Physiology Oct. 7th

Does surface area scale linearly with mass?

• No, surface area doesn’t scale linearly with mass as organisms increase in size

  • Ex. Rotifers are an example due to their high surface area relative to mass

Explain the Nature of Diffusion

• Gas exchange occurs through diffusion from areas of high concentration to low concentration.

• Efficiency of diffusion is time-dependent and correlates with distance.

• Example: Diffusion across a cell membrane (10 nanometers thick) takes about 100 nanoseconds.

• In larger distances, such as those in larger organisms (e.g., human nerve cells), reliance on diffusion alone becomes impractical.

Explain Gas Exchange in Plants

• Getting CO2 out of the external medium (air) and into the leaf

• Within the leaf CO2 needs to get into the chloroplast (Rubisco)

Gas Exchange in Animals

• Getting O2 out of the external medium and into the cells

  • Often via the circulatory system in animals

• Getting CO2 out of the cells and into the external medium

How much CO2 and O2 is found in the air?

• Air (gases)

  • ~21% O2

  • ~0.04% CO2

• Cold Seawater (in solution)

  • ~0.8% O2

  • ~93% of global CO2

What is the ideal gas law?

• Describes how gases behave under various conditions, assuming gas molecules are perfect spheres in constant motion.

• Involves variables like pressure (P), volume (V), and temperature (T).

• Pressure and temperature are directly proportional (as one increases, so does the other).

• Pressure and volume are inversely proportional (as volume decreases, pressure increases).

What is Daltons Law of Partial Pressures?

• Total pressure exerted by a system of gases is the sum of the partial pressures of individual gases.

• Increasing one partial pressure can increase the total pressure without affecting other partial pressures, as each gas contributes independently to the total pressure based on its concentration.

• Example notations for atmospheric gases:

• Atmospheric pressure: 101.3 kPa at sea level.

Ex. What’s the partial pressure of O2 if its 20.95% of air?

What is Henrys Law?

• Describes the relationship between the partial pressure of a gas and its concentration in a liquid.

• Solubility varies by gas (e.g. carbon dioxide is more soluble than oxygen)

What is the diffusion coefficient?

• Unique for each gas and can be influenced by the medium (air vs water).

  • Diffusion occurs faster in air compared to water (10,000 times faster).

What can effect the rate at which a gas diffuses?

• The concentration of a gas directly affects its diffusion rate because diffusion depends on the concentration gradient

  • Larger the concentration difference = faster diffusion and vice vera

• The area available for diffusion effects the the diffusion rate

  • wider the opening of a bottle (or the greater the surface area), the more gas molecules can move across, or out, at once

What is Fick’s Law of Diffusion?

• Governs the net rate of movement of gases through diffusion.

  • Important to note that diffusion rates slow with increased distance.

What are the factors that affect gas solubility?

• Temperature

  • Higher temperatures lead to decreased gas solubility.

  • Cold soda holds fizz better because CO₂ stays dissolved. Warm soda goes flat quickly — the CO₂ solubility drops as temperature rises (CO2 molecules move quicker when heated and won’t be able to stay in solution; want to get out into the air)

• Salinity

  • Increased salinity reduces dissolved gas quantities.

Whats the equation once we put it all together?

What variables of the previous equation can be modified?

• Surface area, Concentration gradients, Distance over which diffusion occurs (A’, C1-C2, and X)

Whats the difference between gases and solutes?

• For gases, its partial pressure, not concentration, that determines the direction of diffusion.

  • diffusion happens from areas of high partial pressure → low partial pressure, not necessarily from high concentration → low concentration.

• For solutes (like salt or sugar in water), diffusion depends on concentration gradients — molecules move from high concentration to low concentration.

How does the beetle example explain this?

• The beetle traps an air bubble under its body while underwater.

• Inside the bubble: O₂ is used up as the beetle breathes, so PO₂ in the bubble drops.

• The water around the bubble still has a higher PO₂, even though the O₂ concentration in water is lower than in the bubble (air).

• Because gases diffuse according to partial pressure, O₂ from the water diffuses into the bubble, replenishing it.

• (O₂ can move into the bubble even if the bubble still has more O₂ molecules (higher concentration)as long as its partial pressure is lower than the surrounding water’s)

Explain Convection vs.Diffusion

• Diffusion

  • more passive of gas molecules from high to low concentration (or partial pressure)

  • its slow over long distances

  • works well for small organisms or across thin membranes (like alveoli in lungs)

• Convection

  • bulk flow of air or fluid, carrying gases with it.

  • It’s much faster than diffusion because it physically moves large amounts of gas.

  • Driven by pressure differences, temperature gradients, or external forces (like stirring or wind).

  • Think of it as air circulation instead of random molecular movement.

    • Blood flow moving oxygen through your body.

• How does the Prairie Dog example help explain this?

• The diagram of the prairie dog burrow shows how convection enhances gas exchange underground:

  • Wind moves over the two burrow openings, creating different pressures (low pressure at one end, high pressure at the other).

    • Upwind (left): Air hits directly, slows down, and “piles up” — this creates higher pressure.

    • Downwind (right): Air speeds up as it flows past, which lowers pressure.

    • Result: Air flows from left → right through the burrow (high → low pressure).

• This pressure difference causes air to flow through the tunnels — that’s convection.

• The flowing air continuously brings in fresh oxygen (O₂) and removes carbon dioxide (CO₂), which keeps the air in the burrow breathable.

• If they relied only on diffusion, gas exchange would be much slower, and oxygen might not reach deeper parts of the burrow.

Explain gas transport in animals 

• Is often a combination of convection and diffusion

• Tidal Convection Ventilates the Lungs:

  • Convection = bulk movement of air (breathing in and out).

  • Air enters the lungs by tidal ventilation — meaning air flows in during inhalation and out during exhalation.

  • This movement brings fresh O₂ to the alveoli and removes CO₂ from the body.

  • Think of this as the “delivery system” that refreshes the air inside the lungs.

• Diffusion into the Bloodstream:

  • Once air reaches the alveoli (tiny air sacs), gas exchange happens by diffusion.

  • O₂ diffuses from high partial pressure in alveoli → low partial pressure in blood.

  • CO₂ diffuses the opposite way (from blood → alveoli).

  • This step is microscopic — gases move only a few micrometers across the alveolar and capillary membranes.

• Unidirectional Flow (Convection) in the Circulatory System:

  • Once gases are in the blood, convection takes over again.

  • The heart pumps blood through vessels — a bulk flow that carries O₂-rich blood to tissues and brings CO₂-rich blood back to the lungs.

  • This is convection on a larger scale, like a delivery route.

• Diffusion from Capillaries into Tissues:

  • When oxygenated blood reaches body tissues, O₂ diffuses from blood to cells where it’s used for metabolism.

  • CO₂ diffuses from cells to blood to be carried back to the lungs.

  • Again, this happens over short distances, driven by partial pressure gradients.