The main function of blood is to transport oxygen from your lungs to every cell in your body and carry carbon dioxide back out. But blood does far more than shuttle gases. It delivers nutrients, removes waste, fights infections, seals wounds, regulates your body temperature, and carries chemical signals between organs. An average adult carries about 1.2 to 1.5 gallons of blood, making up roughly 10% of body weight, and every drop is constantly working to keep you alive.
What Blood Is Made Of
Blood is a mixture of about 55% plasma and 45% cells. Plasma is the liquid portion, mostly water, that carries dissolved proteins, sugars, salts, and hormones. The cellular portion is dominated by red blood cells, which account for 40% to 45% of total blood volume. White blood cells and platelets make up a much smaller fraction but play outsized roles in immune defense and wound repair.
Oxygen Delivery and Carbon Dioxide Removal
Red blood cells contain hemoglobin, a protein built from four interlocking chains, each with an iron-containing core that can grab one molecule of oxygen. What makes hemoglobin remarkably efficient is a feature called cooperative binding: when the first oxygen molecule attaches, the protein’s shape loosens, making it far easier for the second, third, and fourth molecules to latch on. This creates a 500-fold increase in oxygen affinity as hemoglobin loads up. Arterial blood leaving the lungs carries about 20 milliliters of oxygen per 100 milliliters of blood. By the time that blood returns through the veins, it still holds about 15 milliliters per 100 milliliters, meaning tissues extract roughly a quarter of the available oxygen on each pass.
Carbon dioxide, the main waste product of cellular energy production, travels back to the lungs by three routes. About 70% to 80% gets converted into bicarbonate ions inside red blood cells, a chemical transformation that also helps regulate blood acidity. Another 10% to 20% binds directly to hemoglobin’s protein chains. The remaining 5% to 10% simply dissolves in plasma. Once blood reaches the lungs, these processes reverse, releasing carbon dioxide so you can exhale it.
Nutrient Transport and Waste Disposal
Every time you eat, digested nutrients like glucose, amino acids, and fatty acids enter the bloodstream through the walls of the small intestine. Plasma carries these molecules to cells throughout the body, where they serve as fuel or building materials. Vitamins and minerals travel the same route, often bound to carrier proteins in the plasma.
On the return trip, blood collects metabolic waste. Cells produce byproducts like urea (from protein breakdown) that would be toxic if they accumulated. Blood delivers these waste products to the kidneys, which filter them out and send them to the bladder for removal. The liver also processes and neutralizes harmful substances before they can circulate further.
Immune Defense
White blood cells are the body’s mobile defense force, and blood is how they patrol. Neutrophils, the most common type, are first responders that kill bacteria and fungi by engulfing and destroying them. Lymphocytes handle more targeted defense: B cells produce antibodies that tag specific invaders for destruction, while T cells directly attack infected or abnormal cells. Natural killer cells, another type of lymphocyte, specialize in destroying virus-infected cells and certain tumor cells. Monocytes act as cleanup crews, consuming damaged cells and debris left behind after an infection.
These cells don’t just float passively. They can squeeze through blood vessel walls to reach infected tissue, then signal other immune cells to join them. Blood also carries proteins called complement that punch holes in bacterial membranes, along with antibodies produced during past infections that provide lasting immunity.
Wound Sealing and Clot Formation
When a blood vessel is damaged, the body launches a rapid repair sequence. Platelets, tiny cell fragments circulating in the blood, stick to the exposed edges of the wound and clump together to form an initial plug. This buys time for a more durable repair.
The clotting cascade then kicks in. Proteins in the blood activate each other in a chain reaction that ultimately converts a dissolved protein called fibrinogen into fibrin, a sticky, thread-like material. Fibrin weaves itself into a mesh that traps red blood cells and additional platelets, forming a stable clot. A final stabilizing factor strengthens the mesh into a net-like structure that holds the clot firmly in place until the vessel heals underneath. This entire process can begin within seconds of an injury.
Temperature Regulation
Blood acts as your body’s coolant system and heating network in one. When you overheat during exercise or on a hot day, blood vessels near the skin’s surface widen. This sends more blood to the skin, where heat radiates outward into the surrounding air. Skin blood flow can increase severalfold during heat stress, making this one of the body’s most powerful cooling mechanisms.
In cold conditions, the opposite happens. Blood vessels near the skin constrict, reducing blood flow to the surface and keeping warm blood closer to vital organs in the core. This is why your fingers and toes get cold first: the body is deliberately redirecting blood to protect the brain, heart, and lungs from dangerous temperature drops.
Chemical Messaging Between Organs
Blood serves as the body’s internal communication highway. Endocrine glands release hormones directly into the bloodstream, and those hormones travel to distant target organs to trigger specific responses. Insulin from the pancreas tells cells to absorb glucose. Adrenaline from the adrenal glands speeds up the heart. Thyroid hormones adjust metabolism across nearly every tissue. Without blood to carry these signals, organs would have no way to coordinate with each other.
Maintaining pH and Fluid Balance
Your blood must stay within a narrow pH range of 7.35 to 7.45, slightly alkaline. Even small deviations outside this range can disrupt enzyme function and cell chemistry. Blood maintains this balance primarily through a bicarbonate buffer system that neutralizes excess acid or base. The lungs and kidneys back up this system by adjusting how much carbon dioxide you breathe out and how much acid the kidneys excrete in urine.
Plasma proteins, particularly albumin, play a separate but equally important role in fluid balance. Albumin is responsible for about 80% of the osmotic pressure that keeps water inside blood vessels. Without enough albumin, fluid leaks out of the bloodstream into surrounding tissues, causing swelling. This pulling force ensures that the right amount of water stays in circulation to maintain blood pressure and organ perfusion.