Blood performs six essential functions: transporting oxygen and nutrients, removing waste, fighting infections, stopping bleeding, regulating body temperature, and maintaining the chemical balance your cells need to survive. The average adult carries about 4.5 to 5.5 liters (roughly 1.2 to 1.5 gallons) of blood, and every drop is constantly working to keep the body’s systems running.
Carrying Oxygen and Carbon Dioxide
The most immediate job of blood is gas exchange. Red blood cells contain hemoglobin, a protein that picks up oxygen in the lungs and delivers it to tissues throughout the body. Hemoglobin works cooperatively: once the first oxygen molecule binds, the protein changes shape in a way that makes it easier for the next molecules to attach. This design means hemoglobin loads up efficiently in the oxygen-rich environment of the lungs, then releases oxygen readily in tissues where levels are lower.
Carbon dioxide, the waste product of cellular metabolism, travels back to the lungs through three routes. About 70% is converted into bicarbonate ions dissolved in plasma. Another 20 to 25% binds directly to hemoglobin for the return trip. The remaining 5 to 10% simply dissolves in the liquid portion of blood. Once blood reaches the lungs, these processes reverse and the carbon dioxide is exhaled.
Delivering Nutrients and Removing Waste
Plasma, the straw-colored liquid that makes up about 55% of blood volume, acts as the body’s delivery system. It is roughly 90% water by mass, with the remaining fraction composed of proteins, electrolytes, glucose, fats, and other molecules. After you eat, digested nutrients like glucose and fatty acids enter the bloodstream and travel to cells that need fuel. Fats travel packaged inside lipoprotein particles, tiny capsules of protein and fat that keep water-insoluble lipids suspended in the watery plasma.
The same circulation that delivers fuel also collects garbage. Cells constantly produce metabolic waste products, including urea (from protein breakdown) and creatinine (from muscle metabolism). Blood carries these waste molecules to the kidneys, which filter them out for excretion in urine. Without this continuous pickup and disposal service, toxic byproducts would accumulate in tissues within hours.
Defending Against Infection
White blood cells patrol the bloodstream as the body’s mobile immune force. There are five main types, each with a distinct role:
- Neutrophils are the first responders, killing bacteria, fungi, and foreign debris. They make up the largest share of white blood cells.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, T cells coordinate immune attacks and destroy infected cells, and natural killer cells target virus-infected cells and cancer cells.
- Monocytes clean up damaged cells and debris at infection sites, essentially acting as the cleanup crew after the initial fight.
- Eosinophils specialize in destroying parasites and cancer cells, and they assist in allergic responses.
- Basophils trigger allergic responses like coughing, sneezing, and a runny nose by releasing chemical signals that ramp up inflammation.
A healthy adult has between 3.4 billion and 9.6 billion white blood cells per liter of blood. When that count rises or falls outside this range, it often signals infection, inflammation, or an immune system disorder.
Stopping Bleeding
When a blood vessel is damaged, the body launches a three-stage repair process called hemostasis. First, the injured vessel constricts, narrowing its diameter to reduce blood loss from that spot. Second, platelets circulating in the blood stick to the damaged tissue and clump together, forming a temporary plug. This is called primary hemostasis.
Third, a chain reaction known as the coagulation cascade kicks in. Clotting factors in the blood activate one after another in sequence, amplifying the clotting signal at each step. The end result is fibrin, a tough, thread-like protein that weaves through the platelet plug like mortar between bricks. Together, the platelets and fibrin create a stable clot that seals the wound while the tissue underneath heals.
Regulating Body Temperature
Blood acts like a liquid heating and cooling system. Water holds heat well, and since plasma is 90% water, blood is excellent at absorbing heat from active organs (like working muscles) and redistributing it throughout the body.
When your body needs to shed heat, blood vessels near the skin surface widen, a process called vasodilation. This sends more warm blood to the skin, where heat radiates away. During intense exercise or in hot environments, skin blood flow can surge to 6 to 8 liters per minute, a striking increase that works alongside sweating to cool you down. When you need to conserve heat in cold conditions, the opposite happens: skin blood vessels narrow, reducing blood flow to the surface and keeping warmth concentrated around vital organs. Local skin temperature also plays a role. Warming a patch of skin can cause the blood vessels there to open fully, while cooling the skin can shut flow down to minimal levels.
Maintaining pH Balance
Your blood must stay within a very narrow pH range, roughly 7.35 to 7.45, for enzymes and cellular processes to function properly. Even small shifts outside this window can be life-threatening. Blood manages this through three built-in buffer systems that neutralize excess acids or bases before they can shift the pH.
The bicarbonate-carbonic acid system is the most prominent. When a strong acid enters the blood, bicarbonate reacts with it to form carbonic acid, a much weaker acid that has minimal effect on pH. The phosphate buffer works similarly, using two forms of phosphate (one weakly acidic, one weakly basic) to absorb hydrogen ions or release them as needed. Plasma proteins, including albumin, serve as the third buffer. Amino acids in these proteins carry both positive and negative charges, allowing them to bind excess hydrogen ions or hydroxyl ions and keep the pH steady.
Keeping Fluids in Balance
Blood doesn’t just flow through vessels. It constantly exchanges fluid with surrounding tissues through capillary walls. What prevents plasma from leaking out entirely is a force called oncotic pressure, and the main molecule responsible is albumin. This single protein accounts for about 80% of the pressure that holds fluid inside blood vessels, even though it makes up only about half of total plasma protein by weight. The reason is molecular math: albumin molecules are relatively small, so there are far more of them per liter than larger proteins, and it’s the number of particles, not their size, that generates osmotic pull.
Albumin also carries a net negative electrical charge, which attracts sodium and chloride ions into the bloodstream. These extra particles further increase the osmotic pressure inside vessels. Normally, plasma oncotic pressure averages about 28 mm Hg, comfortably higher than the roughly 8 mm Hg of pressure in the surrounding tissue fluid. This difference keeps fluid flowing back into blood vessels rather than pooling in tissues. When albumin levels drop, whether from liver disease, kidney problems, or malnutrition, fluid escapes into surrounding tissue and causes swelling, a condition known as edema.
What Blood Tests Reveal
Because blood touches every organ and performs so many functions, a simple blood draw can reveal a great deal about your health. A complete blood count, one of the most commonly ordered tests, measures several key values. Red blood cell counts typically fall between 4.35 and 5.65 trillion cells per liter in men and 3.92 to 5.13 trillion in women. Hematocrit, the percentage of blood volume occupied by red blood cells, normally ranges from about 38 to 49% in men and 36 to 45% in women. Numbers outside these ranges can point to anemia, dehydration, bone marrow problems, or other conditions that affect how well blood carries out its core functions.