Lecture 8 - Special Circulations Flashcards
Why is the bronchial circulation needed?
- Bronchi too far away for efficient diffusion
- Meets the metabolic requirements of the lungs
Name the two circulations of the lungs?
- Bronchial circulation
- Pulmonary circulation
Is the pulmonary circulation in series or parallel with systemic?
-Series
What is the cardiac output at rest?
5L/min
What is the maximum cardiac output (non-athlete)?
20-25L/min
What is special about the pulmonary pressure and resistance?
-They are both low
How is the resistance low in the pulmonary circulation?
- Many capillaries in parallel with each other
- Short wide vessels
- Arterioloes have relatively little smooth muscle
Name the pressures in the chambers on the heart, aorta and pulmonary artery during systole and diastole?
- RA 0-8mmHg
- LA 1-10mmHg
- LV 100-140mmHg systole, 1-10mmHg diastole
- RV 15-30mmHg systole, 0-8mmHg diastole
- Aorta 100-140mmHg systole, 60-90mmHg diastole
- PA 15-30mmHg systole, 4-12mmHg diastole
Why is the diastolic pressure in the pulmonary artery higher than that in the right ventricle?
-When pulmonary valve closes, the recoil in the artery maintains the diastolic pressure
What is the mean arterial pressure in 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
How is the pulmonary circulation adapted for efficient gas exchange?
Very high density of capillaries and short diffusion distance produces a high O2 and CO2 transport capacity
How does high capillary density help gas exchange in the lungs?
Provides a large surface area over which gas can be exchanged.
What constitutes the diffusion barrier between the air and the blood?
- Epithelium of alveoli (type1 pneumocytes/squamous), attached to their basement membrane
- Endothelium of capillary
What is required for efficient oxygenation?
Ventilation of the alveoli must be matched by perfusion of the alveoli
How is the ventilation perfusion ratio maintained?
By diverting blood away from those alveoli which are not well ventilated, thus those which are ventilated have adequate perfusion
What is the most important mechanism is regulating pulmonary vascular tone?
Hypoxia pulmonary vasoconstriction
What is hypoxia pulmonary vasoconstriction?
Alveolar hypoxia results in the vasoconstriction of pulmonary vessels ensuring that perfusion matches the ventilation, helping to optimise gas exchange
When can hypoxia pulmonary vasoconstriction become problematic?
At high altitude or chronic conditions where there is wide spread reduced ventilation
Why at high altitude/in chronic conditions does hypoxia pulmonary vasoconstriction to cause a problem?
Promotes wide-spread chronic vasoconstriction as oxygen is reduced to all alveoli. This increases the resistance in the lungs and increases the afterload on the right ventricle.
What effect does an increased afterload on the right ventricle have?
Can lead to right ventricle failure as the RV is poorly tolerant to increases in afterload
Describe how the pulmonary vessels in the base of the lungs are influenced by gravity
In upright position there is greater hydrostatic pressure in vessels at the lower part of the lung, this distended the vessels here
Describe how the vessels in the apex of the lungs are effected by gravity
Lower hydrostatic pressure when in upright position so they collapse during diastole
Are the pulmonary vessels at the level of the heart always open or closed?
Open
What effect does exercise habe on pulmonary arterial pressure?
Small increase
What happens to the pulmonary apical vessels when there is a small increase in pulmonary arterial pressure?
They open and gas exchange increases
Why is it important to open the apical vessels of the lungs during exercise?
As oxygen uptake by the lungs is increased, it is important to increase the blood flow to maintain the ventilation perfusion ratio
What happens to capillary transit time in the lungs during exercise?
It can divide by 3 without compromising gas exchange
How is tissue fluid (lung lymph) formed?
Hydrostatic pressure and interstitial oncotic pressure pushes fluid out of the capillary. Plasma oncotic pressure draws fluid back in. The net fluid remaining outside the capillary is the lymph
Where is interstitial oncotic pressure greater, lungs or periphery?
Lungs
Where is capillary hydrostatic pressure greater, lungs or systemic?
Systemic
How does pulmonary oedema occur?
Increased elevated pulmonary venous pressure-> increased hydrostatic capillary pressure-> increasing fluid in the interstitial space-> oedema
How is pulmonary oedema naturally prevented?
