Human blood contains a surprisingly complex mix of components: water, proteins, salts, gases, sugars, fats, minerals, and waste products, all carried alongside living cells. By volume, about 55% of blood is plasma (the liquid portion), 44% is red blood cells, and the remaining 1% is white blood cells and platelets. Within each of those fractions, dozens of individual substances play specific roles in keeping you alive.
Plasma: The Liquid Foundation
Plasma is the straw-colored fluid that carries everything else. It is about 92% water. Proteins make up another 7%, and the final 1% is a mixture of hormones, vitamins, salts, enzymes, and other dissolved compounds.
The proteins in plasma fall into three main groups. Albumin is the most abundant, accounting for roughly 60% of all plasma protein. It helps maintain fluid balance by pulling water into blood vessels and also serves as a transport vehicle for hormones, fatty acids, and certain drugs. Globulins make up about 36% of plasma protein and include antibodies that fight infection. Fibrinogen, the third group at about 4%, is the protein that allows blood to clot when you’re injured. Total plasma protein concentration in a healthy person typically falls between 6 and 8 grams per deciliter.
Electrolytes and Mineral Salts
Dissolved in plasma are charged minerals called electrolytes. These control nerve signaling, muscle contraction, fluid balance, and blood acidity. The major electrolytes and their normal blood ranges are:
- Sodium: 135 to 145 mmol/L, the primary driver of fluid balance
- Potassium: 3.6 to 5.5 mmol/L, essential for heart rhythm and muscle function
- Chloride: 97 to 105 mmol/L, works with sodium to regulate fluid
- Bicarbonate: 22 to 29 mmol/L, buffers blood pH
- Calcium: 8.8 to 10.7 mg/dL, critical for bone health, clotting, and nerve signals
- Magnesium: 1.5 to 2.6 mg/dL, involved in hundreds of enzyme reactions
Even small shifts outside these ranges can cause symptoms. Low potassium, for example, can trigger muscle cramps and irregular heartbeats, while high sodium often reflects dehydration.
Red Blood Cells and Iron
Red blood cells are the most numerous cells in blood, with a healthy count between 4.2 and 5.9 million per microliter. Their primary job is carrying oxygen from your lungs to every tissue in your body, then ferrying carbon dioxide back to the lungs for disposal. They do this through hemoglobin, the iron-containing protein that gives blood its red color.
Iron is central to this process. About two-thirds of all the iron in your body is locked inside hemoglobin, at a concentration of roughly 3.5 milligrams of iron per gram of hemoglobin. Normal hemoglobin levels range from 12 to 16 g/dL for women and 14 to 18 g/dL for men. When iron stores drop too low, your body can’t produce enough functional hemoglobin, which is how iron-deficiency anemia develops. A separate protein called transferrin circulates in plasma and shuttles iron to tissues that need it.
White Blood Cells and Platelets
White blood cells make up a tiny fraction of blood volume but are the backbone of your immune system. A normal count ranges from 4,000 to 11,000 per microliter. Several subtypes exist, each with a different role. Neutrophils are the most common (50% to 70% of white cells) and are first responders to bacterial infections. Lymphocytes (30% to 45%) handle longer-term immunity, including producing antibodies and killing virus-infected cells. Monocytes, eosinophils, and basophils round out the group in smaller numbers, targeting everything from parasites to allergic reactions.
Platelets are tiny cell fragments, numbering 150,000 to 450,000 per microliter. They clump together at wound sites to form the initial plug that stops bleeding, then work with fibrinogen and other clotting factors to build a stable clot.
Dissolved Gases
Blood carries two gases in significant quantities: oxygen and carbon dioxide. In arteries leaving the lungs, oxygen levels reach a partial pressure of about 104 mmHg, while carbon dioxide sits around 40 mmHg. By the time blood returns to the lungs through veins, oxygen has dropped to about 40 mmHg and carbon dioxide has risen to about 45 mmHg. This pressure difference is what drives gas exchange: oxygen flows from areas of high concentration (lungs) to low concentration (tissues), and carbon dioxide does the reverse.
Most oxygen travels bound to hemoglobin rather than dissolved freely in plasma. Carbon dioxide, on the other hand, is transported in three ways: dissolved in plasma, bound to hemoglobin, or converted into bicarbonate ions, which also help buffer blood pH.
Trace Elements
Beyond the major minerals, blood carries small but essential amounts of trace elements. These are present in microgram-per-liter concentrations, yet deficiencies can cause significant health problems.
- Zinc: averages about 6,750 µg/L in whole blood, supporting immune function and wound healing
- Copper: averages around 840 µg/L in whole blood, needed for energy production and connective tissue
- Selenium: averages about 141 µg/L in whole blood, protecting cells from oxidative damage
- Iodine: averages roughly 30 µg/L in whole blood, essential for thyroid hormone production
Other trace elements found in blood include cobalt, manganese, and molybdenum, all present at even lower concentrations but still necessary for specific enzyme functions.
Sugars, Fats, and Nutrients
Blood serves as the body’s delivery system for fuel. Glucose circulates in plasma as the primary energy source for cells, typically maintained between 70 and 100 mg/dL in a fasting state. After a meal, levels temporarily rise before insulin signals cells to absorb the excess.
Lipids (fats) also travel through blood, but because they don’t dissolve in water, they’re packaged inside lipoproteins. These particles carry cholesterol and triglycerides to tissues that need them for energy, hormone production, or cell membrane construction. Vitamins, both fat-soluble (A, D, E, K) and water-soluble (B vitamins, vitamin C), are also transported in plasma, often bound to carrier proteins like albumin.
Waste Products
Blood doesn’t just deliver nutrients. It also collects metabolic waste and carries it to the kidneys and liver for removal. The major waste products circulating in blood include urea (the largest component, making up about 45% of non-protein nitrogen compounds), creatinine (a byproduct of muscle metabolism), uric acid (produced when the body breaks down certain compounds in food), and small amounts of ammonia. Free amino acids and creatine also circulate and are sometimes grouped with these waste compounds, though they still serve useful functions before being filtered out.
Kidney function is often assessed by measuring blood levels of urea and creatinine. When the kidneys aren’t filtering efficiently, these waste products accumulate, which is one of the earliest detectable signs of kidney disease on routine blood work.