Respiratory System Terminology and Overview

• The body's metabolism requires oxygen for biochemical pathways, like oxidative phosphorylation
• Animals need a certain volume of air into the lungs, especially alveoli, each minute to meet oxygen demand

Terminology

• Eupnea = Normal, good, healthy, unlabored breathing
• Dyspnea = Difficult breathing, discomfort breathing
• Hyperpnea = Increased breathing depth, with or without increased respiration frequency as a response to metabolic demand
• Polypnea = Rapid, shallow breathing; Panting to lose excess heat
• Tachypnea = Breathing rate higher than normal, could be physiologic or pathologic
• Bradypnea = Abnormally slow breathing rate
• Apnea = Cessation of breathing, potentially transient

Respiratory System Overview

• The respiratory system transports O2 and CO2 between the environment and the blood
• It provides O2 for tissue metabolism and removes CO2 as a byproduct
• O2 consumption and CO2 production vary with basal metabolic rate, which is a function of metabolic body weight
• Metabolic rate is dependent on an animal's activity level
• VO2max (maximal oxygen consumption) directly relates to mitochondrial mass in skeletal muscle
• The respiratory system:
  • Defense mechanisms
  • Pulmonary fluid exchange
  • Metabolism and elimination of substances
  • Protection against inhaled agents
  • Communication
  • Thermoregulation
  • Aids urination, defecation, and parturition via abdominal pressure

Respiratory System Structure

• The conducting zone, also known as the anatomic dead space, does not participate in gas exchange; it brings air into and out of the respiratory zone
  • Includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles
  • It humidifies, warms, and filters air before it reaches the gas exchange area
• The tracheobronchial tree (trachea and bronchi) is supported by cartilage to prevent closing, and contains mucus-secreting and ciliated cells to filter small particles
  • Walls of conducting airways contain smooth muscle and innervations
    • Sympathetic stimulation using epinephrine allows for dilation of airways
    • Parasympathetic stimulation using ACh allows for constriction of airways
• The respiratory zone is lined with alveoli where gas exchange occurs
  • Includes respiratory bronchioles (transitional structures with cilia), alveolar ducts, and alveolar sacs
  • Alveolar ducts are lined by alveoli and lack cilia
  • Alveoli are thin-walled evaginations covered by capillaries for gas exchange

Alveolar Walls

• Type I pneumonocytes cover 95% for efficient gas exchange
• Type II pneumonocytes cover 5%, synthesize pulmonary surfactant to reduce surface tension, secrete cytokines, and regenerate
• Macrophages are the first line of defense against pathogens and remove inhaled particles

Respiratory Summary

• Trachea - Number 1, Cilia Yes, Smooth Muscle Yes, Cartilage Yes
• Bronchi - Number 2-4, Cilia Yes, Smooth Muscle Yes, Cartilage Yes
• Bronchioles - Number 8-, Cilia Yes, Smooth Muscle No, Cartilage No
• Respiratory Bronchioles - Number -, Cilia Some, Smooth Muscle Some, Cartilage No
• Alveolar ducts - Number -, Cilia No, Smooth Muscle Some, Cartilage No
• Alveolar sacs - Number 6 x 108, Cilia No, Smooth Muscle No, Cartilage No

Respiration

• Consists of four processes:
  • Pulmonary ventilation: moving air in and out of the lungs using the diaphragm and intercostal muscles
  • Pulmonary gas exchange: diffusion of gases in alveoli from the blood in the pulmonary capillaries to cells
  • Gas transport: O2 and CO2 are transported from the lungs to the tissues
  • Peripheral gas exchange: diffusion of gases from blood to cells

Breathing vs Respiration

• External (Breathing)
• Internal (exchange of gases)
• Cellular (oxidation of biological fuels)

Pulmonary Ventilation

• Pulmonary ventilation equals breathing
• Physical movement of air (inhalation and exhalation) occurs as a result of thoracic volume and pressure changes, and low pressure
• It requires muscular energy and lung elasticity

