Special Circulations Flashcards
What circulations do the lungs have?
Bronchial circulation and Pulmonary circulation
What is the bronchial circulation part of?
The systemic circulation
What is the purpose of the bronchial circulation?
It meets the metabolic and oxygen requirements of lungs
What does the bronchial circulation ensure?
That those parts that are not readily perfused with oxygen are close to an oxygen supply
How is the pulmonary circulation related to the systemic circulation?
It is in series
What is the purpose of the pulmonary circulation?
It supplies blood to the alveoli, which is required for gas exchange
What gas exchange needs to occur at the lungs?
Needs to oxygenate blood, and allow for removal of carbon dioxide
What must the pulmonary circulation accept?
The entire cardiac output
Are the equal amounts of blood in the pulmonary and systemic circulations?
No
What is cardiac output at rest?
~5l/min
What does cardiac output have the ability to do?
Increase when exercising
What is the maximum cardiac output?
~20-25l/min
Is flow the same in systemic and pulmonary circulations?
Yes
How does the pulmonary circulation differ from the systemic?
It works with a much lower pressure and resistance
What is the pressure in the right ventricle?
15–30mmHg in systole
0-8mmHg in diastole
What is the pressure in the pulmonary artery?
15-30mmHg systole
4-12mmHg diastolic
Why is pulmonary artery pressure higher in diastole than right ventricle?
Due to elastic recoil and closure of valve
What is the pressure in the left atrium?
1-10mmHg
Why is left atrial pressure a bit higher than right atrial pressure?
Because pulmonary circulation is at low resistance, so you don;t get as much of a drop in pressure
How does the pressure of the left ventricle differ from the right?
It is higher
Why is the pressure in the left ventricle higher than in the right ventricle?
Because the wall of the left ventricle is much thicker so it’s able to squeeze the blood around at a higher pressure
What is the pressure in the aorta?
100-140mmHg systolic
60-90mmHg diastolic
Why is the pressure high in the aorta?
Because the wall is thicker
What is the pressure in the right atrium?
0-8mmHg
Why is the pressure in the RA and RV the same during diastole?
Because during diastole, the tricuspid valve is open, and so RA and RV are continuous
What happens to the right ventricles in systole?
It contracts, and so pressure goes up
What are the features of the pulmonary circulation?
Low pressure
Low resistance
What is the mean arterial pressure in the pulmonary circulation?
12-15mmHg
What is the mean capillary pressure in the pulmonary circulation?
9-12mmHg
What is the mean venous pressure in the pulmonary circulation?
5mmHg
Describe the vessels in the pulmonary circulation
Short, wide
What is the result of the pulmonary circulation having lots of capillaries?
There are many parallel elements, because there is lots of branching. This leads to resistors in parallel, and therefore lower resistance
How do the arterioles in the pulmonary circulation differ from those in the systemic?
They have relatively little smooth muscle
What adaptations does the pulmonary circulation have to promote efficient gas exchange?
Very high density of capillaries in alveoli wall
Short diffusion distance
What is the result of the very high density of capillaries?
Large capillary surface area
What separates the gas phase from the plasma in the pulmonary circulation?
A very thin layer of tissue
What is the combined endothelium and epithelium thickness?
~0.3µm
Why do pressures need to be low in the pulmonary circulation?
Because of the very short diffusion distance- if the pressure gets too high, can rupture the membrane
What do the adaptations of the pulmonary circulation produce?
A high oxygen and carbon dioxide transport capacity
What needs to be matched for efficient oxygenation?
Ventilation of alveoli (V) with perfusion of alveoli (Q)
What is the optional V/Q ratio?
0.8
What determines perfusion?
Cardiac output
What determines ventilation?
Amount you breath
What does maintaining the optimal V/Q ratio require?
Diverting blood from alveoli that are not well ventilated
What ensures the optimal V/Q ratio?
Hypoxic pulmonary vasoconstriction
What is hypoxic pulmonary vasoconstriction important for?
Regulating pulmonary vascular tone
What does alveolar hypoxia result in?
Vasoconstriction of pulmonary vessels
What is the result of hypoxic pulmonary vasoconstriction?
Poorly ventilated alveoli are less well perfuse, and so deoxygenated blood is not returning to the left side of the heart, which helps optimise gas exchange
How is hypoxic pulmonary vasoconstriction different to what happens in the systemic circulation?
It is the opposite effect
What can chronic hypoxic vasoconstriction cause?
Right ventricular failure, as it puts strain on the right side of the heart
Where can chronic hypoxia occur?
At altitude, or as a consequence of lung disease such as emphysema
What happens if someone is it altitude for a long time?
