There is no substitute for blood, and it cannot be manufactured in a lab. The only source for patients in need of a transfusion is volunteer blood donations.

Basic Components That Comprise Human Blood

Plasma

The liquid portion of blood that transports nutrients, hormones, and waste products.

Red Blood Cells (Erythrocytes)

Cells responsible for carrying oxygen from the lungs to the body and returning carbon dioxide to the lungs.

White Blood Cells (Leukocytes)

Part of the immune system, helping the body fight infections and other diseases.

Platelets (Thrombocytes)

Cell fragments that play a key role in blood clotting and wound healing.

Plasma

Plasma is the straw‑colored, liquid portion of blood that makes up about 55% of its total volume. It serves as the primary transport medium for various substances throughout the body. Plasma is composed of approximately 90% water, with the remaining 10% consisting of dissolved proteins, nutrients, hormones, gases, electrolytes, and waste products.

One of the key protein groups in plasma includes albumin, globulins, and fibrinogen. Albumin helps maintain osmotic pressure and fluid balance between blood and tissues. Globulins play a major role in immune function and transportation of substances, while fibrinogen is essential for blood clotting. Plasma also carries nutrients such as glucose, amino acids, and fatty acids absorbed from the digestive system, delivering them to cells throughout the body.

Hormones secreted by endocrine glands travel through plasma to reach their target organs. Electrolytes such as sodium, potassium, calcium, and bicarbonate ions maintain proper pH balance, nerve signaling, and muscle function. Plasma also carries waste products such as urea and carbon dioxide to organs responsible for their elimination. Through its extensive transport and regulatory functions, plasma plays a vital role in maintaining homeostasis in the human body.

Red Blood Cells (Erythrocytes)

Red blood cells (RBCs), also known as erythrocytes, are the most abundant cellular component of blood. They are specially adapted for transporting oxygen from the lungs to tissues and returning carbon dioxide back to the lungs. Their biconcave disc shape increases surface area for efficient gas exchange and allows flexibility to pass through narrow capillaries.

RBCs contain hemoglobin, an iron‑rich protein that binds oxygen molecules. When blood passes through the lungs, oxygen attaches to hemoglobin, forming oxyhemoglobin. As RBCs circulate through tissues with lower oxygen levels, hemoglobin releases oxygen where it is needed for cellular respiration.

The lifespan of an RBC is about 120 days, after which it is removed by macrophages in the liver and spleen. The body continuously produces new RBCs in the bone marrow through a process called erythropoiesis. The hormone erythropoietin, produced by the kidneys, stimulates RBC production in response to low oxygen levels.

RBC count and hemoglobin levels are critical indicators of health. Low RBC levels result in anemia, leading to fatigue and weakness, while abnormally high RBC counts may indicate underlying medical conditions. Overall, RBCs are indispensable for delivering oxygen and maintaining energy production in the body.

White Blood Cells (Leukocytes)

White blood cells (WBCs), or leukocytes, make up less than 1% of total blood volume but are essential for protecting the body against infections, foreign substances, and abnormal cells. Unlike RBCs, leukocytes are true cells with nuclei and organelles, allowing them to perform complex immune functions.

There are several types of WBCs, each with specialized roles:
– **Neutrophils** are the most abundant and act as the first responders to bacterial infections, engulfing and destroying pathogens.
– **Lymphocytes** include B cells, T cells, and natural killer cells. They play key roles in antibody production, cell‑mediated immunity, and destruction of virus‑infected or cancerous cells.
– **Monocytes** mature into macrophages and dendritic cells that engulf pathogens and present antigens to activate other immune cells.
– **Eosinophils** combat parasitic infections and are involved in allergic reactions.
– **Basophils** release histamine during allergic responses and inflammation.

WBCs continuously circulate in the bloodstream and lymphatic system but can migrate into tissues when needed. Their ability to detect and respond to harmful invaders is essential for preventing disease. When infection or inflammation occurs, WBC counts often rise, serving as an important diagnostic indicator.

Platelets (Thrombocytes)

Platelets, or thrombocytes, are small cell fragments derived from large bone marrow cells called megakaryocytes. Although they lack nuclei, platelets play a crucial role in preventing excessive bleeding by forming clots at sites of injury.

When a blood vessel is damaged, platelets rapidly accumulate at the site, adhering to the exposed tissue and each other in a process called platelet aggregation. They release chemical signals that attract additional platelets and activate clotting factors in the plasma. These clotting factors trigger a cascade that converts fibrinogen into fibrin, forming a mesh that stabilizes the platelet plug and seals the wound.

Platelets also help maintain the integrity of blood vessels by repairing small daily wear‑and‑tear injuries. Abnormal platelet levels can lead to health problems: low platelet counts (thrombocytopenia) increase the risk of bleeding, while excessively high platelet counts (thrombocytosis) can cause dangerous clots.

Through their rapid response and involvement in the clotting cascade, platelets are essential for wound healing and preventing blood loss.