Blood has four major components: plasma, red blood cells, white blood cells, and platelets. By volume, plasma makes up about 55% of your blood, while the remaining 45% consists of the cellular elements, with red blood cells accounting for the vast majority of that fraction. Together, these components handle everything from delivering oxygen to fighting infections to sealing wounds.
Plasma: The Liquid Foundation
Plasma is a pale yellow fluid that serves as the transport medium for everything your blood carries. It’s about 92% water. The remaining 8% is a concentrated mix of dissolved substances that keep your body functioning: proteins make up roughly 7%, and the final 1% includes hormones, vitamins, salts, and enzymes.
The most abundant protein in plasma is albumin, which works like a sponge to maintain fluid balance. Albumin keeps plasma from leaking out of blood vessels and into surrounding tissues. Other key plasma proteins include antibodies (which target invaders), fibrinogen (which helps form blood clots), and clotting factors that work alongside platelets to stop bleeding. Plasma also carries glucose, waste products like urea, dissolved gases, and signaling molecules that let distant organs communicate with each other.
Because plasma contains so many critical proteins, donated plasma is used medically to treat burn victims, people with clotting disorders, and patients whose immune systems can’t produce enough antibodies on their own.
Red Blood Cells: Oxygen Carriers
Red blood cells make up 40% to 45% of your blood’s total volume, making them by far the most numerous blood cell. Their single job is gas exchange: they pick up oxygen in your lungs and deliver it to every tissue in your body, then carry carbon dioxide waste back to your lungs so you can exhale it.
This oxygen-carrying ability comes from hemoglobin, a protein packed inside each red blood cell that binds oxygen molecules. Hemoglobin is also what gives blood its red color. When it’s loaded with oxygen, blood appears bright red; when it’s oxygen-depleted and heading back to the lungs, it turns a darker shade.
Red blood cells are shaped like flattened discs with a slight indentation on each side, which gives them a large surface area for gas exchange and enough flexibility to squeeze through the tiniest capillaries. They have no nucleus, which frees up interior space for more hemoglobin. The tradeoff is that they can’t repair themselves, so each red blood cell survives an average of 120 days before it’s broken down and recycled. Your bone marrow constantly produces new ones to replace them.
A normal red blood cell count ranges from about 3.8 to 5.8 million cells per microliter for women and 4.5 to 6.5 million per microliter for men. When red blood cell counts or hemoglobin levels drop too low, the result is anemia, which causes fatigue, weakness, and shortness of breath because tissues aren’t getting enough oxygen.
White Blood Cells: The Immune Defense
White blood cells account for only about 1% of your blood volume, but they’re the backbone of your immune system. A healthy adult carries between 3,600 and 11,000 white blood cells per microliter. Despite their small numbers relative to red blood cells, they patrol your bloodstream constantly, identifying and destroying bacteria, viruses, parasites, and abnormal cells.
There are five main types, each with a distinct role:
- Neutrophils are the most common and act as first responders. When bacteria or other germs enter your body, neutrophils rush to the site and engulf the invaders.
- Lymphocytes include B cells, which produce antibodies against specific threats, and T cells, which can directly destroy virus-infected cells and cancer cells.
- Monocytes kill bacteria and viruses while also clearing away dead or damaged cells, essentially cleaning up after an immune response.
- Eosinophils specialize in fighting parasitic infections and play a role in allergic reactions and inflammation.
- Basophils are the rarest type and release chemical signals during allergic reactions and asthma attacks.
When your white blood cell count rises above normal, it usually signals that your body is fighting an infection or dealing with inflammation. A count that stays abnormally high or low can point to more serious conditions, which is why white blood cell counts are a standard part of routine blood work.
Platelets: Clotting and Wound Repair
Platelets are tiny cell fragments, much smaller than red or white blood cells, that prevent you from bleeding excessively. A healthy person has 150,000 to 450,000 platelets per microliter of blood at any given time.
When a blood vessel is damaged, platelets stick to the injured wall within seconds. Once attached, they go through a rapid activation process: they change shape to make it easier for additional platelets to bind together, release chemicals that cause the blood vessel to narrow (reducing blood flow to the area), and send out signals that recruit more platelets to the wound site. This growing cluster of platelets forms a temporary plug. Clotting proteins in plasma, particularly fibrinogen, then reinforce the plug with a mesh of protein fibers, creating a stable clot that holds until the vessel heals.
If your platelet count drops too low, even minor cuts or bumps can cause prolonged bleeding or bruising. Counts that are too high increase the risk of dangerous clots forming inside blood vessels where there’s no injury.
Where Blood Cells Come From
All three types of blood cells, red cells, white cells, and platelets, originate from the same source: stem cells in your bone marrow. The spongy tissue inside your bones, particularly in the pelvis, spine, and sternum, serves as the body’s blood cell factory from infancy through adulthood.
The process starts with a single type of cell called a hematopoietic stem cell. This stem cell can develop into any blood cell type. With each stage of development, it becomes more specialized, committing to a specific path until it matures into a red blood cell, a particular type of white blood cell, or a platelet. Your body fine-tunes this production constantly, ramping up red blood cell output when oxygen levels are low, boosting white blood cell production during infections, and increasing platelet manufacturing after blood loss.
Blood pH and Internal Balance
Beyond its individual components, blood maintains a tightly controlled pH between 7.35 and 7.45, which is slightly alkaline. Even small deviations outside this range can disrupt the chemical reactions that keep cells alive.
Three systems work together to keep blood pH stable. Buffers in the blood itself, primarily bicarbonate and hemoglobin, neutralize excess acids as they’re produced. Your lungs adjust pH by controlling how much carbon dioxide you exhale (carbon dioxide dissolved in blood forms an acid). Your kidneys handle longer-term regulation by filtering out excess acids and retaining the right amount of alkaline compounds. This three-layered system is why blood pH stays remarkably stable even when your diet, activity level, and metabolism shift throughout the day.