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

Understanding Respiration: A Simplified Overview

• Definition of Respiration:
  • Involves chemical and physical processes for gas exchange with the environment.
  • Mainly focuses on oxygen intake for metabolism and elimination of carbon dioxide.
• Key Gases in Respiration:
  • Oxygen is crucial for oxidative metabolism.
  • Carbon dioxide is a vital byproduct that needs to be expelled from the body.
• External vs. Internal Respiration:
  • External Respiration: Exchange of gases between lungs and blood.
  • Internal Respiration: Exchange that occurs at the tissue level.
• Ventilation vs. Respiration:
  • Ventilation: Act of breathing in and out, not the same as respiration.
  • Respiration: Involves gas exchange between lungs, blood, and tissues.
• Role of Circulatory System:
  • Delivers oxygen to tissues and transports carbon dioxide from tissues to the lungs.
  • Oxygen and carbon dioxide exchange is achieved through physical diffusion.
• Components of Respiratory Apparatus:
  • Includes lungs, airways, thorax, pleural cavity, diaphragm, and associated nerves.
  • Ensures the proper functioning of the respiratory system.
• Structures in the Airway:
  • Nasal cavity's moist and vascular membrane warms and adds moisture to inhaled air.
  • Trachea, kept open by cartilage rings, has ciliated membrane with mucous glands for dust clearance.
• Bronchi and Bronchioles:
  • Similar in structure and function to the trachea.
  • Bronchioles have muscular regulation by bronchoconstrictor and bronchodilator fibers.
• Alveoli and Gas Exchange:
  • Lungs consist of elastic sacs with alveoli, increasing the internal surface area.
  • Gaseous exchange occurs across alveolar walls and capillary endothelium.

Understanding the Respiratory System: Simplified Overview

• Oxygen Diffusion into Blood:
  • Oxygen molecules move from the respiratory air to bind with hemoglobin in a process involving several barriers.
  • These barriers include the alveolar capillary wall, blood plasma layer, and the red cell membrane.
• Thoracic Cavity and Pleura:
  • The thoracic cavity contains the lungs and mediastinal organs, separated from the abdominal cavity by the diaphragm.
  • Pleura, two sacs lining the thoracic cavity, prevent air and fluid passage between them, ensuring lung stability.
• Oxygen Consumption in Animals:
  • The basal oxygen requirement is not directly proportional to body weight but more closely related to body surface area.
  • Small animals have a higher oxygen consumption compared to larger animals.
• Mechanical Considerations in Lung Design:
  • Design factors include the physical properties of air conduits during ventilation and the elastic characteristics of lung tissue.
• Oxygen Movement through Diffusion:
  • Oxygen movement from lung air spaces to pulmonary capillary blood follows the laws of physical diffusion.
  • The total surface area for diffusion exchange in the lung is a crucial factor.
• Lung Capacity and Air Volume:
  • Total lung capacity (maximum inflated air volume) is proportional to body weight.
  • The lung's usable surface area is linked to alveolar surface area densely packed with capillaries.
• Air Volumes in the Lungs:
  • Minimal air refers to the volume that remains in the lung even when removed, associated with collapsed outflow airway channels.
  • Residual volume is the air remaining after a maximal expiration.
  • Vital capacity represents the maximum air expelled after maximal inspiration, with some air always remaining in normal breathing.

