Anatomy and Physiology - Muscle Types and Respiratory System

Muscle Tissue Types and Fundamentals

• Universal Principle of Muscle Function:

  • Muscles can only produce force by contracting.

  • Muscles cannot push; they only exert force through contraction.

  • This principle applies to all muscle tissue types, including skeletal, smooth, and cardiac muscle.

• Skeletal Muscle (Voluntary Muscle):

  • Also known as voluntary muscle because it is under conscious voluntary control.

  • Generally functions in antagonistic pairs: one muscle contracts to generate force while the opposing muscle relaxes.

    • Arm flexion example: When curling an object upward, the bicep contracts to lift the forearm while the tricep relaxes. The tricep cannot push; it simply relaxes to permit bicep action.

    • Arm extension example: When pushing down, the bicep relaxes while the tricep contracts.

  • Although a few minor exceptions exist, skeletal muscle predominantly operates in pairs.

  • Provides four major physiological functions:

    1. Heat Production: Skeletal muscles are the only structures in the body that produce heat, keeping the body warm.

    2. Protection: Shields internal organs and underlying structures.

    3. Structure: Maintains posture and body framework.

    4. Mobility: Enables body movement.

• Smooth Muscle (Involuntary Muscle):

  • Also known as involuntary muscle because it functions automatically without conscious control.

  • Located in three primary anatomical structures:

    1. Vessels: All blood vessels throughout the body, including arteries, capillaries, and veins.

    2. Bronchial Tree: Air passages within the lungs.

    3. Gastrointestinal (GI) Tract: Continuous pathway extending from the esophagus down to the rectum (including esophagus, stomach, small intestine, and large intestine).

• Cardiac Muscle (Myocardium):

  • The specialized muscle tissue of the heart (derived from the prefix myo-, meaning muscle).

  • Distinct physical appearance and microscopic structure compared to skeletal and smooth muscle.

  • Exhibits two key functional features:

    1. High Electrical Conductivity: Highly efficient at conducting electrical signals across cardiac tissue to coordinate heartbeats.

    2. Automaticity: The intrinsic ability of cardiac muscle tissue to generate its own electrical impulse independently.

  • The central nervous system and chemical signals regulate the firing rate, but the electricity itself originates directly within the cardiac tissue.

  • Heart tissue continues to generate electrical impulses even when removed entirely from the body.

Practical Example of Automaticity: Sand Shark Case

• Case Details and Timeline:

  • During a fishing trip, a small sand shark was caught and placed inside a fish box.

  • Barbecue charcoal was prepared using charcoal lighter fluid, requiring a process of $30\text{ to }40\text{ minutes}$ for the coals to become hot, red, and golden.

  • The sand shark had been out of the water for approximately $45\text{ minutes}$ prior to preparation.

  • Being out of the water for $45\text{ minutes}$ induces severe hypoxia in the fish, equivalent to a human being submerged underwater for $45\text{ minutes}$.

• Observation of Cardiac Automaticity:

  • When carving the shark into steaks on the gut table, its heart—approximately the size of a ping-pong ball—was set to the side.

  • Despite being removed from the body and severely hypoxic, the isolated heart continued beating spontaneously at a rate of approximately $6\text{ to }8\text{ times per minute}$.

  • The atrium and ventricles continued their coordinated cycle of contraction and relaxation on the bench, demonstrating cardiac automaticity.

Respiratory System Fundamentals: Ventilation vs. Respiration

• Primary Function:

  • The primary role of the respiratory system is external respiration (exchanging gases with the external environment).

• Definitions and Distinction:

  • Respiration: The metabolic process of gas exchange across cellular or alveolar membranes.

  • Ventilation: The physical movement of air into and out of the body (e.g., performing positive pressure ventilation using a Bag-Valve Mask / BVM on a patient or training mannequin).

  • Clinical Distinction: A biologically dead individual (whose tissues have completely ceased functioning) can be physically ventilated, but zero respiration occurs because metabolic gas exchange is absent.

