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VT 106
Comparative Anatomy and Physiology
Respiratory System
respiration – gas exchange (O2 & CO2)
cellular respiration (aerobic respiration)
mitochondria consume O2 to produce ATP (cells need O2)
and produce CO2 as a by-product (cells must get rid of CO2)
STEPS IN RESPIRATION
1) pulmonary ventilation – breathing
inhalation (inspiration) – breathing in; O2 rich air flows into lungs
exhalation (expiration) – breathing out; CO2 rich air flows out of lungs
2) external respiration – exchange occurring in lungs
3) internal respiration – exchange occurring in systemic tissues
Other Functions of the Respiratory System
conditions air – warms, moisturizes
olfaction – olfactory epithelium
phonation – sound production
protection – prevents debris and microbes from entering body
mucus, cilia, macrophages, tonsils
acid-base homeostasis
STRUCTURAL DIVISIONS OF THE RESPIRATORY SYSTEM
Upper Respiratory Tract
nose, nasal passages, pharynx, larynx, trachea
Lower Respiratory Tract
bronchi, bronchioles, alveolar structures, lungs
ANATOMY OF THE UPPER RESPIRATORY TRACT
1) NOSE – functions in conditioning air, olfaction, phonation
nares - nostrils
nasal passages – internal nose
nasal septum – divides nasal cavity into right & left halves
posterior – vomer and ethmoid bones
anterior – hyaline cartilage
nasal turbinates – increase mucosal surface area to condition air
dorsal and ventral turbinates
nasal meatuses – 3 tunnels formed by the turbinates
nasal mucosa - pseudostratified ciliated columnar epithelium
with goblet cells
many blood vessels and mucous glands in connective tissue
beneath the epithelium
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blood supply warms inhaled air
mucus moisturizes inhaled air and traps debris
cilia sweep debris back into pharynx
olfactory epithelium – sense of smell
openings to paranasal sinuses – resonating chambers
frontal, & maxillary sinuses (ethmoid & sphenoid sinuses)
2) PHARYNX (throat) – passage for respiratory and digestive tracts
funnel of bone and muscle
mucous membrane lines surface
3 Divisions of Pharynx:
1) nasopharynx – respiratory passageway
dorsal to soft palate
pseudostratified ciliated columnar epithelium/goblets
2) oropharynx – digestive passageway from mouth
ventral to soft palate
stratified squamous epithelium – protects from abrasion
3) laryngopharynx – respiratory and digestive passageway
stratified squamous epithelium – protects from abrasion
ventrally, opens into larynx
dorsally, opens into esophagus
tonsils – clusters of MALT (lymphatic tissue)
3) LARYNX
cartilage and ligament tube between laryngopharynx and trachea
anchored to hyoid apparatus at base of tongue
Functions of Larynx:
phonation – produces sound waves
protects airways from aspiration (inhaling foreign material)
thyroid cartilage – large, cranial hyaline cartilage ring
connected to hyoid by ligaments
cricoid cartilage – smaller, caudal ring of hyaline cartilage
epiglottis – leaf-shaped elastic cartilage near base of tongue
glottis – opening into larynx
during swallowing
extrinsic muscles attached to thyroid and cricoid cartilages
elevate the larynx
epiglottis folds down to cover glottis
keeps food and fluids out of trachea
vocal cords – elastic connective tissue bands within the glottis
vibrate to produce sounds
arytenoid cartilages (2) – hyaline cartilages attached to vocal cords
intrinsic muscles move arytenoids and alter tension on vocal cords
alter pitch of voice
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can close glottis to protect airways or hold breath
cough reflex – stimulated by liquid or solid in larynx
vestibular folds – mucosal folds lateral to vocal cords in
nonruminant species
lateral ventricles – pouches between vocal cords and
vestibular folds (roaring in horses)
Epithelium of Larynx:
cranial to vocal cords – stratified squamous
caudal to vocal cords – pseudostratified ciliated columnar/goblets
cilia sweep debris up into pharynx
4) TRACHEA (windpipe) – tube of hyaline cartilage and ligaments that extends
from the larynx to the bronchi
lined by pseudostratified ciliated columnar epithelium/goblets
cilia sweep debris up to the pharynx
C-shaped hyaline cartilage rings – prevent collapse of trachea
trachealis muscle – smooth muscle completes ring dorsally
can adjust diameter of trachea
allows esophagus to compress trachea when swallowing
tracheal collapse – large, loose trachealis muscle
tracheal bifurcation – trachea splits into 2 main stem bronchi
ANATOMY OF THE LOWER RESPIRATORY SYSTEM
BRONCHIAL TREE – branching airways that carry air to the alveoli
1) main (primary) bronchi – right & left branches to R and L lungs
