Blood Composition Explained: Erythrocytes, Plasma, and Immune Components

Overview

This Osmosis explainer walks through blood’s layered structure after centrifugation, detailing how the bottom erythrocyte layer, the buffy coat in the middle, and the top plasma come together to form whole blood. It explains what hematocrit means, how dehydration or disease can shift it, and why plasma proteins and immune cells matter for circulation, clotting, and defense against infection.

• Hematocrit reflects red blood cell mass in blood volume
• Buffy coat contains platelets and white blood cells
• Plasma proteins like albumin and fibrinogen support transport and clotting
• Leukocytes and platelets coordinate immunity and hemostasis

Introduction

This Osmosis video provides a concise tour of blood composition, explaining how centrifugation separates blood into distinct layers and why these layers matter for diagnosis and physiology. It introduces hematocrit, the percentage of blood volume occupied by erythrocytes, and previews how dehydration and disease can shift this balance.

Blood layers after centrifugation

When blood is spun in a centrifuge, the heaviest components settle at the bottom while lighter components rise to the top. The bottom layer consists of erythrocytes, the middle is the buffy coat containing platelets and leukocytes, and the top is plasma. The buffy coat generally accounts for less than 1 percent of blood volume, while plasma makes up about 55 percent and erythrocytes about 45 percent, highlighting how the components contribute to whole blood.

Red blood cells and hematocrit

Erythrocytes are optimized for gas exchange: they are thin, biconcave discs to maximize surface area and lack a nucleus to make room for hemoglobin. They live about 120 days and are continually regenerated in the bone marrow. The hematocrit value—roughly 45 percent in many adults—represents the proportion of the blood volume occupied by erythrocytes. A decreased hematocrit indicates anemia or destruction of red cells, while an increased hematocrit can result from dehydration or overproduction of erythrocytes.

"The main function of erythrocytes is to carry oxygen to the tissues, as well as bring carbon dioxide to the lungs so it can be expired." - Osmosis

Buffy coat and platelets

The buffy coat, the white middle layer just above the erythrocytes, contains platelets and leukocytes. Platelets are small fragments released from megakaryocytes in the bone marrow and are essential for forming a plug that seals damaged vessels and prevents blood loss. Leukocytes, the immune cells, are the only complete cells in blood and are specialized to combat pathogens and support immune defense. Some leukocytes can exit the bloodstream via diapedesis to reach tissues where they are needed.

"Leukocytes are the only complete cells in blood, meaning they have all the usual organelles." - Osmosis

Leukocytes and immunity

Leukocytes include granulocytes (neutrophils, eosinophils, basophils) and non-granulocytes (lymphocytes and monocytes). Neutrophils are the most common granulocytes and are typically the first responders to infection. Eosinophils tackle parasitic infections, basophils participate in allergic responses, lymphocytes drive adaptive immunity with B and T cells and natural killer cells, and monocytes phagocytose pathogens. Leukocytes can migrate from blood to tissues through diapedesis, acting as a mobile immune army throughout the body.

"Leukocytes are the only complete cells in blood, meaning they have all the usual organelles." - Osmosis

Plasma and clotting proteins

Plasma, the top layer of blood, is about 55 percent of whole blood and is largely water with dissolved proteins, electrolytes, and gases. The most abundant protein is albumin, which is produced in the liver and helps maintain oncotic pressure to keep water in the bloodstream. Plasma proteins also include globulins (antibodies and transport proteins) and fibrinogen, which is central to clot formation. Fibrinogen forms a scaffold along with other clotting factors to stabilize clots that seal vascular injuries.

"Fibrinogen is another abundant plasma protein and it's involved in clot formation for damaged vessels." - Osmosis

Serum versus plasma and electrolytes

All clotting factors can be removed from plasma to yield serum, which is acellular. Plasma contains key electrolytes—sodium, potassium, calcium, bicarbonate, and chloride—that regulate acid-base balance and blood osmolarity. These solutes, along with hormones and nutrients, support normal physiological function and gas exchange within the circulatory system.

The video also touches on dissolved gases like oxygen and carbon dioxide and explains their role in maintaining blood chemistry and tissue oxygenation.

"All of these clotting factor proteins, including fibrinogen, can be removed from a sample of plasma and what remains is serum." - Osmosis

"The electrolytes that are found in the plasma include mainly sodium, potassium, calcium, bicarbonate and chloride. These electrolytes play vital roles in maintaining normal acid base physiology in the blood and help regulate blood osmolarity." - Osmosis

Recap and clinical relevance

The video concludes with a quick recap: blood comprises roughly 45 percent erythrocytes and 55 percent plasma, with the buffy coat and platelets making up less than 1 percent. Understanding hematocrit, plasma proteins, and the immune components of blood provides essential context for diagnosing anemia, clotting disorders, immune responses, and overall circulatory health.