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Processes of Respiration (Transport of respiratory gases (Transport via…
Processes of Respiration
Pulmonary Ventilation: also known as breathing, is the delivery of systemic venous blood from heart to lungs for oxygenation.
- It consists of both inspiration and expiration.
- The driving force for pulmonary ventilation is a pressure gradient created by changes in the thoracic volume
It is a mechanical process that depends on volume changes in thoracic cavity. Volume changes lead to pressure changes, which lead to flow of gases to equalize pressure.
Boyle's law gives the relationship between pressure and volume of a gas.
-At constant temperature, the pressure of gas varies inversely with its volume.
-P1V1=P2V2
Physical factors influencing pulmonary ventilation are airway resistance, alveolar surface tension and lung compliance. They influence the ease of air passage and the amount of energy required for ventilation
- Airway Resistance
- It is insignificant for two reasons: diameters of airways in the first part of the conducting zone relative to the low viscosity of air and as the airways get progressively smaller, there are more branches.
- Any resistance usually occurs in medium sized bronchi and disappears at terminal bronchioles where diffusion is what drives gas movement
- As resistance rises, breathing movements become more strenuous
- Alveolar Surface Tension
- The attraction of liquid molecules to each other at a gas-liquid interface produces a state of tension at the liquid surface that draws the liquid molecules close together and reduces their contact with the dissimilar gas molecules and resists any force that tends to increase the surface area of the liquid
- Water with high surface tension coats alveolar walls in a thin film
- Surfactant is the body’s detergent-like lipid and protein complex that helps reduce surface tension of alveolar fluid which prevents alveolar collapse and is produced by type II alveolar cells
- Lung Compliance
- The distensibility (stretchiness) is a measure of change in lung volume that occurs with a given transpulmonary pressure
- CL = △VL / △(Ppul - Pip)
- The higher the lung compliance, the easier it is to expand the lungs at any given transpulmonary pressure
- This is determined by two factors: distensibility of the lung tissue (generally high) and alveolar surface tension (surfactant keep it low), healthy lungs tend to have a high compliance, which favors efficient ventilation
- A lack of surfactant increases surface tension in the alveoli and causes them to collapse between breaths, markedly decreasing lung compliance.
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Inspiration: the period when air flows into the lungs.
- The inspiratory muscles are the diaphragm and external intercostal muscles, they are activated:
- The diaphragm descends and the rib cage rises
- Thoracic cavity volume increases
- Lungs are stretched, intrapulmonary volume increases
- Intrapulmonary pressure drops to -1 mmHg
- Air flows into lungs down its pressure gradient until intrapulmonary pressure is 0 making it equal to atmospheric pressure
Expiration: the period when gases exit the lungs
- The inspiratory muscles are the diaphragm and external intercostal muscles, they are relaxed:
- The diaphragm rises and the rib cage descends due to recoil of costal cartilages
- Thoracic cavity volume decreases
- Elastic lungs recoil passively, intrapulmonary volume decreases
- Intrapulmonary pressure rises to +1 mmHg
- Air flows out of lungs down its pressure gradient until intrapulmonary pressure is 0
Pressure Relationships in the Thoracic Cavity: fluctuates with breathing and is described relative to Patm which is the pressure exerted by air surrounding the body
- Intrapulmonary Pressure (Ppul)
- Pressure in the alveoli which eventually equalizes with Patm
- Pressure inside lung decreases as lung volume increases during inspiration; pressure increases during expiration
- Intrapleural Pressure (Pip)
- Pressure in the pleural cavity
- Always a negative pressure, usually -4 mmHg to keep lungs inflated
- Fluid level must be kept at a minimum
- Pleural cavity pressure becomes more negative as chest wall expands during inspiration. Returns to initial value as chest wall recoils
- Transpulmonary Pressure
- This pressure keeps the lung space open, which keeps them from collapsing
- = (Ppul- Pip)
Measuring Respiratory Volumes and Capacities Help Assess Ventilation:- Respiratory Volumes
- Tidal Volume (TV): amount of air movies into and out of the lung with each breath
- *Inspiratory Reserve Volume (IRV):* amount of air that can be inspired forcibly beyond the tidal volume
- Expiratory Reserve Volume (ERV): amount of air that can be forcibly expelled from lungs
- Residual Volume (RV): amount of air that always remains in lungs; needed to keep alveoli open
Respiratory Capacities: these are combinations of two or more respiratory volumes
- Total Lung Capacity: sum of all lung volumes (TV+ER+IRV+RV)
- Vital Capacity: sum of TV + IRV + ERV
- Functional Residual Capacity: sum of RV + ERV
- Inspiratory Capacity: sum of TV + IRV
Respiratory Zone:
site of gas exchange which consists of microscopic respiratory bronchioles, alveolar ducts, and alveoli
Conducting Zone
** transport gas to and from gas exchange sites and includes all other respiratory structures. Cleanses, warms, and humidifies air.
Dead Space
- Anatomical Dead Space: does not contribute to gas exchange and consists of air that remains in passageways
- Alveolar Dead Space: space occupied by nonfunctional alveoli and can be due to collapse or obstruction
- Total Dead Space: Sum of anatomical and alveolar dead space
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Internal Respiration
Internal Respiration: capillary gas exchange in body tissues
-O2 diffuses from blood to tissue cells
-CO2 diffuses from tissue cells to blood
Exchange Influenced By:
-Tissue cells use O2 for their metabolic activities and produce CO2
-PO2 is lower than PCO2 in tissues
-O2 diffuses rapidly from the blood down its pressure gradient and into tissues
-CO2 moves quickly along its pressure gradient into blood circulation
- Diffusion is driven by partial pressure gradients of O2 and CO2 that exist on opposite sides of exchange membranes
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