Coordinated CV Responses Flashcards
Why will blood flow constantly continue?
Because the pump pressure (heart and aorta) is higher than the outflow pressure (venous pressure).
What does supine mean?
Where there is a mean capillary pressure of approximately 30mmHg.
What is the change in venous and arterial pressure when you go from lying down to standing up?
Venous = 10mmHg -> 100mmHg
Arterial = 96mmHg -> 186mmHg
Why is flow not directly changed across the capillary bed in the feet when you stand up?
Because despite the enormous change in pressure the pressure gradient is unchanged so flow is not affected.
What happens to the blood when you stand up?
The venous valves in the limbs close so for a short time the blood flow out of the heart exceeds the flow in.
The excess CO travels to the veins below the heart causing them to distend.
The increased pressure in these veins re-opens the valves allowing it to flow back to the heart.
What are the two negative consequences of orthostasis (standing up)?
1) Decrease in central venous pressure reducing cardiac output.
2) Loss of blood plasma volume due to increased hydrostatic pressures.
What is the reaction of arteries upon standing up? What mediates this?
Arterial constriction to reduce blood flow.
1) Baroreceptors
2) Local sypathetic axon reflex
What aids venous return in orthostasis?
Skeletal muscle pumping around large veins.
This is capable of lowering foot venous pressure to 20-30mmHg
What causes varicose veins?
Valve failure in tributary superficial veins exposes them to chronic high pressure causing varicose veins.
What happens to venous pressure in veins above the heart when standing?
Pressure falls causing veins outside the cranium to collapse a few cms above the heart. This prevents internal pressures falling below 0 as blood can still flow through collapsed veins.
The arteriovenous pressure gradient driving cerebral perfusion falls.
Veins inside the cranium do not collapse causing their internal pressures to fall to -10mmHg.
This all leads to cerebral blood flow decreasing by 20%.
Why do veins in the cranium not collapse?
Gravity causes a downward displacement of CSF within the subarachnoid space creating a negative intracranial pressure preventing veins collapsing.
What risk is created by negative pressure within cerebral veins?
If one of them is opened during surgery there is a risk of an air embolisim.
What does prolonged standing and postural hypotension lead to?
- Progressive venous pooling
- Progressive fall in pulse pressure
- Progressive rise in heart rate and TPR
This causes a sudden fall in TPR (vasodilation) and heart rate) which leads to a steep fall in BP and cerebral blood flow = syncope.
What is vasovagal syncope?
Syncope caused by vasodilation and vagally mediated bradycardia.
What is VO2 max a measure of?
A person’s ability to do exercise. This is their max ability O2 consumption. This can increase slightly with anaerobic respiration but not much.
How do you calculate oxygen consumption?
Cardiac output x (arterial-mixed venous O2 content) i.e. mls of blood delivered per minute to the tissues minus the mls of O2 removed for each ml of blood.
Both of these factors increase progressively as exercise intensity increases.
What happens to venous O2 content during exercise?
It falls progressively as exercise intensity increases up to a point as some is needed to maintain a capillary PO2 sufficient to drive diffusion.
This improves with training as increased capillary density occurs reducing diffusion distance.
What is the main factor determining VO2 max?
Maximum cardiac output.
What is the difference in the change in TPR between dynamic and isometric exercise?
Dynamic = it falls due to venodilation to increase blood flow.
Isometric = may rise due to the compression of blood vessels in contracting muscle.
What are the effects of acute and chronic blood loss?
Chronic = slow and persistent = Fe deficiency anaemia.
Acute = large loss = reduced circulatory volume causing circulatory shock.
What happens as a result of prolonged circulatory shock?
Tissue damage due to inadequate deliver of oxygen and other nutrients.
What are the symptoms of circulatory shock?
- Neurological = anxiety, restlessness, confusion, aggression, lethargy, coma
- Rapid shallow breathing
- Intense thirst (maybe)
- Nausea
- Rapid weak pulse
- Low BP (not always), pulse pressure (always low)
- Pale, grey or cyanotic with clammy skin
- Reduced urine output
- Acidosis
What happens after 2-5 hours of circulatory shock?
