Lecture 15 - special circulations Flashcards
what are the key special circulations from this lecture ?
pulmonary
cerebral
skeletal
cutaneous
coronary
how is blood supplied to the lungs
The lungs have two circulations
• Bronchial circulation
– part of systemic circulation
– meets the metabolic requirements of the lungs
• Pulmonary circulation
– blood supply to alveoli
– required for gas exchange
the pulmonary circulation must accept the whole cardiac output, it does this by having low pressures and low resistance
outline the features and pressures of the pulmonary circulation - it will help to draw this on a model of the heart and check against lec 15
check against lecture 15 for hear values
Low pressure
– mean arterial pressure ∼ 12-15mmHg
– mean capillary pressure ∼ 9-12mmHg
– mean venous pressure ∼ 5mmHg
• Low resistance
– short, wide vessels
– lots of capillaries (many parallel elements)
– arterioles have relatively little smooth muscle
what adaptations do the lungs have to specialise them ?
efficient gas exchange !
• Very high density of capillaries in alveolar wall
– large capillary surface area
• Short diffusion distance
– very thin layer of tissue
• Large surface area and short
diffusion distance produce high
O2 and CO2 transport capacity
outline the concept of a ventilation perfusion ratio
what are the pro’s and cons
Ventilation – Perfusion ratio (V/Q ratio)
• For efficient oxygenation - need to match
ventilation of alveoli with perfusion of alveoli
• Optimal V/Q ratio = 0.8
• Maintaining this means diverting blood from
alveoli which are not well ventilated - counter intuitive
- Hypoxic pulmonary vasoconstriction is the most important mechanism regulating pulmonary vascular tone - key term
- Alveolar hypoxia results in vasoconstriction of pulmonary vessels
- Ensures that perfusion matches ventilation
- Poorly ventilated alveoli are less well perfused
- Helps to optimise gas exchange
downside
Chronic hypoxic vasoconstriction can cause right
ventricular failure
chronic pulmonary hypertension
– high afterload on right ventricle - can lead to right ventricular heart failure
due to issues such as emphysema
what is the effect of gravity on the lungs vessels ?
what is the effect of exercise
• In the upright position (orthostasis) there is greater hydrostatic pressure on vessels in the lower part of the lung
so apical vessels closed and base vessels disteneded to match the V/Q
• Increased cardiac output
• Small increase in pulmonary arterial pressure
• Opens apical capillaries
• Increased O2 uptake by lungs
• As blood flow increases capillary transit time is reduced
– at rest transit time ~ 1s
– can fall to ~ 0.3s without compromising gas exchange
outline the formation of fluid tissues and oedemas in the lungs, what can cause it , and how the body normally prevents it
what are the consequences of a pulmonary oedema
what drugs will we treat with
check against lecture 15
odema prevent by low capilliary pressure
outline cerebral circulation
The brain has a very high O2 demand for its mass
• Must provide a secure O2 supply - Neurones are very sensitive to hypoxia - ischemic damage occurs within 4 mins
How does the cerebral circulation meet the high demand for O2? • high capillary density – large surface area for gas exchange – reduced diffusion distance (<10μm) • high basal flow rate – X10 average for whole body • high O2 extraction – 35% above average
How is a secure blood supply ensured?
• Structurally
circle of willis - 1 area of brain has multiple blodd supplies
• Functionally
– myogenic autoregulation maintains perfusion during
hypotension (check this against lecture)
– metabolic hyperamia control blood flow
– brainstem regulates other circulations
define meatbolic hyperemia
metabolites will increase vasodilatation and hence blood flow
such as increased Pc02 and low P O2
high H+/ PH
high K+
high adenosine (powerful vasodilator of the brain)
what is cushings reflex ?
check against the lecture 15
outline coronary circulation
coronary arteries must • must deliver O2 at a high basal rate • must meet increased demand – work rate can increase five-fold
a key fact is that flow in the left coronary arteries occurs mainly in diastole due to pressures of systole
- Coronary circulation
- High capillary density facilitates efficient O2 delivery
- Diffusion distance < 9μm
- Continuous production of NO by coronary endothelium maintains a high basal flow
Coronary blood flow increases with myocardial O2 demand
• Extra O2 required at high work load is supplied
mainly by increased blood flow
• Vasodilation due to metabolic hyperaemia
• Vasodilators - adenosine, ↑[K+], ↓pH
what are the issues with coronary circulation ?
- Few aterio-arterial anastomoses
- Prone to atheromas
- Narrowed coronary arteries leads to angina on exercise (increased O2 demand)
• Sudden obstruction by thrombus causes myocardial infarction
outline skeletal muscle circulation
• Must increase O2 and nutrient delivery and removal of metabolites during exercise.
• Resistance vessels have rich innervation by sympathetic vasoconstrictor fibres
– Baroreceptor reflex maintains blood pressure
• Capillary density depends on muscle type
– Postural muscles have higher capillary density
• Very high vascular tone
– Permits lots of dilatation
– Flow can increase > 20 times in active muscle
• At rest only ~ ½ of capillaries are perfused
at any one time
– allows for increased recruitment
Increased flow due to metabolic hyperaemia
• Various agents are thought to act as vasodilators
– ↑[K+], Inorganic phosphates, Adenosine, ↑[H+]
• Adrenaline also acts as a vasodilator at arterioles in skeletal muscle – Acts through β2 receptors
outline cutaneous circulation and explain the concept of AVA’s
honestly this is better done by saying it out loud
then checking what you got right of slides for lecture 15
