The Respiratory System Flashcards
How do the alveoli support gas exchange?
By creating a mass surface area
What are the two major forces that prevent lung collapse?
Negative intra-pleural pressure and surfactants within the alveoli fluid
A slight suction is maintained between the lung (visceral) pleura and the thoracic cavity (parietal) pleura by ___?
A negative pressure gradient (pleural pressure)
What usually causes a collapsed lung?
Neutralisation of pleural pressure
What does the fluid layer covering alveoli allow?
Gases to dissolve and exchange between the alveoli and capillaries
What happens when water interacts with air, e.g. when the fluid-covered alveoli interact with air?
Surface tension develops (water cannot bind with air), so water tries to reduce the amount of surface area that is in contact with air by contracting into a ball, causing the collapse of the alveoli
How is the collapse of the alveoli prevented?
Release of surfactants into the fluid layer, reducing surface tension
How many bonds can one molecule of water make?
Four
How does surfactant bind with the fluid layer of alveoli?
Polar side binds with water, preventing water from contracting into a ball and reducing surface tension
Non-polar side bonds with air
What does the partial pressure of a molecule represent?
The amount of that molecule present per volume (concentration) in a gas mixture
What molecules of air are important for respiration?
Nitrogen, oxygen, carbon dioxide, water
What happens when atmospheric pressure drops?
The number of molecules present are less per set volume, however the ratio of molecules stays the same (i.e. the percentage of molecules stays the same, but the actual number decreases)
Why do the partial pressures of molecules decrease at altitude?
Due to stacking of molecules
How can pressure be changed?
By changing the number of molecules present in one space, or by changing the volume of a space (e.g. in the lungs)
What happens to the volume of/pressure in the lungs during inspiration?
Volume increases, pressure decreases
Why does pressure in the lungs need to be reduced for inspiration to occur?
Because gases move from an area of high concentration (pressure) to low concentration
How is inspiration achieved?
Contraction of the diaphragm (lowering towards pelvic floor, stretches lungs out) and contraction of the external intercostals (raises ribcage up and out, stretches lungs out)
What happens to the volume of/pressure in the lungs during expiration?
Volume decreases, pressure increases
How is expiration achieved during quiet (passive) breathing?
Relaxation of the diaphragm and external intercostals
How is expiration achieved during forced (active) breathing?
As for quiet breathing, plus contraction of the abdominals and internal intercostals
What is tidal volume?
Amount of air inhaled/exhaled in a single quiet breathing step
What is inspiratory capacity?
Tidal volume + inspiratory reserve volume
What is inspiratory reserve volume (IRV)?
Maximum volume of air inhaled in one breath after normal inspiration
What is expiratory reserve volume (ERV)?
Maximum volume of air exhaled in one breath after normal expiration
What is vital capacity?
Maximum volume of air that can be inhaled after maximum expiration (inspiratory capacity + ERV)
What is residual volume?
The amount of air always left in the lungs to prevent collapse
What are the two types of blood flow associated with respiration?
- Pulmonary (high pressure, low flow circulation, supplies oxygenated blood to lungs and trachea)
- Systemic (low pressure, high flow circulation, supplies deoxygenated venous blood from the tissues to the lungs for oxygenation)
Blood flow to the lungs is essentially equal to ____?
Cardiac output (HR x SV)
In general, are the pulmonary vessels passive or active?
Passive - they enlarge in response to increased pressure and narrow in response to decreased pressure
Where is circulation direction for adequate oxygenation?
Areas of the lungs where alveoli are most oxygenated
What happens when oxygen concentrations drop in the alveoli?
Blood vessels feeding that area are constricted, increasing vascular resistance and redirecting circulation to more oxygenated areas
In what direction does the blood pressure gradient in the lungs exist and why?
Vertically, due to hydrostatic pressure (gravity)
Which area of the lungs contains the least pressure and what zone is it known as?
Upper quadrants (above the height of the heart); zone 2
What happens if the pressure in the vessels isn’t greater than or equal to the pressure in the alveoli?
The alveoli squash the vessels
What type of blood flow exists in zone 2?
Intermittent - vessels are only open during peak systolic pressure (i.e. when pressure exceeds alveoli pressure)
What type of blood flow exists in zone 3?
Continuous - vessels are always open as vessel pressure remains higher than alveoli pressure at all times
When is zone 3 pressure seen throughout the body?
When in a lying position or during exercise
What does zone 1 pressure indicate?
An abnormality
How do oxygen and carbon dioxide diffuse between the blood and lungs?
