Cellular Basis of Physiology

Organs - Histology and functions of heart

• The heart is a muscular organ that pumps blood throughout the body.

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• It is composed mainly of cardiac muscle tissue and specialized conducting tissue.

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• The heart is composed of three layers: endocardium, myocardium and epicardium.

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• Histologically, it contains striated, branched cardiac muscle fibers with intercalated discs.

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• Functionally, it acts as a pump, pacemaker and regulator of circulation and pressure, with an additional endocrine role.

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• Histology of the Heart

A. Layers of the Heart Wall

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I. Endocardium

• Innermost layer.
• Lined by endothelium
• (simple squamous epithelium).
• Contains connective tissue and Purkinje fibers (specialized conducting cells).

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II. Myocardium

• Middle, thickest layer.
• Made up of cardiac muscle fibers arranged in spirals.
• Contains intercalated discs (specialized junctions for electrical and mechanical coupling).
• Rich in mitochondria → supports continuous contraction.

III. Epicardium (Visceral layer of serous pericardium)

• Outermost layer.
• Mesothelium with connective tissue, blood vessels, nerves and fat.

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B. Microscopic Features of Cardiac Muscle

• Cardiac myocytes: Branched, cylindrical cells with central nuclei.
• Striated appearance: similar to skeletal muscle, due to sarcomere arrangement.

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• Intercalated discs: specialized junctions with three components:
• Desmosomes – provide mechanical strength.
• Fascia adherens – anchor actin filaments.
• Gap junctions – allow ionic/electrical communication.
• Automaticity: some cells (e.g., SA node) can generate impulses without external stimulation.

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3. Functions of the Heart

• A. Pumping Function
• Maintains circulation of blood through pulmonary and systemic circuits.
• Right heart → pumps deoxygenated blood to lungs.
• Left heart → pumps oxygenated blood to body tissues.

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B. Electrical Conduction System

• Specialized fibers coordinate rhythmic contractions.
• Components:
• SA node (pacemaker) → initiates impulse.
• AV node → delays conduction.
• Bundle of His and Purkinje fibers → distribute impulses to ventricles

C. Other Functions

• Maintains blood pressure – generates hydrostatic pressure for perfusion.
• Ensures unidirectional flow – via heart valves (atrioventricular & semilunar).
• Adjusts cardiac output – according to metabolic needs (exercise, rest, stress).
• Endocrine role – atrial myocytes secrete Atrial Natriuretic Peptide (ANP) → regulates blood volume and pressure.

Histology and Functions of Lung

Histology and Functions of Lung

• The lungs are paired respiratory organs located in the thoracic cavity.
• They provide the structural basis for gas exchange between air and blood.
• The lungs are organized into a branching system from bronchi → bronchioles → alveoli.
• Histologically: lined progressively from pseudostratified columnar epithelium to simple squamous epithelium at alveoli.
• Functions include gas exchange, surfactant secretion, immune defense, metabolic activities and blood filtration.

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Histology of Lung

A. Gross Organization

Each lung is divided into lobes (Right: 3, Left: 2).

Functional unit: Pulmonary lobule (supplied by a terminal bronchiole).

B. Microscopic Structure

Bronchi

Lined by pseudostratified ciliated columnar epithelium with goblet cells.

Wall contains cartilage plates and smooth muscle.

Mucous glands present in submucosa.

• Bronchioles
• Lined by simple columnar/cuboidal ciliated epithelium with Clara (club) cells → secrete protective proteins and surfactant components.
• No cartilage, no glands.
• Prominent smooth muscle for regulation of airway diameter.

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Terminal Bronchioles

Last part of conducting zone.

Epithelium: simple cuboidal with ciliated and Clara cells.

Respiratory Bronchioles

First part of respiratory zone (gas exchange begins).

Epithelium: simple cuboidal; scattered alveoli in walls.

• Alveolar Ducts and Alveoli
• Alveoli: tiny air sacs (about 300 million in both lungs).
• Lining cells:
• Type I pneumocytes (squamous, cover 95% of surface, site of gas exchange).
• Type II pneumocytes (cuboidal, secrete pulmonary surfactant to reduce surface tension, also act as progenitor cells).
• Alveolar macrophages (dust cells) present for phagocytosis.
• Capillary endothelium closely apposed to alveolar epithelium → forms blood–air barrier.

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3. Blood–Air Barrier

• Extremely thin barrier that allows efficient diffusion of gases.
• Composed of:
• Alveolar epithelium (Type I cell).
• Fused basal lamina of alveolar epithelium and capillary endothelium.
• Capillary endothelium.

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4. Functions of Lung

• Respiratory Functions
• Gas exchange: oxygen uptake, carbon dioxide elimination.
• Maintains acid–base balance via CO₂ regulation.
• Non-Respiratory Functions
• Filtration of small thrombi and particulate matter from blood.
• Surfactant secretion (by Type II pneumocytes) → reduces surface tension, prevents alveolar collapse.
• Defense: alveolar macrophages remove pathogens and debris; mucociliary clearance in bronchi.
• Metabolic role:
• Conversion of angiotensin I → angiotensin II by ACE in pulmonary endothelium.
• Inactivation of bradykinin, serotonin, and prostaglandins.
• Reservoir function: acts as a blood reservoir during certain conditions.

Cell classification

Cell classification can be studied in two main ways – based on structural organization and functional specialization

1. Based on Structural Organization

Cells are broadly classified into two types:

(a) Prokaryotic Cells

Primitive cells, simple in structure.

Lack a true nucleus (DNA is free in the cytoplasm as a nucleoid).

No membrane-bound organelles.

Examples: Bacteria, Cyanobacteria (blue-green algae), Archaea.

(b) Eukaryotic Cells

Complex, well-developed cells.

Have a true nucleus enclosed by a nuclear membrane.

Contain membrane-bound organelles (mitochondria, ER, Golgi, etc.).

Examples: Animal cells, Plant cells, Fungi, Protists.

2. Based on Number of Cells in Organisms

Unicellular organisms → single cell performs all functions (e.g., Amoeba, Paramecium, Bacteria).

Multicellular organisms → have many specialized cells performing different functions (e.g., Humans, Plants).

3. Based on Shape and Function

Cells also differ by their role in multicellular organisms:

Epithelial cells – protection and absorption.

Muscle cells – movement and contraction.

Nerve cells (neurons) – transmission of impulses.

Red blood cells – oxygen transport.

White blood cells – immunity.

4. Based on Nutrition (Microbial Classification)

Autotrophic cells → produce their own food (e.g., plant cells with chloroplasts).

Heterotrophic cells → depend on other organisms for food (e.g., animal cells, fungi).