Human Blood: Composition, Function, and Components

Erythrocytes and Oxygen Transport

The most numerous cells in the blood are the erythrocytes, or red blood cells (RBCs). These cells are uniquely engineered for one primary task: transporting oxygen.

Structure and Lifespan

Erythrocytes possess a distinctive biconcave shape, which increases their surface area for efficient gas exchange. Notably, mature RBCs lack a nucleus, a feature that provides more space for their primary functional protein, hemoglobin. However, this lack of a nucleus means they cannot repair themselves, limiting their lifespan to approximately 120 days before they are removed from circulation by the spleen or liver.

Hemoglobin and Gas Exchange

Hemoglobin is the iron-containing protein that binds to oxygen. The efficiency of how oxygen is picked up in the lungs and dropped off in the tissues is governed by two critical physiological phenomena: the Bohr Effect and the Haldane Effect.

Exam Tip: Remember that the Bohr and Haldane effects are "opposites" in terms of what they prioritize. The Bohr effect focuses on how $CO_2$ affects oxygen release, while the Haldane effect focuses on how oxygen affects $CO_2$ release.

The Bohr Effect

The Bohr Effect describes how hemoglobin's affinity for oxygen decreases in environments with high $CO_2$ concentrations or lower pH (increased acidity). In metabolically active tissues (like a working muscle), $CO_2$ levels rise and acidity increases. This triggers hemoglobin to "let go" of its oxygen more readily, ensuring the tissues get the fuel they need.

The Haldane Effect

While the Bohr effect happens at the tissues, the Haldane effect is crucial in the lungs. It describes how the oxygenation of blood displaces $CO_2$ from hemoglobin. When oxygen ($O_2$) binds to hemoglobin ($Hb$), the affinity of $Hb$ for $CO_2$ and hydrogen ions ($H^+$) decreases. This facilitates the release of $CO_2$ into the lungs to be exhaled, helping maintain the body's acid-base balance.

Immune Defense: Leukocytes and Platelets

While erythrocytes handle transport, the other cellular components handle protection and repair.

White Blood Cells (Leukocytes)

Leukocytes are the cells of the immune system. Unlike RBCs, they possess nuclei and are categorized into two main groups:

1. Granulocytes: These contain granules in their cytoplasm.
   • Neutrophils: The most abundant WBC; they are "first responders" that perform phagocytosis to destroy bacteria.
   • Basophils: Large cells that release histamine and heparin to trigger inflammation and allergic responses.
   • Eosinophils: Involved in various immune responses.
2. Agranulocytes:
   • Lymphocytes: Includes B cells (which produce antibodies for humoral immunity), T cells (which coordinate immune responses or kill infected cells), and Natural Killer (NK) cells (which target tumors and viruses).
   • Monocytes: These circulate in the blood and then migrate into tissues to become macrophages or dendritic cells, which are essential for engulfing pathogens and presenting antigens.

Platelets and Hemostasis

Platelets are not full cells but are fragments that play a vital role in hemostasis—the process of stopping bleeding. When a vessel is injured, platelets adhere to exposed collagen, become activated, and form a "platelet plug" to prevent blood loss.

Clinical Note: Several factors can disrupt this process. For example, Vitamin K is necessary for synthesizing clotting factors, and Calcium is required to convert fibrinogen into fibrin. Conversely, medications like Aspirin can make platelets "less sticky" by inhibiting the production of Thromboxane A2, which is used therapeutically to prevent unwanted clots.