Understanding Lung Volumes and Ventilation Mechanics: Simplified Overview

• Excess Air Volume:
  • Expiratory Reserve Volume: Extra air that can be expelled after a normal breath.
• Functional Lung Capacity:
  • Functional Residual Capacity: Sum of Expiratory Reserve Volume and Residual Volume.
  • Represents total air volume in the lungs after a normal exhale.
• Volumes Associated with Breathing:
  • Tidal Volume: Air volume moved in and out with each breath.
  • Inspiratory Reserve Volume: Additional air that can be inhaled after a regular breath.
• Calculating Vital Capacity:
  • Vital Capacity: Sum of Inspiratory Reserve Volume, Tidal Volume, and Expiratory Reserve Volume.
  • Reflects the maximum air capacity the lungs can handle.
• Measuring Air Volumes:
  • Spirometer measures inspired air volume, corrected to BTPS (Body temperature, atmospheric pressure, and saturated with water vapor).
  • Gas volume varies with temperature and pressure, following Charles and Boyle's laws.
• Ventilation Mechanics:
  • Transmural Pressure Gradient: Difference between lung pressure (alveoli) and pleural sac pressure inside the thorax.
  • Normal gradient during relaxed breathing is about 8 cm H₂O.
• Lung Expansion Process:
  • Lungs expand when a force, the transmural pressure gradient, stretches the lung walls.
  • Increasing this pressure gradient, by lowering intra-pleural pressure, allows for lung expansion.

Understanding Breathing and Oxygen Transport Simplified

• Descriptive Breathing States:
  • Eupnea: Normal, effortless breathing at rest.
  • Dyspnea: Labored breathing with increased rate and depth.
  • Polypnea: Rapid, shallow, panting breaths.
• Alveolar Gas and CO₂ Concentration:
  • CO₂ concentration in alveolar air depends on metabolic CO₂ production and alveolar ventilation.
  • Doubling ventilation halves alveolar CO₂, and vice versa.
• Oxygen Transport and Hemoglobin:
  • Oxygen solubility in blood is limited, so most is carried by hemoglobin in red cells.
  • Hemoglobin molecules in red cells can bind 1 to 4 oxygen molecules.
  • Blood's oxygen-carrying capacity depends on hemoglobin concentration.
• Oxygen Saturation (SO₂):
  • SO₂ represents the percentage of hemoglobin binding sites occupied by oxygen.
  • Fully saturated blood is termed oxygen-carrying capacity.
  • Measured by equilibrating blood with room air.

Understanding Blood Oxygenation Simplified

• Blood Position:
  • Arterial blood is marked as 'a,' and venous blood is marked as 'v.'
• Oxyhemoglobin Dissociation Curve:
  • Normal Reaction: Hb + O₂ → HbO₂ (equilibrium content K).
  • Curve Shape: Unlike myoglobin, blood hemoglobin's curve is S-shaped.
  • Multiple Stages: Oxygenation occurs in four stages (K₁, K₂, K₃, and K₄).
• Stages of Oxygenation:
  • Hb + O₂ → HbO₂ (K₁)
  • HbO₂ → HbO₂ (K₂)
  • HbO₂ → HbO₂ (K₃)
  • HbO₂ → Hb (O₂)₃ (K₄)
• Affinity Changes:
  • K₁ and K₂ are similar, but K₃ is slightly greater, and K₄ is 20 times greater than K1.
  • After three O₂ molecules bind, affinity for the last O₂ increases twentyfold.
• Configurational Change:
  • The haemoglobin molecule undergoes configurational changes during the first three steps.
  • This results in the exposure of the last binding site through unfolding and eversion.
• More Hemoglobin, More Capacity:
  • Increasing hemoglobin and red cells in blood enhance its oxygen-carrying capacity.

Oxygen and Carbon Dioxide Exchange in Blood

• Tissue Interaction:
  • As oxygen is released in tissues, carbon dioxide is picked up by capillary blood.
  • The Bohr effect enhances oxygen unloading due to CO₂ and increased hydrogen ions.
• Return to Lungs:
  • When venous blood returns to the lungs, CO₂ is rapidly removed, lowering blood Pco₂.
  • This shifts the oxyhemoglobin dissociation curve up and left, enhancing oxygen loading.
  • This chemical influence is known as the Bohr effect.
• Influence of 2, 3 DPG:
  • Intra-erythrocytic 2, 3 DPG concentration affects the curve's position.
  • Increased 2, 3 DPG enhances oxygen unloading in tissues.
  • Synthesis is stimulated by hypoxia and hypocapnia and inhibited by acidosis.
• Temperature Impact:
  • Increased blood temperature shifts the dissociation curve down and right.
  • This effect, similar to CO₂ or hydrogen ion influence, supports oxygen unloading in tissues.
• Oxygen Diffusing Capacity:
  • During exercise, the diffusing capacity across alveolar capillaries increases significantly.
  • Diseases can reduce this capacity, leading to deficient oxygenation.
• Carbon Dioxide Transport:
  • CO₂ is more soluble than oxygen, but less than 5% is in physical solution.
  • CO₂ combines with water to form carbonic acid, affecting hydrogen ion concentration.
  • Carbonic anhydrase enzyme in red blood cells accelerates this reaction, crucial for CO₂ elimination and uptake.

