Respiratory System: Gas Exchange and Fick's Law

• Quiz 4 on Respiratory Systems will be held on March 11, 2025, at 8:30 AM in N1002.
• The quiz covers lectures from Feb 27, Mar 4, and Mar 6.
• The format includes a short answer definition, 3 True/False questions, and a multi-fill-in-the-blank related to organ circulatory flow patterns.
• Quiz 5 will be March 20, covering lectures from March 11, 13, 18.
• Quiz 6 will be April 3, covering lectures from March 20, 25, 27 and April 1.

The Physics of Respiratory Systems

• The respiratory system facilitates the passive diffusion of gases.
• Diffusion moves molecules down a partial pressure gradient.
• Diffusion alone isn't enough to supply oxygen to all cells and tissues in most organisms.
• A respiratory system consists of a surface across which gas exchange occurs by diffusion between blood and air or water.
• Respiratory surfaces need to be: moist enough to facilitate diffusion; large enough for adequate gas exchange; thin enough for swift diffusion.

Fick's Equation

• Fick's Equation: dQ/dt = D x A x dC/dx gives diffusion rate.
• The diffusion rate = (D * A * ΔP₀₂)/X
• Diffusion is maximized when the diffusion coefficient (D) is large, the area of the membrane (A) is large, gradients (dC/dx) are large, and diffusion distance is small.
• Gas exchange surfaces are typically thin and have a large surface area to help with diffusion.

Gas Dissolution

• Henry's Law: [G] = Pgas x Sgas
• G represents the gas concentration in the solution.
• P corresponds to the partial pressure of the gas in the air.
• S is the solubility of the gas.
• Gas molecules in the air have to dissolve in liquid before diffusing into a cell.

Gas Pressure

• The total pressure exerted by a gas is correlated with the number of moles of the gas and the volume of the chamber.
• Ideal Gas Law: PV = nRT
• P is the pressure [Pa]; V is the volume [m³]; n is the amount of gas [mol]; R is the gas constant 8.3143 m³·Pa·K⁻¹·mol⁻¹; T is temperature in Kelvin [K].
• Volume of a gas varies inversely with pressure.

Composition of Air, Dalton's Law

• Air contains about 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide.
• Dalton's Law explains that each gas in a mixture exerts its own partial pressure. The sum of these partial pressures equals the total pressure of the mixture.
• In atmospheric air, the partial pressure of nitrogen is 600 mm Hg (79%), and the partial pressure of oxygen is 160 mm Hg (21%).

Graham's Law

• Graham's Law explains Diffusion rate is directly related to solubility. Also inversely related to molecular mass (Greater molecular mass = slower diffusion).
• CO₂ mass is 44 g/mol.
• O₂ mass is 32 g/mol.

Diffusion Combined

• The formula for diffusion combines Fick's, Henry's, and Graham's Laws.
• Diffusion rate = (D * ΔP * A * S) / (X * √MolWt)
• At a constant temperature, the rate of diffusion is proportional to these: partial pressure gradient (ΔP), cross-sectional area (A), gas solubility in the fluid (S), diffusion distance (X), and molecular weight of the gas (MolWt).

Summary of Gas Exchange Laws

• Graham's Law: the rate of gas diffusion is proportional to its solubility divided by the square root of its molecular weight.
• Henry's Law: the concentration of a dissolved gas equals its partial pressure times its solubility.
• Dalton's Law: the total pressure is the sum of each gas’s partial pressure.
• Ideal Gas Law: PV = nRT, links pressure to gas amount, volume, and temperature.
• Fick's Law explains how the rate of diffusion relates to distance, area, and gradient.

Factors Affecting Gas Diffusion Effectiveness

• How effectively a gas diffuses in a liquid medium depends on the total gas pressure, pressure gradient (partial pressure), surface area for diffusion, molecular weight, solubility, and distance (membrane thickness).

Air vs Water properties

• Oxygen solubility for air at 20 degrees celcius is 1000ml/L, where as oxygen solubility for water at 20 degrees celcius is 33.1ml/L, implying that at 20 degrees celcius, air is 1:30 times better that water as a medium for oxygen solubility
• Oxygen concentration in air at 1 atm is 8.7mM ,where as oxygen concentration in water at 1 atm is 0.3mM, which is a ratio of 1:30.

Respiratory Strategies

• Diffusion alone is a common strategy among animals less than a few millimeters thick.
• Sponges, cnidarians, and insects circulate the external medium through their bodies.
• Gases diffuse across the body surface, which is accompanied by circulatory transport called "Cutaneous respiration". This is common in aquatic invertebrates, some amphibians, and bird eggs.
• Gases also diffuse across specialized respiratory surfaces such as the gills (evaginations) or lungs (invaginations) accompanied by circulatory transport. This is common in vertebrates.

Surface Area to Volume Ratio

• As organisms grow, the ratio of surface area to volume decreases, meaning surface area is less availible.
• Having less surface area limits the area available for diffusion and increases the diffusion distance needed for oxygen.
• Diffusion alone works for nematodes, horsehair worms and tubellarian flatworms.

Cutaneous Respiration

• Cutaneous respiration is oxygen transfer across the body surface.
• Most aquatic invertebrates as well as terrestrial annelid worms, frogs, and salamanders use this method.
• Plethodontidae Lungless salamanders also use cutaneous respiration.
• Cutaneous respiration requires very thin skin that is prone to damage. This requires them to keep skin moist, limiting their aquatic habitats or wet terrestrial habitats.

