Blood is a mixture of roughly 55% plasma (the liquid portion) and 45% blood cells. An average adult carries about 4.5 to 5.5 liters of it, with the exact volume scaling to body weight: approximately 75 mL per kilogram for men and 65 mL per kilogram for women. Every drop contains four main components: plasma, red blood cells, white blood cells, and platelets, each with a distinct job.
Plasma: The Liquid Carrier
Plasma is a pale yellow fluid that makes up the majority of blood’s volume. It is about 90% water, 8% protein, and just under 1% dissolved salts. That water serves as the transport medium for everything else: nutrients from digestion, hormones from glands, waste products heading to the kidneys, and the cells and proteins described below.
The protein fraction of plasma contains three major types. Albumin is the most abundant, making up roughly 70% of all plasma protein. It helps maintain the fluid balance between your blood vessels and surrounding tissues. If albumin levels drop, fluid leaks out of vessels and causes swelling. Globulins are the second group, and they include antibodies (also called immunoglobulins) that target infections, along with proteins that carry fats and vitamins through the bloodstream. Fibrinogen, the third type, is the raw material your body converts into a mesh of fibers when you need a blood clot.
Plasma also carries dissolved electrolytes like sodium, potassium, calcium, and bicarbonate. These salts keep cells functioning properly and help maintain blood pH within an extremely narrow range. Three buffer systems, the bicarbonate system being the most important, work constantly to prevent blood from becoming too acidic or too alkaline. Even a small shift in pH can disrupt the chemical reactions that keep you alive.
Red Blood Cells
Red blood cells are by far the most numerous cells in your blood. A healthy person has between 4.0 and 6.1 million of them in every microliter, a droplet barely visible to the naked eye. Women typically fall on the lower end of that range (4.0 to 5.4 million) and men on the higher end (4.5 to 6.1 million).
Their sole purpose is carrying oxygen from the lungs to every tissue in the body and ferrying carbon dioxide back. They do this using hemoglobin, an iron-containing protein that gives blood its red color. Normal hemoglobin levels run 13 to 17 grams per deciliter in men and 11.5 to 15.5 grams per deciliter in women. When hemoglobin drops below these ranges, the condition is called anemia, and it shows up as fatigue, shortness of breath, and pale skin because tissues aren’t getting enough oxygen.
Red blood cells are unusual in that they have no nucleus. That design frees up interior space for more hemoglobin, but it also means they can’t repair themselves. Each red blood cell circulates for about 120 days before the spleen filters it out and the bone marrow replaces it with a new one.
White Blood Cells
White blood cells are the immune system’s workforce inside the bloodstream. They’re far less numerous than red blood cells, with a normal total count of 4,000 to 10,000 per microliter, but they punch well above their weight. Five distinct types handle different threats.
- Neutrophils are the most common white blood cell (2,500 to 7,000 per microliter). They respond first to bacterial infections, swarming to the site and engulfing invaders.
- Lymphocytes (1,000 to 4,800 per microliter) include B cells, which produce antibodies, and T cells, which directly kill infected or abnormal cells. They also form the basis of immune memory, which is how vaccines work.
- Monocytes (200 to 800 per microliter) are large cells that move into tissues and become specialized scavengers, cleaning up bacteria, viruses, and dead cells.
- Eosinophils (fewer than 500 per microliter) primarily defend against parasites and also play a role in allergic reactions.
- Basophils (fewer than 300 per microliter) are the rarest type. They release chemical signals during allergic reactions and asthma attacks that trigger inflammation and recruit other immune cells.
When doctors order a “blood differential,” they’re measuring the count of each type. A spike in neutrophils often points to a bacterial infection, while elevated lymphocytes may suggest a viral one. Unusually high eosinophils can signal allergies or a parasitic infection. These patterns help narrow down a diagnosis before more specific tests are run.
Platelets
Platelets are tiny cell fragments, much smaller than red or white blood cells, that circulate in concentrations of 150,000 to 400,000 per microliter. Their job is to stop bleeding. When a blood vessel is damaged, platelets are the first responders: they stick to the injured tissue and clump together to form a temporary plug. This initial response is called primary hemostasis.
That platelet plug is fragile on its own, so the clotting cascade kicks in next. Fibrinogen from the plasma converts into long strands of fibrin that weave through the platelet plug and harden it into a stable clot. Once the vessel heals, the clot dissolves. If your platelet count drops too low, even minor bumps can cause prolonged bleeding or bruising. If it climbs too high, you face a greater risk of unwanted clots forming inside blood vessels.
Where Blood Cells Are Made
All three types of blood cells, red cells, white cells, and platelets, are produced in the bone marrow throughout your entire life. In children, nearly every bone contributes. In adults, production concentrates in the flat bones: the pelvis, sternum, ribs, skull, and vertebrae. Stem cells in the marrow continuously divide and mature into whichever cell type the body needs, a process called hematopoiesis.
These stem cells sit in specialized niches near tiny blood vessels inside the marrow. The body tightly regulates the process with signaling hormones. When oxygen levels drop, for example, the kidneys release a hormone that tells the marrow to ramp up red blood cell production. When an infection takes hold, chemical signals push the marrow to churn out more white blood cells. This constant calibration is why a simple blood count can reveal so much about what’s happening inside the body.
What a Complete Blood Count Tells You
A complete blood count, or CBC, is the most commonly ordered blood test, and it measures nearly every component described above. It reports your red blood cell count, hemoglobin level, hematocrit (the percentage of blood volume occupied by red cells, normally 36% to 55%), white blood cell total and differential, and platelet count.
Results outside the normal range don’t automatically mean something is wrong. Pregnancy, altitude, intense exercise, and even the time of day can shift numbers. Labs also use slightly different reference ranges, so your results should always be compared to the specific range printed on your report rather than a number you found online. Patterns matter more than any single value: a doctor looks at how the numbers relate to each other and to your symptoms before drawing conclusions.