Notes on The Respiratory System in Humans

Introduction to the Respiratory System

  • respiration is a process by which an organism takes up oxygen and uses it to oxidise food substances and release carbon dioxide, water vapour and energy.
  • In humans, the lungs are the primary organs for gas exchange; the respiratory tract conducts air to and from the lungs.
  • The respiratory system includes the lungs plus the respiratory tract and associated muscles.

Parts of the Respiratory System

  • Nose and nasal cavity
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi (singular: bronchus)
  • Bronchioles
  • Alveoli (singular: alveolus)
  • Lungs

Nose

  • Air enters the body through the nose via two external nostrils; internal passage leads to the nasal cavity.
  • The nasal cavity lining secretes mucus that traps harmful organisms; nasal hairs filter entry of particles.
  • Air is warmed and moistened as it passes through the nasal passage.
  • Functions:
    • Prevent entry of harmful substances
    • Warm and moisten air
    • Aid in the sense of smell

Pharynx

  • The nasal cavity leads into the pharynx, which is the common chamber for both food and air.
  • The pharynx leads into the larynx.
  • The opening of the pharynx into the larynx is guarded by the epiglottis.
  • During swallowing, the epiglottis closes the opening to prevent food from entering the windpipe.

Larynx (Voice Box)

  • A rectangular chamber containing vocal cords made of cartilage.
  • When air passes over these cords, they vibrate to produce sound; hence, the larynx is also called the voice box.

Trachea (Wind Pipe)

  • A about 12 cm muscular tube with C-shaped rings of cartilage.
  • The lumen remains open because of these cartilage rings; they support the trachea and allow movement during breathing.

Bronchi

  • The trachea divides into two main bronchi that enter each lung.
  • Cartilaginous rings strengthen the walls of the bronchi.

Bronchioles

  • Each bronchus divides into smaller branches called bronchioles.
  • Cartilage is absent in bronchioles.

Alveoli

  • Bronchioles end in air sacs called alveoli; each alveolus is surrounded by a dense network of capillaries.
  • The wall of the alveolus is very thin and lined with moisture.
  • Gaseous exchange occurs by diffusion across the thin membrane of the alveolar wall.
  • The alveolus is surrounded by blood capillaries carrying blood that is rich in carbon dioxide from body tissues.
  • Oxygen from inhaled air diffuses into the blood through the moist, thin alveolar membrane.
  • The exchange takes place here because the capillaries envelop the alveoli; CO₂ moves from blood into the alveolus to be exhaled; O₂ moves from alveolar air into the blood.
  • The alveolar surface is coated with a thin film of moisture to allow gases to dissolve and diffuse.
  • The alveolar gas exchange is a critical step in respiration.

Cilia and Mucus

  • Cells lining most of the respiratory tract (except the alveoli) have cilia (hair-like projections).
  • Movement of cilia helps to trap and remove unwanted particles from the tract.

Gas Exchange Details

  • Blood vessels bring blood to the lungs that has CO₂ from tissues.
  • The alveolar walls are a single layer thick and envelope by a network of capillaries.
  • CO₂ moves from blood in the capillaries into the alveolus to be exhaled; O₂ from the alveolar air moves into blood.
  • The exchange is driven by differences in partial pressures of O₂ and CO₂ between alveolar air and blood.
  • Inhaled air contains a high O₂ concentration and a relatively low CO₂ concentration compared with the blood arriving at the alveoli.

Lungs and Pleura

  • The lungs are a pair of spongy, air-filled organs located in the chest (thorax) on either side of the heart.
  • The lungs are protected by the rib cage and pleural membranes.
  • Right lung: 3 lobes; Left lung: 2 lobes (to accommodate the heart).
  • The pleura consist of two membranes: the parietal pleura (lining the chest wall) and the visceral pleura (covering the lungs).
  • The pleural cavity between these membranes contains pleural fluid that reduces friction during breathing.
  • The diaphragm and intercostal muscles assist in breathing by changing the volume of the thoracic cavity.

Breathing and Respiration

  • Breathing (inspiration and expiration) is a mechanical process; respiration is a chemical process of gas exchange that occurs at the cellular level.
  • Even a single-celled organism respires, using gas exchange to release energy from nutrients.
  • Inhalation (inspiration) and exhalation (expiration) involve changes in thoracic cavity volume.

Inspiration (Inhalation)

  • Diagrammatic sequence:
    • Intercostal muscles contract.
    • Diaphragm contracts and moves downwards.
    • Rib cage moves up and out.
    • Volume of the thoracic cavity increases.
    • Pressure in the thoracic cavity falls below atmospheric pressure; air flows into the lungs.

Expiration (Exhalation)

  • Diagrammatic sequence:
    • Intercostal muscles relax.
    • Diaphragm relaxes and becomes arched upwards.
    • Rib cage moves down and in.
    • Volume of the thoracic cavity decreases.
    • Pressure in the thoracic cavity rises above atmospheric pressure; air flows out of the lungs.

