Components Of Blood And Their Functions

Blood is a specialized connective tissue with two main parts: a liquid extracellular matrix called blood plasma and various cells and cell fragments known as formed elements.

Table of Contents

I. Centrifuged Blood Composition

Centrifugation separates blood into layers based on density:

Blood Plasma ($≈ 55%$): The top, watery layer containing dissolved substances.
Formed Elements ($≈ 45%$): The bottom layer, made mostly of Red Blood Cells (RBCs).
Buffy Coat ($< 1%$): A thin middle layer containing White Blood Cells (WBCs) and Platelets.

[Image of centrifuged blood showing plasma, buffy coat, and formed elements layers]

II. Blood Plasma

Plasma is $≈ 91.5%$ water and $≈ 8.5%$ solutes.

Plasma Proteins

Plasma proteins make up $7%$ of the plasma and are primarily synthesized by the liver (hepatocytes). They maintain blood osmotic pressure.

Albumins ($≈ 54%$): Most abundant; maintain osmotic pressure.
Globulins ($≈ 38%$): Include transport proteins and Gamma Globulins (antibodies/immunoglobulins) produced during immune responses.
Fibrinogen ($≈ 7%$): Essential for the final step of blood clotting.

Other Solutes

These include electrolytes, nutrients, hormones, gases, and waste products (e.g., urea, bilirubin).

III. Formed Elements

The formed elements are RBCs, WBCs, and Platelets.

A. Hematocrit

Hematocrit is the percentage of blood volume occupied by RBCs.

Normal Ranges:
Adult females $38%-46%$ (avg. $42%$),
Adult males $40%-54%$ (avg. $47%$).
Conditions:
Anemia (low RBC count/low hematocrit) and
Polycythemia (abnormally high RBC count $≥ 65%$, increasing blood viscosity and stroke risk).

B. Red Blood Cells (RBCs) / Erythrocytes

RBCs are responsible for oxygen transport.

Structure: Biconcave discs ($\approx 7–8\ \mu m$); lack a nucleus and organelles to maximize space for Hemoglobin (which makes up $33\%$ of the cell).
Physiology: They generate ATP anaerobically and transport $\text{O}_2$ (bound to $\text{Fe}^{2+}$ in heme groups) and $\text{CO}_2$. Hemoglobin also binds nitric oxide ($\text{NO}$), causing vasodilation.

C. RBC Lifecycle and Erythropoiesis

RBCs have a lifespan of $≈ 120$ days.

Destruction: Worn-out RBCs are phagocytized by macrophages in the spleen, liver, and bone marrow.

Recycling:
Hemoglobin is broken down:
– Globin is reused.
– Iron ($\text{Fe}^{3+}$) is transported by Transferrin and stored as Ferritin.
– The non-iron portion of heme is converted to Bilirubin and excreted via bile.

Erythropoiesis (Production): Occurs in red bone marrow. Proerythroblasts synthesize hemoglobin and eject their nucleus to become Reticulocytes, which mature into RBCs.

Regulation: Hypoxia stimulates the kidneys to release Erythropoietin (EPO), which increases RBC production.

IV. White Blood Cells (WBCs) / Leukocytes

WBCs function in the immune system and have nuclei but lack hemoglobin.
Normal count: $5,000–10,000/\mu L$.
They leave the bloodstream via Emigration to reach infection sites (Chemotaxis).

Classification and Roles

WBCs are classified based on the visibility of cytoplasmic granules:

1. Granular Leukocytes

Neutrophils ($≈ 60%–70%$): Rapid response, perform phagocytosis against bacteria.
Eosinophils ($≈ 2%–4%$): Combat parasitic infections and modulate allergic reactions.
Basophils ($< 1%$): Release Histamine and Heparin to promote inflammation and allergic responses.

2. Agranular Leukocytes

Lymphocytes ($≈ 20%–25%$): Key for adaptive immunity
– $\text{B}$ cells → antibodies
– $\text{T}$ cells → direct killing & coordination
– $\text{NK}$ cells → innate immunity
Monocytes ($≈ 3%–8%$): Highly phagocytic; differentiate into Macrophages in tissues for antigen presentation and debris cleanup.

V. Platelets / Thrombocytes

Platelets are cell fragments vital for hemostasis and clotting.

Formation: Formed by fragmentation of Megakaryocytes in the bone marrow under the influence of Thrombopoietin.

Structure: Small, disc-shaped, lack a nucleus.

Functions: Adhere to damaged vessels to form a temporary plug and release chemicals (ADP, $\text{Ca}^{2+}$) that trigger blood clotting (coagulation).

Lifespan: $≈ 5–9$ days.