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

Chapter 17: Blood

Overview of Blood

Blood is a specialized connective tissue that circulates through arteries, veins, and capillaries, serving vital functions in transport, regulation, and protection. It consists of plasma (the liquid component) and formed elements (cells and cell fragments).

• Transport Function:

  • Delivers nutrients, oxygen, and hormones to body cells.

  • Removes metabolic waste products from cells.

  • Transports hormones from endocrine glands to target organs.

• Regulatory Function:

  • Maintains body temperature.

  • Maintains normal pH in tissues.

  • Maintains fluid volume in the circulatory system via colloid osmotic pressure.

• Protective Function:

  • Prevents blood loss (hemostasis and clot formation).

  • Prevents infection (WBCs, antibodies, and plasma proteins).

Blood Composition

• Plasma: Nonliving fluid matrix, 90% water, contains nutrients, gases, hormones, wastes, proteins, and inorganic ions.

• Formed Elements: Cells and cell fragments, including erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes).

Plasma Proteins

• Albumin (60%): Maintains colloid osmotic pressure.

• Globulins (36%): Alpha, beta (transport), gamma (immune defense).

• Fibrinogen (4%): Blood clotting.

Physical Properties of Blood

• Blood is 5x more viscous than water due to formed elements.

• Alkaline pH: 7.35–7.45.

• Average adult blood volume: 5–6 liters.

Formed Elements

• Erythrocytes (RBCs): Transport oxygen and carbon dioxide.

• Leukocytes (WBCs): Immune defense.

• Platelets (Thrombocytes): Blood clotting.

Red Blood Cells (Erythrocytes)

RBCs are biconcave, anucleate cells specialized for oxygen transport. Their shape increases surface area for gas exchange and allows flexibility in capillaries.

• 97% hemoglobin by volume.

• No mitochondria; rely on anaerobic metabolism.

• Life span: 120 days.

Structure of Hemoglobin

• Composed of 4 polypeptide chains (2 alpha, 2 beta) and 4 heme groups.

• Each heme contains an iron atom that binds one oxygen molecule.

• One hemoglobin molecule can carry up to 4 oxygen molecules.

Hemoglobin & Oxygen Transport

• Oxyhemoglobin: Hemoglobin bound to oxygen; bright red.

• Deoxyhemoglobin: Hemoglobin after oxygen release; dark red.

• Normal Hemoglobin Levels:

  • Men: 13–18 g/100 mL blood

  • Women: 12–16 g/100 mL blood

Production of RBCs (Erythropoiesis)

RBCs are produced in red bone marrow from hematopoietic stem cells. The process is regulated by erythropoietin (EPO), a hormone released in response to low oxygen levels (hypoxia).

• Occurs in axial skeleton, girdles, and proximal humerus/femur.

• It takes about 15 days for a stem cell to mature into a reticulocyte (immature RBC).

Pathway of RBC Production

• Hematopoietic Stem Cell (hemocytoblast) → Proerythroblast → Early erythroblast → Late erythroblast → Reticulocyte → Erythrocyte

Regulation of Erythropoiesis

• Stimulus: Hypoxia (low oxygen levels)

• Response: Kidneys release EPO → stimulates RBC production in bone marrow

• Requirements: Iron, vitamin B12, folic acid

Dietary Requirements for RBC Production

• Iron: 65% stored in hemoglobin; remainder in liver, spleen, and bone marrow as ferritin and hemosiderin.

• B-complex vitamins: Especially B12 and folic acid, required for DNA synthesis.

Hemoglobin Recycling

• RBCs are broken down in spleen and liver.

• Hemoglobin is split into globin (amino acids) and heme.

• Iron is stored or transported by transferrin.

• Heme is converted to bilirubin and excreted in bile.

White Blood Cells (Leukocytes)

WBCs are involved in immune defense and are capable of diapedesis (leaving blood vessels to enter tissues).

• Types of WBCs:

  • Neutrophils

  • Lymphocytes

  • Monocytes

  • Eosinophils

  • Basophils

• Mnemonic: "Never Let Monkeys Eat Bananas" (order of abundance)

Platelets (Thrombocytes)

Platelets are cell fragments derived from megakaryocytes, essential for blood clotting (hemostasis).

• Form temporary plugs in damaged blood vessels.

• Release chemicals that promote clotting.

Blood Disorders

Anemias Due to Decreased RBC Production

• Iron Deficiency Anemia: Insufficient iron → decreased hemoglobin production.

• Pernicious Anemia: Vitamin B12 deficiency, often due to lack of intrinsic factor.

• Renal Anemia: Inadequate EPO production (often due to kidney disease).

• Aplastic Anemia: Bone marrow failure → decreased production of all blood cell types.

Other RBC Disorders

• Sickle Cell Anemia: Genetic mutation in hemoglobin; RBCs become sickle-shaped, leading to vessel blockage and chronic anemia.

• Polycythemia: Excess RBCs; can be primary (bone marrow disorder) or secondary (increased EPO production due to hypoxia, chronic lung disease, or tumors).

Summary Table: Types of Anemia

Type of Anemia	Main Cause	Key Features
Iron Deficiency	Low iron intake, chronic blood loss	Low hemoglobin, microcytic RBCs
Pernicious	Vitamin B12 deficiency	Impaired DNA synthesis, macrocytic RBCs
Renal	Low EPO (kidney disease)	Decreased RBC production
Aplastic	Bone marrow failure	Decreased production of all blood cell types
Sickle Cell	Genetic mutation (HbS)	Sickle-shaped RBCs, vessel blockage
Polycythemia	Bone marrow disorder or increased EPO	Excess RBCs, increased blood viscosity

Key Equations

• Oxygen Transport: $\text{O}_2 \text{ carried by hemoglobin} = \text{Number of RBCs} \times \text{Hemoglobin per RBC} \times 4$

• Blood pH: $\text{Normal blood pH} = 7.35 - 7.45$

Additional info:

• Plasma proteins play a critical role in maintaining osmotic pressure and immune defense.

• Blood disorders can be classified by cause (production, destruction, or loss of RBCs).

• Homeostasis of blood oxygen levels is tightly regulated by feedback mechanisms involving the kidneys and EPO.