Regulation Of Circulation And Specialised Circulations Flashcards
What are the three distinct layers of the blood vessels?
Tunica intima - innermost endothelium
Basal membrane - fenestrated
Adventia - contains more collagen for new smooth muscle production
How do elastic arteries work?
They have elastin proteins that allow them to stretch to take the volume of blood at the pressure generated by the left ventricle
Given an example of an elastic artery
The aorta
Branches coming for the aorta
What is the importance of elasticity?
It maintains the diastolic pressure in the arterial system and contributes to the after load of cardiac output
Where does the majority of the resistance of the arterial system come from?
Arterioles
Why are pressures generated by the right side of the heart lower?
The pulmonary arteries are wider and more compliant
What are conduit arteries?
They are more muscular and feed blood to the smaller arteries of the organs
How do conduit arteries avoid compression at areas such as joint?
They have thick walls that prevent compression and collapse
What is the innervation of conduit arteries?
They have denser noradrengeric innervation of sympathetic vasoconstrictor fibres.
Cause vasoconstriction when active
What are small arteries or arterioles?
They are the resistance vessels where pressure falls sharply from one end to the other - pre-capillary spinsters
What innervation is there to arterioles?
Dense sympathetic noradrenergic innervation causing vasoconstriction
Can effectively determine the number of capillaries in which blood flows
What happens when sympathetic innervation is removed to arterioles?
This causes vasodilation to reduce resistance to blood flow and increase perfusion of capillary beds
What effect do metabolites produced during muscle activation cause?
They dilate local vessels to that area
What are capillaries?
They are the exchange part of the circulation as the walls are extremely thin and comprise of a single layer of endothelium
Why do capillaries provide little resistance to flow?
Because of the sheer number of capillaries in all systemic tissues
What three things that filtration across the capillaries depend on?
Balance of hydrostatic pressure within the capillary
Tissue pressure if surround tissues
Starlings forces - the colloid osmotic pressures in the vessels and extracellular fluid
What is oedema?
Vessels become leaky to water under situations such as inflammation leading to oedema
Where does diapedesis occur?
At capillaries
What is diapedesis?
The movement of WBC from the circulatory system and towards the site of tissue damage/infection
Explain the steps of diapedesis
i) chemotaxins are released bu damaged tissue
ii) They cross the endothelium and attract and activate WBC - neutrophils
iii) Neutrophils then stick to the endothelium and begin separating their cells
iv) A pododcyte of the neutrophil will extend through the gap and others will follow it towards the site of injury
What are arteriovenous anastomoses
They are shunt vessels found mostly in the skin that connect arterioles to venues directly
Bypassing capillaries
What innervation do arteriovenous anastomoses have?
Dense sympathetic innervation from the hypothalamic areas involved in temperature regulation
How is heat lost through the skin?
Removal of sympathetic innervation to the arteriovenous anastomoses so that blood can flow into the deep subcutaneous venous plexus
How are venules different from veins?
They are much smaller and more numerous
What prevents back flow of blood in veins?
Semilunar valves
Absent in large central veins and veins of the head
Do venules offer resistance to blood flow?
No very little resistance is offered
What innervation do venules have?
Sympathetic noradrengeric
When innervated will vasoconstrict to route more blood back to the heart
How do muscle help veins?
Muscle contraction squeezes thin walled veins and pushes the blood along towards the heart
What do lymphatic vessels do?
They help maintain fluid balance in extracellular fluid
They help mop up fat droplets
They also have an immune function
How do lymphatic vessels work?
The endothelial cells are less tightly packed together so they are leaky, this allows fluid to enter from the surround extracellular fluid
They have valves to ensure one way flow
They return the fluid to the vena cava
What is vascular pressure?
The pressure created by the left the ventricle
What is the journey of vascular pressure?
They left ventricle develops a systolic pressure of 120mmHg
From the aorta it drops to 80mmHg - pressure is now pulsatile
In arterioles the pressure drops to 50mmHg - due to high resistance
Pressure continues to fall through capillary beds
By the time it reaches the vena cava it is close to zero
What is pulse pressure?
The difference between systolic and diastolic pressures
What is a normal pulse pressure?
40mmHg
But it increases as you get older
Why does pulse pressure change with age?
Elastin fibres are lost and replaced with collagen fibres, and vessels become stiffer and less complaint
What is mean arterial pressure?