Low capillary hydrostatic pressure normally only permits a small amount of tissue fluid to leave
Why could mitral valve stenosis or LV failure lead to pulmonary oedema?
Blood cannot pass to LV from LA so easy. Pressure in LA increases. Pressure in pulmonary vein increases. Causes increased resistance in pulmonary veins. Increases hydrostatic capillary pressure Causes oedema.
Why is pulmonary oedema problematic?
Impairs gas exchange
What is the difference in pulmonary oedema formation when standing or laying down?
Forms at the base when standing
Forma throughout when laying down
What is used to relieve the symptoms of pulmonary oedema? (Generally speaking)
Diuretics to decrease blood volume
What percent of the cardiac output does the brain receive?
15%
What percent of O2 consumption does grey matter account for at rest?
20%
What is essential about the oxygen supply to the brain?
It must be high and secure.
How does the cerebral circulation meet the high demand for O2?
High capillary density
High basal flow rate
High O2 extraction
How does a high capillary density in the cerebral circulation help meet O2 demand?
Allows a large surface area for gas exchange and a reduced diffusion distance (relative to rest of the body)
How much higher is the basal flow rate of the cerebral circulation compared to the rest of the body?
x10 average for whole body
How much higher is oxygen extraction in the cerebral circulation compared to the rest of the body?
35% above average
Why is a secure oxygen supply to the brain vital?
Neurones are very sensitive to hypoxia and loss of consciousness will occur after only a few seconds of cerebral ischaemia.
After what length of time does irreversible damage to brain neurones occur from hypoxia?
In approximately 4 minutes
What is the result of interruption to blood supply of the brain?
Ischaemic stroke causing neuronal death
How is the blood supply to the brain ensured structurally?
The circle of Willis: Anastomoses between basilar and internal carotid arteries ensures that if one part of the circle becomes occluded/stenosed, blood flow from the other arteries can maintain cerebral perfusion.
How is a secure blood supply to the brain ensured functionally?
- The brain stem regulates other circulations
- Myogenic autoregulation
- Metabolism
What is the purpose of myogenic autoregulation?
To maintain perfusion during hypotension
How does myogenic autoregulation work?
- Cerebral vessel smooth muscle have well developed response to changes in transmural pressure
- increased bp=vasoconstrict
- Decreased bp=vasodilatation
- Maintains cerebral blood flow when bp changes
Why do the cerebral vessels vasoconstrict as bp rises?
In order to maintain intracranial pressure and cerebral blood flow
How does vasodilation of cerebral vessels during hypotension help?
During hypotension, there would be a drop in cerebral perfusion pressure and cerebral blood flow. Vasodilation enhances the blood flow to the brain to maintain perfusion
Does myogenic autoregulation always work?
No, below 50mmHg the mechanism fails as vasodilation is not sufficient to maintain flow
How does metabolic regulation, involving CO2, ensure a secure blood supply?
Cerebral vessels very sensitive to changes in pCO2
- Hypercapnia causes vasodilation of the cerebral vessels to increase blood and oxygen supply
- Hypocapnia causes vasoconstriction of the cerebral vessels
Why does hyperventilating lead to syncope?
Causes Hypocapnia as all CO2 being blown off-> cerebral vasoconstriction-> reduced blood/oxygen supply-> dizziness or fainting
Why does vasoconstriction occur with low CO2?
Low CO2, which is a metabolic vasodilator so vasoconstriction occurs
Why do areas with increased neuronal activity have increased blood flow?
Increased production of metabolic vasodilator a such as Adenosine, CO2, K+ and decreased pO2, this causes vasodilation of the cerebral Arterioloes and thus increased blood flow
What is the aim of cushings reflex?
To maintain cerebral blood flow to the brain
When does cushings reflex occur?
When there is a rise in ICP which exceeds to MABP leading to compression of the brainstem eg haemorrhage or cerebral tumour
Why does raised ICP lead to compression of the brainstem?
Raised ICP increases the pressure in the cerebrospinal fluid located around the skull. As the cranium is rigid, the increased pressure of the CSF gradually meets and exceeds MABP which compresses the brainstem
What are the stages of cushings triad?