  • Documentation Note: While terms such as "respiratory rate" are standard in medical documentation, the physical measurement counts ventilatory cycles.

Gas Composition of Inhalation and Exhalation

• Inhaled Air Composition:

  • Nitrogen ($\text{N}_2$): $78\text{\%}$ (inert gas; acts purely as a space filler/placeholder; not absorbed, produced, or utilized by the body).

  • Oxygen ($\text{O}_2$): $21\text{\%}$.

  • Other gases: $1\text{\%}$.

  • Total: $100\text{\%}$.

• Exhaled Air Composition:

  • Nitrogen ($\text{N}_2$): $78\text{\%}$ (remains unchanged).

  • Oxygen ($\text{O}_2$): $16\text{\%}$.

  • Other gases: $1\text{\%}$.

  • Carbon Dioxide ($\text{CO}_2$): $5\text{\%}$.

  • Total: $100\text{\%}$.

• External Gas Exchange Summary:

  • The body absorbs $5\text{\%}$ of oxygen ($\text{O}_2$) from atmospheric air and releases $5\text{\%}$ of carbon dioxide ($\text{CO}_2$) back into the atmosphere.

Anatomy and Structure of the Airway

• Upper Airway Entry:

  • Airway originates at the tip of the nose (nasopharynx) and mouth (oropharynx).

• Trachea:

  • Rigid tube located inferior to the larynx and Adam's apple.

  • Structurally supported by calcified cartilage rings (tracheal rings) that keep the airway open under external pressure.

• Carina and Main Stem Bronchi:

  • Carina: The anatomical point at the inferior end of the trachea where it bifurcates into the right and left main stem bronchi.

  • Anatomical Asymmetry:

    • The right main stem bronchus extends almost straight down vertically.

    • The left main stem bronchus branches off at a sharper angle to clear the heart.

  • Clinical Significance:

    • If an endotracheal tube is advanced too deep past the carina during intubation, it will almost always enter the right main stem bronchus.

    • Aspirated foreign objects (such as inhaled airsoft pellets or food particles) predominantly enter the right lung.

  • The main stem bronchi are short and reinforced with calcified cartilage rings similar to the trachea.

• Bronchial Tree and Smooth Muscle Spasms:

  • Beyond the main stem bronchi, airways branch repeatedly through extensive miles of passageways within the lung tissue.

  • All lower divisions of the bronchial tree lack calcified rings and consist entirely of smooth muscle.

  • Because the trachea and main stem bronchi contain rigid calcified cartilage, they cannot constrict.

  • The lower miles of smooth muscle airways are susceptible to spasms and constriction (e.g., airway constriction during an asthma attack).

Alveoli and Surface Area

• Alveolar Structure and Function:

  • The bronchial tree terminates at the alveoli, microscopic hollow air sacs.

  • Respiration (gas exchange) occurs exclusively within the alveoli. Zero gas exchange takes place within the conductile airways or bronchial tree.

  • Alveolar walls are semi-permeable, allowing gas molecules to pass back and forth while containing fluid and cellular components.

  • Coffee Filter Analogy: Functionally similar to a coffee filter, which permits fluid, caffeine, oils, and flavor molecules to pass through while keeping solid grounds inside the filter basket.

• Quantity and Surface Area:

  • The average adult human lungs contain approximately $700{,}000{,}000$ alveoli.

  • Lung tissue is lightweight and spongy, composed of open air spaces and vascular networks.

  • If all $700{,}000{,}000$ alveoli were flattened and laid side-by-side, the combined surface area would cover half of a tennis court.

Cilia and Debris Removal

• Definition and Function:

  • Cilia: Microscopic, hair-like fibers lining the internal surface of the bronchial tree and nasal passages.

  • Move continuously in a synchronized, wave-like motion.

  • Function to sweep inhaled dust, pollen, and foreign particulate matter upward out of the $700{,}000{,}000$ alveoli and bronchial tree.

