Control of circulation Flashcards
What is the fick principle?
The oxygen content of the pulmonary vein is derived from the pulmonary artery blood and O2 uptake across the lungs
Q1 = rate of delivery
Q2 = rate of uptake
Q3 = rate of removal
What is Q1?
CO[O2]pa where CO = cardiac output to the lungs
What is Q2?
O2 uptake
What is Q3?
CO[O2]pv
Q1+Q2= Q3
What is the equation for cardiac output?
CO = O2 uptake / ([O2]pv - [O2]pa)
How is [O2]pa measured?
Pulmonary catheter
How is [O2]pv measured?
Peripheral arterial blood
What are the methods used to measure CO?
Inert tracer eg indocyanine green; injected into vein/right ventricle and output is proportional to 1/tracer
Thermodilution; cold saline injected into right atrium, mixes with blood and a sensor in the artery measures temperature change which is used to calculate CO
Ultrasound; measures real time changes in ventricular dimensions, allows stroke volume to be calculated
What is the relationship between CO and stroke volume?
CO = Stroke volume x heart rate
What is the typical stroke volume?
80ml
How much blood is ejected per min at 69bpm?
5.5l
What is the cardiac index?
CO/body surface area
What is the normal rate of perfusion?
2.5 - 3.6 min^-1m^-2
What is Frank Starling’s law of the heart?
The energy of contraction, however measured, is a function of the length of muscle fibre
How is CO controlled?
Varies by heart rate or stroke volume
The rate of heart beat is mainly controlled by innervation
The stroke volume is regulated by innervation and the length/tension relationship
What are the two types of homeometric control?
Intrinsic contractility changes (homeometric) Frank Starling (Heterometric)
End diastolic volume is regulated by what?
Venous return
Venous return is influenced by what (heterometric control)?
Blood volume increase Vascular storage decreases Haemorrhage/blood loss Vascular resistance Muscle pump action Atrial sucking Gravity Inspiration
How much total blood do veins store?
54%
How does haemorrhage cause an increase in CO?
Blood loss = decrease in output and vascular storage
Venous return will increase to attempt to increase CO
What is muscle pump action and what is its effect?
Increases venous return
Some veins are compressed by muscle
Pump blood from lower limbs = better circulation = increase in venous return ie policeman
Where do patients with heart failure feel comfiest?
Sat up - so they can cope with their EDV = reduced venous return
How does inspiration affect cardiac output?
If the ribs move up and out then there is an increase in thoracic volume
If volume increases the pressure decreases so air moves in
Sucks blood into the thorax from lower limb which increases venous return and therefore the preload = increases CO
What is homeometric control?
Positive inotropic effects not related to nervous sstem/ endocrine functioning (10-15%)
Eg Treppe - strength contraction increases when rate increases
What is extrinsic control?
Autonomic nervous system
Humoral factors
What is the effect of the parasympathetic nervous system on cardiac output?
Primary chronotropic - Negative (bradychardia), uses Ach muscarinic receptors, blocked by atropine = reduces firing rate of pacemaker
Vagul input slows node firing
Slows conduction through the AV node
Decrease in force per beat
Which node does the right vagus control?
The SA node
Left = AVN
What effects does the sympathetic nervous system have on cardiac output?
Positive chronotropic and ionotropic effects eg tachycardia
Increases SA firing rate
Increases conduction velocity
Time for systole and diastole increases
Increased contractility as more Ca is available
Noradrenaline B1 receptors can be blocked by what?
Propanalol and atenolol
What do B1 receptors cause?
They are GPCRs and cause an increase in cAMP/Pka
What do Act^nL type calcium channels do?
Increase contractility
What do muscarinic receptors do?
Inhibit cAMP production mainly in the nodes
Which system is usually dominant?
Parasympathetic
Which humoral factors affect CO?
Adrenaline can increase the rate and force but is minor
Thyroid hormones can also increase the rate and force eg hyperthyroidism = high heart rate
Insulin has a positive ionotropic effect unrelated to glucose transport
What is the equation for blood flow?
Blood flow cm^3/s = change in pressure/resistance
What is poiseulle’s law?
Flow = (pie R^4/8) x (Change in P/nL) R = radius P = pressure N = viscosity L = vessel length
When will blood vessels collapse?
If blood pressure falls below the critical closing pressure
Also may be due to extravascular pressure
How could the relationship between pressure and flow be described in biological vessels?
No linear relationship ie line is curved
What is transmeural pressure?
Law of Laplace’s
TP = tension/radius
What is Laplace’s rule concerning ‘balloons’?
If you have a big balloon and a small balloon connected via a tube air will move into the big balloon as the small balloon has a smaller diameter ie it has a bigger pressure
What are the consequences of Laplace’s rule concerning aneurysms?
