Circulation and blood pressure (L13 + 15) Flashcards
What is fluid flow through blood vessels governed by?
Physical laws: Blood flow (cm^3) = change in pressure/resistance
What is Poiseuille’s Law?
Flow = π x r^4 x ΔP / 8 x η x L
The flow of liquid is related to the viscosity of the fluid, the pressure gradient across the tubing, the length of the tubing, the diameter of the tubing.
Why is the diameter of a vessel very important?
Because flow and resistance depend on the change in pressure, but also the radius, especially in small arteries and particularly arterioles which is where blood first meets high resistance. The vessel diameter is a key control point of this resistance.
Explain the idea of collapsing blood vessels.
Blood vessels are not rigid and tissue pressure will collapse vessels is the blood pressure falls (critical closing pressure). The relationship between flow and pressure is not linear in elastic tubes because for flow to increase, the pressure inside must exceed the pressure outside for the tube to be fully open.
What does the law of Laplace state?
Applies to open vessels - states that the transmural pressure (pressure across the wall of the blood vessel) (P) = tension of the blood vessel / radius of the blood vessel. Tension = P x r (the internal radius) / W (wall width)
What are aneurysms and how does Laplace’s law apply?
Ballooning of the blood vessel due to a weakness in the wall. The pressure from within the artery causes it to locally balloon out. The tension across the vessel depends on the radius of the aneurysm. The smaller the aneurysm, the higher the pressure, but the wall is thicker. So as the aneurysm gets bigger, the wall gets thinner and it is more likely to burst
What is compliance? How is it worked out?
Compliance is a measure of give a blood vessel has (how much internal pressure it takes to expand it). Compliance = Change in volume/change in pressure
Why are veins more compliant than arteries?
They have a less thick wall so expand more easily. This means they store blood (capacitance vessels) which can also lead to post-mortem blood pooling.
Why is the fact that your vessels are elastic important?
The fact out arteries and veins are elasticated means that blood can flow continuously. If the tubes were rigid, blood flow would stop with every heartbeat. But because they are elastic, tension is stored when the blood flows through elastic recoil so it can be pulsated continuously in between heart beats.
Explain the different types of flow through vessels and how can they be worked out?
Flow through blood vessels can be laminar or turbulent (similar to in the airways.) The type of flow can be found using Reynold’s number, which is equal to ρ (fluid density) x vessel diameter x mean velocity / η (viscosity)
if Reynolds number is less than 2000 you get a laminar flow and if it’s over 3000 you get turbulent flow.
What happens to blood pressure and velocity at the arterioles and why?
Blood pressure decreases across arterioles because there becomes many more of them. The mean pressure is about 35mmHg compared to 120mmHg in the aorta. Blood velocity also decreases because diametre deceases.
Explain blood flow through the capillaries
5% of total blood volume is contained within capillaries. Velocity through capillaries is low (0.07 cm/s). Capillaries have a large total surface area because they are a major site for nutrient exchange.
Precapillary sphincters direct flow and the vessels can be inactive (collapsed) when less exchange is needed
What is tissue fluid and how is it formed?
Fluid between tissues. Net fluid transfer from capillaries to tissue is by a balance of filtration and absorption. Filtration occurs through cells (aquaporins - transcellular) and between cells (paracellular).
The fluid formation depends on a change in pressure ( a hydrostatic pressure difference between the capillary and interstitial fluid). A difference in COP =a difference in colloid osmotic pressure. CFC = capillary fibrillation coefficient (what dictates the ease of flow across the vessel wall). Blood hydrostatic pressure is normally higher than tissue fluid hydrostatic pressure (which may be negative). Thus, the change in pressure is outwards. COP is caused by impermeable proteins e.g. albumin. If the plasma pressure is high, and the tissue fluid is low (thus the change in COP is about equal to COP of plasma), the change of COP is in. This balance causes a net production of tissue fluid.
What are the similarities and differences between different types of vessels?
Veins have much larger lumens than that of arteries of the same size. All vessels have an endothelium and basement membrane layer. Arteries have a thick elastic layer (tunica media) and veins have a larger tunica externa but not as much elastic tissue. Smaller veins and venuoles contain valves to help with the blood flow in the right direction.
Explain what is meant by a basal tone and autoregulation
Vessels have a resting tension, like with muscles. Their tension is myogenically regulated (stretch evokes a contraction). This low-level control is designed to give a constant flow (auto-regulation). The vessels respond to changes in factors so that they can maintain blood flow to important structures and organs, even if perfusion changes. Local factors like partial pressure of oxygen and co2, temp, potassium conc and lactate conc and things like bradykinins (known as metabolite regulators) can also impact the diameter of the vessel. E.g. increased co2 production makes the vessel dilate to increase blood flow. - no input from medullar needed for autoregulation, the vessel does it all by itself
Explain the role of extrinsic control of vessels through the autonomic system.