Down their concentration gradients, which is determined by partial pressure
What determines a molecule’s partial pressure in fluid (e.g. blood)?
Concentration and solubility
What is the relationship between the solubility of a molecule and its partial pressure in fluid and what is it called?
The more soluble the molecule, the greater the amount it can dissolve without a significant change in partial pressure; Henry’s law
What is the formula for Henry’s law?
Partial pressure = concentration of dissolved gas/solubility coefficient
What has a high partial pressure in fluid, oxygen or carbon dioxide?
Oxygen (weak solubility, i.e. small amount = change in partial pressure)
True or false: The partial pressure of a molecule in gas and liquid is not comparable
True
How does the partial pressure of oxygen, carbon dioxide and water in the alveoli compare to atmospheric pressure?
In the alveoli, partial pressure of oxygen is lower, partial pressure of carbon dioxide and water is higher
Why do the alveoli have a higher PCO2 and a lower PO2?
- Mixing with old air
- Oxygen is continuously being drawn into the blood, carbon dioxide drawn into the alveoli
Why do the alveoli have a higher PH2O?
Humidification of the air as it enters the respiratory system
At rest, what are the partial pressures of oxygen and carbon dioxide in the alveoli and the capillaries prior to gas exchange?
Alveoli:
- PO2 = 104mmHg
- PCO2 = 40mmHg
Pulmonary Capillaries:
- PO2 = 40mmHg
- PCO2 = 45mmHg
During gas exchange at the lungs/tissues, what happens to the partial pressures in the pulmonary capillaries?
They equalise with the partial pressures in the alveoli/tissues
Following gas exchange at the lungs, why does PO2 decrease to 100mmHg?
Because the bronchial veins add in dirty blood from the lungs
At rest, what are the partial pressures of oxygen and carbon dioxide in the tissues and the capillaries prior to gas exchange?
Tissues:
- PO2 = 40mmHg
- PCO2 = 45mmHg
Systemic Capillaries
- PO2 = 100mmHg
- PCO2 = 40mmHg
During exercise, what are the partial pressures of oxygen and carbon dioxide in the tissues?
PO2 = 20mmHg PCO2 = 47mmHg
What are the two ways in which oxygen is transported in the blood?
Dissolved in blood plasma (approx. 2%) and bound to haemoglobin within red blood cells (approx. 98%)
Haemoglobin increases the carrying capacity of oxygen by how many times?
30-100
What is haemoglobin composed of?
4 polypeptide chains, each associated with an iron-containing haem prosthetic group capable of reversibly binding one oxygen molecule
How many oxygen molecules can be bound to a haemoglobin molecule?
Four
What happens when the first molecule of oxygen binds to haemoglobin?
Causes a conformational change in the shape of the other chains (taut state - relaxed state), making the binding of the next two molecules more favourable, which in turn increases an even greater affinity for the final molecule
Why does the relationship between Hb saturation and PO2 show a sigmoidal function, rather than a linear relationship?
Because the affinity of Hb for oxygen changes with the number of molecules bound
What does the Hb dissociation curve show?
- When PO2 is high (i.e. in pulmonary capillaries), Hb has a high affinity for binding O2
- When PO2 is low, Hb has a strong tendency to release O2 (i.e. in tissue capillaries)
True or false: Hb only releases O2 if PO2 dramatically decreases
True
At rest, the PO2 drop of 64mmHg (104-40mmHg) causes what percentage of bound O2 to be released to the surrounding tissues?
25%
During exercise, the PO2 drop of 84mmHg (104-20mmHg) causes what percentage of bound O2 to be released to the surrounding tissues?
75%
The rate at which Hb reversibly binds/releases O2 is dependent on ___?
PO2, temperature, pH, PCO2 and DPG levels
What causes the dissociation curve to shift to the left (i.e. increased affinity for O2?
Increased pH and decreased DPG, temperature and PCO2
What causes the dissociation curve to shift to the right (i.e. decreased affinity for O2?
Decreased pH and increased DPG, temperature and PCO2
How does 2,3-DPG promote release of oxygen?
By binding to Hb and lowering its affinity for oxygen
True or false: Oxygen can be transported in greater quantities than carbon dioxide
False - CO2 can diffuse 20 times faster than O2 and has a significantly higher blood solubility
On entry into the tissue capillaries, what are the three forms in which carbon dioxide is transported?
