Respiratory System Notes

Overview: The Respiratory System

• The respiratory system facilitates gas exchange, primarily taking in oxygen (O₂) and expelling carbon dioxide (CO₂).
• Oxygen is essential for cellular respiration, which produces ATP (energy), while carbon dioxide is a waste product that needs to be eliminated.

Major Structures of the Human Respiratory System

• 1. Nasal Cavity / Nose
  • Warms, moistens, and filters air using cilia and mucus.
  • Hairs and mucus trap dust and microbes.
  • Air can also enter through the mouth, though it's less effective for filtering.
• 2. Pharynx (Throat)
  • A shared passageway for both air and food.
  • Connects the nasal cavity to the larynx.
  • The epiglottis, a flap of cartilage, closes over the trachea during swallowing to prevent choking.
• 3. Larynx (Voice Box)
  • Contains vocal cords that vibrate, producing sound.
  • Made of cartilage and includes the Adam’s apple.
  • Directs air into the trachea.
• 4. Trachea (Windpipe)
  • Carries air from the larynx to the bronchi.
  • Lined with cilia and mucus, forming the mucociliary escalator that traps and moves particles upward.
  • Supported by C-shaped rings of cartilage to maintain its openness.
• 5. Bronchi (Left and Right)
  • Two large tubes that branch from the trachea into each lung.
  • Continue to branch into smaller tubes called bronchioles.
• 6. Bronchioles
  • Small branches of the bronchi that carry air deeper into the lungs.
  • Lack cartilage but are surrounded by smooth muscle.
  • Terminate in clusters of alveoli.
• 7. Alveoli
  • Tiny air sacs where gas exchange occurs.
  • Surrounded by capillaries.
  • Lined with surfactant, a fluid that reduces surface tension and prevents collapse.
  • Thin-walled (one cell thick) to facilitate fast diffusion.
• 8. Lungs
  • Two large organs (the right lung has 3 lobes, and the left has 2 to accommodate the heart).
  • Contain the bronchi, bronchioles, and alveoli.
  • Protected by a double-layered membrane called the pleura.
• 9. Diaphragm
  • A dome-shaped muscle located under the lungs.
  • Contracts (moves downward) during inhalation.
  • Relaxes (moves upward) to aid exhalation.
• 10. Intercostal Muscles
  • Located between the ribs.
  • Assist in breathing by expanding and contracting the rib cage.

Physiology: How Breathing Works (Mechanism of Breathing)

• Inhalation (Inspiration)
  • The diaphragm contracts and flattens.
  • Intercostal muscles contract, lifting the rib cage.
  • This increases the volume of the thoracic cavity.
  • The pressure in the lungs decreases, falling below atmospheric pressure.
  • Air rushes into the lungs.
• Exhalation (Expiration)
  • The diaphragm and intercostal muscles relax.
  • The rib cage drops, and the diaphragm rises.
  • The volume decreases, causing the pressure in the lungs to increase.
  • Air is pushed out of the lungs.
• Mnemonic: "Inhale = diaphragm moves into the abdomen (contracts)."

Gas Exchange

• External Respiration
  • Occurs in the alveoli.
  • O₂ diffuses from the alveoli into the capillaries (into the blood).
  • CO₂ diffuses from the capillaries into the alveoli (to be exhaled).
• Internal Respiration
  • Occurs in body tissues.
  • O₂ diffuses from the blood into the tissues.
  • CO₂ diffuses from the tissues into the blood.
• Both processes rely on concentration gradients: gases move from areas of high concentration to areas of low concentration via diffusion.

Cellular Respiration Connection

• The oxygen inhaled is used in cellular respiration within mitochondria:
  C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + \sim 36 { ATP}
• This equation shows how the body converts food into energy using the O₂ you breathe.

Transport of Gases in Blood

• Oxygen:
  • 98.5% is carried by hemoglobin in red blood cells, forming oxyhemoglobin.
  • 1.5% is dissolved in plasma.
• Carbon Dioxide:
  • 70% is transported as bicarbonate ions (HCO₃⁻).
  • 20% is bound to hemoglobin, forming carbaminohemoglobin.
  • 10% is dissolved in plasma.

