Respiration | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Mechanism of Respiration Simplified

• Overview of Respiration:
  • Respiratory system facilitates the exchange of gases between air and blood.
  • Two main processes: external respiration (between blood and external environment) and internal respiration (between blood and body tissues).
• Lung Structure:
  • Lungs are in the thoracic cavity, covered by visceral and parietal pleurae.
  • Pleural fluid maintains lung expansion through capillary cohesion.
• Thoracic Cavity Movements:
  • Thoracic walls and diaphragm movements change thoracic cavity volume.
  • Diaphragmatic pleura and costal pleura create a potential space called phrenicocostal sinus.
  • Inspiration: Thoracic cavity expands, creating negative intrapleural pressure, causing lungs to expand.
  • Expiration: Thoracic cavity reduces, intrapleural pressure rises, and elastic lungs expel air.
• Muscles Involved:
  • Inspiratory muscles: Expand thorax through cranial rib movement and lateral rib expansion.
  • External and internal intercostal muscles, transversus thoracis, and abdominal wall muscles contribute.
  • Diaphragm is the major muscle for respiration.
• Breathing Process:
  • Inhalation is an active process.
  • Exhalation is primarily passive, driven by elastic lung recoil.

Transport and Exchange of Gases Simplified

Role of Hemoglobin

• Blood can carry more oxygen and carbon dioxide due to hemoglobin in red blood cells.
• Hemoglobin combines reversibly with oxygen and carbon dioxide, enhancing gas transport in blood.
• Without hemoglobin, blood volume would need to be 75 times greater to meet oxygen and carbon dioxide needs.

Oxygen Combination with Hemoglobin

• Oxygen combines with hemoglobin to form oxyhemoglobin, regulated by oxygen partial pressure in blood plasma.
• Each gram of hemoglobin can chemically bind with 1.36 ml of oxygen.
• Oxygen uptake is not directly proportional to oxygen partial pressure; it follows an S-shaped curve influenced by factors like carbon dioxide, pH, ionic concentration, and temperature.

Influence of Carbon Dioxide and pH

• Increased carbon dioxide or blood pH enhances the release of oxygen from oxyhemoglobin.
• Oxygen transfer to tissues is facilitated by carbon dioxide; carbamino-hemoglobin formation reduces oxygen affinity, and lower blood pH promotes oxygen release.

Carbon Dioxide Transport

• Most carbon dioxide combines with other substances, forming carbonic acid, bicarbonate ion, and carbamino-hemoglobin.
• About one-fifth of carbon dioxide in blood is in carbamino-hemoglobin form.
• Special transport mechanisms enable the exchange of plasma carbon dioxide, carbonic acid, and bicarbonate ion within red blood cells.
• Carbon dioxide dissociation is influenced by the presence of oxygen.

Carbon Dioxide Exchange

• Role of Carbonic Anhydrase:
  • Carbonic anhydrase is an enzyme that speeds up the conversion of carbon dioxide to carbonic acid.
  • Adequacy of carbon dioxide exchange in tissues and lungs is determined by this enzyme.
• Tissue Level Exchange:
  • Physically dissolved carbon dioxide moves from tissues to capillaries, increasing plasma carbon dioxide pressure.
  • Simultaneously, oxygen diffuses from blood to tissues.
  • Chloride shift maintains electrical balance in erythrocytes, countering outward diffusion of bicarbonate ion.
• Pulmonary Level Exchange:
  • At the pulmonary level, the process reverses for the release of carbon dioxide and absorption of oxygen.
  • Five percent of absorbed carbon dioxide dissolves in plasma, while the rest forms plasma bicarbonate and carbamino-hemoglobin in erythrocytes.

Neural Control of Respiration

• Muscles Involved:
  • Diaphragm, a key breathing muscle, is innervated by phrenic nerves (from spinal roots C3, C4, C5).
  • Intercostal muscles (ventral branches of thoracic nerves T1-T12) and other muscles assist in respiration.
• Motor Act of Breathing:
  • Breathing starts with diaphragm contraction and muscle actions coordinated by spinal and cranial nerves.
  • Abdominal muscles are innervated by thoracic and lumbar nerves.
• Respiratory Centers in Medulla Oblongata:
  • Rhythm and limits of breathing are controlled by respiratory centers in the medulla oblongata.
  • These centers integrate information from peripheral receptors, chemosensitive areas in the brain, and higher nervous centers.
• Efferent Impulses:
  • Efferent impulses from respiratory centers travel over phrenic and intercostal nerves to control muscle contractions.