Respiratory System Experiment

  • Aim: Demonstrate the role of the diaphragm in breathing.
  • Apparatus: Bell jar, rubber sheet, two rubber balloons, two Y tubes.
  • Procedure: Pull the rubber sheet down and release; observe balloon behavior.
  • Observation: Pulling the sheet down inflates balloons; pushing up causes balloons to deflate.
  • Inference:
    • When the rubber sheet is pulled down, the volume inside the bell jar increases and pressure drops below atmospheric; outside air rushes into the balloons (inspiration).
    • When released, the volume decreases and pressure rises; air is expelled (expiration).
    • This shows that the chest cavity volume and lung pressure drive inspiration and expiration.
  • Correspondence:
    • Chest cavity ↔ Lungs
    • Bell jar ↔ Lungs
    • Y tube ↔ Trachea/Bronchi
    • Rubber sheet ↔ Diaphragm

Some Common Respiratory Diseases

  • Asthma: Narrowed air passages due to spasms of smooth muscles in bronchi/bronchioles and excessive mucus production; coughing, difficulty breathing, wheezing; often related to allergies or psychosomatic factors.
  • Bronchitis: Inflammation of the bronchi; cough with greenish-yellow sputum in later stages; influenced by allergy, smoking, heredity, air pollution, lack of immunity.
  • Pneumonia: Acute infection/inflammation of the lungs, commonly caused by Streptococcus pneumoniae.
  • Tuberculosis: Infectious disease caused by Mycobacterium tuberculosis; bacteria destroy lung tissue, replaced by inelastic fibrous tissue; diffusion becomes harder as fibrous tissue reduces gas exchange.

Summary of Key Points

  • The respiratory system comprises lungs, air passages, and respiratory muscles such as the diaphragm and intercostal muscles.
  • The respiratory tract includes the nose, mouth, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
  • The nose filters, warms, and moistens inhaled air and provides the sense of smell.
  • The pharynx is the shared passage for air and food; the epiglottis prevents food from entering the windpipe during swallowing.
  • The larynx contains vocal cords and serves as the voice box.
  • The trachea is a sturdy tube supported by C-shaped cartilage rings to remain open during breathing.
  • The bronchi branch into bronchioles; cartilage is absent in bronchioles.
  • Alveoli are tiny air sacs where gas exchange occurs; they are enveloped by a dense network of capillaries and lined with moisture for diffusion.
  • Cilia help keep the respiratory tract clean by moving mucus and trapped particles out.
  • Blood in the lung capillaries carries CO₂ from body tissues to the alveoli for exhalation and receives O₂ from the alveoli to supply the body.
  • The lungs differ in lobes: right lung = 3 lobes; left lung = 2 lobes due to heart placement.
  • Pleural membranes surround the lungs, forming the pleural cavity with pleural fluid to reduce friction during breathing.
  • Inspiration increases thoracic volume and decreases pressure, drawing air in; expiration decreases volume and increases pressure, pushing air out.
  • Breathing is mechanical; respiration is chemical gas exchange at the cellular level.
  • Common respiratory diseases include asthma, bronchitis, pneumonia, and tuberculosis.

Chapter Challenge and Study Questions (Key Points)

  • Diagram tasks (pharynx location, epiglottis, diaphragm position, pleural membrane, alveoli, right/left lungs) reinforce anatomy.
  • True/False quick checks:
    • Inspired air is warmer than expired air. (True near tissue warming; but after passing through nasal cavity, inhaled vs exhaled air temperature varies with conditions.)
    • Chest muscles relax during expiration. (True in passive expiration.)
    • Expired air contains more CO₂ than inspired air. (True.)
    • Bronchioles are made of C-shaped cartilage rings. (False; cartilage is absent in bronchioles.)
    • The respiratory tract begins at the mouth. (Partially true; it begins at the nose and mouth entry, but the nasal route is primary for air entry.)
  • Diagram labeling and pathway tracing are useful to test understanding of airflow from nose to alveoli.

Exercises (Key Concepts)

  • Why do organisms respire? To obtain energy from nutrients via oxidative processes and to release CO₂ and H₂O.
  • Name the special organ in mammals that enables breathing: the lungs.
  • Fill in terms:
    • The tubes that lead air into the lungs: bronchi (aka bronchioles as subdivisions).
    • A muscular flap that prevents food from entering the wind pipe during swallowing: epiglottis.
    • The common chamber for air and food: pharynx.
    • The ultimate passages of the respiratory tract: bronchioles and alveoli (gas exchange units in the lungs).
    • The process by which gaseous exchange occurs across the cell membrane: respiration (gas exchange/oxidative metabolism is part of cellular respiration).
    • The protective membranes that envelop the lungs: pleural membranes (parietal and visceral pleura).
  • State one important function of the following:
    • Nose: air filtering, warming, moistening, smell.
  • Differentiate between:
    • Larynx vs Rib cage
    • Respiration vs Breathing
    • Right lung vs Left lung
    • Inhalation vs Exhalation
  • Study the diagram and answer: label the Diaphragm and Rib cage, identify the dotted line X, compare thoracic cavity size in diagrams A and B, indicate air flow direction in diagram B, name the breathing mechanism in diagram B, etc.
  • True/False recap after diagrammatic study:
    • The pharynx is also called the voice box. (False; voice box = larynx.)
    • Cilia help keep unwanted particles out of the respiratory tract. (True.)
    • During expiration, the diaphragm arches upwards. (True.)
    • Breathing is a chemical process. (False; breathing is mechanical; respiration is chemical.)
    • The lungs are protected by pleural membranes. (True.)

Diagram-Related Clarifications

  • Cartilage rings in trachea help keep the airway open and allow flexibility during breathing.
  • The mucus and cilia system traps and removes debris and pathogens from the airways.
  • The alveoli’s thin walls and moist lining facilitate efficient diffusion of gases between air and blood.
  • The pleural membranes create a sealed, lubricated environment that enables smooth lung movement during breathing.

Practical Implications

  • Understanding the respiratory system aids in recognizing symptoms and causes of common diseases (asthma, bronchitis, pneumonia, tuberculosis).
  • Knowledge of how inspiration and expiration work supports understanding of conditions affecting breathing, including how diaphragmatic motion and chest wall mechanics may be impacted by health or injury.
  • Clean airways and minimizing irritants (pollution, cigarette smoke) are important for maintaining healthy gas exchange.