Blood and Circulatory System: Structure, Function, and Gas Exchange

Gas Exchange in Animals

Major Steps in Gas Exchange

Gas exchange is essential for supplying oxygen (O2) to tissues and removing carbon dioxide (CO2) from the body. It involves several coordinated steps:

• 1. Ventilation: Movement of air or water across the respiratory surface (e.g., lungs, gills).

• 2. Diffusion at Respiratory Surface: O2 diffuses into the blood, and CO2 diffuses out.

• 3. Circulation: The circulatory system transports gases throughout the body via blood.

• 4. Diffusion at Tissues: O2 diffuses from blood into tissues; CO2 diffuses from tissues into blood.

• 5. Cellular Respiration: Cells use O2 to produce ATP and generate CO2 as a waste product.

Example: In humans, O2 enters the lungs, diffuses into blood, is transported to tissues, and used in cellular respiration.

Blood: Structure and Function

Functions of Blood

Blood is a specialized connective tissue that acts as a transport medium for the body. Its main functions include:

• Transport: Carries O2 and nutrients to body tissues.

• Waste Removal: Transports metabolic wastes to kidneys for elimination.

• Hormone Distribution: Delivers hormones from endocrine organs to target organs.

• Regulation: Maintains homeostasis of temperature, pH, and fluid volume.

• Protection: Defends against infections and prevents blood loss via clotting.

Blood Composition

Blood is the only tissue that is fluid, consisting of an extracellular matrix (plasma) and cellular components (formed elements).

• Plasma: Straw-colored, sticky fluid; about 55% of blood volume.

• Formed Elements: Red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

• Oxygen Content: Blood color varies with O2 content—brighter when oxygenated, darker when deoxygenated.

• Plasma Composition: Mostly water, dissolved solutes (gases, hormones, wastes, proteins, ions).

Example: A centrifuged blood sample separates into plasma (top), buffy coat (middle, white blood cells and platelets), and erythrocytes (bottom).

Component	Percentage	Description
Plasma	~55%	Water, proteins, nutrients, hormones, wastes
Buffy coat	<1%	Leukocytes and platelets
Erythrocytes	~45%	Red blood cells (hematocrit)

Types of Blood Cells

Red Blood Cells (Erythrocytes)

Red blood cells are specialized for gas transport:

• Structure: Biconcave disc shape increases surface area for gas exchange.

• Function: Contains hemoglobin, which binds O2 and CO2.

• Lifespan: About 120 days in humans.

White Blood Cells (Leukocytes)

White blood cells are complete cells involved in immune defense:

• Types: Neutrophils, eosinophils, basophils, lymphocytes, monocytes.

• Function: Fight infections, remove debris, and produce antibodies.

Platelets (Thrombocytes)

Platelets are cell fragments essential for blood clotting:

• Function: Help stop bleeding by forming clots.

• Origin: Produced from megakaryocytes in bone marrow.

Blood Cell Type	Main Function	Example
Red Blood Cells (Erythrocytes)	Transport O2 and CO2	Hemoglobin binds O2
White Blood Cells (Leukocytes)	Immune defense	Neutrophils fight bacteria
Platelets (Thrombocytes)	Blood clotting	Form platelet plugs

Oxygen and Carbon Dioxide Transport

Role of Hemoglobin

Hemoglobin is a protein in red blood cells that binds and transports oxygen and carbon dioxide:

• Structure: Tetramer with four polypeptide chains, each containing a heme group with an iron atom.

• Function: Each hemoglobin molecule can bind up to four O2 molecules.

Equation: $\text{Hb} + 4\,\text{O}_2 \rightarrow \text{Hb(O}_2)_4$

Cooperative Binding

Hemoglobin exhibits cooperative binding, meaning the binding of one O2 molecule increases the affinity for the next:

• Oxygen Dissociation Curve: S-shaped (sigmoidal) curve shows how hemoglobin saturation changes with partial pressure of O2 ($P_{O_2}$).

• Physiological Importance: Allows efficient loading of O2 in lungs and unloading in tissues.

Equation: $\text{Hb} + \text{O}_2 \rightleftharpoons \text{HbO}_2$

Bohr Effect

The Bohr effect describes how decreases in pH (increased acidity) and increases in temperature cause hemoglobin to release O2 more readily:

• Right Shift: Lower pH and higher temperature shift the oxygen dissociation curve to the right.

• Exercise: During exercise, tissues produce more CO2 and heat, promoting O2 release.

Equation: $\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{carbonic anhydrase}} \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$

Summary Table: Blood Functions and Components

Function	Component Involved	Description
Transport	Plasma, RBCs	O2, CO2, nutrients, hormones
Regulation	Plasma	Maintains pH, temperature, fluid volume
Protection	WBCs, Platelets	Immune response, clotting

Key Terms

• Hemoglobin: Oxygen-carrying protein in red blood cells.

• Plasma: Liquid component of blood.

• Leukocyte: White blood cell involved in immunity.

• Thrombocyte: Platelet, involved in clotting.

• Bohr Effect: Shift in hemoglobin's oxygen affinity due to pH and temperature changes.

Additional info: These notes expand on the provided slides and images, adding definitions, equations, and tables for clarity and completeness.