Flashcards in deck_808335 Deck (65):
Describe the circulation of the lungs
Have two -- pulmonary and bronchial. Bronchial is part of systemic circulation for meeting the metabolic requirements of the lungs. Pulmonary is blood supply to the alveoli for gas exchange.
Define "supply driven"
Characteristic of the pulmonary circulation. Must adapt in order to accommodate the entire cardiac output.
What are the normal pressures in the pulmonary artery, pulmonary capillaries and pulmonary veins?
Pulmonary artery = 12-15 mmHgPulmonary capillaries = 9-12 mmHgPulmonary veins = 5 mmHg
What are the flow resistance characteristics of the pulmonary circulation?
Has minimal flow resistance -- operates at a low resistance, low pressure system
Define colloid osmotic pressure
The osmotic pressure exerted by proteins in the blood plasma that pulls H20 into the circulatory system
What is hypoxic pulmonary vasoconstriction?
Blood is directed away from areas where oxygen uptake is reduced. Allows for efficient oxygenation.
What does hypoxic pulmonary vasoconstriction help to maintain?
The optimal ventilation/perfusion ratio
How is oxygenation made efficient?
Alveolar ventilation needs to be matched with perfusion.
What is alveolar ventilation?
Air flow to the alveoli
What is perfusion?
What is hypoxia caused by?
A mismatch between ventilation and perfusion in the lungs.
What can pulmonary arteries control?
The distribution of cardiac output over the lungs
How does blood reach all areas of the lungs?
Gravity creates hydrostatic pressure in blood column, with a higher transmural pressure in the vessels at the bottom of the lungs. This causes distention of blood vessels at the bottom of the lungs, increasing blood flow to these vessels.
Define physiological shunt
The passing of blood through the lungs without it being properly oxygenated.
When can a physiological shunt occur?
Cardiovascular and respiratory conditions
What features give the pulmonary circulation low resistance?
– short, wide vessels– lots of capillaries (many parallel elements)– arterioles have relatively little smooth muscle
What feature in lungs help promote efficient gas exchange?
High capillary density in alveolar wallShort diffusion distanceLarge SA and short distance lead to high O2 and CO2 capacity
What does alveolar hypoxia cause?
Vasoconstriction of pulmonary vessels.
What can chronic hypoxia cause?
Increases vasoconstriction which increases vascular resistance leading to chronic pulmonary hypertension. This increases the afterload on the RV which can cause right ventricular heart failure
How is tissue fluid formed?
Hydrostatic pressure forces fluid out of the capillaries. -- tends to occur at arterial end of capillaries with the inward movement of fluid occurring at the venous end.
What is ventilation perfusion matching?
The blood flow in the lungs is directed to areas which have a better ventilation/air supply from the alveoli to ensure that the majority of the blood becomes oxygenated
What causes pulmonary oedema?
Increased capillary pressure cause more fluid to filter out of the capillaries. Tends to be cause by L atrial pressure rising to 20-25 mmHg (mitral valve stenosis or LV failure) so left heart pumping is compromised. Filtration > reabsorption
What are the effects of pulmonary oedema and how can you treat it?What is it affected by?
It impairs gas exchange. Use diuretics to relieve symptoms and treat the underlying cause. Posture affects oedema-- base of lungs when upright and throughout lung when lying down.
What does alveolar hypoxia result in?
Vasoconstriction of pulmonary vessels to optimise gas exchange
What is the effect of exercise on pulmonary blood flow?
Increased cardiac output --> small increase in pulmonary arterial pressureOpen apical capillaries --> increased O2 uptake by the lungs
What is the oxygen demand to the heart dependent on?
Depends on how much metabolic work is done
What does the amount of metabolic work of the heart depend on?
External work done Efficiency
Describe the external work done by the heart
Depends on the stroke volume and arterial pressure per heart beat
Describe the differences of high efficiency and low efficiency of the heart
High efficiency -- ventricle is pumping a stroke volume against a low arterial pressureLow efficiency -- the heart is pumping the same stroke volume against a higher arterial pressure
When do the coronary arteries fill?
Coronary blood flow is exclusively diastolic as the coronary arteries are compressed during systole. Is especially seen in the left ventricle.
How does the heart maintain a mean blood flow into the coroner arteries?
Has high blood flow in diastole to compensate for reduced flow in systole.
How does blood flow into the coronary arteries change during an increased heart rate?
The length of diastole shortens, so the peak flow in the coronary circulation must increase quickly to maintain the necessary mean flow.
When can problem with the coronary circulation become apparent?
During high heart rate
How do blockages in the coronary arteries affect blood supply?
They will have a large effect as the CA have few anastomoses. They are end arteries, so total blockage leads to ischaemia which leads to MI
When are partial blockages of the coronary arteries seen?
