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1. CARS
2. Psych/soc
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3.1 Structure and function of proteins and their constituent amino acids
3.2 Transmission of genetic information from the gene to the protein
3.3 Heredity and genetic diversity
3.4 Principles of bioenergetics and fuel molecule metabolism
3.5 Assemblies of molecules, cells, groups of cells
3.6 Structure and physiology of prokaryotes and viruses
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3.8 Structure and functions of nervous and endocrine systems
3.9 Structure and functions of main organ systems
3.9.1 Circulatory system
3.9.2 Digestive system
3.9.3 Excretory and reproductive systems
3.9.4 Muscular and skeletal systems
3.9.5 Respiratory system
3.9.6 Integumentary (skin, hair, nails) system
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4. Chem/phys
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3.9.5 Respiratory system
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3. Bio/biochem
3.9. Structure and functions of main organ systems

Respiratory system

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General function/physiology

The gas exchange process in the lungs depends on diffusion: oxygen moves into the bloodstream, and carbon dioxide moves out of it. This movement is driven by Henry’s law, which states that a gas will dissolve in a liquid until it reaches an equilibrium concentration.

Blood arriving at the lungs has relatively low oxygen because body tissues have used it, so oxygen diffuses from the air in the lungs into the blood. At the same time, this blood carries relatively high carbon dioxide, so carbon dioxide diffuses from the blood into the air spaces of the lungs to be exhaled.

Thermoregulation is also influenced by breathing, because exhaling warm, moist air contributes to body heat loss.

Protection against disease

The respiratory system uses several protective mechanisms against pathogens and particulate matter:

Nostril hair acts as an initial filter, catching larger particles before they travel deeper into the airways.

Once inhaled, mucus produced by Goblet cells - specialized, columnar epithelial cell lining the respiratory tract - traps pathogens and finer particles. The trapped material is then moved out by cilia, tiny hair-like structures that sweep contaminants toward the throat, where they are either expelled or swallowed and neutralized by stomach acid.

Pseudostratified ciliated columnar epithelium of respiratory tract with mucus glands
Pseudostratified ciliated columnar epithelium of respiratory tract with mucus glands

In the alveoli, macrophages provide a final line of defense by engulfing and destroying foreign substances that escape the cilia and reach these delicate air sacs.

Structure of lungs and alveoli

Air enters through the nasal cavity, where it is warmed, humidified, and filtered by hair, mucus, and cilia. It then passes through the pharynx and the larynx into the trachea, a tube about 10-12 cm long. The trachea is supported by hyaline cartilage rings and smooth muscle that can contract during forced exhalation (e.g., coughing).

The trachea splits into two primary bronchi, each entering a lung. Within the lungs, these bronchi branch into secondary and tertiary bronchi, then into smaller bronchioles, which rely on air pressure (rather than cartilage) to stay open.

The lungs differ in size: the right lung has three lobes, whereas the left lung has two. Below them lies the diaphragm, a muscular boundary at the base of the thoracic cavity. When it contracts, it lowers and expands the chest volume, helping drive inhalation.

At the end of each bronchiole, alveolar ducts lead to clusters of alveoli, which resemble tiny bubbles arranged in alveolar sacs. Each alveolus is surrounded by capillaries; both are one cell thick, allowing gas exchange (oxygen in, carbon dioxide out) across their thin walls. With around 300 million alveoli per lung creating a total surface area of ~75 m², this structure maximizes oxygen delivery to the bloodstream and removal of carbon dioxide from it, giving the lungs their soft, sponge-like texture.

Mammalian respiratory system showing airflow from nasal cavity to alveoli
Mammalian respiratory system showing airflow from nasal cavity to alveoli

Breathing mechanisms

Pulmonary ventilation showing inspiration and expiration with thoracic cavity movement
Pulmonary ventilation showing inspiration and expiration with thoracic cavity movement

Diaphragm, rib cage, differential pressure

The diaphragm is a dome-shaped muscle. When it contracts, it moves downward, increasing chest volume and creating negative pressure that draws air into the lungs. At the same time, the rib cage expands outward - supported by intercostal muscles - and helps maintain lung volume even at rest.

