Respiratory System: Organs, Mechanism, Respiration, and Adaptations - A Comprehensive Guide

This note provides a detailed and intuitive understanding of the human respiratory system, covering its structures, the mechanism of breathing, cellular respiration, gas transport, respiratory volumes, and the effects of environmental factors like altitude, as outlined in the syllabus.

1. Structures of the Respiratory System

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the external environment. It is broadly divided into the upper and lower respiratory tracts.

1.1 Upper Respiratory Tract

• Nose and Nasal Cavity: The primary entry point for air. The nasal cavity is lined with mucous membranes and cilia (tiny hairs) that filter, warm, and humidify incoming air. It also contains olfactory receptors for the sense of smell.
• Pharynx (Throat): A muscular tube that serves as a passageway for both air (from the nasal cavity) and food (from the mouth). It connects the nasal cavity and mouth to the larynx and esophagus.
• Larynx (Voice Box): Located at the top of the trachea, it contains the vocal cords, which vibrate to produce sound as air passes through. The epiglottis, a flap of cartilage, covers the opening of the larynx during swallowing to prevent food from entering the airway.

1.2 Lower Respiratory Tract

• Trachea (Windpipe): A rigid tube extending from the larynx to the bronchi. It is supported by C-shaped rings of cartilage that prevent it from collapsing. It is lined with ciliated epithelium and mucus-producing cells to trap foreign particles.
• Bronchi: The trachea divides into two main bronchi (left and right), each leading to a lung. These further subdivide into smaller and smaller bronchioles, forming a branching network called the bronchial tree.
• Bronchioles: Small airways that branch off the bronchi and lead to the alveoli. Their walls contain smooth muscle that can constrict or dilate to regulate airflow.
• Lungs: The primary organs of respiration, located in the thoracic cavity and protected by the rib cage. The right lung has three lobes, and the left lung has two (to accommodate the heart).
• Alveoli (Air Sacs): Tiny, thin-walled air sacs at the end of the bronchioles. They are surrounded by a dense network of capillaries and are the primary sites of gas exchange.
• Pleura: A double-layered membrane that surrounds each lung. The space between the two layers (pleural cavity) contains pleural fluid, which lubricates the surfaces and allows the lungs to slide smoothly against the chest wall during breathing.

2. Mechanism of Breathing (Ventilation)

Breathing, or pulmonary ventilation, is the process of moving air into (inspiration) and out of (expiration) the lungs. It is primarily driven by changes in pressure within the thoracic cavity, which are brought about by the contraction and relaxation of respiratory muscles.

2.1 Role of Diaphragm and Intercostal Muscles

• Diaphragm: A large, dome-shaped muscle located at the base of the thoracic cavity, separating it from the abdominal cavity. It is the primary muscle of respiration.
  • Inspiration: When the diaphragm contracts, it flattens and moves downward, increasing the vertical dimension of the thoracic cavity.
• Intercostal Muscles: Muscles located between the ribs.
  • External Intercostal Muscles: Contract during inspiration, pulling the rib cage upward and outward, increasing the anterior-posterior and lateral dimensions of the thoracic cavity.
  • Internal Intercostal Muscles: Contract during forced expiration, pulling the rib cage downward and inward.

2.2 The Breathing Process

• Inspiration (Inhaling):
  1. The diaphragm contracts and moves downward.
  2. The external intercostal muscles contract, pulling the ribs upward and outward.
  3. These actions increase the volume of the thoracic cavity.
  4. As the volume increases, the pressure inside the lungs (intrapulmonary pressure) decreases, becoming lower than the atmospheric pressure.
  5. Air flows from the higher atmospheric pressure into the lungs (lower pressure) until the pressures equalize.
• Expiration (Exhaling):
  1. During quiet breathing, the diaphragm and external intercostal muscles relax.
  2. The diaphragm moves upward, and the rib cage moves downward and inward due to the elastic recoil of the lungs and chest wall.
  3. These actions decrease the volume of the thoracic cavity.
  4. As the volume decreases, the pressure inside the lungs increases, becoming higher than the atmospheric pressure.
  5. Air flows from the lungs (higher pressure) out into the atmosphere (lower pressure) until the pressures equalize.
  • Forced expiration involves the contraction of internal intercostal muscles and abdominal muscles to further decrease thoracic volume and expel more air.

3. Tissue Respiration (Cellular Respiration) and Heat Production

Tissue respiration, also known as cellular respiration, is the metabolic process by which cells break down organic molecules (primarily glucose) to release energy in the form of ATP (adenosine triphosphate). This process occurs mainly in the mitochondria of cells.

• Process: Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP) + Heat
• Heat Production: Cellular respiration is an exothermic process, meaning it releases energy. While a significant portion of this energy is captured as ATP for cellular activities, a substantial amount (approximately 60%) is released as heat. This heat is crucial for maintaining the body's core temperature (thermoregulation) in warm-blooded animals.

4. Differences Between Anaerobic Respiration in Plants and Humans

Anaerobic respiration occurs in the absence of oxygen and produces less ATP compared to aerobic respiration because glucose is only partially broken down.

• Anaerobic Respiration in Plants (Alcoholic Fermentation):
  • Conditions: Occurs in plant cells when oxygen is scarce (e.g., in waterlogged soils, or in yeast during fermentation).
  • Equation: Glucose → Ethanol + Carbon Dioxide + Small amount of Energy (ATP)
  • End Products: Ethanol (ethyl alcohol) and carbon dioxide. Ethanol is toxic to plant cells if it accumulates.
• Anaerobic Respiration in Humans (Lactic Acid Fermentation):
  • Conditions: Occurs in muscle cells during intense physical activity when oxygen supply cannot meet the demand.
  • Equation: Glucose → Lactic Acid + Small amount of Energy (ATP)
  • End Product: Lactic acid. Accumulation of lactic acid in muscles can lead to muscle fatigue and cramps. It is later transported to the liver and converted back to glucose or oxidized.

