Respiratory system Flashcards
What is respiration?
The process by which O2 is transported to and used by tissues and CO2 produced and eliminated.
Describe the process via which oxygen enters the alveoli.
Air enters the TRACHEA through the nose and mouth:
- flexible airway supported by rings of cartiledge
- produces mucus
- cilia to move dirt-laden mucus towards the mouth
TRACHEA divides into the L and R Bronchus:
- Similar structure to trachea
- Also produces mucus and has cilia
Divides subsequently into smaller BRONCHI and BRONCHIOLES
- walls made of muscle lined with epithelial cells. The muscle allows them to constrict to control air flowing in and out of the alveoli
Final bronchioles end in alveoli:
- major site of gas exchange
- lined with epithelium
- elastic fibres between the alveoli allow them to stretch as they fill with air and spring back with breathing out
- Thin walls reduce diffusion distance (one cell thick)
- Millions of alveoli so large SA
- Rich supply of pulmonary capillaries
- Red blood cells are flattened against the capillary walls reducing the diffusion distance
- Red blood cells are slowed allowing more time for diffusion.
What is Boyle’s Law
The pressure of a given quantity of gas varies inversely with its volume at constant temperature.
P1V1 = P2V2
Describe the neural control of respiration.
- The respiratory muscles (e.g. diaphragm and intercostal muscles) do not contract unless stimulated by nerves.
- Control of neural activity is primarily in the respiratory centre of the medulla. There are two main components of the medullary respiratory centre:
- the dorsal respiratory group (DRG) primarily fire during inspiration
- the ventral respiratory group (VRG):
- upper part contains the respiratory rhythm generator which sets the basal respiratory rate.
- lower part contains nerves that fire both during inspiration and expiration
During inspiration the dorsal respiratory group is active and during exhalation the dorsal respiratory group is inhibited
Describe how gas is exchanged between the alveoli and the blood
- Governed by differenced in partial pressure of oxygen and carbon dioxide on either side of the membrane
- Gas molecules undergo continuous random motion
- The pressure gas exerts is proportional to its temperature and concentration
- The net diffusion of a gas will occur from a region where its PP is high to where it is low
- Plasma entering the lungs has a PO2 of 40mmHg, PCO2 of 46mmHg and Haemoglobin saturation of 75%; alveoli PO2 is 105mmHg and PCO2 is 40mmHg.
- Net diffusion of CO2 from pulmonary capillaries into alveoli and of O2 from alveoli into blood.
- Diffusion ceases when PP on either side of the membrane becomes equal (not oxygen binding to haemoglobin does not count to increase the PO2 in the capillary).
- Opposite occurs at tissues where oxygen flows into tissues from the blood. Here mitochondria are using oxygen and so PO2 is low.
How is oxygen transported in the blood?
- 98% is transported bound to haemoglobin in RBC.
- Oxygen that is bound to haemoglobin doesn’t count in the partial pressure of O2 so doesn’t increase the partial pressure of oxygen in the pulmonary capillary so oxygen can keep crossing the membrane until the haemoglobin is saturated.
Describe the structure of haemoglobin.
- Large protein molecule with 4 subunits.
- The four polypeptide chains are linked together. Each chain is associated with a haem group which contains a ferrous (Fe2+) ion
- Each iron atom can link with one molecule of O2.
- When all 4 binding sites are occupied, haemoglobin is saturated.
Explain the oxygen dissociation curve.
- The oxygen dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure.
- The amount of oxygen bound to haemoglobin depends upon the partial pressure of blood
- When the partial pressure is low, most of the O2 binding sites are empty
- At higher PO2 levels more sites become filled until they are all occupied by O2
- It is S-shaped because the shape of haemoglobin makes it hard for the first oxygen to bind. The binding of the first molecule changes the quaternary structure of the haemoglobin and hence its shape in a way which makes it easier for other oxygen molecules to bind. However, after the binding of the third molecule it gets harder for more oxygen to join because most of the binding sites are occupied. So the curve is steep in the middle where it is easier for oxygen to bind.
What does a shift to the left of the dissociation curve mean and what can cause it?
A greater affinity for oxygen (binds more easily but unloads less easily)
- decrease in PCO2
- decrease in H+ (acidity)
- decrease in temperature
What does a shift to the right of the dissociation curve mean and what can cause it?
A lower affinity for oxygen (loads less easily but unloads more easily)
- increase in PCO2
- increase in H+
- increase in temp.
Exercise increases all these things so shifts the curve right
How is CO2 removed from the body?
a) CO2 carried in RBC - 25-30%
- directly bound to blood proteins e.g. carbaminohaemoglobin
- react reversibly with the amino groups of haemoglobin to form carbamino haemoglobin. This is aided by the fact that deoxyhaemoglobin has a greater affinity for carbon dioxide than oxyhaemoglobin.
b) HCO3- dissolved in plasma as carbonic acid - 60-65%
Occurs very efficiently in RBCs because:
- RBCs contain carbonic anhydrase which is not in plasma
- Deoxyhaemoglobin is a good buffer and can pick up the H+ formed.
c) CO2 dissolved in plasma - 10%
So ultimately CO2 is carried in the blood mainly as HCO3- in plasma
Describe the chemical control of respiration
Blood gas levels are sensed by chemoreceptors in the brain and periphery. These are communicated to respiratory centre of the medulla to cause changes in ventilation to correct gas levels.
There are two types of receptors:
- Central chemoreceptors:
- in medulla
- Stimulated by an increase in brain extracellular fluid CO2 and H+
- Increase firing of medullary inspiratory neurone
- Increase in respiratory muscle contractions
- Increased ventilation - Peripheral chemoreceptors:
- in arteries
- Stimulated by decrease in arterial O2 and increase in H+ and CO2
- Send afferent signals to activate and increase firing of medullary inspiratory neurones
- Increase in respiratory muscle contractions
- Increased ventilation
Central chemoreceptors are much more sensitive
