Overall Function of Respiratory System
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exchange of O2 and CO2 from pulmonary capillaries
thermoregulation
phonation
assistance in regulation of acidity
elimination of water
Anatomy of Respiratory System
• horse cannot breath through the mouth
• upper and lower
– upper
• includes nostrils, nasal passages, pharynx, larynx and trachea
to thorax
• regulates temperature and humidity of air
• phonation
• protection of foreign bodies
– lower - within the thorax
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lungs
left and right bronchi
bronchioles
alveoli - surrounded by blood capillaries
– site of gas exchange
• nostrils - large, wide nostrils desirable
– impaired by facial nerve damage
• nasal cavities - high vascularity
– provide large surface area for heat and water exchange
– cause airflow resistance
• sinuses - air filled cavities that open into nasal cavity
• pharynx - common passage for food and air
– dorsal displacement of soft palate
• larynx - voice box
– laryngeal hemiplagia
• trachea - non-collapsible tube with cartilage rings
• lungs
– large alveolar density compared to other species
– ribcage rigid compared to other species
Control of Breathing
• regulated by 3 factors:
– chemical factors - increase CO2 sends message to
increase breathing frequency and breath more deeply
– nervous control - natural flight response triggers
increased rate of breathing
– mechanical control
• muscle expand and contract ribcage
• force of front legs landing
• chemical and nervous control important at walk and trot
• mechanical control important at canter and gallop
Respiratory Mechanics
• muscular regulation
• inspiration and expiration regulated by muscular effort and
elastic recoil
– diaphragm - main muscle
– contraction - flattened and back in body
• lengthens thorax, increasing volume and stimulating
inspiration
– relaxation - curved into thorax
• abdominal and thoracic muscle contract, decreasing width of
thorax, causing expiration
• resting point (of respiration) - opposing elastic forces are at
equilibrium
• energy is required to increase or decrease volume of lungs
and thorax passed resting point
• recoil returns lungs and thorax to resting point with out
energy
• horse resting point
– biphase - middle of breath
• inspiration - initially passive followed by contraction of
diaphragm
• expiration - passive to resting point followed by abdominal
muscle compression of ribcage (COPD)
Airway Resistance
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nostrils and larynx
reduced by flaring of nostrils
reduced by dilation of larynx
neck and head in straight line
• respiration rate - number of breaths per minute
– rest : 12-20 /min
– intense exercise : 150-180 /min
– changes with exercise, pain and increased body
temperature
• tidal volume - amount of air inhaled and exhaled with each
breath
– rest : 4-7 liters
– exercise : 10 liters
– alveolar ventilation - inspired air that reaches lungs
– dead space ventilation - air that stays in airways and the
part of lungs without gas exchange
– alveolar volume - difference between tidal volume and
dead space
• dead space
– resting horse 60-70% of tidal volume
– intense exercise 20 % of tidal volume
– prolonged steady exercise
• steady increase in dead space with increased respiration rate
– thermoregulation
– percent of dead space in horse at rest, twice as much as
humans and dog
• minute volume - amount of air passing in and out per
minute
– minute volume = respiratory rate X tidal volume
– rest
100 liters/min
– maximal exercise
1500 liters/min
• 7 fold increase in respiratory rate
• 2 fold increase in tidal volume
Functions at Rest and Adaptations of
Respiratory System
• 1) ventilation (air into alveoli) - bulk movement of air into
and out of lungs
– minute volume
• 2) perfusion (how gas is removed from the lungs by the
blood)
– dependent on pressure difference between pulmonary
artery, pulmonary vein, and vascular resistance
• 1% increase PCV results in 4% increase in pulmonary
vascular resistance
• 3) diffusion (how gas gets across the air blood barrier)
– rate of diffusion
• pressure gradient
• diffusability of the gas
– CO2 very soluble, in and out of solution easily
– O2 low solubility, transported by hemoglobin
• thickness of membrane
– all 3 processes increase during exercise to meet O2
demands
Diffusion
• pulmonary diffusion
– humans : 4-5 fold increase in pulmonary blood flow;
expanding capillary blood volume 3 times
– horse : 8 fold increase in pulmonary blood flow; even
greater increase in capillary blood volume
• tissue diffusion
– O2 and CO2 diffuse down the pressure gradient
– PO2 returning from muscle tissue following heavy
exercise, only 16mm Hg
• increased driving pressure of O2 from arterial blood into
muscle
– tissues with high aerobic needs are more vascularized
• greater surface area for exchange