Chapter 18: Gas Exchange and Transport

Introduction to Gas Exchange

• the body needs oxygen and removes CO2

  • hypoxia = too little oxygen

  • hypercapnia = increased concentration of CO2

• to avoid these conditions, body responds to 3 regulated variables

  1. oxygen

  2. CO2

  3. pH

classification of hypoxias

• hypoxic, hypoxia - low arterial oxygen

  • caused by high altitude; alveolar hypoventilation

18.1 Gas exchange in the Lungs and Tissue

• individual gases diffuse along partial pressure gradients until equilibrium

  • gas exchange between alveoli and blood:

    • PO2 alveolar air > PO2 blood

      • alveoli have more oxygen than blood

    • PO2 blood > alveolar air

      • venous blood has more blood than alveolar air

  • gas exchange between blood and tissues

    • PO2 blood > PO2 tissue

      • higher amounts of blood than tissue

    • PCO2 tissue > PCO2 blood

      • cells have more tissue than blood

Lower alveolar PO2 Decreases Oxygen Uptake

• composition of inspired air

  • lower alveolar PO2 if inspired air = abnormally low oxygen content

    • higher altitudes decrease PO2

• alveolar ventilation

  • low alveolar PO2 if ventilation is inadequate (hypoventilation)

    • decreased lung compliance

    • increased airway resistance

    • CNS depression:

      • alcohol poisoning

      • drug overdose

Diffusion Problems Cause Hypoxia

• diffusion rate = surface area x concentration gradient x barrier permeability/distance

• concentration gradient = primary factor affecting gas exchange

• pathological changes that adversely affect gas exchange:

  • surface area

    • decrease in amount of alveolar surface area

  • diffusion (barrier permeability)

    • increase in thickness of alveolar membrane

  • diffusion distance

    • increase in diffusion distance between alveoli and blood

  • airway resistance

    • increase in resistance decreases ventilation

18.2 Gas Transport in the Blood

• Fick equation = estimates oxygen consumption

  • (QO2) = CO x (Arterial [O2] - Venous[O2])

• oxygen binding obeys the law of mass

  • increase PO2 shifts reaction to R (Hb + O2 —> HbO2)

  • decrease PO2 shifts reaction to L (Hb + O2 ←-HbO2)

Several Factors Affect O2-Hb binding

• physiological changes alter O2-binding affinity

  • change in Hb O2 saturation curve reflects O2-binding affinity

    • shift to the right:

      • decreased affinity = more O2 released

      • represents an increase in metabolic activity (decrease PH, increase temp, increase PCO

    • shift to left

      • increased affinity = less O2 released

      • represents a decrease in metabolic activity (increase pH, decrease temp, decrease PCO2)

18.3 regulation of ventilation

• dorsal respiratory group (DRG)

  • to muscles of inspiration- phrenic nerve to diaphragm

  • sensory input from chemoreceptors and mechanoreceptors to pons

    • glossopharyngeal nerve

    • vagus nerve

Neurons in the Medulla Control Breathing

• pontine respiratory groups (PRG)

  • pneumotaxic center = “off” switch

    • controls rate and depth

  • apneustic = “stimulator” for inspiration

  • fine-tune medullary activity to produce normal, smooth respiratory patterns

• ventral respiratory group (VRG)

  • pre-botzinger complex = basic pacemaker activity

  • areas for active expiration and less and normal inspiration

  • innervate muscles of: larynx, pharynx, tongue

• VRG and DRG are both located in the medulla

18.15 neural activity during quiet breathing

• cascade event

CO2, Oxygen, and pH Influence Ventilation

• peripheral chemoreceptors

  • located in carotid bodies

  • senses changes

    • initiate increase in ventilation

      - PO2 - low

      - pH - low

      - PCO2 - high

    • O2 must be <60 mm Hg to trigger reflex at a certain level

• central chemoreceptors

  • located in medulla

  • responds to changes in PCO2

  • arterial increase PCO2, CO crosses into the brain ECF

    • CO2 is converted bicarbonate and H+

    • H+ is actually detected

Protective Reflexes Guard the Lungs

• response to physical injury, irritation, and over inflation

• bronchoconstriction- irritant receptors in airway mucosa send signals through sensory neurons

• Hering Breur inflation reflex

  • prevents over-inflation of lungs

  • activated by pulmonary stretch receptors

  • travels via vagus nerve to brainstem

Higher Brain Centers Affect Patterns of Ventilation

• we can control how we breath

• cerebrum and hypothalamus can change brainstem breath rate and depth

  • higher brain center control is not a requirement for ventilation

• Limbic system (emotion)- bypasses brain stem

• cannot override chemoreceptor reflexes