  • Trapped particulate debris is elevated to the epiglottis, where it is swallowed or coughed out.

  • Cilia in the nasal passages move trapped dust forward, forming nasal mucus (boogers).

Pleural Cavity, Surface Tension, and Lung Dynamics

• Pleural Membranes:

  • Visceral Pleura: A smooth, flexible, glass-like membrane covering the external surface of the lungs.

  • Parietal Pleura: An identical smooth membrane lining the interior wall of the thoracic cavity.

• Mechanism of Surface Tension:

  • A thin layer of pleural fluid coats the space between the visceral and parietal pleura.

  • The surface tension of the fluid adheres the two smooth membranes together, bonding the lungs to the inner chest wall.

  • Glass Plate Analogy: Similar to placing water between two flat, smooth glass plates. The surface tension prevents them from being pulled straight apart, forcing them to adhere, while still allowing them to slide across one another.

• Chest Trauma and Lung Collapse:

  • Lung tissue is inherently elastic and recoils naturally.

  • When a penetrating chest wound breaks the surface tension (e.g., a screwdriver puncturing the chest wall), atmospheric air enters the pleural space.

  • Loss of surface tension causes the elastic lung to shrink and collapse to approximately $40\text{\%}$ of its expanded volume.

Mechanics of Ventilation

• Pressure-Volume Relationship:

  • Altering the volume of a closed container holding gas inversely alters the gas pressure.

  • Decreasing volume increases pressure (e.g., forcing $460\text{ liters}$ of oxygen gas into a small cylinder causes internal pressure to skyrocket).

  • Increasing volume decreases pressure.

• Anatomy of Ventilatory Muscles:

  • Intercostal Muscles: Small muscle groups located between individual ribs.

  • Diaphragm: A dome-shaped muscle approximately $\frac{1}{4}\text{ inch}$ thick that separates the thoracic cavity (lungs) from the abdominal cavity (stomach and liver, located directly inferior).

• Inhalation Sequence (Negative Pressure Ventilation):

  1. The brain signals the respiratory muscles to contract.

  2. The diaphragm contracts and flattens downward from its resting dome shape.

  3. Intercostal muscles contract, pulling the rib cage upward and outward.

  4. Thoracic volume increases, decreasing internal lung pressure below atmospheric pressure (creating negative pressure).

  5. Air flows down the pressure gradient into the lungs until internal and atmospheric pressures equalize.

• Exhalation Sequence (Positive Pressure Ventilation):

  1. The brain stops motor signals, causing the diaphragm and intercostal muscles to relax.

  2. The diaphragm moves upward back into its resting dome shape.

  3. The chest wall recoils inward, aided by the natural elasticity of the lung tissue.

  4. Thoracic volume decreases, raising internal lung pressure above atmospheric pressure (creating positive pressure).

  5. Air flows out of the lungs into the environment until pressures equalize.

• Diaphragmatic Spasms:

  • A hiccough (hiccup) is an involuntary spasm of the diaphragm muscle.

Questions & Discussion

• Q: Do muscles push or pull?

  • A: Muscles can only produce force by contracting. They cannot push. Skeletal muscles operate in opposing pairs, where one contracts and the other relaxes.

• Q: What are the primary locations of smooth muscle?

  • A: Blood vessels (arteries, veins, capillaries), the bronchial tree, and the gastrointestinal tract (from esophagus to rectum).

• Q: What is the exact anatomical path of GI smooth muscle?

  • A: Extends continuously from the esophagus through the stomach, small intestine, large intestine, and rectum.

• Q: What does semi-permeable mean in relation to alveoli?

  • A: It means the membrane allows certain molecules (gases like oxygen and carbon dioxide) to pass through while preventing other substances from crossing, similar to a coffee filter.

• Q: Does the diaphragm flatten during exhalation?

  • A: No. The diaphragm flattens during inhalation when it contracts to enlarge the thoracic cavity and draw air in. During exhalation, the diaphragm relaxes and returns upward to its dome shape.