5. Gaseous Transport

The efficient transport of oxygen and carbon dioxide between the lungs and body tissues is vital for survival. Blood serves as the primary transport medium.

• Oxygen Transport:
  • Hemoglobin: The vast majority (about 97%) of oxygen is transported bound to hemoglobin, a protein found in red blood cells, forming oxyhemoglobin. Each hemoglobin molecule can bind to four oxygen molecules.
  • Dissolved in Plasma: A small percentage (about 3%) of oxygen is transported dissolved directly in the blood plasma.
  • Mechanism: In the lungs, where oxygen concentration is high, oxygen binds to hemoglobin. In the body tissues, where oxygen concentration is low (due to cellular respiration), oxygen dissociates from hemoglobin and diffuses into the cells.
• Carbon Dioxide Transport:
  • Bicarbonate Ions (HCO3-): The most significant way (about 70%) carbon dioxide is transported. CO2 reacts with water in red blood cells to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. Bicarbonate ions are transported in the plasma.
  • Carbaminohemoglobin: About 20-25% of CO2 binds directly to hemoglobin (at a different site than oxygen), forming carbaminohemoglobin.
  • Dissolved in Plasma: About 7-10% of CO2 is transported dissolved directly in the blood plasma.
  • Mechanism: In the tissues, where CO2 concentration is high, CO2 diffuses into the blood. In the lungs, where CO2 concentration is low, these processes reverse, and CO2 diffuses out of the blood into the alveoli to be exhaled.

6. Respiratory Volumes and Capacities

Respiratory volumes and capacities refer to the amount of air that can be moved into and out of the lungs, or the amount of air remaining in the lungs at different points of the respiratory cycle. These measurements are important for assessing lung function.

• Tidal Volume (TV): The volume of air inhaled or exhaled during a normal, quiet breath (approximately 500 mL).
• Inspiratory Reserve Volume (IRV): The maximum volume of air that can be forcibly inhaled after a normal tidal inspiration (above TV).
• Expiratory Reserve Volume (ERV): The maximum volume of air that can be forcibly exhaled after a normal tidal expiration (below TV).
• Residual Volume (RV): The volume of air that always remains in the lungs even after a maximal forced exhalation. This air cannot be exhaled and prevents the lungs from completely collapsing.
• Vital Capacity (VC): The maximum volume of air that can be exhaled after a maximal inhalation (VC = TV + IRV + ERV). It represents the total amount of exchangeable air.
• Total Lung Capacity (TLC): The maximum amount of air the lungs can hold after a maximal inspiration (TLC = VC + RV).
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal tidal expiration (FRC = ERV + RV).

7. Effect of Altitude on Breathing

At high altitudes, the atmospheric pressure is lower, which means the partial pressure of oxygen (PO2) is also lower. This makes it more challenging for the body to take in sufficient oxygen, leading to physiological adaptations.

• Immediate Responses (Acclimatization):
  • Increased Breathing Rate (Hyperventilation): The body responds by increasing the rate and depth of breathing to compensate for the reduced oxygen availability. This helps to increase oxygen intake and carbon dioxide expulsion.
  • Increased Heart Rate: The heart beats faster to circulate blood more rapidly, attempting to deliver more oxygen to the tissues.
• Long-Term Adaptations:
  • Increased Red Blood Cell Production: Over time, the kidneys release erythropoietin, stimulating the bone marrow to produce more red blood cells. This increases the oxygen-carrying capacity of the blood.
  • Increased Capillary Density: The body may develop more capillaries in tissues to improve oxygen delivery to cells.
  • Increased Myoglobin: Muscles may produce more myoglobin, a protein that stores oxygen in muscle cells.

8. Asphyxiation and Hypoxia

Both terms relate to insufficient oxygen, but they describe different aspects of the problem.

• Asphyxiation (Suffocation):
  • Definition: A condition caused by a severe deficiency of oxygen in the blood and tissues, typically resulting from an interference with normal breathing. It implies an interruption of the breathing process itself.
  • Causes: Can be caused by airway obstruction (e.g., choking, strangulation), drowning, being in an environment with insufficient oxygen (e.g., confined spaces, toxic gases), or conditions that prevent respiratory muscle movement.
  • Consequences: Leads to a buildup of carbon dioxide and a lack of oxygen, which can rapidly cause unconsciousness, brain damage, and death if not quickly resolved.
• Hypoxia:
  • Definition: A condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. It refers to the state of low oxygen in tissues, regardless of the cause.
  • Causes: Can result from various factors, including:
    • Hypoxemic Hypoxia: Low oxygen in the blood (e.g., high altitude, lung diseases like pneumonia, emphysema).
    • Anemic Hypoxia: Reduced oxygen-carrying capacity of the blood (e.g., anemia, carbon monoxide poisoning).
    • Ischemic (Stagnant) Hypoxia: Inadequate blood flow to tissues (e.g., heart failure, shock, localized blockages).
    • Histotoxic Hypoxia: Tissues are unable to utilize oxygen effectively (e.g., cyanide poisoning).
  • Symptoms: Can range from mild (headache, dizziness, shortness of breath) to severe (confusion, cyanosis, loss of consciousness, organ damage).

This detailed explanation aims to provide a clear and comprehensive understanding of the respiratory system and related concepts, serving as a thorough study guide.