Carbon Dioxide Transport and Respiratory Gas Exchange

• Equilibrium of CO₂ Reactions:
  • The second equation achieves equilibrium rapidly, not a rate-limiting reaction.
• Buffering Action of Proteins:
  • Third and fourth equations demonstrate the buffering action of proteins, especially hemoglobin.
  • Haemoglobin in red blood cells plays a crucial role in maintaining acid-base balance.
• CO₂ Absorption Curves:
  • Different curves for partially deoxygenated and oxyhemoglobin.
  • Veins have a curve for deoxygenated, and arteries have one for oxygenated blood.
• Balancing CO₂ Carrying Capacity:
  • As oxygen unloads in tissues, CO₂ is loaded onto hemoglobin, balancing CO₂ carrying capacity.
  • In the lungs, as oxygen loads and blood becomes more acidic, CO₂ carrying power decreases, aiding CO₂ removal.
• Respiratory Gas Exchange Assessment:
  • Evaluate respiratory gas exchange by measuring whole-animal oxygen consumption and carbon dioxide production.
  • Respiratory Quotient (RQ): Ratio of expired air volume to differences in concentrations of respiratory gases.
• Respiratory Exchange Ratio:
  • Used for measurements in non-steady-state conditions.
• Respiratory Cycle and CO₂ Gradients:
  • CO₂ pressure gradient between mixed venous blood and alveolar air drives CO₂ out of the blood.
  • Bicarbonate and chloride ions exchange between red cells and plasma during the Chloride or Hamburger Shift.
  • Carbonic anhydrase enzyme speeds up these reactions, establishing a new arterial equilibrium faster than pulmonary capillary transit.

Oxygen and Carbon Dioxide Exchange in the Lungs

• Venous Blood Composition:
  • Venous blood returning to the lungs is 70% saturated with oxygen and has a partial pressure of oxygen of about 40 mm Hg.
  • Alveolar air has a higher partial pressure of oxygen (100 mm Hg), creating a gradient for oxygen of approximately 60 mm Hg, ten times that for CO₂.
• Oxygen Loading in the Lungs:
  • The relatively low solubility of oxygen limits its speed of flow into the plasma.
  • Venous blood unloads carbon dioxide more rapidly than it loads oxygen, shifting the operative oxyhemoglobin dissociation curve up and to the left due to the Bohr effect.
  • This allows more efficient loading of oxygen at the same partial pressure than without CO₂ loss.
• Oxygen Transport in Blood:
  • Oxygen must pass through the plasma and is quickly taken up into the red cell interior, associating with hemoglobin.
  • Saturation of hemoglobin causes a shift in the CO₂ absorption curve through the Haldane effect, leading to extra unloading of CO₂.
• Processes in Tissues:
  • In tissues, these processes occur in reverse but follow the same principles.

Adaptations to High Altitudes

• Long-term residents at high altitudes show chronic hyperventilation and an increased ventilatory response to carbon dioxide.
• Adaptations include increased vital capacity, expiration reserve volume, residual volume, and functional residual capacity.
• During exercise, the hypoxic stimulus becomes more apparent, and adaptations may include an increase in alveolar capillary diffusing area.

Physiological Adaptations to High Altitudes

• Increase in cardiac output.
• Increase in blood viscosity.
• High pulmonary arterial pressure.
• Increase in the number of red blood cells (polycythemia).
• Increase in hemoglobin.
• Expansion of blood volume.
• Increased number of perfused capillaries.
• Increase in glycolysis.
• More ATPase activity.
• Tissues contain higher concentrations of high-energy phosphate stores.