- Dissolved within the blood (7%)
- Bound to haemoglobin (23%)
- Converted to bicarbonate ions (70%; decreases the pH in the lungs)
Within the RBCs, what rapidly catalyses the inter-conversion of carbon dioxide and water to bicarbonate ions (HCO3-)
Carbonic anhydrase
What is the chemical equation for the conversion of carbon dioxide to bicarbonate ions?
CO2 + H2O H2CO3 H+ + HCO3-
What happens to the H+ when HCO3- is released into the blood?
- Large proportion are bound by haemoglobin (strong base)
- Some diffuse out of the RBCs, causing venous blood to be slightly acidic compared to arterial blood
Where is HCO3- converted back to CO2?
At the alveoli epithelium, which contain large amounts of carbonic anhydrase
What is the major hydrogen donor in the body?
Carbonic acid (H2CO3)
What are the three ways in which body pH is regulated?
- Acid-base buffering systems in bodily fluids
- Respiratory system, by regulating removal of COS/H2CO3 from extracellular fluids
- Kidneys excrete acidic/alkaline urine (only system that can remove H+)
What is a fall in blood pH associated with and what does it induce?
Increased CO2 levels, increased exercise/metabolic activity; induces a rise in respiratory rate
What is a rise in blood pH associated with and what does it induce?
Decreased blood CO2 levels; induces a fall in respiratory rate
How does altitude alter oxygen delivery?
Causes decreased atmospheric pressure, which decreases PO2
Above what height does decreased PO2 affect %saturation?
2000m
What happens at low hypoxia?
The urge to reduce the rate/depth of breathing due to a drop in PCO2 negates the drive for deeper/faster breathing to obtain more oxygen
Between what heights does increasing hypoxia override hypocapnia?
2500-4500m
At what height is consciousness lost?
> 6100m
What is acute mountain sickness and what is the cause?
Hypoxia-related problems experienced at >3500m
Increased ventilation causes hypocapnia/alkalosis, inducing cerebral vasoconstriction (increased blood to brain/pressure on brain)
What is high altitude pulmonary oedema (HAPE) and what is the cause?
Hypoxia related problems experienced at >5000m
Hypoxia decreases PO2 in alveoli, causing vasoconstriction and increased permeability, inducing pulmonary vascular resistance, forcing blood from capillaries into the alveoli
How does acclimatisation to altitude occur?
Increased - Pulmonary ventilation - Release of EPO - 2,3-DPG levels - Buffer capacity Reduced vascular response to altitude (reduces effect of hypoxia) Improved muscle use of oxygen Reduced lactate
How does increased pulmonary ventilation cause acclimatisation?
- At first, increased ventilation blows off greater CO2, causing reduced respiration even though PO2 levels are low
- After a while, inhibition fades due to low pH caused by low HCO3-
What two types of control is the rate of oxygen absorption/carbon dioxide excretion from the lungs under?
Local: Controls changes in blood flow and oxygen delivery (moves blood to areas of high oxygenation in alveoli)
Neuronal: Modulates rate/depth of breathing (voluntary and involuntary)
What three regions is the respiratory centre composed o?
Medulla
- Dorsal respiratory group (regulates inspiration)
- Ventral respiratory group (regulates expiration)
Pons
- Apneustic and pneumotaxic centre (modulates rate/depth of breathing)
How does the DRG regulate quiet breathing?
By stimulating contraction of the external intercostals and diaphragm
How does the VRG regulate forced breathing?
By stimulating the contraction of the abdominals and internal intercostals
What occurs during the quiet breathing cycle?
1) DRG active (2 secs)
2) Inspiration
3) DRG inactive (3 secs)
4) Expiration
What occurs during the forced breathing cycle?
1) DRG active (2 secs)
2) Inspiration
3) DRG inactive (3 secs) and VRG active
4) Expiration + contraction of abdominals and internal intercostals
What do the apneustic and pneumotaxic centres appear to do?
Apneustic: Increase rate/depth of breathing
Pneumotaxic: Decrease rate/depth of breathing
How does sensory information modify the activity of the respiratory centre?
Chemoreceptors: Changes in PO2, PCO2 and pH
Baroreceptors: Changes in blood pressure
Mechanoreceptors: Changes in lung size (prevent over/under-expansion)
Irritants: Physical/chemical stimuli that irritate the respiratory tract
Where are central chemoreceptors located and what do they detect?
Medulla; changes in PCO2 and pH
Where are peripheral chemoreceptors located and what do they detect?
Aortic arch and carotid body; changes in PO2 and pH
What does a decrease in blood pressure cause?
Increased cardiac output and respiratory rate
