Homeostatic Mechanisms 1 (function)

Example 2: Gas Exchange

Example 1: Gas Exchange in Plants

A developmental response of maize roots to flooding and oxygen deprivation

Vascularcylinder

Air tubes

Epidermis

100 m 100 m(a) Control root (aerated) (b) Experimental root (nonaerated)

Gas Exchange in Plants: Stomates

By controlling stomate opening/closing, terrestrial plants control gas exchange.

Great for land….…What about aquatic plants?No/few stomates on submerged leaves. Water is the exhange medium

Quick Check: Make Sure You Can1. Explain why gas exchange is necessary in

plants.2. Explain how stomates regulate the exchange

of gases in plants.3. Predict the effects of changing

concentrations of gases on the rate of gas exchange in plants.

Example 2: Gas Exchange in Animals

Gas Exchange in AnimalsGas exchange in animals depends on a respiratory surface:Moist: easy diffusion, functional cell membranes are wet.High surface area: Increase the rate of gas exchange. Diffusion moves O2 & CO2

High surface area?High surface area!

Where have we heard that before?

Gas exchange in many forms…one-celled amphibians echinoderms

insects fish mammals

endotherm vs. ectothermsize

cilia

water vs. land ••

Evolution of gas exchange structures

external systems with lots of surface area exposed to aquatic environment

Aquatic organisms

moist internal respiratory tissues with lots of surface area

Terrestrial

Gas Exchange in Water: Gills

Counter current exchange system• Water carrying gas flows in one direction,

blood flows in opposite direction

just keepswimming….

Why does it workcounter current?

Adaptation!

• Blood & water flow in opposite directions– maintains diffusion gradient over whole length of gill

capillary– maximizing O2 transfer from water to blood

water

blood

How counter current exchange works

front back

blood

100%15%

5%90%

70% 40%

60% 30%

100%

5%

50%

50%

70%

30%

watercounter-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 in air –respiratory surfaces exposed to air do not have to

be ventilated as thoroughly as gills• air is much lighter than water & much easier to

pump–expend less energy moving air

• Disadvantages– keeping large respiratory surface moist

causes high water loss• reduce water loss by keeping lungs internal

Why don’t land animals

use gills?

Terrestrial adaptations

• air tubes branching throughout body• gas exchanged by diffusion across

moist cells lining terminal ends, not through open circulatory system

Tracheae

Lungs Exchange tissue:spongy texture, honeycombed with moist epithelium

Why is this exchangewith the environment

RISKY?

Alveoli• Gas exchange across thin epithelium of

millions of alveoli– total surface area in humans ~100 m2

Negative pressure breathing• Breathing due to changing pressures in lungs

– air flows from higher pressure to lower pressure– pulling air instead of pushing it

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

don’t wantto have to think

to breathe!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

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

Breathing and Homeostasis• Homeostasis

– keeping the internal environment of the body balanced

– need to balance O2 in and CO2 out– need to balance energy (ATP) production

• Exercise– breathe faster

• need more ATP• bring in more O2 & remove more CO2

• Disease– poor lung or heart function = breathe faster

• need to work harder to bring in O2 & remove CO2

O2

ATP

CO2

Diffusion of gases

• Concentration gradient & pressure drives movement of gases into & out of blood at both lungs & body tissue

blood lungs

CO2

O2

CO2

O2

blood body

CO2

O2

CO2

O2

capillaries in lungs capillaries in muscle

Inhaled air Exhaled air

160 0.2O2 CO2

O2 CO2

O2 CO2

O2 CO2 O2 CO2

O2 CO2 O2 CO2

O2 CO2

40 45

40 45

100 40

104 40

104 40

120 27

CO2O2

Alveolarepithelial

cells

Pulmonaryarteries

Blood enteringalveolar

capillaries

Blood leavingtissue

capillaries

Blood enteringtissue

capillaries

Blood leaving

alveolar capillaries

CO2O2

Tissue capillaries

Heart

Alveolar capillaries

of lung

<40 >45

Tissue cells

Pulmonaryveins

Systemic arteriesSystemic

veinsO2CO2

O2

CO2

Alveolar spaces

12

43

Loading and unloading of respiratory gases

Hemoglobin• Why use a carrier molecule?

– O2 not soluble enough in H2O for animal needs• blood alone could not provide enough O2 to animal cells • hemocyanin in insects = copper (bluish/greenish)• hemoglobin in vertebrates = iron (reddish)

• Reversibly binds O2

– loading O2 at lungs or gills & unloading at cells

cooperativity

heme group

Cooperativity in Hemoglobin • Binding O2

– binding of O2 to 1st subunit causes shape change to other subunits

• conformational change– increasing attraction to O2

• Releasing O2 – when 1st subunit releases O2,

causes shape change to other subunits

• conformational change– lowers attraction to O2

O2 dissociation curve for hemoglobin

Bohr Shift increase in

temperature lowers affinity of Hb for O2

active muscle produces heat

PO2 (mm Hg)

0102030405060708090

100

0 20 40 60 80 100 120 140

More O2 delivered to tissues

20°C

43°C37°C

% o

xyhe

mog

lobi

n sa

tura

tion

Effect of Temperature

Transporting CO2 in blood

Tissue cells

Plasma

CO2 dissolvesin plasma

CO2 combineswith Hb

CO2 + H2O H2CO3

H+ + HCO3–

HCO3–

H2CO3

CO2

Carbonicanhydrase

Cl–

• Dissolved in blood plasma as bicarbonate ion

carbonic acidCO2 + H2O H2CO3

bicarbonateH2CO3 H+

+ HCO3–

carbonic anhydrase

Releasing CO2 from blood at lungs

• Lower CO2 pressure at lungs allows CO2 to diffuse out of blood into lungs

Plasma

Lungs: Alveoli

CO2 dissolvedin plasma

HCO3–Cl–

CO2

H2CO3

H2CO3Hemoglobin + CO2

CO2 + H2O

HCO3 – + H+

Adaptations for pregnancy

• Mother & fetus exchange O2 & CO2 across placental tissue

Why wouldmother’s Hb give up its O2 to baby’s Hb?

Fetal hemoglobin (HbF)

What is the adaptive advantage?

2 alpha & 2 gamma units

• HbF has greater attraction 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