Blood performs a surprisingly long list of jobs, from ferrying oxygen to every cell in your body to fighting infections, sealing wounds, and carrying chemical messages between organs. An average adult carries about 1.2 to 1.5 gallons of it, roughly 10% of total body weight, and every drop is constantly at work keeping you alive.
Delivering Oxygen to Every Cell
The most essential job of blood is oxygen delivery. Red blood cells contain a protein called hemoglobin, and each hemoglobin molecule can pick up four oxygen molecules at once. In the lungs, where oxygen is plentiful and carbon dioxide levels are low, hemoglobin eagerly grabs onto oxygen. When that blood reaches your muscles, brain, and other tissues, the environment flips: carbon dioxide is higher and the chemistry is more acidic. Hemoglobin responds by releasing its oxygen right where cells need it most. This elegant switching behavior means the same molecule that loads up in the lungs automatically unloads in the tissues without any conscious effort on your part.
Each gram of hemoglobin can carry about 1.3 milliliters of oxygen. That may sound tiny, but with roughly 750 grams of hemoglobin circulating in a typical adult, the total carrying capacity is enormous. Your red blood cells live about 115 days on average (though that can range from 70 to 140 days), and your bone marrow constantly produces new ones to replace the old.
Hauling Away Waste
Cells generate waste products as they burn fuel, and blood is the cleanup crew. The two biggest waste products are carbon dioxide and nitrogen-containing compounds from protein breakdown. Carbon dioxide dissolves into the blood and travels back to the lungs, where you simply breathe it out. Nitrogen waste follows a different path: the liver converts ammonia, a toxic byproduct of protein metabolism, into a much less harmful substance called urea. Blood carries that urea to the kidneys, which filter it out so it leaves your body in urine.
Fighting Infection
White blood cells are the immune system’s frontline soldiers, and blood is how they patrol the body. There are five main types, each with a distinct specialty:
- Neutrophils kill bacteria, fungi, and foreign debris. They’re the most abundant and usually the first to arrive at an infection.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, proteins that latch onto invaders and mark them for destruction. T cells coordinate attacks and kill infected cells directly.
- Monocytes clean up damaged and dead cells, acting as the body’s waste disposal service after an immune battle.
- Eosinophils target parasites and certain cancer cells.
- Basophils trigger allergic responses like sneezing, coughing, and runny nose, which help expel irritants from your airways.
When you cut your finger or catch a cold, these cells flood the affected area through the bloodstream, producing antibodies and directly attacking whatever doesn’t belong.
Sealing Wounds
When a blood vessel is damaged, a rapid chain reaction stops the bleeding. First, tiny cell fragments called platelets rush to the injury site and clump together, forming a temporary plug. That plug alone isn’t strong enough to last, so a chemical cascade kicks in. Platelets provide a surface where clotting proteins assemble and amplify the signal. The cascade ultimately produces large amounts of a protein called thrombin, which converts a dissolved blood protein into solid threads of fibrin. Those fibrin threads weave through the platelet plug and cross-link into a tough mesh, creating a stable clot that holds until the tissue underneath can heal.
Regulating Body Temperature
Blood acts as your body’s coolant system and heater. When you’re too warm, blood vessels near the skin’s surface widen, increasing blood flow to the skin by several times the normal rate. This pushes heat from your core outward, where sweat evaporating on the surface cools the blood before it circulates back inside. When you’re cold, the opposite happens: blood vessels near the skin constrict sharply, reducing heat loss and keeping warm blood closer to your vital organs. If your internal temperature drops further, your body adds heat generation through shivering.
Carrying Hormonal Messages
Your endocrine glands, including the thyroid, adrenal glands, and pancreas, release hormones directly into the bloodstream. These chemical messengers travel through the blood to reach specific target cells throughout the body, locking onto receptors the way a key fits a lock. Insulin traveling from your pancreas to your muscle cells, adrenaline surging from your adrenal glands to your heart, growth hormone reaching your bones: none of these signals could arrive without blood as the delivery system. Your body continuously monitors hormone levels in the blood and adjusts production to keep everything in balance.
Maintaining Chemical Balance
Your blood holds its pH in a remarkably tight range, between 7.35 and 7.45. Even a small drift outside that window can disrupt cell function throughout the body. The primary system that prevents this is the bicarbonate buffer, a chemical partnership between carbon dioxide and bicarbonate ions that neutralizes excess acid or base almost instantly. Hemoglobin itself doubles as a buffer by absorbing extra hydrogen ions. Proteins like albumin and phosphate compounds in cells and kidneys provide additional backup. Together, these systems keep your blood chemistry stable even as your metabolism constantly produces acids.
What Blood Is Made Of
About 55% of blood is plasma, a pale yellow liquid that is 92% water and 8% dissolved substances. Proteins make up around 7% of plasma. The most abundant, albumin, accounts for about 60% of those proteins and helps maintain fluid balance so water doesn’t leak out of your blood vessels. Globulins, the second most abundant group, include the antibodies your immune system produces. The remaining 45% of blood consists of cells: overwhelmingly red blood cells, with white blood cells and platelets making up a small but critical fraction.
Every component plays a role. Plasma carries nutrients, hormones, and waste. Red blood cells handle gas exchange. White blood cells run immune defense. Platelets manage clotting. Remove any one piece, and the system breaks down. Blood isn’t just a fluid filling your veins. It’s a living tissue performing dozens of tasks simultaneously, circulating through your body roughly once every minute to keep every organ supplied, protected, and in communication with the rest.