blood is a living tissue made of two main parts: a pale yellow liquid called plasma and billions of tiny cells suspended in it. Plasma accounts for about 55% of your blood volume, while cells and cell fragments make up the remaining 45%. An average adult carries roughly 5.5 liters (about 1.5 gallons) of blood, all of it produced deep inside your bones.
Plasma: The Liquid Half
Plasma is mostly water, about 90% by weight, but the other 10% is what makes it so useful. Dissolved in that water are proteins, hormones, vitamins, salts, sugars, fats, and waste products on their way to the kidneys. Plasma is essentially the body’s delivery and removal service, carrying nutrients to cells and hauling away what they don’t need.
Three types of protein do most of the heavy lifting in plasma. Albumin, the most abundant, acts like a transport truck, ferrying hormones, vitamins, and even medications through the bloodstream. It also keeps fluid balanced between your blood vessels and surrounding tissues by maintaining the right amount of osmotic pressure. Without enough albumin, fluid leaks out of blood vessels and causes swelling.
Globulins are the second major group. Some globulins carry fats and metals, but the most well-known are immunoglobulins, or antibodies. These proteins recognize and latch onto bacteria, viruses, fungi, and parasites, tagging them for destruction by your immune cells. The third key protein is fibrinogen, a clotting factor. When you cut yourself, fibrinogen converts into long, sticky threads that weave together to form a mesh over the wound.
Beyond proteins, plasma carries dissolved electrolytes like sodium, potassium, and calcium that keep your muscles firing and your heart beating in rhythm. It also buffers your blood’s pH, keeping it in a narrow range that your cells need to function.
Red Blood Cells: Oxygen Carriers
Red blood cells are by far the most numerous cells in your blood, making up about 44% of total blood volume. Their sole job is gas exchange: picking up oxygen in the lungs and delivering it to every tissue in the body, then carrying carbon dioxide back to the lungs to be exhaled.
They pull this off thanks to hemoglobin, an iron-rich protein that gives blood its red color. Each red blood cell is packed with roughly 270 million hemoglobin molecules. Normal hemoglobin levels run between 14 and 18 grams per deciliter for men and 12 to 16 g/dL for women. When hemoglobin drops too low, you have anemia, which is why it often shows up on routine blood tests.
Red blood cells have an unusual shape: a flattened disc pinched in the center, like a donut without a hole. This design maximizes surface area so oxygen can move in and out quickly. They’re also flexible enough to squeeze through capillaries narrower than they are. A single red blood cell circulates for an average of about 115 days, though individual cells can last anywhere from 70 to 140 days before the spleen and liver break them down and recycle the iron.
White Blood Cells: The Immune Defense
White blood cells and platelets together account for only about 1% of your blood, but white blood cells punch well above their weight. They are the core of your immune system, patrolling the bloodstream and slipping into tissues to hunt down threats. There are five main types, each with a distinct role.
- Neutrophils are the most common and the first responders. They kill bacteria and fungi by engulfing and digesting them.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, T cells coordinate attacks and destroy infected cells, and natural killer cells target virus-infected cells and cancer cells.
- Monocytes act as cleanup crews. They migrate into tissues, transform into larger cells, and devour dead cells, debris, and pathogens.
- Eosinophils specialize in fighting parasites and also play a role in allergic reactions and targeting certain cancer cells.
- Basophils are the rarest white blood cells. They trigger allergic responses like sneezing, coughing, and runny nose by releasing chemicals such as histamine.
Platelets: Plugging the Leaks
Platelets are not full cells. They’re small fragments that break off from much larger cells in the bone marrow. A healthy adult has between 140,000 and 440,000 platelets per microliter of blood, and their primary purpose is stopping bleeding.
When a blood vessel is damaged, the exposed tissue sends out chemical signals that platelets recognize immediately. They stick to the damaged surface (adhesion), change shape and release signaling chemicals (activation), then clump together (aggregation) to form a temporary plug. This process, called primary hemostasis, happens within seconds. The platelet plug buys time for the clotting proteins in plasma, especially fibrinogen, to build a stronger, more permanent seal over the wound.
Where Blood Cells Come From
All blood cells originate in the bone marrow, the spongy tissue inside your larger bones like the pelvis, spine, and femur. The process is called hematopoiesis. It starts with a single type of stem cell that can develop into any blood cell lineage. These stem cells divide and progressively specialize, eventually becoming red blood cells, each of the five white blood cell types, or the large cells that shed platelet fragments.
Your body produces blood cells constantly. Because red blood cells die after a few months and platelets last only about 10 days, the marrow churns out billions of replacements every day to keep counts stable.
Blood Types
Not everyone’s blood is interchangeable. The surface of every red blood cell carries molecular markers called antigens, and the combination you inherit determines your blood type. The ABO system sorts people into four groups: A, B, AB, and O, depending on whether red blood cells carry the A antigen, the B antigen, both, or neither.
A second major system is the Rh factor. If your red blood cells carry the Rh-D antigen, you’re Rh positive; if not, you’re Rh negative. Combining the two systems gives the familiar labels: O positive, A negative, AB positive, and so on. These distinctions matter most during blood transfusions and pregnancy, because receiving mismatched blood can trigger a dangerous immune reaction in which your antibodies attack the foreign red blood cells.