Blood is a liquid connective tissue made up of two main parts: plasma, a pale yellow fluid that accounts for roughly 55% of total blood volume, and formed elements (red blood cells, white blood cells, and platelets) that make up the remaining 45%. An average adult carries about 4.5 to 5.5 liters of blood, all of it maintained at a slightly alkaline pH of around 7.40. Understanding what’s actually in that blood, and what each component does, helps make sense of everything from routine lab results to why a small cut eventually stops bleeding.
Plasma: The Liquid Portion
Plasma is about 92% water. The remaining 8% is a mix of proteins (roughly 7%) and a final 1% composed of hormones, vitamins, salts, enzymes, and dissolved gases. Despite making up a small fraction, those solutes do enormous work. They carry nutrients to tissues, shuttle waste products to the kidneys and liver, transmit chemical signals between organs, and keep the blood’s chemistry stable.
Three protein groups dominate plasma. Albumin is the most abundant, making up about 55 to 60% of all plasma proteins. Its main job is maintaining osmotic pressure, the force that keeps fluid inside blood vessels rather than leaking into surrounding tissue. When albumin drops too low, fluid seeps out and causes swelling.
Globulins come next, divided into several subtypes. The most familiar are gamma globulins, better known as antibodies. These are the proteins your immune system produces to tag and neutralize bacteria, viruses, and toxins. Other globulins help transport fats, iron, and hormones through the bloodstream.
Fibrinogen is the third major plasma protein, present in smaller amounts but essential for clotting. When a blood vessel is injured, fibrinogen converts into long, sticky strands called fibrin that weave together into a mesh, trapping blood cells and sealing the wound.
Red Blood Cells
Red blood cells, or erythrocytes, are by far the most numerous cells in your blood. A normal count ranges from about 4.0 to 5.4 million cells per microliter in women and 4.5 to 6.1 million per microliter in men. Each cell is a tiny, flexible disc with a concave center, a shape that maximizes surface area for gas exchange and lets the cell squeeze through capillaries narrower than itself.
Their primary cargo is hemoglobin, an iron-containing protein that binds oxygen in the lungs and releases it in tissues that need it. Hemoglobin also picks up some carbon dioxide on the return trip, though most CO₂ travels dissolved in plasma as bicarbonate. Healthy hemoglobin levels run roughly 11.5 to 15.5 g/dL in women and 13 to 17 g/dL in men. When hemoglobin falls below these ranges, the result is anemia, which causes fatigue and shortness of breath because tissues aren’t getting enough oxygen.
A related measurement you’ll see on lab work is hematocrit, the percentage of blood volume occupied by red blood cells. Normal hematocrit is about 36 to 48% for women and 40 to 55% for men. Because red blood cells make up the vast majority of formed elements, hematocrit is essentially a measure of how packed your blood is with oxygen-carrying cells.
White Blood Cells
White blood cells, or leukocytes, are the immune system’s mobile defense force. They’re far less numerous than red blood cells. A healthy adult has between 4,000 and 10,000 white blood cells per microliter of blood. What they lack in numbers, they make up for in specialization. Five distinct types handle different threats.
Neutrophils are the most common, typically numbering 2,500 to 7,000 per microliter. They’re first responders, rushing to the site of a bacterial or viral invasion and engulfing the invaders. Lymphocytes (1,000 to 4,800 per microliter) include B cells, which produce antibodies, and T cells, which can directly destroy infected or cancerous cells. These two cell types form the backbone of your adaptive immune system, the branch that “remembers” past infections and responds faster the second time around.
Monocytes (200 to 800 per microliter) are larger cells that consume bacteria and dead tissue and help coordinate the broader immune response. Eosinophils (fewer than 500 per microliter) specialize in fighting parasites and play a role in allergic reactions. Basophils (fewer than 300 per microliter) are the rarest, releasing chemical signals during allergic responses and asthma attacks that trigger inflammation and recruit other immune cells to the area.
Platelets and Clotting
Platelets, or thrombocytes, aren’t full cells. They’re tiny fragments shed from large cells in bone marrow, and they circulate in concentrations of 150,000 to 400,000 per microliter. When a blood vessel is damaged, platelets stick to the exposed tissue and to each other, forming a temporary plug within seconds. They then work alongside fibrinogen and other clotting factors in plasma to build a stable clot that seals the break.
A platelet count below 150,000 (thrombocytopenia) increases the risk of excessive bleeding, while a count above 400,000 (thrombocytosis) can raise the risk of abnormal clots forming inside blood vessels. Both directions matter, which is why platelet count is a standard part of routine blood work.
Dissolved Gases
Blood constantly shuttles oxygen from the lungs to every tissue and returns carbon dioxide to be exhaled. In healthy arterial blood at sea level, the partial pressure of oxygen ranges from 75 to 100 mm Hg, while carbon dioxide sits at 38 to 42 mm Hg. Most oxygen rides on hemoglobin inside red blood cells, but a small amount dissolves directly in plasma. Carbon dioxide travels three ways: bound to hemoglobin, dissolved in plasma, and, most significantly, converted to bicarbonate ions that double as a pH buffer.
Electrolytes and pH Balance
Dissolved in plasma is a precise mix of charged minerals that keep your cells, nerves, and muscles working. Sodium (135 to 145 mmol/L) is the dominant electrolyte in blood and controls fluid balance. Potassium (3.6 to 5.5 mmol/L) is critical for heart rhythm and muscle contraction. Calcium (8.8 to 10.7 mg/dL) supports bone health, clotting, and nerve signaling. Chloride (97 to 105 mmol/L) helps regulate fluid and acid-base balance alongside sodium.
Bicarbonate (22 to 29 mmol/L) deserves special attention because it’s the body’s main chemical buffer. It neutralizes excess acid to keep blood pH locked in the narrow range of 7.35 to 7.45. Even small deviations outside this window disrupt enzyme function and cellular chemistry. A pH below 6.8 or above 7.8 is life-threatening. The kidneys and lungs continuously adjust bicarbonate and carbon dioxide levels to keep the balance steady.
How It All Works Together
No single component operates in isolation. Red blood cells depend on plasma to carry them through vessels, and on the iron and nutrients dissolved in that same plasma to build hemoglobin. White blood cells use chemical signals carried in plasma to find infections. Platelets rely on fibrinogen and other plasma proteins to turn a fragile plug into a durable clot. Electrolytes maintain the electrical gradients that let every cell in the body, including blood cells themselves, function properly.
This is why a complete blood count, the most common blood test ordered worldwide, measures so many values at once. A shift in any single component often signals a change in the others. Low red blood cells may reflect iron deficiency, chronic disease, or blood loss. A spike in white blood cells usually points to infection or inflammation. Abnormal electrolytes can reveal kidney problems, dehydration, or hormonal imbalances. Blood composition is, in many ways, a real-time report card on the body’s overall health.