Most vessels (not capillaries and venules) have tonic sympathetic adrenergic constrictor input (alpha adrenergic receptors). Precapillary vessels in skeletal muscle, heart, lung and kidneys have sympathetic ACh neurones, which cause vasodilation and increase blood flow to a tissue. The exception is erectile tissue/glands which have para. ACH vasodilators. Sympathetic constrictors are very important, they divert blood into the capillary beds.
Explain how innervation from the autonomic system varies between vessels.
Innervation to vessels int eh brain is low because you never really need to decrease blood to this area. Muscles with a weaker sympathetic tine cause smooth muscle relaxation and the blood pressure dilates the vessel. Whereas, the innervation to cutaneous vessels is high, so blood flow to those areas can be controlled. , e.g. to direct blood to the surface of the skin to reduce heat
Explain the humoral ways blood vessels are regulated.
Hormones made by the adrenal medulla e.g. adrenaline
- Causes vasoconstriction in the skin/viscera, but vasodilation in the skeletal muscle and liver- this is so blood is concentrated to certain areas in the fight/flight response when it’s needed most. (muscle, liver, heart etc) In this response, however, it is the nervous system response rather than autoregulation that has the biggest influence because it is faster. Kinins e.g. bradykinin is a vasodilatory peptide (released when using ACE inhibitors to treat hypertension)
Angiotensin II are vasoconstrictors which increas BP: they’re formed by enzymes e.g. renin in the kidney acting on precuroses when BP falls.
What local agents help control blood vessels?
Prostaglandins can be produced by tissues in response to hypoxia (causes vasodilation), Serotonin (produced by platelets) causes vasoconstriction (helps reduce bleeding during a cut). Histamine released from mast cells during allergic reactions causes vasodilation (BUT bronchoconstriction) - why you go red when you have an allergic reaction - its aim is to flush blood into the area to hopefully get rid of the allergen.
Explain how Endothelium-derived relaxing factor causes relaxation.
It is another local agent involved in the extrinsic control of blood vessels. its basically NO produced by endothelial cells after stimulation e.g. ACh. It stimulates cGMP in the muscle which causes relaxation of smooth muscle. Sildenafil (Viagra) inhibits cGMP breakdown, which increases relaxation in the penis. Nitroglycerin causes an increase of NO which causes vasodilation - this increases flow and blood supply (used to treat angina - pain caused by decreased blood flow to the heart) endothelium-derived hyperpolarizing factors are also released on stimulation by ACh. They do the same thing as EDRFs, but their pathway is very complex.
Explain the central control of blood vessels.
Done by interlinked CNS centres in the brain - especially the medulla. It receives multiple inputs from the body and will alter blood flow accordingly. The emotional response also plays a part in this. And your brain also ‘knows’ when you’re going to exercise. Neurones in the medullary centres regulate cardioexcitation, cardioinhibition, and vasomotor output.
E.g. the carotid sinus is innervated by the glossopharangeal nerve. Stretch of the sinus will result in activation of the nerve. This signal will be sent to the cardioinhibitory centre. It will the be passed down parasympathetic efferents to causes a decrease in the heart rate. A similar signal is also sent to the vessels to cause vasodilation. This reduces blood pressure
Explain the kind of things medullary neurones react to.
- Local environment:
- respond to pH change, increase in CO2, decrease in O2.
- Increase in intercranial pressure e.g. due to swelling after brain injury - decreases blood flow to the brain
- Cushing’s reflex leads to increase in vasoconstriction, this causes an increase in BP but also brachycardia (decrease in heart rate) - so risk of death in these patients is high. - Peripheral proprioceptors
- Baroreceptors (stretch receptors)
- Stretch receptors - in the carotid sinus / aortic arch detect high pressure
- A sudden decrease in arterial pressure decreases baroreceptor firing, which activates sympathetic neurones and inactivates vagal neurones in the medulla. This increases cardiac output and systemic vascular resistance increases. This acts to increase arterial pressure.
- this is short term regulation of BP - long term is by the kidneys that regulate blood volume
What is the prevalence of hypertension and what is it?
Kills up to 1 mill people per year. It is when blood pressure is abnormally high compared to the average for the age/sex of the patient.
What are the different types of hypertension?
Essential hypertension
Secondary hypertension - a consequence of a clinical condition (so basically caused by another disease)
