Blood keeps every cell in your body alive. It delivers oxygen and nutrients, hauls away waste, fights infections, seals wounds, and regulates your body temperature. An average adult carries about 1.2 to 1.5 gallons of it, making up roughly 10% of their body weight. That fluid is constantly circulating, performing dozens of jobs at once to keep your organs functioning and your internal environment stable.
What Blood Is Made Of
Blood is a mixture of about 55% plasma and 45% cells. Plasma is the liquid portion, a pale yellow fluid made mostly of water along with dissolved salts, enzymes, hormones, and proteins. It serves as the transport medium, carrying dissolved substances to wherever they’re needed and picking up cellular waste for disposal.
The remaining 45% is made up of three types of cells. Red blood cells are by far the most numerous, accounting for 40% to 45% of total blood volume. White blood cells make up only about 1%, but they’re the backbone of immune defense. Platelets, the smallest components, are cell fragments responsible for clotting.
Delivering Oxygen to Every Cell
The most critical job blood performs is oxygen delivery. When you inhale, oxygen crosses from your lungs into tiny capillaries, where it binds to a protein inside red blood cells called hemoglobin. Each hemoglobin molecule can carry four oxygen molecules at once. These loaded red blood cells then travel through arteries to tissues throughout your body.
Once blood reaches tissues that are actively burning energy (your muscles during a run, your brain while you think), the local environment is more acidic and warmer. Those conditions cause hemoglobin to release its oxygen. At the same time, carbon dioxide, the main waste product of metabolism, loads onto hemoglobin for the return trip to the lungs. This two-way exchange is sometimes called the Bohr and Haldane effect: conditions in the lungs promote oxygen pickup and carbon dioxide release, while conditions in active tissues promote the reverse. It’s an elegant, self-regulating system.
Transporting Nutrients and Clearing Waste
Oxygen isn’t the only cargo. Plasma carries glucose, amino acids, fatty acids, vitamins, and minerals from your digestive system to cells that need fuel and building materials. Hormones released by glands also travel through the bloodstream. When a hormone enters the blood, it circulates past most cells until it reaches its specific target, where it locks onto a receptor and triggers a response. This is how your thyroid, adrenal glands, and pancreas communicate with distant organs without any direct physical connection.
Cells constantly produce metabolic waste as they work. They dump these byproducts into the plasma, which carries them to the kidneys and liver for filtering and disposal. Carbon dioxide exits through the lungs, urea leaves through the kidneys, and various toxins are broken down in the liver. Without this continuous cleanup, waste products would accumulate to toxic levels within minutes.
Fighting Infection
White blood cells are your circulating immune force, and different types handle different threats. Neutrophils are the first responders, arriving quickly at sites of infection to kill bacteria and fungi. Lymphocytes, which include T cells and B cells, handle viral infections and produce antibodies, proteins that tag invaders for destruction. Monocytes clean up damaged and dead cells after an infection or injury. Eosinophils target parasites and certain cancer cells. Basophils trigger allergic responses like sneezing, coughing, and nasal congestion, part of the body’s attempt to expel foreign substances.
These cells patrol the bloodstream constantly. When they detect chemical signals from damaged or infected tissue, they migrate out of the blood vessels and into the affected area. This is why infected wounds become red and swollen: blood flow increases to the site, flooding it with immune cells.
Sealing Wounds and Stopping Blood Loss
When a blood vessel is damaged, your body launches a rapid three-step repair process. First, the injured vessel constricts, narrowing itself to reduce blood flow to the area. Second, platelets circulating in the blood stick to the damaged tissue, piling up to form a temporary plug. Think of it like a cork in a bottle: it keeps blood in and germs out.
Third, a chain reaction called the coagulation cascade kicks in. Clotting proteins in the blood activate one after another, amplifying the response until a tough protein called fibrin weaves through the platelet plug. Platelets and fibrin together form a stable clot, like bricks and mortar. As the tissue heals over the following days, normal tissue gradually replaces the clot.
Regulating Body Temperature
Blood acts as your body’s cooling and heating system. When you exercise or spend time in the heat, blood vessels near the skin’s surface widen, allowing more blood to flow close to the outside of your body. This transfers heat from your core to your skin, where it dissipates into the air. To support this increased skin blood flow, your heart pumps harder and redirects blood away from internal organs temporarily.
In cold conditions, the opposite happens. Blood vessels near the skin constrict, pulling warm blood deeper into your body to protect vital organs from heat loss. This is why your fingers and toes get cold first: your body is deliberately restricting blood flow to your extremities to keep your core warm. Even local temperature changes matter. Warming a small patch of skin can open blood vessels in that area to their maximum, while cooling it can reduce flow to nearly zero.
Maintaining Chemical Balance
Your blood’s chemistry stays within remarkably tight limits. Healthy blood has a pH between 7.35 and 7.45, just slightly alkaline. Even small deviations outside this range can disrupt the chemical reactions your cells depend on. The body’s primary defense is a buffering system involving carbonic acid (formed from dissolved carbon dioxide) and bicarbonate. These two substances work together to neutralize excess acid or base, keeping pH steady even as your cells constantly produce acidic waste.
Blood also maintains a careful balance of electrolytes like sodium, potassium, and calcium, all of which are essential for nerve signaling, muscle contraction, and fluid balance. The kidneys fine-tune these levels continuously, adjusting what gets reabsorbed and what gets excreted based on signals carried, once again, by the blood itself. It’s a feedback loop: blood delivers the information the kidneys need to keep the blood in balance.