Human blood is a mixture of roughly 55% plasma (the liquid portion) and 45% cells and cell fragments. That simple split, liquid and solid, is the starting point for understanding everything blood does, from delivering oxygen to fighting infections to sealing wounds. Each component has a distinct job, and together they keep every organ in your body functioning.
Plasma: The Liquid Foundation
Plasma is a pale yellow fluid that makes up just over half your blood volume. It’s more than 90% water, but that remaining fraction carries a surprisingly complex mix of proteins, salts, nutrients, hormones, and dissolved gases. Think of plasma as the river that carries everything else where it needs to go.
Proteins make up about 7% of plasma by weight and do most of the heavy lifting. The most abundant is albumin, a relatively small protein whose main job is to act like a sponge, holding water inside your blood vessels so fluid doesn’t leak into surrounding tissues. Globulins are a broader family of proteins that include antibodies (immunoglobulins), which your immune system produces to target specific invaders like viruses and bacteria. Plasma also contains clotting proteins like fibrinogen, which become active when you’re injured.
Beyond proteins, plasma carries fats packaged inside protein shells called lipoproteins, which shuttle cholesterol and other fats through the bloodstream. Minerals dissolved in plasma include sodium and chloride (essentially salt), along with smaller amounts of potassium, calcium, magnesium, and trace elements like iron, copper, and zinc. Iron and copper each travel bound to a dedicated transport protein. Plasma also carries bicarbonate, which plays a critical role in keeping blood at its normal pH of 7.2 to 7.4, and phosphate, which helps buffer acidity and supports calcium metabolism.
Red Blood Cells: Oxygen Carriers
Red blood cells are by far the most numerous cells in your blood, accounting for 40% to 45% of total blood volume. A healthy adult male has about 4.35 to 5.65 million red blood cells in a single microliter of blood (roughly the size of a pinhead). For females, the range is 3.92 to 5.13 million per microliter. Children fall somewhere in between, at 4 to 5.5 million.
Each red blood cell is packed with hemoglobin, the protein responsible for picking up oxygen in your lungs and ferrying it to tissues throughout your body. About 98% of the oxygen in your blood travels bound to hemoglobin rather than dissolved freely in plasma, because oxygen simply isn’t soluble enough to travel on its own in meaningful quantities. Hemoglobin also helps transport carbon dioxide back to the lungs for removal. Roughly 15% to 25% of carbon dioxide binds directly to hemoglobin, while about 70% gets converted into bicarbonate inside red blood cells before dissolving in plasma. Only about 7% of carbon dioxide travels dissolved in plasma on its own.
Red blood cells have an unusual shape: a flattened disc with a dimple on each side, which maximizes surface area for gas exchange. They lack a nucleus and most internal structures, leaving more room for hemoglobin. This stripped-down design means they can’t repair themselves, so each red blood cell survives an average of 120 days before being broken down and recycled, primarily in the spleen and liver.
White Blood Cells: The Immune Defense
White blood cells make up only about 1% of your blood volume, but they’re essential for fighting infection and clearing damaged cells. A normal count ranges from 3.4 to 9.6 billion per liter of blood. There are five main types, each with a different specialty.
- Neutrophils (55% to 70% of white blood cells) are the first responders. They rush to sites of infection and engulf bacteria and debris. They’re the reason pus is white; it’s largely spent neutrophils.
- Lymphocytes (20% to 40%) include T cells and B cells. B cells produce antibodies, while T cells directly attack infected or abnormal cells. These are the cells behind your long-term immunity after a vaccination or infection.
- Monocytes (2% to 8%) are the cleanup crew. They migrate into tissues, mature into larger cells called macrophages, and consume pathogens, dead cells, and other waste.
- Eosinophils (1% to 4%) specialize in fighting parasites and play a role in allergic reactions. Elevated eosinophil counts often signal an allergy or parasitic infection.
- Basophils (0.5% to 1%) are the rarest type. They release chemicals like histamine during allergic responses and help direct other immune cells to trouble spots.
Platelets: Wound Sealers
Platelets are not full cells but tiny cell fragments, much smaller than red or white blood cells. A normal platelet count ranges from about 135 to 371 billion per liter of blood, with the range varying slightly between males and females. Despite their size, platelets are the reason a paper cut stops bleeding in minutes.
When a blood vessel is damaged, platelets respond in three rapid stages. First, they stick to the exposed edges of the wound (adhesion). Next, they activate: changing shape, releasing chemicals that narrow the blood vessel to slow blood flow, and signaling more platelets to the site. Finally, the accumulated platelets clump together (aggregation) to form a temporary plug over the break. Clotting proteins in plasma then reinforce this plug with a mesh of fibrin strands, creating a stable clot.
Where Blood Cells Come From
All blood cells, red and white alike, along with platelets, originate from a single type of stem cell in your bone marrow. These stem cells are multipotent, meaning they can develop into any blood cell type depending on the chemical signals they receive. Proteins called colony-stimulating factors circulate through the blood and bone marrow, instructing stem cells to multiply and mature into whichever cell type is needed most. During an infection, for example, these signals ramp up production of immune cells and release them into the bloodstream to fight off the invader.
This production system is remarkably prolific. Your body generates millions of new blood cells every second to replace those that wear out, are consumed fighting infections, or are used in clotting. Red blood cells take about seven days to mature in the bone marrow before entering circulation, while certain white blood cells can be deployed within hours when the body detects a threat.
Dissolved Gases and pH Balance
Blood constantly carries two gases in opposite directions: oxygen from the lungs to your tissues, and carbon dioxide from your tissues back to the lungs. Oxygen relies almost entirely on hemoglobin for transport, while carbon dioxide uses multiple pathways, mostly conversion to bicarbonate, with smaller portions binding to hemoglobin or dissolving directly in plasma.
This gas exchange is tightly linked to blood pH. Carbon dioxide is acidic when dissolved, so the bicarbonate buffering system acts as a chemical thermostat, neutralizing excess acid to keep blood within its narrow pH window of 7.2 to 7.4. Even small shifts outside this range can disrupt the function of enzymes and cells throughout your body. The lungs and kidneys work together to fine-tune the balance: your lungs adjust how much carbon dioxide you exhale, while your kidneys regulate bicarbonate levels over longer time frames.