Chapter 42.
Gas Exchange
gills
alveoli
elephantseals
Gas exchange
O2 & CO2 exchange
exchange between environment & cells
provides O2 for aerobic cellular respiration
need moist membrane
need high surface area
Respiration for respiration!
Optimizing gas exchange
Why high surface area?
maximizing rate of gas exchange
CO2 & O2 move across cell membrane by diffusion
rate of diffusion proportional to surface area
Why moist membranes?
moisture maintains cell membrane structure
gases diffuse only dissolved in water
Gas exchange in many forms…
one-celled
amphibians
echinoderms
insects
fish
mammals
endotherm vs. ectotherm
water vs. land
water vs. land
Evolution of gas exchange structures
external systems with lots of surface area exposed to aquatic environment
Aquatic organisms
moist internal respiratory surfaces with lots of surface area
Terrestrial
Gas Exchange in Water: Gills
Function of gills
out-foldings of body
surface suspended in water
Counter current exchange system
Water carrying gas flows in one direction, blood flows in opposite direction
What is the adaptive value?
How counter current exchange works
Blood & water flow in opposite directions
Maintains diffusion gradient over whole length of gill capillary
maximizing O2 transfer from water to blood
front
back
blood
100%
15%
5%
90%
70%
40%
60%
30%
100%
5%
50%
50%
70%
30%
water
counter-current
concurrent
Gas Exchange on Land
Advantages of terrestrial life
air has many advantages over water
higher concentration of O2
O2 & CO2 diffuse much faster through air
respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills
air is much lighter than water & therefore much easier to pump
expend less energy moving air in & out
Disadvantages
keeping large respiratory surface moist causes high water loss
Terrestrial adaptations
air tubes branching throughout body
gas exchanged by diffusion across moist cells lining terminal ends, not through open circulatory system
Tracheae
How is this adaptive?
Lungs
exchange surface, but also creates risk: entry point for environment into body
spongy texture, honeycombed with moist epithelium
Alveoli
Gas exchange across thin epithelium of millions of alveoli
total surface area in humans ~100 m2
Mechanics of breathing
Air enters nostrils
filtered by hairs, warmed & humidified
sampled for odors
Pharynx  glottis  larynx (vocal cords)  trachea (windpipe)  bronchi  bronchioles  air sacs (alveoli)
Epithelial lining covered by cilia & thin film of mucus
mucus traps dust, pollen, particulates
beating cilia move mucus upward to pharynx, where it is swallowed
Negative pressure breathing
Breathing due to changing pressures in lungs
air flows from higher pressure to lower pressure
pulling air instead of pushing it
Positive pressure breathing
Frogs
draw in air through nostrils, fill mouth, with mouth & nose closed, air is forced down the trachea
Autonomic breathing control
Medulla sets rhythm & pons moderates it
coordinate respiratory, cardiovascular systems & metabolic demands
Nerve sensors in walls of aorta & carotid arteries in neck detect O2 & CO2 in blood
Don’t have to think to breathe!
Medulla monitors blood
Monitors CO2 level of blood
measures pH of blood & cerebrospinal fluid bathing brain
CO2 + H2O  H2CO3 (carbonic acid)
if pH decreases then increase depth & rate of breathing & excess CO2 is eliminated in exhaled air
Diffusion of gases
Concentration & pressure drives movement of gases into & out of blood at both lungs & body tissue
blood
lungs
blood
body
capillaries in lungs
capillaries in muscle
Pressure gradients
Lungs
Hemoglobin
Why use a carrier molecule?
O2 not soluble enough in H2O for animal needs
hemocyanin in insects = copper (bluish)
hemoglobin in vertebrates = iron (reddish)
Reversibly binds O2
loading O2 at lungs or gills &unloading in other parts of body
Hemoglobin
Binding O2
loading & unloading from Hb protein depends on cooperation among protein’s subunits
binding of O2 to 1 subunit induces remaining subunits to change shape slightly increasing affinity for O2
Releasing O2
when 1 subunit releases O2, other 3 quickly follow as shape change lowers affinity for O2
Heme group
O2 dissociation curves for hemoglobin
drop in pH lowers affinity of Hb for O2
active tissue (producing CO2) lowers blood pH& induces Hb to release more O2
Bohr shift
Dissolved in blood plasma
Bound to Hb protein
Bicarbonate ion (HCO3-) & carbonic acid (H2CO3) in RBC
enzyme: carbonic anhydrase reduces CO2
Transporting CO2 in blood
Adaptations for pregnancy
Mother & fetus exchange O2 across placental tissue
why would mothers Hb give up its O2 to baby’s Hb?
Fetal hemoglobin
What is the adaptive advantage?
2 alpha & 2 gamma units
HbF has greater affinity to O2 than Hb
low O2% by time blood reaches placenta
fetal Hb must be able to bind O2 with greater attraction than maternal Hb
Any Questions??