In total ~ 0.5µm thick Estimated 300 million alveoli = immense surface area for gas exchange (~2450m 2 )
Accessory Structures
Diaphragm is a large cranial facing, dome-shaped sheet of skeletal muscle separating the thoracic and abdominal cavities. When it contracts and flattens it enlarges the chest cavity and causes an air pressure change to facilitate inhalation. It has perforations for the aorta, caudal vena cava and esophagus. Gutteral Pouches are unique to the horse and are thought to equalize air pressure within the middle ear. Gutteral pouches are a large caudoventral out-pocketing of the auditory tube and sit between the middle ear & throat. It is located ventral to the occiput between the mandibles, dorsal to the pharynx and laterally bound by the parotid salivary glands. Capacity of the pouches is anywhere between 250-300 ml. The pouches fill with air during exhalation and swallowing to prevent food materials upwards toward the pouches. The pouches are lined with mucus membranes which is continuous with the lining of the pharynx. Important structures that pass through the GP include the internal carotid artery and multiple cranial nerves. The paranasal sinuses are parallel to the nasal passageways which are air-filled spaces which act to reduce the weight of the skull. The sinuses remain in communication with the nasal cavity through apertures. The frontal sinuses occupy the dorsal part of the skull between the eyes. The maxillary sinuses are within the maxilla above the tooth roots. The sphenopalatine sinuses are located medial to the caudal maxillary sinuses. Each sinus consists of a rostral & caudal component. They too are lined with mucus membranes.
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