Blood is a mixture of liquid and cells, roughly split into two parts: plasma (the liquid portion, making up about 55% of blood volume) and formed elements (the cells and cell fragments, making up the other 45%). An average adult carrying 150 to 180 pounds has approximately 1.2 to 1.5 gallons circulating through their body at any given moment. Every drop contains millions of cells suspended in a protein-rich fluid, each with a specific job.
Plasma: The Liquid Half
Plasma is 91% to 92% water. The remaining 8% to 9% is a concentrated mix of proteins, electrolytes, dissolved gases, nutrients, hormones, and waste products. That small solid fraction does enormous work.
The three most important plasma proteins are albumin, globulins, and fibrinogen. Albumin is the most abundant. It acts like a sponge, pulling water into blood vessels to maintain blood pressure and volume. Globulins include antibodies, the proteins your immune system uses to tag and neutralize invaders. Fibrinogen is the raw material for blood clots. When you cut yourself, fibrinogen converts into long sticky strands called fibrin that weave together to seal the wound.
Dissolved in that water you’ll also find electrolytes: sodium, potassium, chloride, calcium, and bicarbonate. These charged particles keep your blood at a tightly controlled pH between 7.35 and 7.45, slightly alkaline. Even a small shift outside that range disrupts the chemical reactions your cells depend on. Electrolytes also help nerves fire, muscles contract, and fluid move between tissues.
Plasma carries other cargo too. Glucose and fatty acids travel through it as fuel for cells. Hormones like insulin and cortisol use plasma as a highway to reach distant organs. Waste products like urea and creatinine ride the plasma to the kidneys for disposal.
Red Blood Cells: Oxygen Carriers
Red blood cells are by far the most numerous cells in your blood. A healthy man has about 4.3 to 5.9 million per cubic millimeter; a healthy woman has about 3.5 to 5.5 million. Your bone marrow produces 2 to 3 million new red blood cells every second to replace old ones that wear out after roughly 120 days.
Their job is gas exchange. Each red blood cell is packed with hemoglobin, a protein that binds oxygen in the lungs and releases it in tissues that need it. About 98% of the oxygen in your blood rides on hemoglobin inside red blood cells. Only about 2% floats dissolved directly in the plasma. Red blood cells also help carry carbon dioxide, a metabolic waste product, back to the lungs so you can exhale it.
Red blood cells have an unusual shape: a flattened disc with a dimple on each side, like a donut without the hole. This maximizes surface area for gas exchange and lets them flex through capillaries narrower than the cells themselves.
White Blood Cells: The Immune System
White blood cells are far less numerous, with a normal count between 4,500 and 11,000 per cubic millimeter. But their impact is outsized. They are the immune system’s mobile workforce, circulating through the blood until they detect signals from damaged or infected tissue. When that happens, they squeeze through tiny gaps in blood vessel walls and migrate to the problem site.
There are five types, each with a distinct role:
- Neutrophils are the most common and act as the body’s first responders against bacteria, viruses, and other germs.
- Lymphocytes include B cells, which produce antibodies against specific threats, and T cells, which can destroy virus-infected cells and cancer cells directly.
- Monocytes engulf and digest bacteria and dead cells, and they help coordinate the broader immune response.
- Eosinophils target parasites and play a role in allergic reactions and inflammation.
- Basophils release chemical signals during allergic reactions and asthma attacks.
All five types originate from stem cells in the bone marrow, but they mature along different paths and end up looking and behaving quite differently from one another.
Platelets: Damage Control
Platelets are not true cells. They’re small, irregularly shaped fragments that break off from large cells in the bone marrow. A healthy person has between 150,000 and 450,000 platelets per microliter of blood at any given time.
Platelets circulate for about 8 to 10 days, scanning for damage. When they encounter a torn or injured blood vessel wall, they stick to the damaged spot, clump together, and activate a chain reaction involving proteins called clotting factors. Those proteins work together to produce fibrin, which forms a mesh over the platelet plug and hardens into a stable clot. This process stops bleeding in seconds to minutes for minor injuries. Platelets that never encounter damage are eventually filtered out by the spleen.
Dissolved Gases
Blood constantly carries two gases in opposite directions. Oxygen travels from the lungs to every tissue in the body, while carbon dioxide makes the return trip from tissues back to the lungs. Most oxygen binds to hemoglobin, but a small fraction dissolves directly in plasma. The amount of dissolved oxygen follows a simple physical law: the higher the oxygen pressure in the lungs, the more dissolves into the liquid.
Carbon dioxide travels three ways: some dissolves in plasma, some binds to hemoglobin, and the majority converts into bicarbonate, one of the electrolytes that helps buffer blood pH. This system is remarkably efficient. Your blood completes a full loop through your body in about 60 seconds, picking up and dropping off gases continuously.
Other Substances in Blood
Beyond cells, proteins, and gases, blood contains a long list of smaller molecules. Glucose is the primary fuel source for most cells and is tightly regulated by hormones. Lipids, including cholesterol and triglycerides, travel bundled in protein packages. Amino acids from digested food circulate to cells that need building material for new proteins. Vitamins and minerals hitch rides on carrier proteins or dissolve freely in plasma.
Blood also carries waste. Urea, produced when the liver breaks down proteins, travels to the kidneys for excretion. Bilirubin, a yellowish byproduct of old red blood cells being recycled, passes through the liver and into bile. Creatinine, a byproduct of muscle metabolism, filters out through the kidneys. Measuring these waste products in a blood test gives doctors a snapshot of how well your liver and kidneys are functioning.