Blood keeps every organ and tissue in your body alive by performing half a dozen jobs simultaneously. It delivers oxygen and fuel, hauls away waste, fights infections, seals wounds, carries chemical messages between organs, and helps hold your body temperature steady. The average adult carries about 1.2 to 1.5 gallons of it, and losing even a fraction of that supply can become life-threatening within minutes.
Whole blood is roughly 55% plasma, a pale yellow liquid made mostly of water, and 45% cells. Red blood cells make up the vast majority of that cellular portion (40 to 45% of total blood volume), while white blood cells account for about 1% and platelets fill in the rest. Each component has a distinct set of responsibilities.
Delivering Oxygen and Removing Carbon Dioxide
The single most urgent thing blood does is ferry oxygen from your lungs to every cell in your body. Red blood cells contain hemoglobin, a protein that picks up oxygen molecules when blood passes through the lungs, where oxygen concentration is high, and releases them when blood reaches tissues where oxygen concentration is low. This pickup-and-release behavior follows a curve: hemoglobin grabs oxygen eagerly at first, and each oxygen molecule it binds makes the next one attach even more easily. The reverse happens during release.
Several factors fine-tune this system. When tissues are working hard, they produce more carbon dioxide and acid, both of which cause hemoglobin to loosen its grip on oxygen right where it’s needed most. Carbon dioxide then hitches a ride back to the lungs on the same hemoglobin molecules (and dissolved in plasma), where you exhale it. This two-way exchange happens continuously with every heartbeat.
Transporting Nutrients, Hormones, and Waste
Plasma acts as the body’s liquid highway. After you eat, digested nutrients like glucose, amino acids, fatty molecules, and minerals enter the bloodstream and travel to cells that need them. Calcium, sodium, and phosphate all circulate in plasma and must be kept within tight ranges for your muscles, nerves, and bones to function properly.
Blood also carries hormones, the chemical signals your glands use to communicate with distant organs. Some hormones dissolve easily in the watery plasma and travel freely. Others, like thyroid and steroid hormones, are nearly insoluble in water. These get bound to carrier proteins in the blood, which act as both a transport vehicle and a reservoir. Only the tiny “free” fraction of hormone floating unbound in the plasma actually enters cells and triggers a response, while the protein-bound portion stays in reserve, buffering against sudden surges.
On the return trip, plasma picks up metabolic waste. Urea (a byproduct of protein breakdown), creatinine (from normal muscle metabolism), ammonia, and excess potassium all travel through the blood to the kidneys, where they’re filtered out and leave the body in urine.
Fighting Infection
White blood cells are your immune system’s mobile workforce. Though they represent only about 1% of blood volume, they patrol the entire body and respond to threats within hours.
- Neutrophils are the first responders, killing bacteria and fungi by engulfing and digesting them. They’re the most abundant white blood cell type.
- Lymphocytes include T cells, B cells, and natural killer cells. B cells produce antibodies, proteins that tag specific invaders for destruction. T cells coordinate the broader immune response and can kill virus-infected cells directly.
- Monocytes clean up damaged and dead cells, acting as the body’s garbage collectors after an infection or injury.
- Eosinophils target parasites and also play a role in fighting certain cancers.
- Basophils drive allergic responses, triggering symptoms like sneezing, coughing, and a runny nose when you encounter an allergen.
Sealing Wounds Through Clotting
When a blood vessel is damaged, the body launches a rapid, multi-step repair process called hemostasis. Platelets, tiny cell fragments circulating in the blood, are the first to arrive. They stick to the exposed edges of the wound and clump together to form a temporary plug.
That plug alone isn’t strong enough to hold. A chain reaction of clotting proteins in the plasma (often called the coagulation cascade) activates on the surface of those clustered platelets. The end result is a protein called thrombin, which converts a dissolved plasma protein called fibrinogen into long, sticky strands of fibrin. These strands weave through and around the platelet plug like reinforcing mesh, creating a stable clot that stops the bleeding. Eventually, as the vessel heals underneath, the clot dissolves.
Regulating Body Temperature
Blood acts as a liquid coolant and heating system. When your core temperature rises, whether from exercise, fever, or a hot environment, blood vessels near the skin’s surface widen. This sends more warm blood toward the skin, where heat radiates outward into the surrounding air. Local warming of the skin alone can push those vessels open to their maximum diameter.
When you’re cold, the opposite happens. Blood vessels in the skin constrict, redirecting blood away from the surface and deeper into the body’s core. This reduces heat loss and protects vital organs from dropping in temperature. It’s the reason your fingers and toes go pale and numb in freezing weather: your body is deliberately rationing warm blood to protect your heart, brain, and lungs.
Maintaining pH Balance
Your blood must stay within an extremely narrow pH range of 7.35 to 7.45, just slightly basic. A pH below 7.35 is called acidosis; above 7.45 is alkalosis. Either direction can disrupt enzyme function, nerve signaling, and muscle contraction.
The body’s primary tool for keeping blood pH stable is the bicarbonate buffer system, the largest chemical buffer in the blood. It works through a simple, reversible reaction. Carbon dioxide from metabolism combines with water to form carbonic acid, a weak acid that can split into a hydrogen ion and a bicarbonate ion. When something pushes blood toward being too acidic, bicarbonate ions soak up the excess hydrogen ions. When blood drifts too basic, carbonic acid donates hydrogen ions back. This back-and-forth reaction runs continuously, absorbing chemical shocks before they can shift pH outside the safe zone. The lungs and kidneys back up this system by adjusting how much carbon dioxide you exhale and how much bicarbonate the kidneys retain or excrete.