Control of tidal volume and respiratory frequency

Question 1

What are the features of a dead space?

Answer 1

• Dead space: component of tidal volume that does not participate in gas exchange
• Need to assume the primary role of the cardiorespiratory system is to support metabolism and promote homeostasis, where the support systems are O2 delivery to tissues and CO2 removal
• Anatomic dead space/ conducting zone (150ml) is permanent and exists due to lack of alveoli for gas exchange, it is affected by: body size, age, head position and drugs
• Alveolar dead space (0ml) exists due to alveoli lacking ventilation or perfusion
• Physiological dead space is the total volume in respiratory system where no gas exchange is taking place
• physiological dead space = anatomic dead space + alveolar dead space
• Fowler’s method to measure anatomical dead space and Bohr’s method to measure physiological dead space (if values are close together then there is no significant alveolar dead space)

Question 2

What do the generations in gas velocity show?

Answer 2

• As airways branch and form the next generation (until 23), each individual airway is small, but total number of airways increases leading to increase in total-cross sectional area within each generation
• Increase in cross sectional area = reduced gas velocity
• By generation 17, alveoli start to appear and gas velocity has reduced to speed of diffusion
• Flow = velocity x cross section area
• Before generation 17, gas moves through quicker than for diffusion to occur
• At the largest cross sectional area in the capillaries. there is the largest decrease in velocity of blood, as blood drained into venous side, total cross sectional area decreases and velocity increases

Question 3

Define tidal breathing

Answer 3

• Minute ventilation is how much air can be breathed in/out in a minute
• Minute ventilation = tidal volume x respiratory frequency
• Gas exchange only occurs in alveoli so alveolar ventilation needs to be matched to metabolism to maintain homeostasis
• Dead space ventilation cannot be eliminated as air must flow through the conducting zone in order to reach alveoli and so do not take part in gas exchange

Question 4

Relationship between alveolar gases

Answer 4

• if alveolar ventilation is matched to metabolism, then alveolar gases (PAO2 & PACO2) will remain constant
• constancy of alveolar gases maintains diffusion gradient with blood leading to constant arterial blood gases (PAO2 & PACO2) after equilibrium with alveolar gases
• systemic venous blood has a lower oxygen level and a higher carbon dioxide level coming into the lungs but when it leaves lungs to go to heart it has a higher oxygen level and lower carbon dioxide level
• ventilation adds oxygen, metabolism removes it
• PAO2 & PACO2 are inversely related, matching ventilation to metabolism means alveolar partial pressures can remain constant