Airflow is driven by differential pressure. The pressure inside the lungs (intrapulmonary) moves toward atmospheric levels, while the intrapleural pressure remains lower. This lower intrapleural pressure helps pull the lungs open and prevents collapse.

Resiliency and surface tension effects

A key feature of lung resiliency is elastic recoil. Without the outward support of the rib cage, the lungs would collapse further after exhalation.

Surface tension in the alveoli also promotes collapse. To counter this, the alveoli produce surfactants that reduce surface tension and help keep the air sacs open.

Alveolar gas exchange and pH control

In alveolar gas exchange, oxygen diffuses into blood and carbon dioxide diffuses out. This is guided by differences in partial pressures and can be described quantitatively by Henry’s Law.

Gas movement also connects to pH control, because elevated CO2​ in the blood can lower pH.

Nervous system control

The body’s nervous control responds by adjusting breathing rate, reflecting its CO2​ sensitivity.

Together, these coordinated mechanisms support oxygen delivery and carbon dioxide removal while also helping regulate acid-base balance.

General function/physiology

  • Gas exchange via diffusion: oxygen in, carbon dioxide out
  • Driven by Henry’s law (gas dissolves until equilibrium concentration)
  • Breathing aids thermoregulation (exhalation releases heat and moisture)

Protection against disease

  • Nostril hair filters large particles
  • Mucus (from Goblet cells) and cilia trap and move contaminants out
  • Alveolar macrophages engulf and destroy foreign substances in alveoli

Structure of lungs and alveoli

  • Air pathway: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
  • Trachea: supported by hyaline cartilage rings and smooth muscle
  • Lungs: right (3 lobes), left (2 lobes); diaphragm below for inhalation
  • Alveoli: clusters at bronchiole ends, surrounded by capillaries (one cell thick) for gas exchange
    • ~300 million alveoli per lung, ~75 m² surface area

Breathing mechanisms

  • Diaphragm, rib cage, differential pressure

    • Diaphragm contraction increases chest volume, creates negative pressure
    • Rib cage expands with intercostal muscles
    • Intrapleural pressure lower than intrapulmonary, prevents lung collapse

  • Resiliency and surface tension effects

    • Elastic recoil provides lung resiliency
    • Surface tension in alveoli countered by surfactant to prevent collapse

  • Alveolar gas exchange and pH control

    • Oxygen diffuses into blood, carbon dioxide out (partial pressure differences, Henry’s Law)
    • Blood CO2​ levels influence pH (high CO2​ lowers pH)

  • Nervous system control

    • Breathing rate adjusted by nervous system (sensitive to CO2​)
    • Maintains oxygen delivery, carbon dioxide removal, and acid-base balance

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Respiratory system

General function/physiology

The gas exchange process in the lungs depends on diffusion: oxygen moves into the bloodstream, and carbon dioxide moves out of it. This movement is driven by Henry’s law, which states that a gas will dissolve in a liquid until it reaches an equilibrium concentration.

Blood arriving at the lungs has relatively low oxygen because body tissues have used it, so oxygen diffuses from the air in the lungs into the blood. At the same time, this blood carries relatively high carbon dioxide, so carbon dioxide diffuses from the blood into the air spaces of the lungs to be exhaled.

Thermoregulation is also influenced by breathing, because exhaling warm, moist air contributes to body heat loss.

Protection against disease

The respiratory system uses several protective mechanisms against pathogens and particulate matter:

Nostril hair acts as an initial filter, catching larger particles before they travel deeper into the airways.

Once inhaled, mucus produced by Goblet cells - specialized, columnar epithelial cell lining the respiratory tract - traps pathogens and finer particles. The trapped material is then moved out by cilia, tiny hair-like structures that sweep contaminants toward the throat, where they are either expelled or swallowed and neutralized by stomach acid.

In the alveoli, macrophages provide a final line of defense by engulfing and destroying foreign substances that escape the cilia and reach these delicate air sacs.