Protective Mechanisms

• Cilia and mucus trap and sweep out dust/microbes.
• Coughing/sneezing clear the airways.
• Alveolar macrophages destroy foreign particles in the alveoli.

Regulation of Breathing

• Controlled by the medulla oblongata in the brainstem.
• The medulla oblongata senses CO₂ levels in the blood (not O₂).
• High CO₂ levels signal the diaphragm and intercostal muscles to increase the breathing rate.
• Hyperventilation, which lowers CO₂ too much, slows breathing.

Common Disorders

• Asthma: Inflammation and narrowing of the bronchioles, causing difficulty breathing and wheezing.
• Emphysema: Damage to the alveoli, reducing surface area and impairing gas exchange.
• Bronchitis: Inflammation of the bronchi, leading to excess mucus production and coughing.
• Pneumonia: Infection causing fluid accumulation in the alveoli, reducing gas exchange.
• Lung Cancer: Uncontrolled cell growth in the lungs, which may block airways or spread.

Comparative Respiratory Systems

• Earthworm:
  • Gas exchange occurs through the skin (cutaneous respiration).
  • Requires moist skin.
  • No lungs; blood transports gases directly.
• Frog:
  • Three methods of respiration:
    1. Skin (cutaneous respiration) – major method when underwater.
    2. Lungs – used when active or on land.
    3. Buccopharyngeal respiration – across the lining of the mouth.
• Human:
  • Specialized lung-based system with internal alveoli.
  • Involves negative pressure breathing.
• Frogs and earthworms both use their skin for respiration, unlike humans who rely entirely on their lungs.

Key Vocabulary

• Alveoli
• Bronchi/Bronchioles
• Diaphragm
• Epiglottis
• Pleura
• Hemoglobin
• Oxyhemoglobin
• Bicarbonate ion
• Mucociliary escalator
• Inhalation / Exhalation
• Gas exchange
• Diffusion
• External vs. Internal respiration
• Medulla oblongata

Sample Questions and Answers

• 1. Describe how oxygen and carbon dioxide are exchanged in the alveoli.
  • Gas exchange occurs by diffusion across thin, moist membranes.
  • Oxygen (O₂) concentration is higher in the alveoli than in the blood, so it diffuses into the blood.
  • Carbon dioxide (CO₂) concentration is higher in the blood than in the alveoli, so it diffuses into the alveoli to be exhaled.
  • Alveoli are surrounded by capillaries and have very thin walls (one cell thick), a large surface area, a moist lining, and surfactant to keep them open. This is called external respiration.
• 2. Explain the role of the diaphragm during inhalation.
  • The diaphragm is a dome-shaped muscle located beneath the lungs.
  • During inhalation, the diaphragm contracts and flattens downward, increasing the volume of the thoracic cavity.
  • As a result, the pressure inside the lungs drops below atmospheric pressure, and air flows into the lungs to equalize the pressure. This is called negative pressure breathing.
• 3. Compare gas exchange in frogs and humans.

*   Frogs can breathe through their skin and mouth lining when inactive or in water. Humans rely entirely on their lungs and have alveoli for fast, efficient gas exchange.

• 4. What happens to CO₂ in the blood?
  • Carbon dioxide (CO₂) produced by body cells is transported in the blood in three main forms:
    1. 70% is converted into bicarbonate ions (HCO₃⁻) by the enzyme carbonic anhydrase inside red blood cells.
    2. 20% binds to hemoglobin, forming carbaminohemoglobin.
    3. 10% dissolves directly in the plasma.
  • In the lungs, the bicarbonate is converted back into CO₂, which diffuses into the alveoli and is exhaled.
• 5. Why does breathing rate increase during exercise?
  • During exercise, muscle cells use more oxygen and produce more carbon dioxide.
  • The increase in CO₂ levels causes the blood to become more acidic (lower pH).
  • Chemoreceptors in the brainstem (medulla oblongata) detect the rising CO₂ levels.
  • The brain signals the diaphragm and intercostal muscles to increase breathing rate and depth.
  • This helps bring in more oxygen to support cellular respiration and remove extra carbon dioxide from the blood.