Decreased coagulation time and increased neutrophils.
What is the WHO haemorrhage classification system?
- Class 1 = Minimal = <15% = blood loss unlikely to elicit shock.
- Class 2 = Mild = 20-30% = blood loose induces shock and depressed BP. Not life threatening.
- Class 3 = Moderate = 30-40% = severe shock and profound fall in BP and CO. This may become irreversible (refractory).
- Class 4 = Severe = >50% = death.
What is it important to remember about blood loss?
Severity of blood loss is related to how rapid it is lost. A rapid loss of 30% can be fatal, but a loss of 50% over 24h may be survived.
What is the venous response to blood loss?
Veins shrink to maintain venous pressure and venous return.
This starts after 10 mins. and takes an hour for it to develop fully.
What are the nervous system responses to blood loss?
- Powerful peripheral vasoconstriction.
- Gut and renal perfusion is severely reduced (dangerous if sustained).
- Activation of RAAS (electrolyte and water retention, vasoconstriction, thirst, increase in ADH).
What have animal experiments shown about the association of the SNS and blood loss?
If the SNS is blocked, 15-20% blood loss in 30 mins leads to death.
30-40% blood loss can be survived if the SNS is intact.
What happens in the heart and large pulmonary vessels in acute blood loss?
Mechanoreceptors in the heart and large pulmonary vessels respond to the drop in blood volume. These send nervous impulses to the hypothalamus and brainstem which increases ADH and adrenaline levels.
The adrenaline and ADH cause vasoconstriction. ADH also causes thirst and water reabsorption.
How is blood flow to organ systems affected in moderate haemorrhage?
Blood flow to skin, gut, muscle and kidneys are reduced via sympathetic vasoconstriction.
Blood flow to coronary system and brain remains normal.
How long does it take to recover blood volume following 25% blood loss?
3 days
What is internal transfusion?
Due to vasoconstriction and a fall in venous pressure, capillary hydrostatic pressure is reduced. This means that as blood flows through the capillary bed, fluid is drawn into the blood from the tissues increasing blood volume.
How is the kidney stimulated to restore blood volume and how does it do this?
- Baroreceptors and a decrease in atrial stretch send signals to the hypothalamus and brainstem.
- The brain increases thirst, ADH release and sympathetic renal activity.
- The kidneys decrease diuresis, and triggers the RAAS system causing water and Na retention.
How long does it take to recover heamoglobin and erythrocyte levels following 25% blood loss?
Hb = 4 weeks
Erythrocytes = 4+ weeks
Can take up to 6 weeks.
How long does it take to recover plasma proteins following 25% blood loss?
1 week
What is haemodilution?
Where after 12-24 hours of blood loss blood volume is restored but the RBC population and Hb levels have not been restored.
What is non-progressive shock?
Shock from blood loss that gets better without treatment. E.g. if a fit person lost <20% blood volume (i.e. donating blood).
Blood volume and cardiac output is restored in 16-24 hours.
What is the golden hour?
A person treated with a blood transfusion within an hour of haemorrhage has a good chance of survival whereas after an hour if the patient is still in shock, transfusion may only give a temporary respite due to irreversible cardiac damage.
What happens to systolic and diastolic pressure as a person ages?
Systolic progressively increases whereas diastolic peaks at 50-60 years and then starts to decrease.
What are the vascular changes as a result of ageing?
- Reduced elasticity of large arteries (arterial dilation) and increased collagen content (stiffness) leads to arteriosclerosis.
- Increase in SNS activity to vasculature, NO release decreases leading to increased TPR.
- Decreased muscle flow during exercise.
What is the overall effect of the vascular changes as a result of ageing?
- Increase in systolic and pulse pressures.
- Increase in afterload.
What happens to the heart’s ability to increase CO when stressed in the elderly? Why?
It becomes less able.
1) Max attainable HR falls (220-age).
2) Fall in cardiac contractility reduces stroke volume.
What is responsible for the heart unable to increase CO when stressed?
Decreased response to β1 receptor activation and loss of myocytes.
Increased afterload.
