Blood is the fluid that circulates through your heart, arteries, veins, and capillaries, delivering oxygen and nutrients to every cell in your body while carrying away waste. An average adult carries about 1.2 to 1.5 gallons of it. Whole blood is roughly 55% plasma (a yellowish liquid made mostly of water) and 45% blood cells, and every component plays a distinct role in keeping you alive.
What Blood Is Made Of
Plasma, the liquid portion, carries dissolved sugars, fats, proteins, hormones, and electrolytes throughout your body. It also transports waste products to your kidneys and liver for removal. Think of plasma as the river; the blood cells are the cargo floating in it.
Red blood cells are by far the most abundant cells in blood, making up 40% to 45% of its total volume. Their job is singular and critical: picking up oxygen in your lungs and delivering it to tissues and organs everywhere else. They accomplish this through hemoglobin, a protein that contains iron atoms. Each iron atom binds an oxygen molecule in the lungs, then releases it when the red blood cell reaches oxygen-hungry tissue. The surrounding structure of hemoglobin accelerates this binding process enormously, making oxygen pickup almost instantaneous with each breath.
White blood cells account for only about 1% of blood volume, but they form the backbone of your immune defense. Platelets, even smaller cell fragments, float alongside them and are responsible for clotting when you’re injured.
Why Blood Is Red
Oxygenated blood flowing through your arteries is bright red. Blood that has already delivered its oxygen and is returning through your veins is dark red, not blue. Veins look blue through your skin because your skin and fat layers scatter red light before it can bounce back to your eyes, leaving mostly blue wavelengths to reach you. But if you’ve ever had blood drawn from a vein, you’ve seen the truth: it comes out dark red.
How Blood Fights Infection
White blood cells come in several specialized types, each handling a different kind of threat. Neutrophils are the most common, making up 50% to 70% of all white blood cells. They’re your body’s first responders, rushing to the site of a bacterial infection and swallowing invaders whole through a process called phagocytosis.
Lymphocytes, the second most common type at 20% to 40%, are part of your adaptive immune system. These are the cells that learn to recognize specific threats and remember them for future encounters, which is the principle behind vaccination. Monocytes leave the bloodstream and settle into tissues, where they mature into larger cells that consume bacteria and debris. Eosinophils specialize in parasitic infections and help moderate allergic reactions. Basophils, the rarest type, trigger inflammation and release histamine during allergic responses.
How Blood Clots Work
When a blood vessel is damaged, your body launches a three-stage repair process. First, within about 30 minutes, the damaged vessel constricts to reduce blood flow to the area. Second, platelets rush to the injury site, sticking to exposed tissue and to each other, forming a temporary plug. As they pile up, they change shape and release chemical signals that recruit even more platelets.
Third, a cascade of proteins in your blood activates in sequence, ultimately converting a substance called prothrombin into thrombin. Thrombin then creates fibrin, a tough protein mesh that reinforces the platelet plug into a stable clot. This entire chain reaction involves dozens of clotting factors working in precise order. A deficiency in any one of them can lead to excessive bleeding or, conversely, dangerous clots forming where they shouldn’t.
Blood Types and Why They Matter
Your blood type depends on which marker proteins (called antigens) sit on the surface of your red blood cells. The ABO system has two key antigens: A and B. If your cells carry the A antigen, you’re type A. If they carry B, you’re type B. If they carry both, you’re type AB. If they carry neither, you’re type O (from the German word “Ohne,” meaning “without”).
Here’s the critical part: your immune system produces antibodies against whichever antigens your own blood cells lack. A person with type A blood carries anti-B antibodies. Someone with type O carries both anti-A and anti-B antibodies. Type AB individuals carry neither, which is why AB is sometimes called the universal recipient. If you receive a transfusion of the wrong blood type, your antibodies attack the foreign red blood cells, which can be life-threatening.
The Rh factor adds another layer. If your red blood cells carry the Rh protein, you’re Rh-positive; if not, Rh-negative. Combining ABO and Rh gives the eight common blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
What Blood Tests Reveal About Your Health
Blood is one of the most useful diagnostic tools in medicine because it reflects what’s happening throughout your entire body. The most common screening is a complete blood count (CBC), which measures the levels of your different blood cells. Normal ranges for adults include:
- White blood cells: 4,000 to 10,000 cells per microliter. Higher counts often signal infection or inflammation. Lower counts can indicate immune suppression.
- Hemoglobin: 11.5 to 15.5 g/dL for those on estrogen or 13 to 17 g/dL for those on testosterone. Low hemoglobin means anemia, which explains fatigue, weakness, and shortness of breath.
- Hematocrit: 36% to 48% or 40% to 55%, depending on hormonal profile. This is simply the percentage of your blood volume occupied by red blood cells.
- Platelets: 150,000 to 400,000 cells per microliter. Too few means bleeding risk; too many can mean clotting risk.
Blood glucose testing is another routine check. A fasting blood sugar of 99 mg/dL or below is normal. Between 100 and 125 mg/dL falls in the prediabetes range, and 126 mg/dL or above indicates diabetes. These numbers come from a simple blood draw after an overnight fast and are often the earliest signal that your body’s ability to regulate sugar is changing.
Beyond these basics, blood tests can measure cholesterol, thyroid hormones, liver enzymes, kidney function markers, vitamin levels, and inflammatory proteins. A few tablespoons of blood, drawn in seconds, can paint a remarkably detailed picture of your overall health.