Lecture 17 - Diffusive Oxygen Transport into Muscle Flashcards
How is oxygen transferred from the blood to the muscle?
- binds to hemoglobin in the red blood cell
- hemoglobin releases oxygen to separate the two, this oxygen is diffused into thee tissue
- diffuses into the plasma then the mitochondria
how is O2 used in the mitochondria?
- must be present to create water by accepting electrons
- drives the flow of electrons along the chain
- facilitates proton pumping to create a gradient (for ATP synthase)
- drawn to complex IV where is accepts electron pairs
what is the flow of O2 from the blood to the muscle tissue?
- blood plasma
- capillary endothelium
- interstitial space
- sarcoplasm
- mitochondria
- complex IV
What are the two ways to deliver O2 from the blood to the tissues?
- convection
- diffusion
what is convection?
- transfer of O2 by movement of RBC within arterial blood to the tissue
- the flow of a fluid (blood)
what is diffusion?
- transfer of O2 from an area of high pressure to low pressure
- only the gas flows
- two locations (within the lung and within the tissues)
how does diffusion occur in the lungs?
- O2 diffuses from alveoli into the pulmonary capillaries
- driven by the gradient between PPO2 in the alveolar space and in the deoxygenated pulmonary capillary blood
how does diffusion occur in the tissues?
- O2 diffuses from the systemic capillaries into the skeletal muscle mitochondria
- driven by the gradient between PPO2 in muscle capillaries and the mitochondria
What does the variable Q represent?
- flow of O2 rich blood
- aka oxygen delivery
what is the equation for convection?
VO2 = Q x a-vO2 difference
- aka Ficks principle
what is the equation for diffusion?
VO2 = DMO2 x (PO2 capillary - PO2 mitochondria)
- decreasing the DMO2 will also raise VO2
What are the two steps of diffusive O2 delivery?
- diffusion of O2 from blood to sarcoplasm (cytosol)
- diffusion of O2 from sarcoplasm to mitochondria
what is the relation between partial pressure and hemoglobin?
- hemoglobin has an affinity for O2 and blood PO2 which creates a pressure gradient between the capillary and the myoglobin
- the bond between Hb and O2 weakens when O2blood reaches the muscle cells
- O2 is released from Hb as RBCs pass through capillaries –> it then diffuses into the muscle cells
why is the fall of PO2 in the tissues based on luck/chance?
- fall in PO2 because it reduces the affinity for HBO2 from oxygen desaturation
- oxygen needs to be released so it can be used by the tissues
How does PO2 change in arterioles, capillaries and veins?
- PO2 lowers in the capillary as dissolved O2 diffuses out of the plasma
- this facilitates off-loading of O2
- more O2 leaves the plasma the further along the capillary (nearer to venule) you get
- during exercise more CO2 is released increasing capillary PCO2 towards the venular end
- Bohr shift (rightward shift) in HBO2 dissociation curve
- therefore “end-capillary” PO2 may be quite low relative to “mean capillary”
- the “driving pressure” is reduced progressively from the arterial to venous end
How is O2 diffused into the mitochondria?
- dissolved O2
- myoglobin-facilitated O2 diffusion
- oxygen is now in the interstitial space
What is myoglobin?
- an iron containing globular protein in skeletal and cardiac muscle fibres
what is the role of myoglobin?
- O2 diffusion from blood to sarcoplasm
- provide an extra O2 source to release O2 at low PO2
- resembles Hb but differs because Mb molecules contain only 1 iron atom compared to 4
- Hb can hand off O2 to Mb inside the capillaries through the equation Mb + O2 –> MbO2
how is the dissociation curve for Mb different from Hb?
- Mb saturation abruptly increases in percentage, there is little change in saturation following this
- The Hb line is much less abrupt
- Mb affinity for O2 is not dependent on other variables
what are the 3 factors that affect diffusive O2 delivery?
- the ability of the muscle to extract O2 from the blood to the muscle mitochondria
1. pressure gradient b/w capillary and mitochondria
2. contact area b/w RBC and myocyte
3. distance between capillary and mitochondria
what is Fick’s Law of Diffusion?
- muscle diffusion capacity
1. pressure gradient b/w capillary and mitochondria (“p1-p2”)
2. contact area b/w RBC and myocyte (“area”)
3. distance between capillary and mitochondria (“thickness”)
What is the effect of driving pressure on VO2 max?
- VO2 max is 25% less in hypoxia
- decreased pressure = decrease diffusive delivery = decreased vo2 max
- low oxygen reduces VO2 max
Effect of driving pressure on VO2max
- decreased driving pressure = decreased VO2 max
- training increases diffusion capacity such that a greater VO2max can be achieved for the same pressure
What is RBC Spacing?
- during exercise, greater convective blood flow increases number of blood cells in a given capillary (less RBC spacing)
- increased hematocrit = increased functional surface area for diffusion = increased diffusive O2 delivery
- RBC hemodynamics at onset of electrically stimulated rhythmic contraction of rat spinotrapezius muscle
what is capillarization?
- increase capillaries in contact with muscle fibres and decrease diffusion distance
what are the 7 stages of diffusive O2 delivery during exercise?
- when oxygenated blood reaches muscle capillaries, the bond between o2 and Hb molecules weakens
- as RBCs pass single file through the tiny capillaries that surround muscle cells, O2 is released from Hb
- capillary PO2 > cytosolic PO2 so O2 diffuses into the sarcoplasm
- once inside the sarcoplasm, O2 can either diffuse directly into mitochondria or bind to myoglobin
- myoglobin shuttles O2 from cytosol to the mitochondria when cytosolic PO2 falls to critical levels during exercise
- once in the mitochondria, O2 is drawn to cytochrome c oxidase (complex IV) where it accepts electrons delivered by oxidation of CHO or FAT
- reduced O2 then picks up two H+ from the surrounding medium to form water