2) lobar (secondary) bronchi – branches to individual lung lobes
histology of bronchi is similar to trachea except cartilage is reduced as
bronchi get smaller
3) bronchioles – tiny branches of smallest bronchi
terminal bronchioles – end of bronchioles
histology of bronchioles changes as they get smaller:
epithelial cells get shorter and lose cilia & goblet cells
proportion of cartilage decreases
proportion of smooth muscle in walls increases
ANS regulates diameter of airways (mainly bronchioles)
sympathetic – bronchodilation increases airflow
parasympathetic – bronchoconstriction decreases airflow
asthma – irritants or allergies cause bronchoconstriction and inflammation
of airways, making breathing difficult
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ALVEOLAR STRUCTURES
alveolar ducts – final branches of respiratory tree where gas exchange begins
alveolar sacs – grape-like clusters of alveoli
alveoli – tiny sacs at the ends of airways, surrounded by capillary beds
main site for external respiration (gas exchange)
Histology of an Alveolus
respiratory membrane – very thin membrane where gas
exchange occurs
layers of respiratory membrane:
1) alveolar wall – simple squamous epithelium (type I cells)
2) thin basement membrane containing elastic fibers
3) endothelium of capillaries – simple squamous epithelium
type II alveolar cells – cuboidal secretory cells
surfactant – oily secretion that lubricates alveolar surface
lipids reduce surface tension (attraction of water
molecules for each other), which prevents
alveoli collapsing
LUNGS – bronchial tree + alveolar structures
Thoracic Cavity – divided by mediastinum into 2 pleural cavities
pleural membranes – serous membranes
parietal pleura – lines cavity
visceral pleura – lines lungs
pleural cavities – contain pleural fluid (lubricates lung surface)
pleural effusion – excess fluid in pleural cavity
pleuritis – inflammation of pleural membranes
causes friction and pain
Gross Anatomy of Lungs
base – broad end near diaphragm
apex – narrow, cranial end
hilus – entry point for bronchi, nerves, blood & lymph vessels
fissures divide lungs into lobes
right lung (4) – cranial, middle, caudal, accessory lobes
left lung (2) – cranial, caudal lobes
[horse – left lung (1 lobe), right lung (1 lobe) + accessory]
Blood Supply to Lungs
Pulmonary Circuit (branches follow respiratory tree)
right & left pulmonary arteries (deoxygenated blood)
pulmonary capillaries surround alveoli
right & left pulmonary veins (oxygenated blood)
Ventilation – Perfusion Coupling
hypoxia (low O2) in lung tissues causes vasoconstriction
diverts blood to lung regions with good oxygenation
(opposite effect occurs in other body tissues)
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PULMONARY PHYSIOLOGY
PULMONARY VENTILATION
air flows from high pressure to low pressure
air pressure is due to movement of air molecules
760 mmHg at sea level
(Boyle’s Law) – air pressure varies inversely with volume
increase volume of container = air pressure decreases
decrease volume of container = air pressure increases
(ventilation occurs due to changing size of thoracic cavity)
3 Regions of Pressure
1) atmospheric pressure – air pressure around body
2) alveolar pressure – air pressure inside lungs
3) intrathoracic pressure – air pressure in thoracic cavity
slightly less than alveolar pressure (partial vacuum)
visceral pleura adheres to parietal pleura
surface tension due to pleural fluid
inhalation (inspiration) – air flows into lungs
occurs when alveolar pressure < atmospheric pressure
1) size of thoracic cavity increases – active process
diaphragm contracts – flattens to increase length of cavity
external intercostal muscles contract – swivels ribs cranially
increases diameter of cavity
2) volume of thoracic cavity increases = intrathoracic pressure decreases
3) lungs adhere to thoracic wall, stretch and expand
lung volume increases = alveolar pressure decreases
4) air flows into lungs until alveolar pressure = atmospheric pressure
forced inhalations – other muscles of neck and shoulders help elevate
ribs, making the thoracic cavity larger, and more air is inhaled
exhalation (expiration)– air flows out of lungs
occurs when alveolar pressure > atmospheric pressure
1) size of thoracic cavity decreases – passive process
diaphragm relaxes – rounds cranially
external intercostals relax – ribs swivel caudally
2) thoracic cavity & lungs decrease in volume
(elastic recoil of lung tissue & chest aids in decreasing volume)
alveolar pressure increases above atmospheric pressure
3) air flows out of lungs until alveolar pressure = atmospheric pressure
forced exhalations – active
internal intercostal muscles – pull ribs caudally