Get constriction of pulmonary vessels that increases blood pressure in the pulmonary artery
What can result from the increase in blood pressure in the pulmonary artery caused by prolonged altitude?
Pulmonary hypertension
What can lung disease lead to?
Poor ventilation, and then hypoxic vasoconstriction, leading to an increase in pulmonary artery pressure
What can high afterload on the right ventricle lead to?
Right ventricular heart failure
What are pulmonary vessels strongly influenced by?
Gravity
Why are pulmonary vessels strongly influenced by gravity?
Because they are low pressure and relatively thin vessels
What happens to pulmonary vessels when in the upright position?
There is greater hydrostatic pressure on the vessels in the lower part of the lung, and so the vessels near the base are distended
Why are the vessels near the base of the lung distended?
Due to increased hydrostatic pressure, because of column of liquid and effect of gravity
What happens to vessels near the apex of the lung?
They collapse during diastole
What happens to vessels at the level of the heart?
They are continuously patent
What effect does exercise have on cardiac output?
It increases it
What is the effect of exercise on the pulmonary blood flow?
There is a small increase in pulmonary arterial pressure
Capillary transit time reduced as blood flow increases
What is the effect of the increase in pulmonary arterial pressure?
It opens apical capillaries, and so means capillaries are better perfused so there is better matching of ventilation and perfusion
What is transit time?
The time it takes for RBCs to get through the capillaries
What is transit time in the pulmonary circulation at rest?
~1s
How far can transit time in the pulmonary circulation fall without compromising gas exchange?
~0.3s
Where can fluid move?
Through capillaries, and through capillaries into tissues
What determines tissue formation?
Starlings forces
What are Starlings forces?
Hydrostatic pressure of blood within capillary Oncotic pressure (colloid osmotic pressure)
What does hydrostatic pressure of blood within the capillary do?
Pushes fluid out of capillary
What is oncotic pressure?
The pressure exerted by large molecules such as plasma proteins
What does oncotic pressure do?
Draws fluid into the capillary, because there aren’t many proteins outside
What is capillary hydrostatic pressure influenced by?
More venous pressure than arterial pressure
Why is capillary pressure influenced more by venous pressure?
Because between arteries and capillaries, there are very thick arterioles with lots of resistance, and so there is a drop of pressure across this, meaning that it’s not influenced as much by arterial pressure
Does hypertension result in peripheral oedema?
No
What does low capillary pressure in the lungs minimise?
Formation of lung lymph
What drives fluid into the capillary?
Plasma oncotic pressure
What drives fluid out of the capillary?
Interstital oncotic pressure and hydrostatic pressure
Why does not much lung lymph form?
Because filtration is roughly equal to reabsorption- they balance out well
Why is the minimisation of lung lymph formation important?
Because if you get too much fluid going out, get pulmonary oedema, which can impair gas exchange
How does oncotic pressure differ in the tissue fluid of the lungs compared to the periphery?
It is higher
How does the capillary hydrostatic pressure differ in the lungs compared to the periphery?
It is less
How does plasmic oncotic pressure differ in the lungs compared to the periphery?
It doesn’t
What does increased capillary pressure lead to?
Oedema
Why can an increase in arterial pressure have an influence on capillary pressure in the lungs?
Because of the low resistance
What prevents pulmonary oedema?
Low capillary pressure
What is pulmonary capillary pressure normally?
9-12mmHg
How much fluid leaves the capillaries in the lungs at normal pressure?
Only a small amount
What can happen if capillary pressure in the lungs rises?
Can get pulmonary oedema
How high does pressure have to rise to get pulmonary oedema?
If left atrial pressure rises to 20-25mmHg
When can left atrial pressures reach levels that may give pulmonary oedema?
Mitral valve stenosis
Left ventricular failure
Why does mitral valve stenosis cause an increase in left ventricular pressure?
More difficult for blood to move from left atrium to ventricle, so build up in left atrial pressure
Why does an increase in left atrial pressure lead to pulmonary oedema?
Harder for pulmonary veins to drain
Why does left ventricular failure cause pulmonary oedema?
If left ventricle can’t pump out as much, higher pressure in left ventricle, so harder for blood to move in from left atrium, so harder for pulmonary veins to drain
What is the problem with pulmonary oedema?
It impairs gas exchange
What affects pulmonary oedema?
Posture
Why does posture affect pulmonary oedema?
Due to changes in hydrostatic pressure due to gravity
Where does pulmonary oedema form when upright?
Mainly at bases
Where does pulmonary oedema form when lying down?
Throughout the lungs
What does the formation of pulmonary oedema throughout the lungs when lying down cause?
Symptoms that are worse at night
How are the symptoms of pulmonary oedema relieved?
Diuretics, which reduce blood volume
Treat underlying cause if possible
How much of the cardiac output does the brain receive?
