Blood: Structure, Function, and Cellular Components – Anatomy & Physiology Study Notes

Blood: Overview and Composition

Definition and General Properties

Blood is a connective tissue that makes up about 8% of total body weight and circulates through blood vessels in all tissues. It serves as the primary medium for transporting nutrients, gases, wastes, and other molecules throughout the body.

• Plasma: The liquid extracellular matrix of blood, comprising about 55% of its volume. Plasma is mostly water, with dissolved proteins, nutrients, hormones, and waste products.

• Formed elements: The cellular components of blood, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments).

• Blood can be separated into three layers by centrifugation:

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

  • Buffy coat (middle layer): Contains leukocytes and platelets, ~1% of blood volume.

  • Hematocrit (bottom layer): Contains erythrocytes, ~44% of blood volume.

Functions of Blood

• Gas exchange: Transports oxygen and carbon dioxide between lungs and tissues.

• Distribution: Carries nutrients, hormones, and wastes to and from cells.

• Immune function: Leukocytes and immune proteins defend against pathogens.

• Temperature regulation: Blood redistributes heat generated by metabolism.

• Sealing damaged vessels: Platelets and clotting proteins form blood clots to prevent blood loss.

• pH stabilization: Blood buffers maintain pH between 7.35 and 7.45.

• Blood pressure regulation: Blood volume influences blood pressure.

Plasma: Composition and Functions

Plasma Proteins

Plasma is about 90% water and contains several important proteins:

• Albumin: Maintains osmotic pressure and transports substances.

• Immune proteins (antibodies): Defend against pathogens.

• Transport proteins: Carry lipid-based molecules incompatible with water.

• Clotting proteins: Help form blood clots to stop bleeding.

The remaining 1% of plasma consists of dissolved small molecules, which are exchanged between blood and interstitial fluid.

Erythrocytes (Red Blood Cells) and Oxygen Transport

Structure and Function

Erythrocytes are specialized for oxygen and carbon dioxide transport. Their biconcave disc shape increases surface area for gas exchange.

• Hemoglobin (Hb): The main protein in erythrocytes, responsible for binding and transporting oxygen and carbon dioxide.

• Each hemoglobin molecule consists of four polypeptide chains (two alpha, two beta) and four iron-containing heme groups.

• Oxygen binds to iron in the heme group, forming oxyhemoglobin ($HbO_2$).

• Carbon dioxide binds to hemoglobin to form carbaminohemoglobin ($HbCO_2$).

• Carbon monoxide binds more strongly to hemoglobin than oxygen, forming carboxyhemoglobin ($HbCO$), which is toxic.

Lifespan and Production

• Erythrocytes have a lifespan of 100–120 days.

• They are produced in the red bone marrow by hematopoietic stem cells (HSCs) through a process called erythropoiesis.

• Erythropoietin (EPO), a hormone from the kidneys, stimulates erythropoiesis.

• Developing erythrocytes lose their nucleus and become reticulocytes before entering the bloodstream.

Regulation of Erythropoiesis

• Regulated by a negative feedback loop:

  • Stimulus: Low blood oxygen.

  • Receptor: Kidneys detect low oxygen.

  • Control center: Kidneys release more EPO.

  • Effector/response: Increased erythrocyte production.

  • Homeostasis: Restored blood oxygen levels.

Erythrocyte Death and Recycling

• Old erythrocytes are trapped in the spleen and destroyed by macrophages.

• Hemoglobin is broken down into amino acids, iron, and bilirubin (a waste product).

• Iron is recycled and transported by transferrin to the bone marrow.

• Bilirubin is sent to the liver for excretion.

Anemia and Blood Disorders

Types and Causes of Anemia

Anemia is a condition characterized by decreased oxygen-carrying capacity of blood due to low hemoglobin, decreased hematocrit, or abnormal hemoglobin.

• Decreased hemoglobin: Often due to iron deficiency or chronic disease.

• Decreased hematocrit: Caused by blood loss, bone marrow failure, or destruction of erythrocytes.

• Abnormal hemoglobin: Genetic disorders such as sickle-cell anemia.

Common Symptoms

• Pale skin

• Fatigue

• Weakness

• Shortness of breath

Special Types of Anemia

• Pernicious anemia: Due to vitamin B12 deficiency, affecting rapidly dividing cells.

• Hemolytic anemia: Caused by destruction of erythrocytes due to infection, toxins, or autoimmune reactions.

• Sickle-cell anemia: Genetic disorder causing abnormal hemoglobin (HbS) and sickle-shaped erythrocytes.

Leukocytes (White Blood Cells) and Immune Function

Types of Leukocytes

Leukocytes are larger than erythrocytes and play key roles in immune defense. They are divided into two main categories:

• Granulocytes: Contain cytoplasmic granules.

  • Neutrophils: Most common; phagocytize bacteria and release granules to kill pathogens. Nucleus has 3–5 lobes (polymorphonuclear).

  • Eosinophils: Bilobed nucleus; respond to parasitic infections and allergies.

  • Basophils: S-shaped nucleus; release histamine to mediate inflammation.

• Agranulocytes: Lack visible granules.

  • Lymphocytes: Large nucleus; two types:

    • B lymphocytes (B cells): Produce antibodies.

    • T lymphocytes (T cells): Directly attack infected or abnormal cells.

  • Monocytes: Largest leukocytes; U-shaped nucleus; become macrophages in tissues.

Summary Table: Blood Components and Functions

Component	Main Function	Key Features
Plasma	Transport, osmotic balance, immunity, clotting	90% water, proteins (albumin, antibodies, clotting factors)
Erythrocytes (RBCs)	Oxygen and CO2 transport	Biconcave disc, hemoglobin, no nucleus
Leukocytes (WBCs)	Immune defense	Granulocytes (neutrophils, eosinophils, basophils), agranulocytes (lymphocytes, monocytes)
Platelets	Clot formation	Cell fragments, initiate clotting cascade

Key Equations and Concepts

• Oxygen binding to hemoglobin: $Hb + O_2 \rightarrow HbO_2$

• Carbon dioxide transport: $Hb + CO_2 \rightarrow HbCO_2$

• Carboxyhemoglobin formation (toxic): $Hb + CO \rightarrow HbCO$

• Hematocrit calculation: $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100\%$

Additional info:

• Blood also contains dissolved gases, nutrients, hormones, and metabolic wastes.

• Platelets are derived from megakaryocytes in the bone marrow.

• Blood disorders can affect any component, leading to clinical symptoms and requiring diagnostic evaluation.