Blood transports oxygen, carbon dioxide, nutrients, hormones, waste products, immune cells, clotting factors, electrolytes, and heat throughout your body. It functions as a delivery and removal system, carrying essential substances to every tissue and hauling away what cells no longer need. Understanding what blood moves, and how, helps explain why so many health conditions trace back to problems with circulation.
Oxygen and Carbon Dioxide
The most immediate job of blood is gas exchange. Red blood cells pick up oxygen in the lungs and deliver it to tissues throughout the body, where cells use it to produce energy. Hemoglobin, the protein inside red blood cells, binds oxygen molecules and releases them when it reaches tissues that need fuel.
Carbon dioxide, the waste product of that energy production, travels back to the lungs for removal, but it takes a more complex route than oxygen. Only about 10% of carbon dioxide dissolves directly in the plasma. Another 10% binds to hemoglobin itself. The remaining majority gets converted inside red blood cells into bicarbonate, a dissolved ion that travels through the plasma to the lungs. There, the process reverses: bicarbonate converts back to carbon dioxide, which you exhale. This bicarbonate system also plays a key role in keeping your blood’s pH stable.
Nutrients and Fuel
After you eat, your digestive system breaks food down into molecules small enough to cross the intestinal wall and enter the bloodstream. Glucose, amino acids, and fatty acids all travel through blood plasma to reach the cells that need them. Glucose is the primary energy source for most cells, and blood keeps a steady supply circulating at all times. Amino acids serve as building blocks for proteins, while fatty acids provide energy and raw material for cell membranes.
Vitamins and minerals also hitch a ride. Fat-soluble vitamins (A, D, E, and K) travel bound to carrier proteins, while water-soluble vitamins like B and C dissolve directly in plasma. Iron travels bound to a transport protein and gets delivered primarily to bone marrow, where new red blood cells are assembled.
Metabolic Waste
Cells constantly produce waste that would become toxic if it accumulated. Blood carries these byproducts to the organs that filter and remove them. The two most important nitrogenous wastes are urea and creatinine.
Urea forms in the liver. When your body breaks down dietary protein, the process strips nitrogen from amino acids, and the liver packages that excess nitrogen into urea. Blood then carries roughly 10 grams of urea per day to the kidneys, where it’s filtered out and excreted in urine. A small amount leaves through sweat, especially during exercise.
Creatinine comes from muscle tissue. Your muscles store a compound called creatine phosphate for quick energy, and about 2% of those stores convert into creatinine each day. Blood carries creatinine to the kidneys, where it’s freely filtered and excreted. Because creatinine production is relatively constant, doctors use its blood levels as a reliable marker of kidney function.
Hormones and Chemical Signals
Blood is the body’s communication highway. Glands release hormones into the bloodstream, and blood carries them to distant target organs where they trigger specific responses. Insulin travels from the pancreas to muscles and fat cells. Thyroid hormones reach virtually every tissue to regulate metabolism. Adrenaline surges from the adrenal glands to the heart, lungs, and muscles during stress.
How hormones travel depends on their chemistry. Water-soluble hormones like insulin dissolve freely in plasma. Steroid hormones, which include cortisol, estrogen, and testosterone, don’t dissolve well in blood. The liver produces carrier proteins that bind these hormones and shuttle them through the bloodstream. At any given moment, most steroid hormones in your blood are bound to these carriers and inactive. Only the small “free” fraction can enter cells and produce effects. This balance matters clinically. For example, when levels of the carrier protein for sex hormones rise, more testosterone gets locked up, leaving less free testosterone available. In men, this shift can sometimes cause breast tissue enlargement because the ratio tips in favor of estrogen’s effects.
Immune Cells and Antibodies
White blood cells use blood as a transit system. They circulate through the bloodstream until chemical signals from damaged or infected tissue recruit them to a specific location. Neutrophils, the most abundant type, are first responders that attack bacteria. Lymphocytes, which include T cells and B cells, coordinate more targeted immune responses and produce antibodies. Monocytes travel through blood and then enter tissues where they mature into macrophages, large cells that engulf debris and pathogens.
Antibodies themselves are proteins dissolved in plasma. Once B cells produce them in response to an infection or vaccine, antibodies circulate throughout the blood, tagging viruses, bacteria, and other invaders for destruction. This is why a blood sample can reveal whether you’ve been exposed to a particular pathogen: the antibodies remain in circulation as a record of past immune responses.
Clotting Factors and Platelets
Blood carries its own repair kit. Platelets, small cell fragments produced in bone marrow, circulate continuously and respond within seconds when a blood vessel is damaged. The repair process follows a sequence: the injured vessel first narrows to limit blood loss, then platelets stick to the exposed tissue and clump together to form a plug. Finally, a cascade of clotting proteins in the plasma produces fibrin, a tough mesh that reinforces the platelet plug into a stable clot.
Platelets are more than passive plugs. They carry their own internal stores of clotting factors, including factor V, von Willebrand factor, and fibrinogen, packed into tiny compartments called granules. When platelets activate at a wound site, they release these factors and provide a surface where the clotting cascade can proceed efficiently. The platelet supply of factor V is significant enough that even people born with a genetic deficiency of this protein can often avoid severe bleeding because their platelets still release enough to get the job done.
Electrolytes
Blood plasma carries dissolved electrolytes that cells depend on for basic functions like nerve signaling, muscle contraction, and fluid balance. The body maintains these within tight ranges. Sodium, the most abundant electrolyte in blood, normally sits between 135 and 145 mmol/L and controls how much water your body retains. Potassium, which ranges from 3.6 to 5.5 mmol/L, is critical for heart rhythm and muscle function. Calcium, kept between 8.8 and 10.7 mg/dL, drives muscle contractions, nerve transmission, and bone maintenance.
Even small deviations outside these ranges can cause serious symptoms. Low potassium can trigger dangerous heart rhythms. Low calcium causes muscle cramps and tingling. Blood constantly circulates these minerals between the gut (where they’re absorbed), the organs that use them, and the kidneys (which fine-tune how much stays or goes).
Heat Distribution
Blood is the body’s primary tool for temperature regulation. As blood flows through active muscles and organs, it absorbs heat. That warmed blood then circulates to other areas, distributing heat evenly or releasing it at the skin’s surface. Blood flow to the skin is the most effective mechanism for transferring heat from the body’s core to the environment.
Your hypothalamus, the brain’s thermostat, controls this process by adjusting blood vessel diameter. When you’re overheating, blood vessels near the skin dilate, increasing blood flow to the surface so heat can escape. You flush visibly because more blood is flowing just beneath the skin. When you’re cold, those same vessels constrict, redirecting blood toward your core to protect vital organs and conserve heat. This is why your fingers and toes go pale and numb in freezing temperatures: your body is deliberately reducing blood flow to your extremities to keep your core warm.