The primary function of blood is to transport oxygen from your lungs to every cell in your body and carry carbon dioxide back to your lungs for removal. This gas exchange keeps your tissues alive and your metabolism running. But blood does far more than shuttle gases. It also delivers nutrients, removes waste, fights infections, seals wounds, and regulates your body temperature and chemical balance. An average adult carries about 5.5 liters of blood, and roughly 55% of that volume is plasma, a yellowish fluid that is itself about 92% water.
Oxygen Delivery and CO2 Removal
Red blood cells contain a protein called hemoglobin, which is responsible for picking up oxygen in the lungs and releasing it where your body needs it. Each hemoglobin molecule can bind up to four oxygen molecules at once, and it does so cooperatively: once the first oxygen attaches, the remaining binding sites become easier to fill. This cooperative binding makes hemoglobin remarkably efficient at loading oxygen in the lungs, where oxygen levels are high, and unloading it in tissues where levels are low.
The return trip matters just as much. Carbon dioxide, a waste product of cellular metabolism, needs to get back to the lungs so you can exhale it. Blood handles this in three ways. About 90% of carbon dioxide travels as bicarbonate, a dissolved ion that forms when CO2 reacts with water in your blood. Another 5% binds directly to proteins like hemoglobin, and the remaining 5% dissolves in the plasma as a gas. This system ensures that CO2 doesn’t accumulate to dangerous levels in your tissues.
Nutrient and Hormone Transport
Every time you eat, digested nutrients enter your bloodstream for delivery throughout the body. Water-soluble molecules like glucose, minerals, and certain vitamins move freely in the watery plasma. Fat-soluble substances, including cholesterol, triglycerides, and fat-soluble vitamins, can’t dissolve in water on their own, so they get packaged into tiny protein-wrapped particles called lipoproteins that allow them to travel through the blood without clumping together.
Blood also serves as the body’s internal messaging system. Hormones released by glands travel through the bloodstream to reach distant target organs. Many of these hormones hitch a ride on albumin, the most abundant protein in plasma, which acts as a transport carrier for hormones, drugs, and other molecules that can’t circulate independently.
Immune Defense
White blood cells patrol the bloodstream looking for threats. There are five main types, each with a distinct role. Neutrophils, the most numerous, kill bacteria, fungi, and foreign debris. They’re typically the first responders at the site of an infection. Lymphocytes handle longer-term defense: B cells produce antibodies, T cells attack infected cells directly, and natural killer cells target viruses and abnormal cells. Monocytes clean up damaged tissue and dying cells. Eosinophils identify and destroy parasites and certain cancer cells. Basophils trigger allergic responses like sneezing, coughing, and nasal congestion.
These cells work together to recognize, contain, and eliminate threats before they can spread. When you develop a fever or notice swelling around a cut, that’s your blood-based immune system in action.
Wound Sealing and Clot Formation
When a blood vessel is damaged, your body launches a three-step repair process called hemostasis. First, the damaged vessel constricts, narrowing itself to slow blood loss from the injury site. Second, platelets circulating in your blood rush to the wound and stick to the damaged tissue, forming a temporary plug. Third, the coagulation cascade kicks in: a chain reaction of clotting factors amplifies the process, and a tough protein called fibrin weaves through the platelet plug like mortar between bricks. The result is a stable clot that holds until the tissue underneath can heal.
Fluid Balance and Albumin’s Role
Albumin does more than carry hormones. It’s responsible for about 80% of the pressure that keeps fluid inside your blood vessels rather than leaking into surrounding tissues. This pressure, called oncotic pressure, averages about 28 mmHg in a healthy person. The albumin molecule carries a negative electrical charge, which attracts positively charged ions like sodium into the bloodstream. Those extra ions pull water with them, adding even more fluid-retaining force on top of what albumin alone provides.
When albumin levels drop, as happens in severe liver disease, this pressure falls and fluid seeps out of blood vessels into the surrounding tissue. The visible result is generalized swelling, or edema, particularly in the abdomen and legs.
pH Regulation
Your blood maintains a tightly controlled pH between 7.35 and 7.45. Even small deviations outside this range can disrupt the chemical reactions your cells depend on. The main tool for this balancing act is the bicarbonate buffer system. Carbon dioxide from metabolism reacts with water in your blood to form carbonic acid, a weak acid that can release or absorb hydrogen ions depending on whether the blood is becoming too basic or too acidic. An enzyme in your red blood cells speeds this reaction in either direction almost instantly.
Your lungs and kidneys work alongside this buffer. Breathing faster removes more CO2 and shifts blood toward a more basic pH. Your kidneys adjust how much bicarbonate they retain or excrete, handling slower, longer-term corrections.
Temperature Regulation
Blood plays a direct role in keeping your core temperature stable. When your body heats up during exercise or in hot environments, blood vessels near the skin surface widen, allowing more warm blood to flow close to the surface where heat can radiate away. Your brain’s temperature-sensing region detects the rise in internal or skin temperature and triggers this response automatically.
In cold conditions, the opposite happens. Blood vessels near the skin constrict, reducing blood flow to the surface and keeping warm blood deeper in the body where it protects vital organs. This is why your fingers and toes get cold first: your body is deliberately routing blood away from your extremities to preserve core heat.