Only during high heart rate as there is an increased O2 demand due to poorer blood supply. This ischaemia leads to angina upon exercise.
How is blood flow controlled into the myocardium to the coronary arteries?
Local vasodilator metabolites.
How important is the blood supply to the brain?
Very important -- any reduction for a few seconds leads to syncope. Brain damage or death can occur after 3-4 minutes. Receives about 15% of the totla cardiac outputInteruptions e.g. stroke leads to neuronal death
How is the high oxygen demand in the brain met?
High capillary density -- large SA for gas exchange and small diffusion distanceHigh basal flow rate -- 10 times higher than the body's average rateHigh O2 extraction -- 35% above average for body
What structures in the brain are most susceptible to hypoxia?
What structures in the brain ensure a good blood supply?
Anastomoses between basilar and internal carotid arteriesKnown as the circle of Willis: if one artery is blocked, blood supply is not greatly affected
What function does the brain have to ensure a good blood supply?
Brainstem regulates other circulations so brain circulation is sufficiently perfused. Myogenic autoregulationMetabolic factors
What is myogenic auto regulation?
Cerebral resistance vessels respond to changes in transmural pressure. Increased BP --> vasoconstrictionDecreased BP --> vasodilatationEnsures cerebral blood flow stays constant(fails below 50mmHg)
What metabolic factors help to regulate cerebral blood flow?
The pCO2 Increased pCO2 (hypercapnia) --> vasodilatationDecreased pCO2 --> vasoconstriction
What does panic hyperventilation cause?
Panic hyperventilation can cause hypocapnia and cerebral vasoconstriction leading to dizziness or fainting
What substances cause dilation of cerebral arterioles?
Increased pCO2Increased K+Increased adenosine (powerful vasodilator)Decreased O2
What areas of the brain have the highest blood flow?
Areas with increased neuronal activity
What can small alterations to cerebral blood flow cause?
Small alterations can have larger effects-- headaches-- other disturbances in cerebral function
What vessels does blood flow though in the skin?
aterio-venous anastomoses -- blood flow through skin tends not to be nutritive
What does the cutaneous circulation play a major role in?
Temperature regulation-- skin is the main heat dissipating surface
What is a major controlled of heat loss through the skin?
Sympathetic nervous system-- controls the blood flow into the skinVery little change occurs due to local metabolites other than mediators released from sweat glands
What does a decrease in core temperature cause?
Increase tone in AVAs, decreasing blood flow to apical skin due to sympathetic innervation
What does an increase in core temperature lead to?
Dilation of AVAs, low resistance shunt to venous plexus arises which is close to the surface and allows for the dissipation of heat.
What happens to most skeletal muscle capillaries at rest?
They are shut off due to contraction of pre-capillary sphincters. About half are "shut off" to allow for increased recruitment during exercise
What does the capillary density of skeletal muscle depend on?
Type of muscle -- postural muscles have a higher capillary density
What is a particular characteristic of skeletal muscle?
It has very high vascular tone which permits lots of vasodilatation– Flow can increase > 20 times in active muscle
How are increases in blood flow to skeletal muscle brought about?
Opening more capillaries by vasodilator nervous activity and local metabolites. These reduce tonic vasoconstrictor tone. The increase in capillary number also lowers diffusion distance.
What acts as vasodilators in skeletal muscle?
– increased [K+]– increased osmolarity– Inorganicphosphates – Adenosine– increased [H+]adrenaline acts on arterioles though beta2 receptors (vasoconstriction is due to noradrenaline on alpha1 receptors)Metabolites are the main vasodilators
Why is blood flow through the coronary arteries important?
Need a high basal rate due to constant work undertaken by the heart. Has to be able to increase to match the workload of the heart
How is a high basal flow maintained in coronary endothelium?
Constant production of nitric oxide (NO) which is a vasodilator
Why are coronary arteries prone to atheromas?
The are few arerio-arterial anastomoses as the coronary arteries are functional end arteries. Sudden obstruction by a thrombus will cause a myocardial infarction
What are artereovenous anastomoses?
Vessel between arterioles and venules bypassing capillaries which are specialised to allow for heat dissipation. Are highly convoluted. Can be known as glomus body. Are innervated sympathetically.
What can cause vasodilatation to promote heat loss in non-apical skin?
Sympathetic innervation of sweat glands by ACh can also produce bradykinin. This causes vasodilatation of capillaries for heat loss in non-apical skin.
Describe cushings reflex
Rigid cranium does not allow for distension so haemmorhage and tumour lead to vasoconstriction of arteries, impairing blood flow. Vasomotor control regions of the brain increase their sympathetic vasomotor activity (vasoconstriction) to increase arterial blood pressure therefore establishing sufficient cerebral blood flow
What makes up the blood brain barrier?