Overall Function of Respiratory System

1. Overall Function of Respiratory System • exchange of O2 and CO2 from pulmonary capillaries • thermoregulation • phonation • assistance in regulation of acidity • elimination of water

2. 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 • lungs • left and right bronchi • bronchioles • alveoli - surrounded by blood capillaries • site of gas exchange

3. 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

4. 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

5. 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

6. 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)

7. Airway Resistance • nostrils and larynx • reduced by flaring of nostrils • reduced by dilation of larynx • neck and head in straight line

8. 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

9. 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

10. 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

11. 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

12. 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