When the aneurysm is small it is not a big problem as it is hard to ‘blow up’ however once it gets bigger it becomes easier so the walls get thinner and it can burst
What is the equation for complaince?
Change in volume/ change in pressure
Which vessels are more compliant?
Veins as they have less smooth muscle
At rest how much blood is stored in veins?
54% - this is why they are called capacitance vessels
Where will blood pool after death?
The underside of the body if they have died on their back
This is important for forensic analysis
What is the windkessel effect?
Describes how blood vessels store a little bit of pressure ie further away from the heart vessels do not have a pulsative drop to zero?
Which vessel is pulsative continuous?
The aorta
What is the equation for Reynold’s number?
Nr = pDV/ n p= fluid density D = vessel diameter V = mean velocity n = viscosity
Where is turbulent flow required in the cardiovascular system and why?
In the ventricles - laminar flow would increase the formation of clots
Ventricle walls have papillary muscles and crevices to create turbulent flow
Where in the cardiovascular system is turbulent flow not possible?
In capillaries - cells have to travel in a single file as the lumen is 7um and RBCs are 5 um
What happens to RBCs as a consequence of turbulent flow?
They get damaged and they burst
What is the life span of a red blood cell?
120 days - reproduced by the spleen and bone marrow
What is the percentage of blood volume in the capillaries?
5%
What is the velocity of blood flow in capillaries?
0.07cm/s - very slow
What closes capillaries?
The sphincter
How is tissue fluid formed?
Pressure in capillaries means plasma and serum is pushed out of gap junctions in the capillary
Where does tissue fluid drain to?
The lymphatic system - back into the heart via the right subclavian vein which joins the vena cava
How does malnutrition cause swollen stomachs?
Low albumin hydrostatic pressure causes fluid to be pushed out
Where is pitting oedama dangerous?
The brain - the skull can’t expand
The lungs - alveolar fill with fluid = dry drowning (hypoxia due to no gas exchange)
The swelling of ankles can be a sign of what?
Heart failure - diuretics may be prescribed
What does fluid formation depend on?
Hydrostatic pressure difference between the capillary and Interstitial fluid
Difference in colloid osmotic pressure
Capillary filtration coefficient
What is the effective of histamine?
Makes blood vessels leaky = swelling
How much does the lymphatic system drain per day?
2-4L
What is the structure of large arteries?
Endothelial lining regulates blood flow
Basement layer
Tunica medium
Vasa Vasorum (vessels of vessels)
What is the main difference between large and small arteries?
The tunica media is thinner
What is the main differences between arteries and veins?
The veins have valves to prevent blood backflow
Which veins do not have valves?
In the head or neck - valves would cause bidirectional flow
What is myogenic tone?
The stretch of an artery evokes a muscle contraction which prevents the stretch from occuring and causes the vessel to return to its diameter in healthy tissue
What can cause vessels to dilate?
pH lowered Reduced oxygen Increase in body temperature Lactate Increase in potassium
Why is innervation density low in the cerebral vasculature?
A high blood flow to the vein is always required
What are kinins?
Vasodilatory peptides
What is angiotensin ii?
A vasoconstrictor which increases blood pressure - formed by enzymes eg renin
What causes serotonin to be produced locally?
If platelets are damaged they produce serotonin to initiate the clotting cascade
What is endothelium derived relaxing factor?
Released after stimulation eg by Ach
NO is released from endothelial cells
Stimulates cGMP in muscles = relaxation
What is the action of viagra?
Inhibits the breakdown of cGMP = no muscle relaxation
What is nitroglycerin?
A treatment for angina which is sprayed under the tongue = NO is released which causes vasodilation so flow and O2 supply increases
Also reduces venous return so contractility decreases and the heart can rest
What does the cardioaccelatory centre do?
Increases heart rate
Where do the cardioacceloratory and cardioinhibitory centers send signals to?
Spinal cord levels at T1 and T4
What is the action of the vagus nerve?
It is inhibitory
The enviroment around medullary neurons respond to what?
pH, pCO2 (increase), and PO2 (decrease)
What is coning?
Where the brainstem becomes distorted - increase CO2 so they can breathe more quickly
What does the cushing reflex lead to?
Vasoconstriction so BP increases but also causes bradcardia = high risk of death
Where are baroreceptors located?
In the corotid sinus and the aortic arch (detect high pressure)
What is the valsave manoevre?
Forced expiration against a close glottis
Increases thoracic blood pressure
Venous return is reduced so blood pressure decreases = decrease in barorecptor activity = tachycardia and vasoconstriction
What does the hypothalamus control?
The parasympathetic NS
Linked to the cortex and the limbic system
Ermergency situations/ responses
What does the medial preoptic area control?
Sexual responses
What does the anterior cingulate control?
Coordination between autonomic and emotional responses
eg in some animals under threat BP drops so they can play dead