Structure of lungs and alveoli

Air enters through the nasal cavity, where it is warmed, humidified, and filtered by hair, mucus, and cilia. It then passes through the pharynx and the larynx into the trachea, a tube about 10-12 cm long. The trachea is supported by hyaline cartilage rings and smooth muscle that can contract during forced exhalation (e.g., coughing).

The trachea splits into two primary bronchi, each entering a lung. Within the lungs, these bronchi branch into secondary and tertiary bronchi, then into smaller bronchioles, which rely on air pressure (rather than cartilage) to stay open.

The lungs differ in size: the right lung has three lobes, whereas the left lung has two. Below them lies the diaphragm, a muscular boundary at the base of the thoracic cavity. When it contracts, it lowers and expands the chest volume, helping drive inhalation.

At the end of each bronchiole, alveolar ducts lead to clusters of alveoli, which resemble tiny bubbles arranged in alveolar sacs. Each alveolus is surrounded by capillaries; both are one cell thick, allowing gas exchange (oxygen in, carbon dioxide out) across their thin walls. With around 300 million alveoli per lung creating a total surface area of ~75 m², this structure maximizes oxygen delivery to the bloodstream and removal of carbon dioxide from it, giving the lungs their soft, sponge-like texture.

Breathing mechanisms

Diaphragm, rib cage, differential pressure

The diaphragm is a dome-shaped muscle. When it contracts, it moves downward, increasing chest volume and creating negative pressure that draws air into the lungs. At the same time, the rib cage expands outward - supported by intercostal muscles - and helps maintain lung volume even at rest.

Airflow is driven by differential pressure. The pressure inside the lungs (intrapulmonary) moves toward atmospheric levels, while the intrapleural pressure remains lower. This lower intrapleural pressure helps pull the lungs open and prevents collapse.

Resiliency and surface tension effects

A key feature of lung resiliency is elastic recoil. Without the outward support of the rib cage, the lungs would collapse further after exhalation.

Surface tension in the alveoli also promotes collapse. To counter this, the alveoli produce surfactants that reduce surface tension and help keep the air sacs open.

Alveolar gas exchange and pH control

In alveolar gas exchange, oxygen diffuses into blood and carbon dioxide diffuses out. This is guided by differences in partial pressures and can be described quantitatively by Henry’s Law.

Gas movement also connects to pH control, because elevated CO2​ in the blood can lower pH.

Nervous system control

The body’s nervous control responds by adjusting breathing rate, reflecting its CO2​ sensitivity.

Together, these coordinated mechanisms support oxygen delivery and carbon dioxide removal while also helping regulate acid-base balance.

Key points

General function/physiology

  • Gas exchange via diffusion: oxygen in, carbon dioxide out
  • Driven by Henry’s law (gas dissolves until equilibrium concentration)
  • Breathing aids thermoregulation (exhalation releases heat and moisture)

Protection against disease

  • Nostril hair filters large particles
  • Mucus (from Goblet cells) and cilia trap and move contaminants out
  • Alveolar macrophages engulf and destroy foreign substances in alveoli

Structure of lungs and alveoli

  • Air pathway: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
  • Trachea: supported by hyaline cartilage rings and smooth muscle
  • Lungs: right (3 lobes), left (2 lobes); diaphragm below for inhalation
  • Alveoli: clusters at bronchiole ends, surrounded by capillaries (one cell thick) for gas exchange
    • ~300 million alveoli per lung, ~75 m² surface area

Breathing mechanisms

  • Diaphragm, rib cage, differential pressure

    • Diaphragm contraction increases chest volume, creates negative pressure
    • Rib cage expands with intercostal muscles
    • Intrapleural pressure lower than intrapulmonary, prevents lung collapse

  • Resiliency and surface tension effects

    • Elastic recoil provides lung resiliency
    • Surface tension in alveoli countered by surfactant to prevent collapse

  • Alveolar gas exchange and pH control

    • Oxygen diffuses into blood, carbon dioxide out (partial pressure differences, Henry’s Law)
    • Blood CO2​ levels influence pH (high CO2​ lowers pH)

  • Nervous system control

    • Breathing rate adjusted by nervous system (sensitive to CO2​)
    • Maintains oxygen delivery, carbon dioxide removal, and acid-base balance