The diastolic pressure + 1/3 systolic pressure
Normally it is around 93mmHg
Why are pulmonary pressures lower than systemic pressures?
Resistance in pulmonary flow is lower
Because the vessels are wider and more compliant
Systemic pressures must get around the whole body, pulmonary pressures only need to reach the lungs
Why are veins more compliant than arteries
They have less muscle mass
Greater distension and blood volume carried
What is the difference in veins between low pressure and high pressure?
Low pressures - veins are relatively collapsed
High pressures - they distend and become rounder
What innervation do veins have?
Moderate degree of sympathetic
This increases activity of smooth muscle contractions and stiffen the vessel
What is the Frank-Starling mechanism?
More in - more out
How are smooth muscle cells arranged?
Small dense bodies anchor the actin while myosin fibres suspend between them
What are the difference between smooth muscle cells and skeletal muscle cells?
They are smaller, long (20-500 micrometers) and thin (1-5 micrometers)
They are interconnected by gap junctions - allows for rapid conduction
How does smooth muscle function?
It has underlying rhythmical contractions, that produce phasic, slow rhythmical contractions
Forces generated are smaller and can be maintained for longer periods (hours / days)
Steps to smooth muscle contraction
i) sympathetic nerves release noradrenaline
ii) G-protein coupled receptors release second messengers (DAG) which open calcium channels - rapids calcium increase
iii) Depolarisation can cause calcium to come in from the extracellular fluid as well
iv) calcium induced calcium release - calcium binds of receptors on sarcoplasmic reticulum
v) Calcium or 2nd messengers may also interact with ion channels to cause depolarisation
vi) increased calcium binds to calmodulin this complex activated enzyme myosin light chain kinase
vii) Myosin is then phosphorylated and cross bridging commences
viii) contraction can maintain even after dephosphorylation
What is Poiseuille’s law?
That flow of a fluid is directly proportional to the viscosity of blood and the length of the vessel
While being inversely proportional to the 4th power of the radius of the vessel
What effect does radius have on resistance to flow?
A small decrease in radius will cause a large increase in resistance and consequently reduction in flow
Define viscosity
Viscosity of blood is the resistance of fluid against flow
What is the equation for arterial pressure?
Pa = CO x TPR
What is the equation of cardiac output?
CO = SV x HR
Why is the equation for arterial blood pressure important?
It shows the three elements that medications target to alter arterial BP
Why is vascular resistance important?
Without resistance there would be maximal dilation and maximal flow, and this would exceed the maximal CO we are capable of by a factor of two
What is basal tone?
The balance of dilator and constrictor influences acting on smooth muscle
What are the extrinsic influences?
Nervous control
Circulating hormones
What are the intrinsic factors ( local within organ)?
Myogenic response
Temperature and pressure
Tissues metabolites
Vasoactive chemicals and autocoids
Endothelial secretions (e.g. nitric oxide)
What are the two sources of calcium for smooth muscle use?
Influx from surrounding extracellular fluid
Internal stores of the sarcoplasmic reticulum
What is active vasoconstriction?
Sympathetic nerves nerves release noradrenaline which bind to adrenoreceptors and cause vasoconstriction
What is passive vasodilation?
Sympathetic nerve activity decreases
What causes active vasodilation?
Some sympathetic nerves will cause vasodilation by the release of ACh
How does the sympathetic neuroeffector junction work?
Adrenergic receptors are distributed all over the smooth muscle so when transmitter is released it diffuses and interacts with receptors all around the varicosities
Where does acetylcholine work during sympathetic innervation?
The sympathetic ganglionic synapse
Where does noradrenaline work during sympathetic innervation?
At the neuroeffector junction
Where do cotransmitters work?
On the postsynaptic membrane of the target tissues
Where do neuromodulators work?
On the sympathetic nerve terminals from which they’re released
What receptors does noradrenaline have a greater affinity for?
Alpha receptors
Coupled to G protein and Gq
What receptors does adrenaline have a greater affinity for?
Beta receptors
Coupled to Gs G protein
How does sympathetic innervation reach the blood vessels?
Promoter neurones travel to the spinal cord,
synapse with preganglionic neurones, exit the spine and travel to sympathetic ganglia then
synapse with postganglionic neurones