About 15%
Why does the brain receive so much of the cardiac output?
Because it has high oxygen demands
What % of oxygen consumption does grey matter account for at rest?
20%
What is important due to the high oxygen demands of the brain?
Must provide a secure oxygen supply
How are the brains oxygen demands met?
High capillary density
High basal flow rate
High oxygen extraction
What does high capillary density provide?
A large surface area for gas exchange, and a reduced diffusion distance
What is the diffusion distance in the brain?
~10µm
How does the basal flow rate in the brain compare to the rest of the body?
It’s about 10x the average
How does oxygen extraction in the brain differ from the rest of the body?
35% above average
Why is secure oxygen supply to the brain vital?
Neurones are very sensitive to hypoxia
Interruption to blood supply causes neuronal death
How long can neurones be without oxygen?
Loss of consciousness occurs after a few seconds of cerebral ischaemia
Begin to get irreversible damage to neurones after about 4 minutes
What happens when you get neuronal death?
Stroke
How is a secure blood supply to the brain ensured?
Structurally
Functionally
How is blood supply secured structurally?
Circles of Willis
What are circles of Willis?
Anastomoses between basilar and internal carotid arteries
How do circle of Willis ensure a secure blood supply?
Means there’s more than one route for blood to get there, so if you get a blockage in one, doesn’t prevent flow to the whole brain
How is secure blood supply to the brain ensured functionally?
Myogenic autoregulation maintains perfusion during hypertension
Metabolic factors control blood flow
What is myogenic autoregulation generated by?
Smooth muscle cells
What does myogenic autoregulation respond to?
Changes to transmural pressure
How does the brain respond to changes in transmural pressure?
High blood pressure causes vasoconstriction
Low blood pressure causes vasodilation
What does myogenic autoregulation serve to do?
Maintain cerebral blood flow when BP changes
When does myogenic autoregulation fail?
Between 50mmHg
What are cerebral vessels very sensitive too?
Changes in arterial P CO2
When does CO2 rise?
When theres a lot of activity in the brain, and therefore more demand
What does hypercapnia cause?
Vasodilation
What does hypocapnia cause?
Vasoconstriction
What can cause hypocapnia?
Panic hyperventilation
What can panic hyperventilation cause?
Dizziness or fainting
How is panic hyperventilation treated?
Give bag to rebreathe into, to prevent blowing off too much CO2
What regulates circulations?
Brainstem
What is the result of the brainstem regulating other circulations?
The brain can prioritise its own circulation
What does regional activity cause?
Local increases in blood flow
Why does regional activity cause local increases in blood flow?
Things that are produced through breakdown of ATP cause vasodilation
What metabolic factors cause vasodilation?
Increase in P CO2 Increase in [K+]
Increase in adenosine
Decrease in P O2
What is the result of the rigid cranium?
It protects the brain, but does not allow for volume expansion
What do increases in intracranial pressure do?
Impair cerebral blood flow
What can cause increases in intracranial pressure?
Cerebral tumour or haemorrhage
What is the result of the impairment of cerebral blood flow?
It makes it more difficult to perfuse the brain
What is Cushing’s reflex?
The impaired blood flow to the vasomotor control regions of the brainstem due to an increase in intracranial pressure increases sympathetic vasomotor activity, which increases arterial blood pressure due to vasoconstriction in peripheral vessels
What is the result of Cushing’s reflex?
Helps maintain cerebral blood flow
What do the cerebral capillaries form?
A tight blood-brain barrier
What can diffuse through the blood-brain barrier?
Lipid soluble molecules such as oxygen and carbon dioxide
What can’t diffuse through the blood-brain barrier?
Lipid insoluble molecules such as K and catecholamines
Why doesn’t the blood-brain barrier allow lipid insoluble molecules through?
Don’t want them, as they may cause too much activity
What must the coronary circulation deliver?
Oxygen at a high basal rate
What must the coronary circulation meet?
An increased demand, as work rate can increase five-fold
What do the right and left coronary arteries arise from?
The right and left aortic sinuses
When does blood flow in the left coronary artery mainly occur?
During diastole
Why does left coronary artery flow mainly occur during diastole?
Because in systole, the left ventricle is contracting, which squeezes capillaries and small vessels, so difficult for coronary blood flow to take place
When is it most difficult for coronary blood flow to take place?
At the start of isovolumetric contraction
What happens to coronary blood flow at the start of isovolumetric contraction?
It drops to basically 0
What is cardiac muscle adapted to do?
Receive lots of oxygen
How is the cardiac muscle adapted to receive oxygen?
Muscle fibres 18µm in diameter
Capillary density 3000/mm 2
Capillaries continuously perfused
What diameter are the muscle fibres in skeletal muscle?