Brain Control of Respiration

• Brain Stem Respiratory Centers:
  • Brain stem components, particularly the pons and medulla, play a crucial role in respiratory regulation.
  • Pons contains respiratory centers—pneumotaxic and apneustic—that influence breathing rate and rhythm.
  • Medulla houses inspiratory and expiratory centers, acting as pacemakers for the respiratory cycle.
• Interaction between Pneumotaxic and Apneustic Centers:
  • Pneumotaxic center inhibits apneustic center, leading to the cessation of the inspiratory phase.
  • Apneustic center facilitates the medullary inspiratory center during normal respiration.
  • Vagus afferents and pneumotaxic center modulate the role of the apneustic center.

Chemoreceptors

• Chemoreceptor Network:
  • Chemoreceptors are sensors in the carotid and aortic bodies responsible for monitoring blood's chemical composition.
  • Stimulated by changes in partial pressures of carbon dioxide, oxygen, and pH.
• Types of Hypoxia and Chemoreceptor Activity:
  • Four types of hypoxia affect chemoreceptor activity, except silent anemic hypoxia.
  • Hypoxic chemoreflex becomes significant when the partial pressure of oxygen drops to around 66 mm Hg.
• Excitatory Locus for Carbon Dioxide:
  • The main stimulator for carbon dioxide is in the medulla's central chemoreceptor (C1).
  • Superficial cells near the fourth ventricle are chemosensitive, sensing changes in cerebrospinal fluid.
• Chemoreceptor System as a Backup Mechanism:
  • Chemoreceptor system acts as a reserve mechanism supporting the higher nervous system.
  • Chemoreceptors in arterial blood serve as error correctors, particularly those in the brain sensitive to cerebrospinal fluid composition.
  • They don't regulate ventilation during acute episodes but contribute significantly over minutes and hours.

Hypoxia and Its Types

Definition: Hypoxia occurs when the partial pressure of oxygen is below normal, while anoxia is the absence of oxygen.

Types of Hypoxia

• Ambient Hypoxia:
  • Low oxygen levels in the environment affecting the entire body.
  • Found in high altitudes, enclosed spaces, or with certain anesthetic agents.
  • Diseases causing reduced breathing lead to arterial hypoxia.
• Anaemic Hypoxia:
  • Caused by insufficient functional hemoglobin.
  • Seen after bleeding, in venous anemias, and when hemoglobin binds with carbon monoxide.
• Stagnant Hypoxia:
  • Result of general or local circulation failure.
• Histotoxic Hypoxia:
  • Tissues unable to use oxygen in normal processes (e.g., cyanide poisoning).

High Altitude Stress

• Animals at high altitudes face significant respiratory challenges.
• Economic impact on domestic animals, and physiological responses to hypoxia can be studied.

High Altitude Risks for Cattle

• Cattle at high altitudes may develop conditions like risket disease.
• Chronic hypertension in the pulmonary circuit leads to right ventricular hypertrophy and congestive failure.
• Survival may require returning to lower altitudes.

Response to Low Oxygen

• Lower arterial oxygen pressure stimulates carotid and aortic bodies (chemoreceptors).
• Increased chemoreceptor output, heightened phrenic nerve discharge, and boosted ventilation.
• Hypoxia-induced ventilation response initially causes a rapid drop in alveolar CO2 pressure, leading to temporary inhibition of ventilation.
• Overall, a balance between hypoxic stimulus and hypocapnic inhibition results in a relatively minor increase in ventilation.

Bird Respiration Simplified

Unique Features of Avian Respiration

• Small lungs attached to ribs, limiting expansion.
• Presence of large, poorly vascularized air sacs.
• Active contraction during both inhalation and exhalation.
• Unidirectional air flow through primary, secondary, and tertiary bronchi.
• Gaseous exchange occurs in air capillaries branching off tertiary bronchi.

Respiratory System Overview

• Begins with external nares leading to nasal cavities.
• Trachea divides into two primary bronchi with the syrinx (vocal organ) at the division.
• Syringeal muscles control vocal sounds by manipulating membranes in the trachea and bronchi.
• Primary bronchi connect to lungs, further branching into secondary and tertiary bronchi.
• Air sacs outside the lungs function as airways with limited gas exchange.
• Pneumatic bones of birds are not connected to air sacs.

Ventilation and Lung Function

• Lungs compressed during inhalation, expanded during exhalation by sternum and rib movements.
• Air sacs are filled during inhalation, emptying the lungs.

Respiratory Characteristics

• Avian lung lacks structures similar to the mammalian diaphragm.
• Respiratory rates vary, from about 5 per minute in the ostrich to over 100 in small birds.
• Resting and flight tidal volumes differ, with increased rates during flight.

Gas Exchange and Regulation

• Gaseous exchange in avian lungs governed by diffusion, similar to mammals.
• Respiratory center in the medulla regulates respiration, sensitive to factors like pH and blood temperature.
• Pulmonary receptors in the lungs play a crucial role in carbon dioxide regulation.
• Other receptors in the upper respiratory tract respond to external stimuli, influencing respiration.