Unconventional Respiration

• Lake Titicaca frogs (Telmatobius coleus) and hairy frogs use cutaneous respiration.

Specialized Respiratory Surfaces

• Gills are outpockets, mostly found in water environments.
• Lungs are infoldings, typically situated in terrestrial environments.
• Ventilation helps the movement of an external medium and the ventilation rate can change with oxygen demand. Ventilation can do this with movement that is unidirectional or tidal.

Ventilation and Gas Exchange

• Animals need to use different ventilation strategies based on on the medium they occupy because of the different physical properties of air and water.
• Oxygen is 30x greater in air than water.
• Water is more dense and viscous than air.
• Evaporation is mainly a concern for air breathers.
• Most water-breathers use unidirectional ventilation.
• Air-breathers use tidal ventilation.
• Air is also able to be transported through Air filled tubes, which is common in insects.

Ventilation with Water

• Water respiration may involve circulating the external medium through an internal cavity or several strategies to ventilate internal/external gills.

Sponges and Cnidarians

• Sponges and Cnidarians circulate the external medium through an internal cavity.
• In sponges, flagella move water in through openings called "ostia" and out through the "osculum".
• Cnidarians move water in and out through the mouth using muscle contractions.

Molluscs: Gills and Mantle Cavity

• One molluscs strategy forventilating their gills and mantle cavity is Beating cilia on gills to move water across gills unidirectionally.
• Snails and clams use beating cilia on gills to move water across gills unidirectionally, which countercurrent to the flow of blood.
• Second strategy for ventilating gills and mantle cavity involves muscular contractions to move water unidirectionally through it.
• Cephalopods (squid etc) use muscular contractions to move water unidirectionally through it, which flow of blood is countercurrent to.
• The squid are able to ventilate the gill with muscle power in a muscle-driven water stream for swimming, where as the pulmonate land snail has a lack of gills, but a lung derived from the mantle cavity.

Polychaete Annelids

• Polychaete Annelids also known as tentacular fans use tentacles to provide ventilation, and collect food.

Crustaceans

• Filter feeding (barnacles) or small species (copepods) lack gills and rely on diffusion
• Shrimp, crabs, and lobsters, have gills derived from modified appendages located within a branchial cavity
• Movements of the gill bailer scaphognathite propels water out of the branchial chamber; the negative pressure sucks water across the gills

Echinoderms

• Most sea stars and sea urchins use tube feet for gas exchange
• Water is sucked in and exits through the madreporite
• Sea stars also have external gill-like structures (respiratory papulae); cilia move water over the surface
• Brittle stars used cilia to move water into bursae in order to ventilate internal invaginations.
• Sea cucumbers use muscular contractions of the cloaca and the respiratory tree to breathe water tidally though the anus.

Jawless Fishes

• Lamprey and hagfish have multiple pairs of gill sacs
• Hagfish use a muscular pump (velum) to propels water through the respiratory cavity, causing ventilation.

Lampreys (Jawless fish)

• Lamprey ventilation is similar to hagfish when not feeding.
• When feeding, the mouth is attached to a prey (parasitic), meaning the ventilation is tidal though the gill openings.

Elasmobranchs (sharks etc)

• These are some steps for Elasmobranchs ventilation: expand the buccal cavity, which causes an increased volume to suck fluid into the buccal cavity via the mouth and spiracles. Also, mouth and spiracles close as muscles around the buccal cavity contact, forcing water past the gills and out the external gill slits to allow Blood flow to be countercurrent.

Teleost Fishes (ray-finned fishes)

• Teleost Fishes have gills located in the opercular cavity, which is protected by the flaplike operculum. Ventilation is aided by the dual pump.
• Active fish can also use ram ventilation.
• With the mouth open, the floor of the buccal cavity lowers, which causes a volume increase to decrease pressure and water to flow in from outside.
• Volume increases in the opercular cavity as the operculum closes.
• Pressure decreases and water flows into the buccal cavity.

Fish Gill Arches

• Fish gills contain gill arches arranged for countercurrent flow that contain primary and secondary lamellae to act as a medium where water and blood flow.

Ram Ventilation

• Ram ventilation is flow of respiratory medium across the respiratory surface by the forward movement of the organism, which is common in tuna, Mackerel, billfish

Ventilation and Gas Exchange in Air

• Two major lineages have colonized terrestrial habitats, which are Vertebrates and Arthoropds.
• Arthopods include Crustaceans, Chelicerates, and Insects

Terrestrial Crabs

• Terrestrial crabs use the processes of ventilation that are similar to its marine relatives.
• Their gills have to be stiff so they do not collapse in air.
• Scaphognathite also helps with air intake.
• Branchial cavity is highly vascularized acting as the primary site of gas exchange.
• Variability and structures vary across groups of the Terrestrial crabs making it harder to generalize facts.
• Globonautes macropus are tree climbing freshwater crab that have gills and a pseudolung.

Chelicerates

• Chelicerates are spiders and scorpions, and horseradish crab that consist of four book lungs
• These lungs consist of 10-100 lamellae that open to the outside with Spiracles and gas diffuses in and out
• Some spiders also have a tracheal system of air-filled tubes in order to survive, especially when combined with the book lungs because they also have only a tracheal system.