CHAPTER 11_Gas Exchange in Humans

1) Features of Gas Exhange

Gas exchange surfaces vary across organisms, adapted to their size and environment. Despite differences, all gas exchange surfaces share common features that maximize gas exchange efficiency:

• Large surface area, for fast gas diffusion

• Thin walls to minimize diffusion distance → making it shorter

• Good ventilation, with air, to maintain diffusion gradient

• Good blood supply to sustain high concentration gradients for faster diffusion

2) The breathing System

1. Nasal Cavity

2. Trachea

3. Pleural Cavity (filled with fluid)

4. Lung

5. Bronchus

6. Intercostal muscles

7. Ribs

8. Bronchioles → alveoli

9. Diaphragm

3) Investigating differences in Inspired & Expired air

Investigation to inspect differences between inspired and expired air:

• Wehn we breath in, air drowns through the boiling tube A

• When we breath out, air is blown into the boiling tube B

• Lime water is clear but becomes cloudy/ milky when carbon dioxide is bubbled through it.

  • Lime water in boiling tube A remains clear but limewater in boiling tube B becomes cloudy.

→ This shows us that the percentage of CO2 in the exhaled air is higher than in inhaled air.

Differences in Inspired and Expired air

• Gas exchange in alveoli changes the composition of air.

• Inhaled air has 20-21% oxygen

  • of which only 4-5% is absorbed

  • so exhaled air has 16% oxygen

• CO2 in inhaled air is 0.04%

  • exhaled air holds 4% CO2 due to diffusion from the blood

• Exhaled air has more water vapour & is warmer than inhaled air

4) Investigating effects of Physical Activity on Breathing

Exercise increases breathing rate (frequency) & also the depth (amount)

1. At rest: Count breaths per minute & measure the average chest expansion, over 5 breaths with a tape measure

2. Exercise for at least 3 minutes

3. After exercise: repeat the breath count & chest expansion measure again

   → Both will have increased post-exercise

Explaining the Link between Physical Activity

• During exercise, breathing rate & depth increases, because muscles need more oxygen for aerobic respiration & to remove CO2.

• If oxygen demand isn’t met, muscles respire anaerobically, producing lactic acid.

  • Lactic acid lowers cell pH & can denature enzymes, so it must be removed

    → it’s removed using oxygen: called repaying oxygen depth

• The longer it takes for breathing to return to normal, the more lactic acid was made & the greater the oxygen depth.

Mechanism for increasing breathing during exercise

During heavy exercise, respiration in muscle cells increases, producing more Co2 as a by-product

→ this CO2 diffuses out of muscle cells into blood plasma

→ the increase in CO2 lowers blood pH slightly, making it more acidic.

• As blood circulates to brain, chemoreceptors in medulla oblongata detects the raised CO2 levels & drops in the pH.

  • Chemoreceptors are specialized cells that monitor chemical changes like gas levels & pH in the blood.

• In response, the brain sends nerve impulses to diaphragm & intercostal muscles, causing them to contract more frequently & forcefully.

  • This increases both the rate & depth of breathing, improving oxygen intake & CO2 removal to support higher respiration rate in muscles.

5) Identifying Intercostal muscles

• Muscles can only pull, not push, so two sets are needed to move the rib cage.

• One set pulls the rib cage up: the other pulls it down

  • like a push and pull factor

• External Intercostal muscles are located on the outside of the ribcage

• Internal Intercostal muscles are located on the inside of the ribcage

  

6) Function of Cartilage in Trachea

• Rings of cartilage surround the trachea (and also bronchi)

• The function of the cartilage is to support the airways & keep them open during breathing.

• If they were not present, then the sides could collapse inwards, pressure breathing and the pressure inside the tubes drops.

7) Volume & Pressure changes in the Lungs

The Diaphragm, a thin muscle separating chest and abdomen, controls lung ventilation:

A → Inhalation: Diaphragm contacts and flattens

I. Thorax volume increases

II. Air pressure inside the lungs drops

III. Air is drawn in

B → Exhalation: Diaphragm relaxes

I. Thorax volume decreases

II. Air pressure rises

III. Air is forced out

Intercostal muscles work as antagonistic pairs

A → External intercostals: contract during inhalation (ribs go up and out)

I. Thorax volume goes up

II. Pressure inside decreases

III. Air goes in

→ they relax during exhalation (ribs go down and in)

I. Thorax Volume decreases

II. Pressure inside increases

III. Air goes out

B → Internal Intercostals: help during forced exhalation (e.g. during exercise) by pulling ribs further down and in

I. Thorax Volume decreases even more

II. Air goes out faster

→ this removes excess CO2 & increases gas exchange

INHALATION:

1. External intercostal muscles contract

2. Ribcage moves up & out

3. Diaphragm contracts & flattens

4. Volume of thorax increases

5. Pressure inside thorax decreases

6. Air is drawn in

EXHALATION:

1. External Intercostal muscles relax

2. Ribcage moves down & in

3. Diaphragm relaxes & becomes dome-shaped

4. Volume of thorax decreases

5. Pressure inside the thorax increases

6. Air is forced out

Forced exhalation:

• Internal intercostal muscles contract

• Pulls ribcage further downwards & more inwards.

8) Protecting the Breathing System

• The airways to lungs are lined with ciliated epithelial cells that have tiny hair-like structures called cilia.

• These Cilia beat to push mucus up towards the nose & throat, where it can be removed.

• Goblet cells (named after their cup shape) produce the mucus.

• The mucus traps dust, pathogens & particles, preventing them from reaching & damaging the lungs.