Blood: Structure, Function, and Disorders (Anatomy & Physiology Study Notes)

Blood: Overview

Introduction to Blood

Blood is a specialized liquid connective tissue that interacts with all other body systems. It plays a vital role in maintaining homeostasis and supporting physiological processes throughout the body.

• Three main functions:

  • Transportation: Delivers gases (O2, CO2), nutrients, and wastes.

  • Regulation: Maintains pH, temperature, and water content of cells.

  • Protection: Provides immune defense and enables blood clotting.

Characteristics & Components of Blood

Physical and Chemical Properties

• Normal blood is more viscous than water and appears opaque.

• Color: Ranges from bright scarlet (high O2) to brick red (low O2).

• pH: 7.35–7.45; Temperature: ~38°C.

• Volume: ~8% of body weight (5–6 L in males, 4–5 L in females).

Components of Blood

• Cells & Formed Elements:

  • Red blood cells (RBCs): ~44% of blood volume.

  • White blood cells (WBCs) & Platelets: ~1% ("buffy coat").

• Matrix (Plasma): ~55% of blood volume.

  • Water: ~90% of plasma volume.

  • Proteins: ~9% (e.g., albumin, globulins, fibrinogen).

  • Solutes: ~1% (nutrients, wastes, gases, electrolytes).

Red Blood Cells (Erythrocytes)

Structure and Function

• Anucleate, biconcave disc: Increases surface area for gas exchange.

• Concentration: ~5.4 million/μL of blood.

• Lack organelles: Maximizes space for hemoglobin (Hb).

Hemoglobin (Hb)

• Each Hb molecule consists of:

  • 4 polypeptide chains (globins).

  • Each globin contains a heme group with an iron (Fe) atom.

  • Each heme binds one O2 molecule.

RBC Lifespan and Recycling

• Functional lifespan: Up to 120 days.

• Recycling: Old RBCs are phagocytosed (mainly in the spleen); heme and amino acids are retained, while waste is excreted via digestive and urinary systems.

Hematopoiesis: Formation of Blood Cells

Process and Regulation

• Location: Occurs in red bone marrow from adult stem cells (hematopoietic stem cells, HSCs).

• Produces: RBCs, WBCs, and platelets.

• Hormonal stimulation:

  • Erythropoietin (EPO): Stimulates RBC production.

  • Cytokines: Regulate WBC production.

  • Thrombopoietin: Stimulates platelet production.

Erythropoiesis

• Balance between RBC production and destruction is crucial.

• Too few RBCs: Leads to hypoxia (low O2), stimulating EPO release from kidneys and increasing RBC production.

• Requirements: Adequate iron, amino acids, and B vitamins are essential for effective erythropoiesis.

Anemia

Definition and Types

• Anemia: Reduced oxygen-carrying capacity of hemoglobin.

• Iron-deficiency anemia: Most common; insufficient iron for Hb synthesis.

• Pernicious anemia: Decreased RBC production due to vitamin B12 deficiency (often from lack of intrinsic factor).

Sickle Cell Disease

• Caused by abnormal hemoglobin (HbS) that distorts RBC shape, leading to sickling and rupture.

• Genetics:

  • Carriers with one mutated allele have sickle cell trait.

  • Individuals with two mutated alleles have sickle cell anemia.

• Sickled cells can block small blood vessels, causing pain and organ damage.

Summary Table: Blood Components

Component	Percentage of Blood	Main Function
Plasma	~55%	Transport of nutrients, wastes, hormones, and heat
Red Blood Cells (RBCs)	~44%	Oxygen and carbon dioxide transport
White Blood Cells (WBCs)	~1% (with platelets)	Immune defense
Platelets	~1% (with WBCs)	Blood clotting (hemostasis)

Key Equations

• Hematocrit (Hct): Proportion of RBCs in blood volume. $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

Additional info:

• Blood also contains dissolved gases (O2, CO2), electrolytes (Na+, K+, Ca2+), and hormones.

• Disorders such as anemia can have multiple causes, including genetic, nutritional, or chronic disease factors.