Gas Exchange in Humans

Gas Exchange in Humans

Features of Gas Exchange Surfaces

• Gas exchange surfaces vary across organisms, but share common features that maximize gas exchange efficiency.
• These features facilitate the maximum exchange of gases in the shortest amount of time.
• Key features include:
  • Large Surface Area: Presence of millions of alveoli to allow faster diffusion of gases. More molecules can diffuse simultaneously.
  • Thin Walls: Ensures diffusion distances remain short for faster gas exchange.
  • Good Blood Supply: Maintains a high concentration gradient, accelerating diffusion.
  • Good Ventilation: Continuous movement of fresh air to maintain a diffusion gradient.
  • Ventilation is the movement of fresh air into the lungs through inspiration and stale air out of the lungs through expiration.

Structure of Lungs

• Nasal Passage: Air enters; hairs trap dust and pollen; mucus moistens the air; blood vessels warm the air.
• Larynx: Contains vocal cords that vibrate to create sounds as air flows from the lungs.
• Trachea:
  • Made of C-shaped cartilage to keep it open and prevent collapse due to pressure changes during breathing.
  • Inner walls consist of ciliated epithelial cells.
  • Goblet cells produce mucus to trap dust and bacteria.
  • Cilia move mucus towards the throat to be swallowed or spat out, protecting lungs from pathogens.
• Bronchus: Divides from the trachea, composed of cartilage, smooth muscles, and ciliated epithelial cells.
• Bronchioles: Branch from the bronchus and consist of smooth muscle.
• Alveoli:
  • Site of gas exchange.
  • Thin-walled; oxygen crosses just two cells (one alveolar and one capillary) before entering the blood.

Function of Cartilage in the Trachea

• Keeps the trachea open, preventing collapse or bursting during breathing.
• Inner walls contain ciliated epithelial and goblet cells.
• Goblet cells produce mucus that traps dust and bacteria, protecting the lungs.
• Cilia sweep mucus away from the breathing tubes towards the throat.
• Smooth muscles change the diameter of the trachea, widening it during exercise.

Gaseous Exchange

• Refers to the exchange of oxygen (O₂) and carbon dioxide (CO₂) between the air and blood vessels in the lungs.
• Oxygen combines with hemoglobin in red blood cells (RBCs) to form oxyhemoglobin.
• Carbon dioxide in the plasma is released when hydrogen carbonate ions break down into CO₂ and H₂O.

Differences in Inspired and Expired Air

• Inspired air (breathed in) and expired air (breathed out) have different gas compositions due to gas exchange in the alveoli.
• Atmospheric air contains about 20-21% O₂, with about 4-5% absorbed, resulting in expired air containing ~16% O₂.
• Normal CO₂ content in air is about 0.04%, while air exhaled contains around 4% CO₂.
• Exhaled air has more water vapor and is at a higher temperature than inhaled air.

Composition of Inspired and Expired Air

• Expired air has less O₂, more CO₂, more moisture, and a higher temperature than inspired air.

Lung Capacity and Breathing Rate

• Total lung volume when fully inflated is about 5 liters in an adult.
• There is a residual volume of 1.5 liters that cannot be expelled, which is important to keep the alveoli open at rest and prevent them from collapsing. Without the residual volume they would remain closed and prevent you from inhaling.
• Normal breathing rate is about 12 times per minute at rest.
• During exercise, the breathing rate may increase to over 20 breaths per minute and the depth of breathing also increases.

Breathing Rate and Exercise

• Increased breathing rate and depth during exercise allow more oxygen to dissolve in the blood, supplying active muscles.
• Extra CO₂ from the muscles is detected by the brain, instructing intercostal muscles and the diaphragm to contract and relax more rapidly, increasing breathing rate.
• The faster, deeper breathing removes CO₂ more efficiently.

Link Between Physical Activity & Breathing

• Frequency and depth of breathing increase during exercise.
• Muscles work harder and respire aerobically, needing more oxygen and producing more carbon dioxide.
• If energy demand cannot be met aerobically, muscles respire anaerobically, producing lactic acid.
• Post-exercise, lactic acid needs to be removed by combining with oxygen, repaying the "oxygen debt."
• The time taken for breathing rate and depth to return to normal indicates the amount of lactic acid produced and the oxygen debt.

Practical Work: Investigating Differences in Inspired & Expired Air

Oxygen in Exhaled Air

• A candle burns longer (about 15-20 seconds) in a large jar of ordinary air compared to exhaled air (about 5 seconds) because exhaled air contains much less O₂.
• Burning requires O₂; the flame goes out when O₂ is used up.

Carbon Dioxide in Exhaled Air

• The limewater test: inspired is drawn through boiling tube A. When we breathe out, the air is blown into boiling tube B.
• Limewater (calcium hydroxide) is clear but becomes cloudy (or milky) when carbon dioxide (CO_2) is bubbled through it.
• Lime water in boiling tube A remains clear; limewater in boiling tube B becomes cloudy.
• This indicates a higher percentage of CO_2 in exhaled air than in inhaled air.

Further Notes on Limewater Test

• If breathing process is carried out too long, limewater that turned milky will turn colorless because calcium carbonate (CaCO3) formed reacts in water with CO2 to form calcium hydrogen carbonate (Ca(HCO3)2), which is soluble and colorless.

• Hydrogen carbonate indicator changes from red to yellow when CO_2 is bubbled through it.

Investigating the Effect of Exercise on CO_2 Production

• Half-fill two clean boiling tubes with limewater.
• Blow into one tube with a relaxed breath; count how many breaths are needed for the limewater to turn milky.
• Exercise for 1-2 minutes.
• Blow into the second tube; count the number of breaths needed.
• The number of breaths needed after exercise will be less than before exercise due to rapid production of CO_2.

Investigating the Effect of Exercise on Rate and Depth of Breathing

• Done using a spirometer.
• Breathing rate increases from ~12 breaths per minute to more than 20 breaths per minute.

Ventilation of the Lungs

• The lungs contain no muscle fibers and expand/contract through the movements of ribs and diaphragm.
• The diaphragm is a sheet of tissue separating the thorax from the abdomen.

The Mechanism of Breathing

Bell Jar Demonstration

1. Pull the rubber sheet down (like the diaphragm contracting).
2. This increases the volume inside the bell jar, decreasing the pressure.
3. The drop in pressure causes air to rush into the balloons (like breathing in).
4. Release the rubber sheet (like the diaphragm relaxing).
5. The volume in the jar shrinks. This increases the pressure, so air rushes out (like breathing out).

Inspiration (Breathing In)

1. External intercostal muscles and diaphragm contract; internal intercostal muscles relax.
2. The ribs move upwards and outwards.
3. The volume of the thorax increases.
4. The pressure inside the thorax decreases, drawing air in.

Expiration (Breathing Out)

1. Internal intercostal muscles contract; external intercostal muscles and diaphragm relax.
2. The ribs move downwards and inwards.
3. The volume of the thorax decreases.
4. The pressure inside the thorax increases, forcing air out.

Difference Between Inhaling and Exhaling