- first increased blood pressure
- second reduced heart rate
- third irregular breathing
What causes the increased blood pressure during cushings reflex?
Increase in sympathetic activity causes activation of a1 adrenoreceptors which cause vasoconstriction of the bodies arteries and increases bp
What causes an increase in sympathetic activity in cushings reflex?
Compression of the arterioles, and thus reduction of blood flow in the vessels which serve the brainstem. Brainstem contains vasomotor control regions and thus the sympathetic and parasympathetic nervous systems become activated.
Why is there an increase in bp if both branches of the and are activated?
Sympathetic tone is greater than parasympathetic in the first stage of the reflex
What is the aim of increasing the blood pressure within the systemic circulation in cushings reflex?
To restore blood flow to the brain which is becoming ischaemic
Why does the brain become ischaemia due to a raised ICP?
Pressure compromises the cerebral blood flow
What happens in the second stage of cushings reflex?
Baroreceptors in aortic arch detect increased bp, further activate parasympathetic through vagus nerve and causes a decrease in HR (bradycardia)
Why does blood pressure remain high in cushings reflex if the parasympathetic nervous system is activated?
The ICP is still high and thus the compression on the brain is still there, which maintains sympathetic vasomotor tone.
Why does cushings reflex result in irregular breathing?
The brainstem controls involuntary breathing and changes in its homeostasis causes irregular breathing
What forms the blood brain barrier?
Cerebral capillaries
Where do the left and right coronary arteries arise from?
The aortic sinuses
When does the blood flow in the coronary artery mainly occur?
During diastole
Why is blood flow to the left coronary artery reduced during systole?
Due to changes in pressure in the aorta
Why is the right coronary artery filling less effected by systole/diastole?
Right heart has less muscle mass and less pressure
How is fibre diameter of cardiac muscle different from skeletal muscle?
The fibre diameter is just over half the size
Why is there a high capillary density in cardiac muscle?
Diffusion distance for the cardiac fibres is reduced
Why are coronary capillaries Always open?
The endothelial cells of cardiac capillaries continually produce NO which is a potent vasodilator
How is coronary blood flow increased during increased myocardial O2 demand?
Vasodilation due to metabolic hyperaemia
Small increase in O2 extraction
What is the relationship between coronary blood flow and increased myocardial O2 demand?
Almost linear until there is a very high O2 demand
Why do narrowed coronary arteries lead to angina on exercise?
The narrowing means there is reduced supply to the heart anyway, There is an increased O2 demand of the heart during exercise, the coronary arteries are mainly filled during diastole, diastole reduced as heart rate increases-> blood flow further reduced
What environmental factors can cause sympathetic coronary vasoconstriction and angina?
Stress and cold
Sudden obstruction of a coronary vessel by a thrombus causes…
Myocardial infarction
How does skeletal muscle circulation help to regulate arterial blood pressure?
The resistance vessels (arterioles) have rich innervation by sympathetic vasoconstriction fibres, allowing communication with the cns and baroreceptors maintain the bp
In skeletal muscle circulation, what does capillary density depend on?
The type of muscle, eg postural muscles have a higher capillary density
What are the advantages of the vessels in skeletal muscle circulation having very high vascular tone?
Permits a lot of vasodilation so flow can increase (Upto 20x in active muscle)
Why is it possible for skeletal muscle to have recruitment?
At rest, only half of capillaries are perfused at any one time
What is the advantage of recruitment in skeletal muscle circulation?
Opening of more capillaries increases blood flow and reduces the diffusion distance from fibres
Why does increased blood flow occur in skeletal muscle?
Increased metabolic activity, increased production of metabolic vasodilators
Increased adrenaline production acts as a vasodilator through b2 adrenoreceptors
What is the main special role in cutaneous circulation?
Temperature regulation
What are artereovenous anastomoses?
Special structures which allow blood to flow straight from arterioles to venule and venous plexus (bypasses capillaries)
Why are artereovenous anastomoses useful?
Allow rapid heat dissipation when there is a high body temperature
Where are artereovenous anastomoses found?
In acral skin
What causes dilation of artereovenous anastomoses?
Reduction in sympathetic vasoconstrictor tone when core temperature is increased
