2: Gas Exchange

Question 1

alveoli

Answer 1

bronchioles open into alveolar duct which have alveolar sacs
150 million alveoli in each lung
140m squared exchange surface
elastic fibres in connective tissue
most cells are type I pneumocytes which have one cell thick simple squamous epithelium walls
type II pneumocytes secrete surfactant - keeps alveoli open as it reduces surface tension
macrophages digest dust particles and other foreign particles
fuse of capillary endothelium and alveolar epithelium
capillaries on surface of approx. 70% alveoli

Question 2

blood air barrier

Answer 2

gas exchange takes place across the blood air barrier which has three layers:
alveolar cell layer
capillary endothelial layer
fused basement membrane (in between alveolar and endothelial cells)

pulmonary embolism = blockage of a branch of a pulmonary artery which stops blood flow to a group of lobules or alveoli

Question 3

Henry’s law

Answer 3

when gas under pressure comes in contact with liquid, the gas dissolves in liquid until equilibrium is reached
amount of gas in a solution is proportional to partial pressure of that gas
amount of oxygen and carbon dioxide that dissolves in the liquid lining of lungs is directly proportional to partial pressure in the air

Question 4

5 reasons gas exchange is efficient

Answer 4

1. substantial differences in partial pressure across the respiratory membrane
2. distances involved in gas exchange are very short
3. O2 and CO2 are lipid soluble so they can move through cells of target tissues
4. surface area for diffusion is large
5. blood flow and airflow are coordinated (HR increased due to inhalation)

Question 5

partial pressure

Answer 5

partial pressure is the pressure contributed by a single gas
PP in alveolar air and alveolar capillaries determine rate of diffusion
blood arriving in pulmonary arteries has low PO2 and high PCO2 so oxygen enters blood and CO2 leaves at lungs
interstitial fluid: PO2 = 40mmHg PCO2 = 45mmHg

Question 6

gas pickup and delivery

Answer 6

plasma cannot transport enough O2 or CO2 to meet human needs
so red blood cells transport gases from peripheral tissues
RBC reactions are both temporary and completely reversible

Question 7

red blood cells

Answer 7

98.5% oxygen in arterial blood binds to haemoglobin Hb
each Hb has 4 heme sites to bind to 4 molecules of oxygen
active tissues have a lower PO2 so Hb releases more oxygen there
each oxygen that binds to Hb makes it easier to bind another molecule (allows for more binding when O2 levels low)

Question 8

haemoglobin activity

Answer 8

increased temp = Hb releases more oxygen
decreased temp = Hb holds oxygen more tightly
temp effects only significant in active skeletal muscle that are generating large amounts of heat
Bohr effect: describes how affinity of Hb for oxygen decreases as pH decreases
more CO2 diffused into RBC = lower pH = releases more oxygen
BPG is a bi-product made by ATP glycolysis which affects oxygen binding
more BPG = more oxygen released by Hb
BGP levels rise when pH increases or when stimulated by certain hormones
BPG levels too low = Hb will not release oxygen

Question 9

CO2 transportation

Answer 9

there are 3 pathways or CO2 molecules in the bloodstream which are all reversible:
dissolve into plasma
bind to haemoglobin
converted to H2CO3 (carbonic acid)

Question 10

transportation

Answer 10

7% of CO2 remains dissolved in plasma as it gets saturated rapidly
other 93% diffuses into RBCs
23% then binds to HB to from carbaminohaemoglobin
other 70% converted to carbonic acid by enzyme carbonic anhydrase
each carbonic acid molecule immediately dissociates into a hydrogen and a bicarbonate ion
the hydrogen is removed by buffers (especially Hb) and bicarbonate moves into the plasma in exchange for a chloride ion (chloride shift) until a reverse reaction occurs when blood reaches the lungs

Question 11

control of respiration

Answer 11

normal conditions: rates of oxygen absorption and carbon dioxide production is the same at lungs
normality removed: changes in blood flow and oxygen delivery under local control, changes in respiration under control of respiratory centres in brain

