Non-Learning Objective Flashcards
Velocity of signal conduction
0.3-0.5m/s along both atrial and ventricular muscle fibres.
Purkinje fibres: 4m/s
BPM: SA Node, AV Node, Purkinje Fibres
SA Node: 70-80 bpm
AV Node: 40-60 bpm
Purkinje fibres: 15-40 bpm
Autonomic Nervous System - Control of Heart Rhythmicity
Parasympathetic:
- vagus, nerves mainly to the SA and AV node.
- Release of ACh by post-ganglionic neurons onto muscarinic receptors causes decrease in SA node rhythm and slows down conduction through the AV node.
- ACh opens ligand-gated K+ channels. Efflux of K+ ions hyperpolarises the nodal cells, taking them further from threshold. So it takes longer to generate an action potential.
- Same mechanism in AV node.
SYMPATHETIC:
-Distributed to all parts of the heart including AV and SA nodes.
- Release of norepinephrine by post-ganglionic neurons causes:
- increased SA node rhythm, AV node conduction speed and force of contraction.
- Norepinephrine binds to beta-1-adrenergic receptors which mediate effects on heart rate.
CURRENT THEORY: Increases Na+ perm. in SA/AV nodal cells so closer to threshold and increases Ca2+ perm. in cardiomyocytes so increases contractile strength.
Venous Pressures:
Where is central venous pressure?
What is pressure in standing still feet?
Where can you get negative pressures?
Central venous pressure is in right arium = 0mmHg
Standing Feet: +90mmHg
Negative pressures can be found in dural sinuses of head (veins in skull are in a non-collapsible chamber and don’t collapse)
What % of blood is usually in the veins?
> 60%
Most important means by which substances are transferred between plasma and interstitial fluid:
diffusion
Exchange between blood and interstitial fluid: lipid v. water soluble
lipid soluble substances diffuse directly through the cell membranes of the capillary endothelium
water soluble substances diffuse through intercellular pores in the capillary membrane
Special cases - acute blood flow control: kidneys
mainly through the tubuloglomerular feedback mechanism
Special cases - acute blood flow control:
Brain
- In addition to tissue oxygen, [H+] and [CO2] also play a role.
- Increase in either causes dilation of cerebral vessels to rapidly wash out excess.
- Important because level of excitability in the brain is dependent on appropriate control of CO2 and H+ concentration.
Special cases - acute blood flow control:
Skin
Closely linked to body temperature,
controlled largely by CNS via sympathetic nerves
Blood supply to the lungs:
High-pressure, low-flow circulation: Systemic arterial (oxygenated) blood to the lungs and trachea
Low-pressure, high-flow circulation:
Venous blood to the lungs for oxygenation.
Blood flow to the lungs is essentially equal to:
CO = HR x SV
In general, the pulmonary vessels enlarge/narrow in response to:
pressure
Blood circulation is directed to which areas of the lungs? Why?
Most oxygenated alveoli
So the blood is adequately oxygenated
What happens when oxygen concentrations drop in the alveoli?
Blood vessels feeding that area are constricted, increasing the vascular resistance (this resistance is associated with the development of pulmonary oedema)
Pressure Gradients in Lungs
- A blood pressure gradient exists vertically through the lungs due to hydrostatic pressure (gravity)
- Least Pressure: Upper quadrants (above the heart). Zone 2 - Intermittent blood flow, only at peak systolic pressure, exceeds alveoli pressure.
- Greatest pressure: Lower quadrants (below the level of the heart)
Zone 3 flow, capillary pressure always higher than alveoli pressure. - When lying down or during exercise, Zone 3 blood flow is seen throughout the lungs.
PP in alveoli as opposed to ATM air:
Why the difference?
Nitrogen: 74.9
O2: 13.6
CO2: 5.3
H2O: 6.2
Why?
- Higher PH2O due to humidification of air as it enters the respiratory system.
- Changes in CO2 and O2 due to exchange.
Transportation of O2
2 ways
- Dissolved in plasma (~2% due to poor solubility)
- Bound to Hb in RBCs (98%). Hb increases our O2 carrying capacity 30-100x.
During exercise, tissue demand for O2 rises by how much? And how is this met?
20x
Increased cardiac output (7x)
Increased O2 release (3x)
At rest, Hb passing through tissue capillaries gives up what % of its bound O2? During exercise, what %
At rest: 25%
During exercise: 75%
- Greater drop in PO2 during exercise, steep drop in Hb-O2 dissociation curve
How does Hb affinity for O2 change with PO2?
