Respiratory System and Gas Exchange — Comprehensive Notes Summary & Study Notes

🫁 Overview

Breathing (ventilation) is the physical movement of air into and out of the lungs. Respiration is the biochemical breakdown of nutrients to release energy in cells. They are linked because organisms breathe to obtain $O_2$ for aerobic respiration and to remove the toxic product $CO_2$.

🧭 Body regions and location

The breathing system is located in the thorax (chest). The main cavities involved are the nasal/oral cavities, pharynx, trachea, and the two lungs which sit inside the thorax and are protected by the ribs.

🫳 Pathway of air

Air enters through the nostrils or mouth, passes through the nasal cavity (where it is warmed, filtered, moistened and smelled), moves through the pharynx, then down the trachea. The trachea splits into two bronchi, each entering a lung and dividing into smaller bronchioles, ending in alveoli.

🔧 Key structures and functions

• Trachea: windpipe; conducts air to bronchi.
• Bronchi / Bronchioles: branching tubes distributing air through the lungs.
• Alveoli: tiny air sacs where gas exchange occurs.
• Diaphragm: muscular sheet at the base of the thorax that changes thoracic volume.
• Intercostal muscles: muscles between ribs that move the ribcage.
• Pleural membranes: moist membranes that lubricate and prevent lung collapse.

🎈 Mechanics of breathing — Inhalation

During inhalation: the diaphragm contracts and moves down; external intercostal muscles contract to raise the ribs. Thoracic volume increases, pressure in the lungs decreases relative to atmospheric pressure, and air flows in.

🧾 Mechanics of breathing — Exhalation

During exhalation (at rest): the diaphragm relaxes and moves up; intercostal muscles relax so ribs lower. Thoracic volume decreases, pressure in the lungs increases, and air flows out. Forced exhalation uses abdominal and internal intercostal muscles.

⚖️ Alveolar adaptations for efficient gas exchange

Alveoli have several structural specialisations:

• Large surface area: many alveoli provide a huge total area (roughly half a tennis court) to increase diffusion.
• Thin diffusion distance: alveolar walls and capillaries are one cell thick, giving a very short path for gases.
• Moist surface: inner surfaces are moist so gases can dissolve and diffuse more easily.
• Rich capillary network: dense blood supply maintains rapid uptake/removal of gases.

🔬 Diffusion and concentration gradients

Gas movement across the alveolar-capillary membrane follows the law of diffusion: gases move from regions of higher to lower partial pressure. A steep concentration (partial pressure) gradient for $O_2$ and $CO_2$ is maintained by continuous breathing (removing high $CO_2$ air and bringing fresh $O_2$) and continuous blood flow (carrying away oxygenated blood and replacing it with deoxygenated blood).

📊 Composition of inspired vs expired air (key points)

• Atmospheric air: approx. $N_2$ $78$, $O_2$ $21$, $CO_2$ $0.04$.
• Exhaled air: $O_2$ falls (e.g. to about $17$) and $CO_2$ rises (e.g. to about $4.04$).
  Explanation: $O_2$ is absorbed into blood at alveoli, while $CO_2$ is released from blood into alveoli and then exhaled.

📏 Surface area : volume and why specialised surfaces are needed

Small unicellular organisms have a large surface area to volume ratio, so diffusion across the body surface meets needs. Large multicellular organisms have a small surface area to volume ratio and therefore require specialised exchange surfaces (lungs) with large surface area and short diffusion distances.

🧪 Designing an investigation: effect of exercise on breathing rate (CORMS)

• C (Change?): type/intensity of exercise (e.g. rest, moderate, intense).
• O (Organism?): the participant(s) (same person or matched group).
• R (Repeat): perform multiple trials or use multiple participants for reliability.
• M1 (Measure?): breathing rate (breaths per minute).
• M2 (Time period?): measure for a fixed time (e.g. 1 minute after exercise or every minute during recovery).
• S1/S2 (Same?): keep temperature, posture, and measurement method constant.

✅ Summary

• Breathing moves air; respiration releases energy.
• Air travels: nostril/mouth → nasal/oral cavity → pharynx → trachea → bronchi → bronchioles → alveoli.
• Efficient gas exchange requires large surface area, thin walls, moist surfaces, and a good blood supply maintained by ventilation and circulation.