Question 12

tissues during activity

Answer 12

decrease in PO2 and increase in PCO2
changes in gas exchange
increased blood flow
rising PCO2 levels causes smooth muscle in walls of arterioles to relax, increasing blood flow

Question 13

neural control of respiration

Answer 13

respiratory centres in brain: when oxygen demand rises, increases cardiac output and respiratory rates under neural control
involuntary: regulation of respiratory muscles, frequency, depth, response to info from lungs
voluntary: activity in cerebral cortex affects output of respiratory centre and motor neurons
respiratory centres are three pairs of nuclei in the reticular formation of the pons and medulla oblongata

Question 14

respiratory centres

Answer 14

medulla oblongata - the reflex centres, establishes pace and depth
pons - apneustic and pneumotaxic centres = paired nuclei that adjust output of respiratory rhythmicity centres, respond to sensory stimuli, emotional states, speech patterns
dorsal respiratory group = inspiratory centre, functions in quiet and forced breathing
ventral respiratory group = inspiratory and expiratory centre, function only in forced breathing

Question 15

DRG vs VRG

Answer 15

DRG’s inspiratory centre functions in every respiratory cycle, neurons that control inspiratory muscles (external intercostal muscles and diaphragm), stimulation for 2 seconds, silent for next 3 seconds when passive exhalation takes place
VRG functions only during forced breathing, expiratory centre that contains neurons that control accessory respiratory muscles involved in active exhalation. Inspiratory centre contains neurons involved in maximal inhalation

Question 16

respiratory inhibition

Answer 16

takes place between the neurons involved with inhalation and exhalation
when inspiratory neurons are active, expiratory neurons are inhibited and vice versa

Question 17

respiratory reflexes

Answer 17

respiratory reflexes = when activities of the reparatory centre are modified by sensory information from receptors, altering the pattern of respiratory
chemoreceptors = sensitive to change in PCO2, PO2, pH of blood or pH of CSF
baroreceptors = sensitive to change in blood pressure
stretch receptors = change in lung volume
physical or chemical stimuli in nasal cavity, larynx or bronchial tree
other sensations including pain, changes in body temp, abnormal visceral senses

Question 18

mechanoreceptors

Answer 18

baroreceptors in aortic or carotid sinuses = sensitive to change in BP, when BP falls, respiratory rate increases and vice versa
stretch receptors = respond to changes in lung volume

Question 19

Hering Breuer relfexes

Answer 19

2 mechanoreceptors reflexes involved in forced breathing
inflation reflex = as lung volume increases, DRG is inhibited and VRG stimulated to prevent overexpansion of lungs
deflation reflex = inhibits expiratory centres so lungs aren’t completely empty during forced exhalation

Question 20

chemoreceptor relfexes

Answer 20

they respond to chemical changes in blood and CSF
their stimulation results in increased depth and rate of respiration
respiratory centres are strongly influenced by chemoreceptor input from: glossopharyngeal nerve (carotid bodies), vagus nerve (aortic bodies) and central chemoreceptors that monitor CSF
drop in PO2 to around 40mmHg increases respiration by 50-70%
rise in 10% arterial PCO2 increase respiration by 100%
chemoreceptor stimulation subjected to adaption - decreased sensitivity due to chronic stimulation

Question 21

summary of control

Answer 21

Basic pace of respiration is established by the interplay between respiratory centres in the pons and medulla oblongata
That pace is modified in response to input from chemoreceptors, baroreceptors and stretch receptors
In general, CO2 levels, rather than O2, are the primary drivers for respiratory activity
Respiratory activity can also be interrupted by protective reflexes and adjusted by the conscious control of respiratory muscles

Question 22

respiration changes with age

Answer 22

before birth: pulmonary vessels are collapsed, lungs contain no air
at birth: new-born overcomes force of surface tension to inflate bronchial tree and alveoli and take first breath
elderly changes: deterioration in elastic tissue, arthritic changes and decreased flexibility, emphysema (smoking)