Blood is a mixture of liquid and cells, roughly split between a pale yellow fluid called plasma and three types of blood cells: red cells, white cells, and platelets. The average adult carries about 5 liters of blood, though the exact amount varies with body size. Every component has a distinct job, from delivering oxygen to fighting infections to sealing wounds.
Plasma: The Liquid Half
Plasma makes up about 55% of your total blood volume. It’s mostly water, around 91% to 92%, with the remaining 8% to 9% made up of dissolved proteins, salts, hormones, and nutrients. Think of it as the river that carries everything else where it needs to go.
Three protein groups do most of the heavy lifting in plasma. Albumin, the most abundant, acts like a sponge that pulls water into your blood vessels so fluid doesn’t leak into surrounding tissues. Globulins include antibodies that target bacteria, viruses, and other threats. Fibrinogen is the protein responsible for clotting; when you cut yourself, fibrinogen converts into long sticky threads that help form a solid plug over the wound.
Plasma also carries dissolved electrolytes, including sodium, potassium, calcium, chloride, and bicarbonate. These salts keep your blood at a stable pH and help nerves and muscles fire correctly. Smaller amounts of glucose, hormones, vitamins, and waste products like carbon dioxide hitch a ride in plasma as well.
Red Blood Cells: Oxygen Carriers
Red blood cells are by far the most numerous cells in your blood. Their single purpose is transporting oxygen from your lungs to every tissue in your body and carrying carbon dioxide back to the lungs to be exhaled. They pull this off thanks to hemoglobin, an iron-rich protein that binds oxygen molecules. It’s also what gives blood its red color.
A healthy red blood cell is shaped like a flexible disc, thinner in the middle than at the edges. That shape lets it squeeze through capillaries narrower than the cell itself. Each cell circulates for roughly 115 days before the body breaks it down and recycles its iron. In some people that lifespan ranges from 70 to 140 days, but the body constantly produces replacements to keep the count stable.
The proportion of your blood occupied by red blood cells is measured as hematocrit. Normal hematocrit runs 41% to 50% in men and 36% to 44% in women. A number below that range can signal anemia, meaning your blood isn’t carrying enough oxygen. A number above it can make blood thicker and harder to pump.
White Blood Cells: The Immune Defense
White blood cells make up less than 1% of your blood by volume, but they’re the core of your immune system. Five distinct types patrol the bloodstream, each specialized for a different threat.
- Neutrophils are the first responders. They kill bacteria, fungi, and foreign debris and are the most common white blood cell in circulation.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, T cells coordinate attacks on infected cells, and natural killer cells destroy virus-infected or cancerous cells on contact.
- Monocytes act as cleanup crews, engulfing damaged or dead cells and clearing debris from infection sites.
- Eosinophils target parasites and cancer cells and play a role in allergic reactions.
- Basophils trigger allergic responses like sneezing, coughing, and a runny nose by releasing chemicals that promote inflammation.
White blood cells have much shorter lifespans than red blood cells. Some neutrophils last only hours to a few days, while certain memory lymphocytes can survive for years, ready to recognize a pathogen you encountered long ago.
Platelets: The Repair Crew
Platelets are tiny cell fragments, much smaller than red or white blood cells. A normal count falls between 140,000 and 440,000 per microliter of blood. Their job is to stop bleeding whenever a blood vessel is damaged.
The process works in three quick steps. First, platelets stick to the exposed collagen fibers at the wound site, a step called adhesion. Next, they activate, changing shape and releasing chemical signals that recruit more platelets. Finally, the arriving platelets clump together (aggregation) to form a temporary plug. Clotting proteins in the plasma, including fibrinogen, then reinforce that plug into a stable clot. Meanwhile, the damaged vessel constricts to slow blood flow to the area, giving the clot time to solidify.
Platelets circulate for about 8 to 10 days before the spleen filters them out and the bone marrow sends new ones into the bloodstream.
Where Blood Cells Are Made
All blood cells originate in the bone marrow, the spongy tissue inside the central cavity of your bones. The process starts with hematopoietic stem cells, a small population of master cells that live in specialized regions of the marrow called niches. These stem cells can divide and develop into any type of blood cell: red cells, each of the five white cell types, or platelets.
In children, nearly every bone produces blood cells. In adults, production concentrates in the larger bones: the pelvis, spine, ribs, and sternum. The marrow churns out hundreds of billions of new cells every day to replace the ones that wear out. Red blood cells alone require roughly 200 billion replacements daily to keep oxygen delivery on schedule.
How the Components Work Together
Blood’s power comes from the balance between its parts. Plasma distributes nutrients, hormones, and waste products while maintaining the fluid pressure that keeps blood flowing. Red blood cells handle gas exchange, picking up oxygen and offloading carbon dioxide with each pass through the lungs. White blood cells patrol for threats and mount immune responses. Platelets and clotting proteins stand ready to seal any breach in a vessel wall.
When one component falls out of balance, the effects ripple through the whole system. Too few red blood cells means less oxygen reaching your muscles and brain, leaving you fatigued and short of breath. A low platelet count makes you bruise easily and bleed longer from minor cuts. A drop in white blood cells leaves you vulnerable to infections that a healthy immune system would handle without trouble. Blood’s composition is simple enough to describe in a few categories, but the coordination between those categories is what keeps every organ in your body running.