Respiratory Structures

Respiratory tract is the conducting portion or airway consists of nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.

Pharynx is a common passageway for air and food. The opening from the pharynx leading to the continuation of the airway is the larynx, the organ of phonation (sound production) in mammals.

It is produced by the controlled passage which causes vibration of vocal cords of larynx.

The organ of phonation in birds called the syrinx which is located at the junction where trachea divides into bronchi.

The glottis is the slit-like opening between the vocal cords. Extending craniad from the larynx is the epiglottis.

It is a leaf shaped plate of cartilage covered with mucous membrane.

It is located at the root of the tongue, which is passively bent over the larynx during the act of swallowing, thereby preventing the entrance of food into the trachea.

Pleural membranes

The lungs are surrounded by a serous membrane called as pleural membrane. The superficial layer lining the thoracic cavity is the parietal pleura and the layer closely adhering to the lungs is the visceral pleura.

The narrow parietal space between these two layers contains a small amount of pleural fluid that allows the two layers to slide over one another during breathing.

The lungs have an almost friction-free movement within the thorax because of the pleura. Inflammation of the pleural membrane is called pleurisy.

After the first breath, the lungs become less dense. If the lung floats in water, it indicates that the animal took at least one breath, and therefore was born alive. This fact allows one to determine whether a newborn animal is stillborn or not.

The pressure inside the intrapleural space is negative due to continuous removal of fluid that is secreted . This negative pressure is vital for the expansion of the lungs.

If an injury to the chest wall punctures the pleural membrane, it can allow air to enter the intrapleural space, resulting in a condition called as “pneumothorax“.

Respiratory membrane

The respiratory membrane is where O₂ and CO₂ diffuse across the alveolar and capillary walls. It is a very thin membrane about 0.5 µm thick and consists of four layers. Exchange between alveoli and capillaries.

• Gas moving from alveolus to blood must pass through:
  1. Thin layer of pulmonary surfactant
  2. Alveolar epithelium : A layer of type I and type II alveolar cells, and alveolar macrophages.
  3. The epithelial basement membrane.
  4. The capillary basement membrane.
  5. Capillary endothelium
  6. Plasma
  7. Red blood cell membrane
  8. Red blood cell to reach hemoglobin

The pulmonary artery carries deoxygenated blood, while oxygenated blood returns to the left atrium via the pulmonary veins. The darker purple color of venous blood becomes bright red arterial blood during the saturation of hemoglobin with oxygen that has freshly diffused from the alveoli. 

During exercise, cardiac output can increase by as much as eightfold. During this time, blood flow to the lungs must increase. Pulmonary blood vessels dilate. In the horse, pulmonary arterial pressure can be so high as to cause erythrocytes to leak from pulmonary capillaries, a condition called as exercise-induced pulmonary hemorrhage, in athletic horses.

Pulmonary ventilation

• Respiration, the process of gaseous exchange, occurs in three steps-
  1. Pulmonary ventilation, or breathing, is the mechanical movement of air into (inspiration) and out (expiration) of the lungs.
  2. External respiration is the exchange of gases between the lungs and the pulmonary capillaries, which occurs across the respiratory membrane. The blood gains O₂ and loses CO₂.
  3. Internal respiration is the exchange of gases between systemic capillaries and tissue. The blood gains CO₂ and loses O₂.