Blood performs six major functions: delivering oxygen to every cell, fighting infections, sealing wounds, transporting nutrients and hormones, regulating body temperature, and maintaining the chemical balance your organs need to work. The average adult carries about 5 liters of blood, roughly 55% of which is plasma, a straw-colored liquid that carries red blood cells, white blood cells, and platelets throughout the body.
Delivering Oxygen and Removing Carbon Dioxide
The most essential job of blood is gas exchange. Red blood cells contain hemoglobin, a large protein built from four folded chains, each holding an iron-containing molecule called heme. Each iron ion binds one oxygen molecule, so a single hemoglobin molecule carries four oxygen molecules at a time. In the lungs, oxygen latches onto these iron ions, turning hemoglobin bright red. As blood reaches tissues that need oxygen, hemoglobin releases it and shifts to a darker red color.
Carbon dioxide, the main waste product of cell metabolism, travels back to the lungs by three routes. About 76% dissolves directly in plasma (some as dissolved gas, the rest converted into bicarbonate). Another 23 to 24% binds to hemoglobin itself, hitching a ride back to the lungs for exhalation. The remaining small fraction stays dissolved in plasma as CO₂. This two-way shuttle runs continuously: oxygen in, carbon dioxide out, every time blood completes a loop through your circulatory system.
Fighting Infection
White blood cells are the immune workforce circulating in your blood. There are five main types, each with a distinct role. Neutrophils are the first responders, killing bacteria, fungi, and foreign debris. Lymphocytes, which include T cells, B cells, and natural killer cells, handle viral infections and produce antibodies. Monocytes clean up damaged and dead cells. Eosinophils target parasites and certain cancer cells. Basophils trigger allergic responses like sneezing, coughing, and a runny nose.
White blood cells make up a tiny fraction of total blood volume, but their impact is enormous. They patrol the bloodstream constantly, and when they detect a threat, they can migrate through blood vessel walls into infected tissue to mount a concentrated attack.
Sealing Wounds
When a blood vessel is damaged, the body launches a rapid repair process called hemostasis. Platelets, small cell fragments circulating in plasma, are the first to arrive. They stick to the exposed tissue at the wound site and clump together to form a temporary plug. This initial plug slows bleeding within seconds.
Next, a chain reaction of clotting proteins amplifies the response. Activated platelets provide a surface where these proteins assemble into complexes that ultimately convert a dissolved protein called fibrinogen into fibrin. Fibrin strands weave through the platelet plug and cross-link into a stable mesh, turning a fragile clump into a durable clot. This layered system ensures that tiny nicks seal quickly while larger injuries trigger a proportionally bigger response.
Transporting Nutrients, Hormones, and Waste
Plasma is the liquid carrier for almost everything your cells need. After digestion breaks food down into usable molecules, those nutrients enter the bloodstream and plasma ferries them to tissues throughout the body. Salts, enzymes, and proteins all travel in plasma as well. At the same time, cells dump their metabolic waste products into the surrounding fluid, which drains into plasma for removal by the kidneys and liver.
Hormones depend on blood for long-distance signaling. Water-soluble hormones, like adrenaline, dissolve easily in plasma and travel freely. Fat-soluble hormones, such as thyroid and steroid hormones, are nearly insoluble in water and would barely make it past the gland that produced them without help. These hormones bind to carrier proteins in the blood, which act as both a transport vehicle and a reservoir. Only a tiny “free” fraction of these hormones floats unbound at any moment. For thyroid hormone T4, just 0.05% circulates in its free, active form. The protein-bound reserves keep levels stable and prevent sudden surges from overwhelming target tissues.
Regulating Body Temperature
Blood acts as a liquid coolant (and heater) by redistributing heat throughout the body. At rest in a comfortable environment, skin blood flow runs at about 250 milliliters per minute, shedding roughly 80 to 90 kilocalories of heat per hour, which matches the heat your resting metabolism produces.
When you exercise or sit in the heat, blood vessels near the skin’s surface widen dramatically. This vasodilation can push skin blood flow from that baseline of 250 mL/min up to 6 to 8 liters per minute during serious overheating. The surge of warm blood to the skin’s surface allows heat to radiate outward, and sweating cools the skin further, chilling the blood in those dilated vessels before it cycles back to the core. In cold conditions, the opposite happens: blood vessels near the skin constrict, reducing blood flow to the surface and trapping heat deeper in the body to protect vital organs.
A region of the brain called the hypothalamus coordinates this entire process, reading core and skin temperature data and adjusting blood vessel diameter accordingly. The active vasodilator system controlled by sympathetic nerves accounts for 80 to 90% of the skin’s blood flow increase during heat stress.
Maintaining pH Balance
Your blood must stay within a narrow pH range of 7.35 to 7.45. Even small shifts outside that window can disrupt enzyme activity and organ function. The primary tool blood uses to hold this balance is the bicarbonate buffer system. In normal conditions, bicarbonate ions outnumber carbonic acid in the blood by a ratio of about 20 to 1. That heavy tilt toward bicarbonate makes the system especially effective at neutralizing acids, which is useful because normal metabolism constantly generates acidic byproducts.
When a strong acid enters the bloodstream, bicarbonate reacts with it and converts it into carbonic acid, a much weaker acid that the body can handle. When a strong base shows up, carbonic acid neutralizes it, producing bicarbonate and water. The lungs and kidneys fine-tune this system in real time: the lungs adjust how much CO₂ is exhaled (which shifts carbonic acid levels), and the kidneys control how much bicarbonate is retained or excreted. Together, these mechanisms keep blood chemistry stable enough for every other function on this list to work properly.