Regulation of Breathing

• Breathing is initiated by rhythmic contractions of the diaphragm and intercostal muscles.
• Inspiratory activity exceeds expiratory activity under normal resting conditions, with the diaphragm and intercostal muscles being active during both inspiration and expiration.

Mechanism of Breathing

Control Centers in the Brain

• The rhythm and limits of breathing are regulated by neuronal pools in the medulla oblongata.
• Neuronal pools, collectively called respiratory centers, receive information from peripheral receptors, chemosensitive areas, and higher nervous centers, including the cerebral cortex.
• Efferent impulses from these centers flow through phrenic and intercostal nerves.

Pneumotaxic Center and Respiratory Cycle

• The pneumotaxic center in the upper part of the brain's pons converts continuous inspiratory center discharge into rhythmic patterns.
• Both inspiratory and expiratory centers are located in the medulla.
• There's a feedback circuit between the medulla and pons involving the inspiratory, expiratory, and pneumotaxic centers.
• The inspiratory center sends impulses to the pneumotaxic center, and vice versa, forming a regulatory loop.

Lung and Chest Receptors

• Vagus nerve carries impulses from lung tissue receptors sensitive to stretch.
• The Hering-Breuer reflex, triggered by vagus activity, inhibits the inspiratory center, influencing the normal rhythm of respiration.
• An expiratory-excitatory reflex, also involving the vagi, promotes inspiration, especially at lung volumes below functional residual capacity.
• Intercostal muscles with muscle spindles act as length receptors and contribute to reflexes that affect inspiration.

Chemical Control of Breathing

• The partial pressures of CO₂ and oxygen are crucial stimuli for regulating ventilation.
• Peripheral chemoreceptors in carotid and aortic bodies sense low oxygen levels.
• CNS, specifically the medulla, responds to CO₂ concentration and hydrogen ions, particularly if there's accompanying low oxygen.
• Carotid and aortic bodies act as noniterating devices, sensing oxygen and CO₂ levels in arterial blood.
• Negative feedback control system ensures ventilation increases if levels deviate from normal, bringing them back to a balanced state.

Carotid Body and Ventilatory Control

• Structure of Carotid Body:
  • The carotid body consists of epithelioid cells surrounded by a network of sinusoidal blood vessels.
  • It has a rich blood supply compared to other organs.
• Nervous Control and Inhibition:
  • Chemoreceptor cells in the carotid body are under efferent nervous control.
  • The major effect of nervous control is inhibitory.
• CSF as a Determinant:
  • Cerebrospinal fluid (CSF) plays a role in ventilatory control.
  • Changes in hydrogen ion concentration in CSF lead to ventilatory responses.
  • Chemoreceptive sites on or beneath the fourth ventricle's floor respond to CSF changes.
• CSF as Brain Fluid:
  • CSF is in equilibrium with cerebral blood, likely venous, acting as the brain's extracellular fluid.
  • Its constant hydrogen ion concentration despite blood acid-base changes suggests robust control.
• Chemoreceptor System as Reserve Mechanism:
  • The chemoreceptor system acts as a reserve mechanism supporting the higher nervous system.
  • Chemoreceptors in the arterial blood stream serve as "error correctors" in maintaining physiological balance.
• Role of Chemoreceptor Areas in the Brain:
  • Chemoreceptor areas in the brain, especially those sensitive to CSF composition, don't regulate ventilation in acute episodes.
  • Their response is a crucial part of the overall ventilatory control process.

Symbols and Abbreviations

• P: Pressure
• V: Volume of blood or air
• PA: Alveolar pressure
• VO₂: Oxygen consumption
• VCO₂: Carbon dioxide production
• R: Respiratory exchange ratio

Volumes and Pressures

• STPD: Standard temperature and pressure, dry
• BTPS: Body temperature and pressure saturated with water vapor
• ATPS: Ambient temperature and pressure saturated with water vapor
• Gas Concentrations:
• PACO₂: Alveolar partial pressure of CO₂
• FIO₂: Fractional concentration of O₂ in inspiration
• CvCO₂: Mixed venous blood CO₂ content