50µm
What is the result of the small muscle fibres in cardiac muscle?
The diffusion distance is always >9µm
What is the capillary density in skeletal muscle?
3000/mm 2
What is the result of the high capillary density in cardiac muscle?
It facilitates efficient oxygen delivery
Are all capillaries perfused at rest in skeletal muscle?
No
How are all capillaries perfused at rest in cardiac muscle?
Continuous production of nitric oxide by coronary endothelium keeps the capillaries open, which maintains a high basal flow
What does coronary blood flow increase to meet?
Myocardial demand
How is extra oxygen required at high work load supplied?
By increased blood flow
What kind of relationship exists between oxygen demand and blood flow?
Almost linear relationship until very high oxygen demand, and then only a small increase in amount of oxygen extracted
What can cause vasodilation of cardiac vessels?
Extra blood flow in response to metabolites
What metabolites can cause myocardial vasodilation?
Increase in [K+]
Decrease in pH
What kind of arteries are coronary arteries?
Functional end arteries
What is the result of the coronary arteries being functional end arteries?
Few arterio-arterial anastomoses
What is meant by their being few artero-arterial anastomoses?
Different arteries don’t join with each other, as they do in some tissues
What are coronary arteries prone to?
Atheromas
What do narrowed coronary arteries lead to?
Angina on exercise
Why do narrowed coronary arteries lead to angina on exercise?
Due to increased oxygen demand
Made worse because coronary blood flow mainly during diastole, but diastole duration reduced as heart rate increases, compromising blood flow even further
What does stress and cold cause?
Sympathetic coronary vasoconstriction and angina
What does sudden obstruction by thrombus cause?
Myocardial infarction
Why may skeletal muscle circulation need to increase?
To meet oxygen and nutrient delivery demands, and remove metabolites during exercise
What does skeletal muscle have an important role in?
Regulating arterial pressure
Why does skeletal muscle circulation help regulate arterial pressure?
Because it constitutes 40% of adult body mass, and so there is a lot of circulation going to skeletal muscle
What do resistance vessels in skeletal muscle have?
Rich innervation by sympathetic vasoconstriction fibres
What does the baroreceptor reflex do?
Maintains blood pressure
What does capillaries density depend on?
Muscle type
What kind of muscle has a higher capillary density?
Postural muscle
Why do skeletal muscles have very high vascular tone?
Permites lots of dilation, and so flow can increase >20 times in active muscle
How much of the capillaries are perfused at any one time at rest?
Only half
What does only half the capillaries being perfused at rest allow for?
Increased recruitment
What allows more capillaries to be perfused?
Opening of precapillary sphincters
What is the result of more capillaries being perfused?
Increased blood flow and reduced diffusion distance
What does flow in skeletal muscle increase in response to?
Metabolic hyperaemia
What agents are thought to act as vasodilators?
Increase in [K+] Increase in osmolarity Inorganic phosphates Adenosine Increase in [H+] Adrenaline
How does adrenaline act in arterioles in skeletal muscle?
Through ß2 receptors
What effect does noradrenaline have?
Vasoconstrictor response
How does NA exert its vasoconstrictor response?
Via α1 receptors
Does the skin have a high metabolic requirement?
No
Where does the cutaneous circulation have a special role?
In temperature regulation
What is core temperature maintained around?
Around 37 degrees
What is core temperature a balance between?
Heat production and heat loss
What is the main heat dissipating surface?
Skin
What is heat dissipation from the skin regulated by?
Cutaneous blood flow
Where does the cutaneous circulation have a role?
In maintain blood pressure
How does the cutaneous circulation maintain blood pressure?
Vasoconstriction in cutaneous circulation can maintain blood pressure
What specialised structures does acral (apical) skin have?
Arteriovenous anatomoses (AVAs)
Where do AVAs tend to be?
In skin where you have a high surface area to volume ratio
Give 4 examples of skin that has a lot of AVAs?
Ears
Nose
Fingers
Toes
What kind of structure do AVAs have?
Coiled like
What control are AVAs under?
Sympathetic
How can heat be lost rapidly?
Bypass capillary bed and get blood to venous supply through low resistance shunt to venous plexus, which allows a large increase in blood flow just below the skin, allowing the skin temp to rise and so dissipating heat
What effect does a decrease in core temperature have?
Increase in sympathetic tone in AVAs, and so decreases blood flow to the skin
How can heat be lost through apical skin?
Due to dilation of capillary beds
What happens if you’re hot?
Get increase in sympathetic outflow to sweat glands, which is thought to cause vasodilation
What may sweat glands release?
Bradykinin
What does bradykinin do?
Cause vasodilation