abdominal muscles – pull ribs caudally & compress abdominal cavity
abdominal organs push diaphragm cranially
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Factors Affecting Pulmonary Ventilation:
airway resistance – friction of air against airways
depends on diameter of airways
obstructive disorders – decreased diameter of airways
COPD (chronic obstructive pulmonary disease), asthma,
bronchitis (inflammation of bronchial tree)
compliance – ability of thorax or lungs to expand
restrictive disorders – prevent expansion of thorax or lungs
decreased elasticity of lungs – fibrosis, inflammation
pneumonia – infection of lungs
physical compression
pleural effusion – fluid in pleural cavity
pneumothorax – air in pleural cavity
skeletal or muscular disorders
increased surface tension in alveoli
deficiency of surfactant
pulmonary edema – fluid in lungs
Respiratory Volumes
tidal volume – volume of 1 breath at rest
at rest about 70% of tidal volume reaches alveoli
anatomic dead space – 30% is in airways where no gas exchange
occurs
minute volume – volume inhaled/minute
tidal volume X respiratory rate (breaths/min)
residual volume – air remaining in lungs after forced exhalation
(all air in lungs can never be exhaled)
vital capacity – maximum volume that can be forcefully inhaled
following forced exhalation
(maximum volume that can be exchanged)
RESPIRATION – EXCHANGE OF O2 & CO2
ATMOSPHERIC GAS
gas
nitrogen
oxygen
carbon dioxide
water vapor
others
atmospheric pressure = 760mmHg
% of atmosphere
partial pressure
78.6
597.4 mmHg
20.9
158 mmHg
0.04
0.3 mmHg
0.4
3.0 mmHg
0.06
0.5 mmHg
(Dalton’s Law) – each gas exerts its own pressure proportional to its concentration
partial pressure – pressure exerted by one gas in a mixture of gases
high concentration = high partial pressure
gases diffuse from high pressure (concentration) to low pressure (concentration)
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(Henry’s Law) – the quantity of gas that will dissolve in a liquid is proportional to its
partial pressure
high partial pressure = more gas in solution
EXTERNAL (PULMONARY) RESPIRATION – occurs in lungs
1) air in alveoli (high PO2 / low PCO2)
2) blood in pulmonary capillaries (low PO2 / high PCO2)
O2 diffuses into capillaries / CO2 diffuses out of capillaries
INTERNAL (TISSUE) RESPIRATION – occurs in systemic tissues
1) blood in systemic capillaries (high PO2 / low PCO2)
2) interstitial fluid in systemic tissues (low PO2 / high PCO2)
O2 diffuses into tissues / CO2 diffuses out of tissues
the amount of a gas in solution also depends on the gas’s solubility(molecular structure)
O2 – moderately soluble in body fluids
CO2 – 24X more soluble than O2
N2 – very little solubility in body fluids
OXYGEN TRANSPORT IN BLOOD
1) free O2 (dissolved in plasma) – 1.5%
2) bound to hemoglobin (Hb) – 98.5%
oxyhemoglobin – bound to O2 (up to 4 molecules); bright red
deoxyhemoglobin – no bound O2; darker, purplish
Hb saturation – percentage of O2-binding sites occupied
pulse oximeter measures Hb saturation of blood
carbon monoxide (CO) – binds to O2 binding sites on Hb
CO has 200X the affinity for Hb binding-sites (blocks O2 binding)
CARBON DIOXIDE TRANSPORT IN BLOOD
1) free CO2 (dissolved) – 7%
2) carbaminohemoglobin – 23% (reversibly bound to globin portion of Hb)
high PCO2 (tissues) - increases formation
low PCO2 (lungs) - CO2 released and diffuses out of blood
3) bicarbonate ions – 70%
CO2 + H2O <-----------> H2CO3 <--------------> H+ + HCO3- (reversible)
carbonic acid
bicarbonate ion
as CO2 increases, body fluids become more acidic
some H+ binds to Hb (hemoglobin helps buffer acids in body)
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CONTROL OF RESPIRATION
oxygen demands vary with metabolic activity
exercise – 15-20X higher demand
cardiovascular changes – increase tissue perfusion
respiratory changes – increase ventilation rate and volume
CNS REGULATION – RESPIRATORY CENTER
located in medulla oblongata; sends signals via phrenic and intercostal nerves
inhalation – stimulates diaphragm and external intercostals to contract at regular
intervals
exhalation – periods of no stimulation
also regulates muscles for forced inhalation and exhalation
rate and depth of breathing are set in response to ANS reflexes
RESPIRATORY REFLEXES – ANS
Stretch Reflexes (mechanical control) – stretch receptors in bronchioles and
alveoli
set inflation and deflation limits for lungs
Chemical Reflexes – triggered by levels of CO2, H+, O2
central chemoreceptors – medulla oblongata
detect H+ and CO2 levels in CSF and blood
peripheral chemoreceptors – detect CO2, O2 and H+ levels in blood
aortic bodies (aortic arch) – vagus nerve (CN X)
