Respiratory Physiology

• Components of the respiratory system

  • Lungs
  • Air passageways
  • Other structures that facilitate the uptake of gases from the atmosphere (e.g. air sacs, the rib cage, nostrils etc.)

  
• Mammalian respiratory system


  • Different from avian respiratory system
  • Birds have air-sacs and 2 different areas of the lungs (Paleopulmo and neopulmo parabronchi) but mammals DON'T
  • Mammals have diaphragm and collapsible lungs but birds DON'T
• Why do we need to learn the normal functions and the characteristics of the Respiratory System?
• Reasons to learn about the respiratory system

  • Diagnosis - Respiratory pattern, Auscultation/percussion
  • Performance evaluation/assessments - particularly horses
  • Respiratory failure is a certain cause of death
  • Understanding the importance of ventilation in anaesthesia/surgery and critical disease conditions. Endotracheal Intubation
  • Respiratory signs are commonly observed in many diseases, especially those related to heart failure and certain drug poisonings
  • Usage of gases in supportive therapy, e.g. Acetaminophen Poisoning in cats
  • Understanding the pathophysiology that affects the lungs

  
• Classification of the respiratory system

  • Upper Respiratory Tract - nostrils, nasal passages, pharynx, larynx and trachea
  • Lower Respiratory Tract - bronchi, bronchioles, alveolar ducts, alveoli

  
• Lung lobes by species

  • Cat, cow, dog, goat - Cranial & Caudal Lobes
  • Pig, sheep, humans - Cranial, Middle, Caudal & Accessory Lobes
  • Horse - One Lobe + Accessory Lobe

  
• Tracheal bronchus (extra bronchus before the carina) in the pigs, cattle, sheep and other ruminants can be a useful landmark during bronchoscopy & X-ray, as it indicates the RIGHT LUNGS and their airways

  
• Tracheal bronchus (bronchus suis) also occurs as a congenital anomaly in humans with an incidence of about 1-3% and could be associated with respiratory problems

  
• Main functions of the respiratory system

  • Gas exchange - removal of carbon dioxide and oxygen intake
  • Regulation of the acid-base balance
  • Regulation of the body temperature
  • Regulation of the body water

  
• Secondary functions of the respiratory system

  • Phonation - or voice production
  • Olfaction - sense of smell
  • Metabolic - activation or inactivation of substances regulating other bodily functions

  
• Olfaction capabilities varied across animal species due to their anatomical adaptations and ability to develop "olfactory wisdom"

  
• Dogs have up to 300 million olfactory receptors, and 100 cm2 of nasal epithelium for olfaction, while humans only have 6 million olfactory receptors and 5 cm2 of olfactory epithelium

  
• Dogs are able to discern smell based on MHC to determine mate-compatibility, weakness in the immune system, and even disease

  
• Major physiological events of respiration

  • Pulmonary ventilation
  • Diffusion of gases between alveoli and the blood
  • Transport of dissolved gases in body fluids to cells
  • Regulation of the respiration process itself

  
• Components of respiration in mammals

  • Nostrils
  • Air passageways - no gas exchange takes place and therefore known as Dead Space Volume, VD
  • Alveoli - alveolar volume, VA

  
• Functions of the respiratory system

  • Filtration of the air
  • Humidifying the air
  • Warming-up the air
  • Defence against air-borne pathogens

  
• In conditions where air is breathed in directly into the lungs (e.g. after tracheostomy), the functions of filtration, humidification, and warming are not performed, which can lead to lung crusting, vocalization difficulties, and probable lung infections

  
• Functions of the alveoli

  • Site for gaseous exchange
  • Final defense site against probable air-borne pathogens
  • Alveolar macrophages and Pneumocytes type I and II
  • Secretion of surfactants/serous to aid breathing and entrapping particles (0.1 – 1.0 m)
  • Macrophages ingest particles 0.1 – 0.5 m in size

  
• The respiratory process accounts for approximately 3 – 5 % of total body energy at rest, but energy requirement may increase 50 fold in strenuous exercise, related to the Athletic Potential of an individual

  
• Components of the blood-gas barrier

  • Alveolar fluid with surfactant
  • Alveolar wall - type I and II pneumocytes, and alveolar macrophages
  • Epithelial basement membrane
  • Capillary basement membrane
  • Endothelial cells of the capillary

  
• The alveoli are interconnected by a series of pores (Pores of Kohn) which help in gas distribution during pulmonary ventilation, by allowing collateral movement of air

  
• Factors affecting the rate of gaseous exchange during external respiration

  • Concentration of a particular gas
  • Presence of water vapour
  • Partial pressure differences of the inspired & expired gases
  • Surface area for gas exchange
  • Diffusion distance
  • Solubility and molecular weight of gases

  
• The alveolar gas equation is used to calculate the alveolar oxygen partial pressure (PAO2)

  
• The Alveolar Gas Equation

  PAo2 = [(PB – PH2O) x FIo2] – Paco2 / R

  
• PB


  Barometric pressure (760 mmHg at sea level)
• PH2O


  Partial pressure of water vapour (50 mmHg at 100 RH, 38.2 oC)
• FIo2


  Fraction of oxygen in inspired air (0.21 in atmospheric air)

  
• Paco2


  Arterial partial pressure of CO2
• R


  Respiratory exchange ratio, ratio or CO2 production to O2 consumption (taken as 0.8 in normal resting animals)
• At least 70 to 80 mmHg of PAo2 is required to almost saturate Hb in a normal animal
• Oxygen Transport


  1. Formation of oxyhemoglobin

  2. Hb (Deoxyhemoglobin) + O2 D HbO2 (Oxyhemoglobin)

  
• Factors affecting oxygen-hemoglobin dissociation

  • Oxygen partial pressure (h PO2 h HbO2)
  • Acidity of blood (ipH i HbO2) – the Bohr's effect, a right shift
  • CO2 partial pressure (hPCO2 i HbO2) – adaptation to exercise
  • Temperature (hTemp i HbO2) – within limits
  • BPG or 2,3, biphosphoglycerate - decreases the affinity of Hb to oxygen and thus helps unload oxygen from Hb, BPG formed during glycolysis. Adaptation to exercise and high altitude
  • Types of hemoglobins - Fetal Hb has more affinity to oxygen. Hb in small animals are able to unload their O2 at higher Po2 to satisfy their higher metabolic demand
  • Presence of toxic agents – CO, Nitrites

  
• Oxygen-hemoglobin dissociation curve


  Graphical representation of the relationship between oxygen partial pressure (PO2) and the percentage saturation of hemoglobin with oxygen

  
• Right-shift of the curve

  Oxygen released from blood

  
• Left-shift of the curve

  Oxygen retained in blood

  
• 1 g of saturated Hb can carry 1.39 mL of O2. In a typical animal with 10 – 15 g/dL Hb
• When Po2 dips below 60 mmHg


  The dissociation curve becomes steeper
• When pH decreases


  The entire Hb-O2 dissociation curve is shifted to the right, meaning that MORE oxygen is being released

  
• When PCO2 increases


  The Hb releases oxygen readily

  
• Bohr shift/effect


  Shift in HbO dissociation curve resulting from a change in Pco2