High PO2 = higher affinity (e.g. in pulmonary capillaries where it loads oxygen)
Low PO2 = lower affinity (e.g. tissue capillaries where it unloads oxygen)
This is the basis for reversible transport of oxygen.
The binding of the first O2 to Hb causes what?
Conformational change in the shape of the other chains from taut T shape to relaxed R shape, increases their affinity for O2.
The rate at which haemoglobin reversibly binds/releases O2 is dependent on:
PO2 Temperature pH PCO2 DPG levels
What happens to the dissociation curve during exercise, as a result of elevated levels of PCO2, body temp, drop in pH?
Right-shift in the dissociation curve. Promotes O2 release.
What is the effect of DPG on Hb?
2,3-DPG (or 2,3-BPG) binds to Hb lowering its affinity for O2, thus promoting its release.
What shifts Hb-O2 curve left?
Increased O2 affinity:
Inc. pH
Dec. DPG, Temp, PCO2
What shifts Hb-O2 curve right?
Decreased O2 affinity:
Dec. pH
Inc. DPG, Temp, PCO2
Why can CO2 be transported in greater quantities than O2?
- Can diffuse 20 times faster than O2.
2. Blood solubility is significantly higher.
How is CO2 transported o the lungs?
- Dissolved within the blood (7%)
- Bound to Hb (23%)
- Converted to bicarbonate ions (70%)
What catalyses the inter-conversion of CO2 and water to bicarbonate ions:
(include equation)
carbonic anhydrase
CO2 + H2O <> H2CO3 <> H+ + HCO3-
Why is venous blood returning from tissue capillaries slightly acidic compared to arterial blood?
Some H+ diffuses out of RBCs when CO2 is converted for transport.
Where are the bicarbonate ions converted back to CO2?
At the alveoli epithelium which contain large amounts of carbonic anhydrase
How does the respiratory system regulate body pH?
regulates the removal of CO2, and hence H2CO3 from the extracellular fluid.
Fall in blood pH - effect on respiratory rate?
Induces a rise, increased removal of CO2
A rise in blood pH is associated with what CO2 levels and has what effect on respiratory rate?
- associated with decreased blood CO2 levels (hence a decrease in [H+])
- Induces a fall in respiratory rate (decreased removal of CO2)
What is polycythaemia and what causes it to increase?
- Greater release of erythropoietin, therefore RBC mass and Hb mass.
- Acclimatisation to altitude.
Respiratory Centre
3 major regions
MEDULLA:
Dorsal Respiratory Group: regulates inspiration
Ventral Respiratory Group: regulates expiration
PONS:
Apneustic & Pneumotaxic centres which regulate rate/depth of breathing.
How does DRG regulate breathing?
Regulates inspiration by stimulating expansion of lungs through contraction of inspiratory muscles.
How does VRG regulate breathing?
Regulates expiration by stimulating compression of lungs through contraction of expiratory muscles:
abdominal muscles
internal intercostal muscles
Quiet Breathing Cycle
Inspiration (Active): stimulation of DRG - expansion of lungs (2 seconds)
Expiration (Passive): inhibition of DRG - slow depression of lungs (3 seconds)
Forced Breathing Cycle
Inspiration (Active): stimulation of DRG - expansion of lungs
Expiration (Passive/Active): inhibition of DRG, activation of VRG - causes rapid/greater depression of lungs
Apneustic Centre - Function
APPEARS TO increase the rate and depth of breathing.
Pneumotaxic Centre - Function
APPEARS TO decrease the rate and depth of breathing
Respiratory Chemoreceptors:
CENTRAL CHEMORECEPTORS:
- located in medulla
- detect changes in PCO2 & pH
- analyse CSF
PERIPHERAL CHEMORECEPTORS:
- located in carotid body & aortic arch
- detect changes in PO2, pH, indirectly PCO2 via pH
- analyse arterial blood supply
Baroreceptors - relation to respiratory system
Drop in BP: Increase CO and resp. rate
Rise in BP: Decrease CO and resp. rate
Lung - protective reflex - Mechanoreceptors
Hering-Breuer: Prevents over/under Inflation of the lungs:
Inflation Reflex: over-inflation is detected by stretch receptors in the smooth muscle surrounding the bronchioles. Information is transmitted to the DRG/VRG: inhibits DRG and stimulates VRG.
Deflation Reflex: under-inflation is detected by stretch receptors in the alveoli. Inhibits VRG, Stimulates DRG.