carotid bodies (carotid sinus) – glossopharyngeal nerve (CN IX)
hypercapnia (high CO2) – main stimulus for respirations
also causes acidosis (more H+), which stimulates respirations
respiratory rate is important in regulating acid-base balance
hypoxia (low O2) – also a stimulus for respirations
(severe hypoxia depresses all brain functions)
Baroreceptor Reflexes – triggered by changes in blood pressure
low BP stimulates respirations
high BP inhibits respirations
Protective Reflexes – irritation of airways stimulate sneezing & coughing
Cerebrocortical Regulation – allows limited voluntary control of breathing
(eg. holding breath, sniffing, phonation)
buildup of CO2 and H+ eventually stimulate respiratory center and override
voluntary control
hyperventilation – ventilation exceeding O2 needs of tissues
can lead to hypocapnia – low CO2; inhibits respirations
hypoventilation – ventilation inadequate to meet O2 needs of tissues
leads to hypercapnea and acidosis
apnea – absence of breathing
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AVIAN RESPIRATORY SYSTEM
Upper Respiratory Tract
choana – no soft palate; nasal passage opens into roof of mouth
larynx – no vocal cords
syrinx – enlargement of trachea at tracheal bifurcation
contains vibrating membranes and muscles to adjust tension
functions as “voicebox” of bird
Lower Respiratory Tract
birds do NOT have a diaphragm
lungs do NOT inflate and deflate
9 air sacs are connected to bronchi
warm and moisturize inhaled air
expand and contract to force air through respiratory tract
pneumatic bones – extensions of air sacs are found within largest bones of the
pectoral and pelvic girdles and limbs
(eg. synsacrum, sternum, femur, humerus)
Respiratory Physiology
airflow is unidirectional
thoracoabdominal cavity expands to inhale, contracts to exhale
2 inhalations are needed to cycle air completely through respiratory tract
inhalation 1 – air flows into caudal air sacs
exhalation 1 – air forced into lungs (composed of air capillaries)
inhalation 2 – air flows into anterior air sacs
exhalation 2 – air forced out through trachea
gas exchange is very efficient
“fresh” inhaled air never mixes with “used” exhaled air
air sacs also function in evaporative cooling
respiratory rate can increase dramatically when birds are hot
ACID-BASE BALANCE
acid – dissociates in solution to form H+ (proton)
base – dissociates in solution to form a proton acceptor
decreases acidity by binding H+
pH scale measures H+ concentration of a solution
7 = neutral (acid=base)
<7 = acidic (more hydrogen ions)
>7 = basic or alkaline (fewer hydrogen ions)
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normal blood pH is vital for normal metabolic functions
proteins (structural, enzymes) denature at high or low pH
pH changes disrupt cell membranes
pH changes alter balance of reversible reactions
acidosis – physiological state when pH is too low
CNS – disorientation, coma
heart – decreases contractility, causes arrhythmias
alkalosis – physiological state when pH is too high
CNS – convulsions, seizures
muscles – spasms, tetanus
Acid-Base Disturbances
respiratory acidosis – low pH due to inadequate respiration
CO2 builds up in blood and forms carbonic acid
respiratory alkalosis – high pH due to excessive respiration
CO2 is exhaled too rapidly and less carbonic acid is formed
metabolic acidosis – low pH due to altered metabolic processes
high metabolic rate causes mores CO2 production – carbonic acid
catabolizing proteins forms acid by-products
hypoxia causes lactic acid formation
starvation – breaking down fats and proteins forms ketone bodies
metabolic alkalosis – high pH due to altered metabolic processes
loss of acid – severe vomiting (HCl lost)
excess alkaline drugs – antacids
Regulation of Body pH
3 Mechanisms:
1) buffer systems – molecules that can temporarily bind excess H+
or donate H+ when they are low
protein buffers (eg. hemoglobin)
carbonic acid – bicarbonate buffer system (in ECF)
carbonic acid (H2CO3) – donates H+ when acidity decreases
bicarbonate (HCO3-) – binds excess H+ when acidity increases
2) respiratory compensation – altering ventilation to adjust pH
CO2 + H2O <----> H2CO3 <----> H+ + HCO3(CO2 diffuses out of blood in lungs and is exhaled)
increasing respirations = more CO2 exhaled = blood is less acidic
decreasing respirations = less CO2 exhaled = blood is more acidic
3) renal compensation – altering kidney activity to adjust pH
the DCT & collecting duct usually secrete excess H+ and reabsorb HCO3acidosis causes more H+ secretion and more HCO3- reabsorption
alkalosis causes less H+ secretion and less HCO3- reabsorption
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