Functional RespiratoryAnatomy �of �Mammals and Birds

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Dr. Amar Chaudhary, ^{DVM, MS}

Assistant Professor

Department of Physiology and Biochemistry

Agriculture and Forestry University,

Rampur, Chitwan, Nepal

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Respiratory System

• Exchange of gases, mainly oxygen (O₂) and carbon dioxide (CO₂), between the organism and the environment
• In animals, the system’s primary function is to facilitate this gas exchange for cellular metabolism, maintenance of homeostasis, and acid-base balance
• It also plays a role in vocalization, thermoregulation, and defence against pathogens
• Components of the Respiratory System

Upper Respiratory Tract (URT)

• Nose (Nasal Cavity):
• Nostrils (External Nares): The entry point for air, equipped with vibrissae (nasal hairs) to filter particles
• Nasal Passages: Lined with a mucosa that warms, moistens, and filters the incoming air
• Turbinates (Nasal Conchae): These scroll-like structures increase the surface area for air conditioning and olfaction
• Pharynx:
• Nasopharynx: The area behind the nasal cavity that connects to the oral cavity and the trachea
• Oropharynx: The portion shared by both the digestive and respiratory systems
• Laryngopharynx: Located posteriorly to the larynx, serves as a passageway for both food and air
• Larynx (Voice Box):
• Functions:
• Passageway for air, vocalization (via vocal cords), and protection of the lower airways (via the epiglottis, which prevents aspiration)
• Cartilages: Including the thyroid cartilage, cricoid cartilage, and arytenoid cartilages
• Vocal cords: Produce sound by vibrating as air passes through

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Respiratory System

Lower Respiratory Tract (LRT)

• Trachea (Windpipe):
• A tubular structure made of C-shaped cartilage rings that keep the airway open
• It runs from the larynx to the bifurcation into the bronchi
• Tracheal mucosa: Ciliated epithelium that helps trap debris and pathogens
• Bronchi and Bronchioles:
• Primary Bronchi:
• The trachea bifurcates into two primary bronchi, one for each lung
• Secondary (Lobar) Bronchi:
• Branches further into smaller bronchi that lead into lobes of the lungs
• Bronchioles:
• Smallest airways without cartilage; lined with smooth muscle and ciliated epithelium
• They branch further into terminal bronchioles
• Alveolar Ducts and Alveoli:
• Alveolar Ducts:
• Small, tube-like structures leading to alveolar sacs
• Alveoli:
• Microscopic air sacs where gas exchange occurs
• They are surrounded by a network of capillaries for efficient O₂ and CO₂ exchange

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Respiratory System

• The lungs
• are the principal organs of respiration, located in the thoracic cavity
• Each lung is divided into lobes (e.g., 3 in the right lung, 2 in the left lung for dogs, cats, and most mammals)
• The lungs are protected by the pleura, a double-layered serous membrane
• Pleura:
• Two layers of serous membrane (visceral and parietal) that encase the lungs and line the thoracic cavity
• Thoracic Cavity:
• The space within the rib cage that houses the lungs
• The diaphragm serves as the primary muscle for respiration

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Respiratory Apparatus and Mechanism of Breathing

• Ventilation (Breathing) :
• refers to the physical process of moving air in and out of the lungs, consisting of two phases:
• Inhalation (Inspiration):
• The diaphragm contracts and moves downward, expanding the thoracic cavity
• Intercostal muscles assist by lifting the ribs, decreasing internal pressure and allowing air to flow in
• Exhalation (Expiration):
• The diaphragm relaxes and moves upward, while the intercostal muscles relax, causing the thoracic cavity to decrease in volume
• This increase in pressure expels air from the lungs
• Gas Exchange and Diffusion
• Alveolar-capillary membrane:
• The thin membrane separating the alveoli and capillaries allows for the exchange of gases via diffusion
• Oxygen (O₂) moves from the alveoli into the blood, while carbon dioxide (CO₂) moves from the blood into the alveoli to be exhaled

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Control of Respiration

Respiration is regulated by both voluntary and involuntary mechanisms:

• Central Control (Brainstem):
• The medulla oblongata and pons in the brainstem are responsible for automatic control of respiration
• They monitor blood levels of CO₂ and adjust the rate and depth of breathing accordingly
• Chemoreceptors in the carotid and aortic bodies detect changes in blood O₂ and CO₂ levels, sending signals to the respiratory centres
• Mechanoreceptors:
• Located in the lungs and airways, they provide feedback on lung stretch and airway resistance, helping to regulate the depth and rhythm of respiration
• Proprioception:
• Muscle stretch receptors (in diaphragm and intercostals) and receptors from the larynx also contribute to the regulation of breathing

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�Clinical Relevance�

• Understanding the functional anatomy of the respiratory system is essential for diagnosing and treating respiratory disorders in animals
• Some conditions include:
• Upper respiratory infections (e.g., kennel cough in dogs, feline viral upper respiratory disease)
• Obstructive disorders (e.g., laryngeal paralysis, brachycephalic airway syndrome in dogs)
• Restrictive lung diseases (e.g., pleural effusion, pulmonary fibrosis)
• Inflammatory diseases (e.g., pneumonia, bronchitis)
• Trauma: Rib fractures, pneumothorax (air in the pleural cavity), or hemothorax (blood in the pleural cavity)

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Functional anatomy Of the Avian Respiratory system

Birds have a highly efficient respiratory system designed to meet the oxygen demands of flight

Some of the key features are:

• Respiratory Structures:
• Nostrils (Nares):
• Located at the beak, these allow air to enter and are lined with a mucous membrane to filter and humidify the incoming air
• Trachea:
• The trachea is the windpipe through which air travels from the external environment to the lungs
• It is supported by cartilaginous rings to prevent collapse
• Lungs:
• The lungs in birds are relatively rigid and do not expand and contract significantly like mammalian lungs
• They are more like a series of interconnected tubes where gas exchange takes place

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Functional anatomy of the Avian Respiratory system

• Air Sacs:
• Birds have air sacs (anterior and posterior) that do not participate in gas exchange but act as reservoirs to help move air through the lungs
• There are 9 air sacs in most birds
• Anterior air sacs:
• Includes the cervical, anterior thoracic, and interclavicular sacs
• Posterior air sacs:
• Includes the abdominal and posterior thoracic sacs.
• Function of Air Sacs:
• Non-Respiratory Role:
• Air sacs do not directly participate in gas exchange but store and pump air through the lungs
• Continuous Airflow:
• Air sacs allow birds to maintain a continuous flow of air through the lungs, which is essential for efficient gas exchange, especially during flight when oxygen demand is high

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Mechanisms of Avian respiration

• Avian respiration differs significantly from that of mammals due to the presence of air sacs
• Which facilitate a unidirectional flow of air through the lungs
• Respiration in birds involve two respiratory cycles to fully process a single breath of air
• Unidirectional Airflow:
• In birds, air flows in one direction through the lungs, unlike the bidirectional flow seen in mammals
• This unidirectional flow increases the efficiency of gas exchange by minimizing the mixing of fresh and stale air
• The air flows through the lungs from posterior to anterior, moving into the air sacs, then back through the lungs during the second inhalation
• Two Phases of Breathing:
• Inhalation Phase 1:
• Air enters the posterior air sacs, and the air that has been exhaled from the lungs is pushed out into the external environment
• Exhalation Phase 1:
• The air moves from the posterior air sacs into the lungs for gas exchange
• Inhalation Phase 2:
• During the second inhalation, fresh air moves into the anterior air sacs, and the air from the lungs moves into the posterior air sacs
• Exhalation Phase 2:
• The air in the anterior air sacs is exhaled into the environment
• In total, it takes two cycles of inhalation and exhalation for air to travel from the beak to the lungs and back out

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Gas exchange in Birds

• Gas exchange in birds occurs primarily in the parabronchi, which are tiny tubes within the lungs
• The design of the avian lung and the air sacs facilitates highly efficient gas exchange
• Parabronchi:
• These are the functional units of the avian lungs where gas exchange occurs
• Unlike mammalian lungs, where air flows in and out of alveoli, birds have a continuous flow of air through the parabronchi, which allows for more efficient oxygen extraction
• The parabronchi are lined with capillaries where oxygen from the air diffuses into the blood, and carbon dioxide diffuses from the blood into the air to be exhaled
• Counter current Exchange Mechanism:
• Gas exchange in the parabronchi is enhanced by a counter current exchange system
• The flow of air through the parabronchi is in the opposite direction to the flow of blood in the capillaries, which maintains a gradient for oxygen to diffuse from the air into the blood and for CO₂ to diffuse out

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Regulation of Respiration in Birds

Respiratory regulation in birds is similar to that of mammals but adapted for the specific needs of flight.

• Neural Regulation:
• The medulla oblongata and pons in the brainstem control the basic rhythm of breathing through the respiratory centres.
• Chemoreceptors in the blood vessels detect changes in carbon dioxide (CO₂) levels and pH
• An increase in CO₂ or a decrease in pH signals the need for faster and deeper breathing to remove excess CO₂
• Stretch receptors in the lungs and air sacs prevent overinflation by sending inhibitory signals to the brainstem, adjusting the depth of breathing
• O₂ and CO₂ Sensing:
• Peripheral chemoreceptors located in the carotid arteries and aortic arch monitor oxygen levels in the blood
• A decrease in oxygen (hypoxia) leads to increased ventilation
• Central chemoreceptors in the brainstem sense CO₂ levels and help adjust the respiratory rate accordingly
• Ventilation and Activity:
• During flight, oxygen demand increases significantly due to muscle activity
• The respiratory system compensates by increasing the rate and depth of breathing
• Birds have a high metabolic rate, which is supported by their efficient respiratory system that ensures continuous oxygen supply even during intense physical activity

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Advantages of the Avian Respiratory System

The avian respiratory system is uniquely adapted for the high metabolic demands of flight:

• Efficient Gas Exchange:
• The unidirectional flow of air and the counter current exchange system in the parabronchi maximize oxygen uptake and carbon dioxide removal
• Continuous Airflow:
• The air sacs allow for a constant supply of fresh air through the lungs, ensuring efficient oxygen extraction
• Increased Oxygen Supply:
• The structure of the avian respiratory system supports high oxygen consumption, which is crucial for flight

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Difference between Mammlain and Avian respiration

• Unidirectional vs. Bidirectional Airflow:
• In birds, air flows in one direction through the lungs, while in mammals, air flows in and out of the lungs
• Air Sacs:
• Birds have air sacs that act as reservoirs and facilitate continuous airflow, while mammals lack air sacs
• Gas Exchange Sites:
• In mammals, gas exchange occurs in the alveoli; in birds, it occurs in the parabronchi

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Clinical relevance

Understanding the physiology of bird respiration is crucial for diagnosing and treating respiratory diseases in avian species

Some clinical considerations include:

• Air Sac Infections:
• Infections can impair the function of air sacs, leading to reduced ventilation efficiency
• Aspiration Pneumonia:
• Birds are prone to inhaling food or liquid into their airways, leading to lung infections
• Respiratory Distress:
• Birds may show rapid or laboured breathing when experiencing respiratory issues such as tracheal obstructions, infectious diseases, or toxin exposure

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