Pulmonary Ventilation

• Inhalation requires active contraction (muscular energy) of:
  • Diaphragm
  • External intercostal muscles
  • Abductor muscles attached to external nares, pharynx and larynx
• Exhalation is relaxation of inspiratory muscles
  • Airways are normally passive at rest
  • Elastic energy stored in the stretched lungs and thorax decreases volume
  • Assisted during exercise and in some disease conditions

Physical Factors

• Physical factors that influence pulmonary ventilation:
  • Frictional resistance is the major nonelastic source of resistance to airflow
    • Flow = ΔP / R, with R (resistance) determined primarily by the radius of the airways
  • Elasticity is how readily the lungs rebound after being stretched
    • Connective tissue causes lungs to return to original shape
  • Compliance is the ability to stretch.
    • Influenced by:
      • Distensibility of the lung tissue
      • Surface tension of the alveoli

Surface tension of the alveoli

• It results from forces acting on the liquid surfaces causing it to reduce the alveoli to the smallest possible size
• Surfactant
  • A detergent-like complex made by Type II pneumocytes
  • It reduces surface tension and helps keep the alveoli collapsing

Velocity of Airflow

• The velocity of airway flow diminishes progressively from the trachea toward the bronchioles
• The branching pattern of the tracheobronchial tree causes the total cross-sectional area increases dramatically which results in decreased velocity of air
• Turbulent velocity causes high velocity in the trachea to lung sounds, and low velocity in the bronchioles produces no sound

Lung Volumes

• Tidal Volume: Air breathed in or out during one cycle - Can increase or decrease from normal depending on ventilation requirements - Dogs: 10-15ml/Kg
• Minute Ventilation: The total volume of air breathed per minute
  • Determined by tidal volume and number of breaths
  • VE = VT x f
• It is the sum of alveolar ventilation and dead space ventilation

Blood flow

• Pulmonary and bronchial circulation - Pulmonary circulation carries blood from the right ventricle to the alveolar capillaries for gas exchange, then returns it to the left side of the heart - Pulmonary Arteries carry deoxygenated blood from the ventricle to the capillaries - Bronchial circulation from the systemic circulation provides blood and supplies oxygen. - Bronchial Arteries carry oxygenated blood

Blood flow Lung

• Distribution of pulmonary blood flow within the lung differs between quadrupeds and bipeds - Bipedal pulmonary circulation is gravity dependent and blood flow is highest at the bottom. - Quadrupeds blood flow is distributed to the cuadodorsal region of the lungs - Branching pattern of vessels and relative resistance are the major determinants

Pulmonary System

• The total cardiac output of the right side of the heart passes through the pulmonary circulation and is 10-20% of blood volume - It can accommodate for exercise without a Large increase in work and has a lower pulmonary vascular resistance - The blood flow regulates the blood via hypoxic pulmonary vasoconstriction

Pulmonary System Part 2

• The main pulmonary arteries are elastic - Smaller arterioles are muscular and thinner than systemic - The arterioles lead to capilleries form a blanketing for the alveolar surface for gas exchange
  • Amount of smooth muscle influences the vasculature to hypoxia and neural and humoral stimili

Pulmonary System Part 3

• Pulmonary veins have thin walls and conduct oxygenated blood to pulmonary capillaries to left atrium
• Blood helps to serve as a reservoir of blood - Available for sudden increases in cardiac output - i.e. exercise

Alveolar System

• Depends on local mechanical properties of the lung
• gas exchange cannot occur, - alveolus receives blood but no ventilation - Ventilation / perfusion mismatch
• Distribution of ventilation is uneven but can worsen with - local decreases in lung compliance (pulmonary edema) - local airway obstructions (i.e. mucus) - Alveolar Atelectasis (recumbency under anestesia)

Hypoxic System

• Distribution of air can get reduced by presence of fluids and airway obstructions
• Alveolar Hypoxia includes Pulmonary Arteies
• reduces bloodflow and reduces pulmonary shunt. -Generalized Pulmonary Hypoxia can increase arterial pressure