Blood is a liquid tissue that circulates through your body, delivering oxygen and nutrients to every cell while carrying away waste. The average adult carries a volume of blood equal to roughly 10% of their body weight. It looks like a simple red fluid, but blood is a complex mix of liquid, cells, and proteins that keeps you alive by performing three essential jobs: transportation, regulation, and protection.
What Blood Is Made Of
Blood has two main parts. About 55% is plasma, a pale yellow liquid. The remaining 45% is made up of cells and cell fragments: red blood cells, white blood cells, and platelets.
Plasma itself is 91% to 92% water. The rest is a mix of dissolved proteins, salts, hormones, and nutrients. The proteins in plasma include albumin (which helps maintain fluid balance), clotting factors like fibrinogen, and immune proteins called immunoglobulins that help fight infection. Dissolved salts like sodium, potassium, and calcium keep your blood at the right pH so your cells can function properly.
Red Blood Cells and Oxygen Delivery
Red blood cells are by far the most numerous cells in your blood. Each one is shaped like a disc that’s thinner in the center than at the edges, roughly 8 to 10 micrometers across. That shape maximizes surface area, making them extremely efficient at picking up and releasing oxygen.
The key to their job is hemoglobin, an iron-rich protein packed inside each red blood cell. When blood passes through your lungs, hemoglobin binds oxygen. As blood reaches tissues that need it, hemoglobin releases that oxygen for your cells to use. On the return trip, red blood cells carry carbon dioxide back to the lungs so you can exhale it. A single red blood cell lives about 120 days before it’s broken down and recycled by your body, which means your bone marrow is constantly producing new ones.
White Blood Cells and Immune Defense
White blood cells make up a much smaller fraction of your blood, but they’re your body’s defense force. There are five main types, each with a different role.
- Neutrophils are the most common, making up 50% to 70% of all white blood cells. They’re the first responders to bacterial infections, swallowing and destroying invaders on contact.
- Lymphocytes are part of your adaptive immune system. They learn to recognize specific threats and mount targeted attacks, which is how vaccines work.
- Monocytes leave the bloodstream and settle in tissues, where they become macrophages. These large cells engulf bacteria, dead cells, and debris.
- Eosinophils specialize in fighting parasitic infections and play a role in allergic reactions and chronic inflammation.
- Basophils release histamine and other chemicals during allergic responses, triggering inflammation that helps recruit other immune cells to the area.
Platelets and Blood Clotting
Platelets are tiny cell fragments, not full cells. A healthy adult has between 150,000 and 400,000 platelets per microliter of blood, and their job is to stop bleeding.
When a blood vessel is damaged, the body responds in three rapid steps. First, the injured vessel narrows to reduce blood flow to the area. Then platelets rush to the wound, stick to the vessel wall, activate, and clump together to form a plug. Finally, a cascade of clotting proteins in the plasma produces fibrin, a tough protein mesh that reinforces the platelet plug and seals the wound. This whole process can happen in seconds for a minor cut.
The Three Jobs of Blood
Everything blood does falls into three categories. Transportation is the most obvious: blood carries oxygen from the lungs to cells, shuttles nutrients from digestion to where they’re needed, delivers hormones from glands to target organs, and hauls waste products to the liver and kidneys for disposal.
Regulation is subtler. Blood helps maintain a stable body temperature by absorbing heat from active muscles and organs and distributing it or releasing it through the skin. Plasma absorbs and gives off heat, and the speed of blood flow adjusts to help warm or cool different areas. Blood also keeps its own pH tightly controlled, which is critical for normal cell function.
Protection covers both clotting and immunity. Platelets and clotting proteins prevent dangerous blood loss, while white blood cells and immune proteins patrol for infections, foreign substances, and damaged cells.
Where Blood Cells Are Made
All blood cells originate in the bone marrow, the spongy tissue inside your bones. The process starts with a single type of precursor called a hematopoietic stem cell. These stem cells are remarkably versatile. Depending on the signals they receive, they can develop into red blood cells, any of the five types of white blood cells, or platelet-producing cells. This production runs continuously throughout your life to replace cells that wear out, get damaged, or are used up fighting infections.
Blood Types and Compatibility
Your blood type is determined by markers (antigens) on the surface of your red blood cells. In the ABO system, there are four basic types: A, B, AB, and O. If you have type A blood, your red cells carry the A antigen and your plasma contains antibodies against the B antigen. Type B is the reverse. Type AB carries both antigens and has neither antibody, while type O carries neither antigen but has antibodies against both A and B.
This matters enormously for transfusions. If you receive blood with antigens your immune system doesn’t recognize, those antibodies attack the donated red cells, destroying them rapidly. This can trigger shock, kidney failure, and death. Most fatal transfusion reactions are caused by ABO-incompatible blood, which is why blood typing and crossmatching are done before every transfusion.
Beyond the ABO system, there’s the Rh factor. About 85% of people are Rh-positive, meaning their red blood cells carry the Rh antigen. The remaining 15% are Rh-negative. Combined with the ABO types, this gives you the familiar labels like O-positive or AB-negative.
What Blood Tests Reveal
One of the most commonly ordered lab tests is a complete blood count, or CBC. It measures your red blood cells, white blood cells, platelets, hemoglobin, hematocrit (the percentage of your blood volume occupied by red cells), and the average size of your red blood cells.
Low red blood cell counts, hemoglobin, or hematocrit can point to anemia, dehydration, iron deficiency, or heart disease. A high white blood cell count often signals an active infection or a reaction to medication, while an unusually low count may indicate an autoimmune disorder or a problem with the bone marrow. Because so many conditions leave their fingerprint in the blood, a simple CBC can serve as an early warning system for a wide range of health issues.