Session 8 ILOs - Specific circulations and peripheral and arterial disease Flashcards
State the major differences between the properties of the systemic and pulmonary circulations
Systemic circulation:
- Carries blood throughout the body and back
- Higher pressure, high resistance
- Hypoxia = vasodilation (no ventilation:perfusion)
Pulmonary circulation:
- Carries blood to the lungs and back
- Lower pressure
- Low resistance
- Adaptations to promote efficient gas exchange (alveoli, large surface area etc.)
- Hypoxia = vasoconstriction (important for ventilation:perfusion)
State the normal pressures in the pulmonary artery, pulmonary capillaries and pulmonary veins
Pulmonary artery: 15-30mmHg/4-12mmHg
Mean Pulmonary arteries: 12-15mmHg
Mean Pulmonary capillaries: 9-12mmHg
Mean Pulmonary veins: 5mmHg
Explain the concept of ventilation perfusion matching in the pulmonary circulation
In order to have efficient oxygenation, need to match the ventilation of the alveoli with their perfusion
Perfusion is regulated by hypoxic pulmonary vasoconstriction e.g. if there is a hypoxic area in the alveoli, vasoconstriction of pulmonary vessels to make sure that the perfusion matches the poor ventilation (opposite to what happens in the systemic circulation!)
However, chronic hypoxic pulmonary vasoconstriction can because right-sided heart failure (e.g. due to emphysema) as there is an increase in resistance which backs up to the heart
Describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation
Starling’s forces of the capillary:
The four Starling’s forces are:
1. Hydrostatic pressure in the capillary (Pc) = pushes fluid out
2. Hydrostatic pressure in the interstitium (Pi)
3. Oncotic pressure in the capillary (pc) = draws fluid in
4. Oncotic pressure in the interstitium (pi)
Important: capillary hydrostatic pressure is influenced more by venous pressure in the systemic circulation (hypertension doesn’t usually result in peripheral oedema)
Describe the relationship between the mechanical work and oxygen demand of the myocardium
The oxygen demand of the myocardium is determined by how much metabolic work is done - which depends on how much external work is done and the efficiency of this (stroke volume and arterial pressure).
Describe the particular features of the coronary circulation
- Metabolic demands are continuous, therefore blood flow through coronary circuit must be very good
- Heart contraction compresses the coronary vessels, so filling is almost exclusively diastolic - therefore at rest there tends to be no problems as diastole is long but as diastole begins to shorten during exercise, problems can arise (e.g. angina)
- Efficiency of contraction is determined by pressures the heart is pumping against (resistance)
- Heart must be quick to respond to changes in demands of the body
- Control of flow through myocardium is almost all down to local vasodilator metabolites
- Highly capillary density and short diffusion distance
Describe the consequences of partial or total occlusion of coronary arteries
Partial occlusion (angina):
- Generally due to a gradual occlusion, likely due to atherosclerosis
- May not be a problem at rest, as metabolic demands of the heart are low and there is plenty of time for diastole
- But can get ischaemia on exercise, where diastole shortens and metabolic demands increase = angina
- This results in certain areas of the myocardium being underperfused during exercise
Total occlusion:
- Generally due to a sudden occlusion, either rupture of an atherosclerotic plaque or thrombus
- Total occlusion leaves an area of myocardium ischaemic (depending on the size of the coronary artery blocked)
- This can lead to a myocardial infarction (NSTEMI or STEMI) and damage to the myocardium which may cause a cardiac arrest
Describe the factors which influence blood flow through the brain
- Continuous cerebral blood flow is incredibly important, neurones can’t survive for long otherwise = high o2 demand
- If blood flow is reduced for even a few seconds, then syncope occurs (for 3-4 minutes, brain damage or death can occur!)
- Cerebral circulation has many anastomoses (circle of Willis)
1. Cerebral circulation is generally auto-regulated by local metabolites acting upon resistance vessels (high Co2, high K+, adenosine, low O2) Carbon dioxide is a potent modulator e.g. a high pp carbon dioxide = vasodilation, low pp Co2 = vasoconstriction
2. Also cerebral resistance vessels are sensitive to pressure changes e..g high blood pressure = vasoconstriction, low blood pressure = vasodilation to maintain flow - Small changes to cerebral blood flow can have large effects e.g. headaches and other disturbance of cerebral function
- Cushing’s Reflex, impaired blood flow to vasomotor control regions of brainstem increases sympathetic vasomotor activity (to maintain cerebral blood flow)
Describe in broad outline the factors which influence blood flow through skin and skeletal muscle
Skin:
- Most blood flow through skin is not to provide nutrition, it’s main function is to maintain a constant body temperature
- Most of the blood flows through arterio-venous anastomoses
- Generally controlled by the sympathetic NS (little influence from local metabolites generally)
Muscle:
- Metabolic activity of muscle varies massively, so does the blood flow!
- At rest, most capillaries are shut off (by pre-capillary sphincters)
- Increase blood flow by opening the pre-capillary sphincters and more capillaries
- Influenced by local metabolites (high K+, adenosine, high H+) and vasodilator NS activity (adrenaline acts at B2 AR to reduce sympathetic constrictor tone) - resistance vessels have a rich sympathetic innervation
Describe the arterial supply and venous drainage of the lower limbs
Arterial supply:
- Arteries run close to the venous supply
- Main artery of the lower limb is the femoral artery - a continuation of the external iliac artery (terminal branch of the abdominal aorta)
- Damage can occur on gradual or sudden occlusion of the vessels
Venous drainage:
- Veins store a lot of blood, tendency for blood to pool in lower limb veins (gravity)
- Veins have valves and calf pumping effect to increase venous return
- Veins are superficial (short and long saphenous veins) or deep (femoral and popliteal veins)
- Blood travels from superficial -> deep
- Damage can occur to valves and the walls of superficial veins (inflammation can be a risk factor for DVT also)
Interpret some causes, symptoms and signs of peripheral venous disease
Varicose veins (most commonly saphenous veins):
Causes:
- Ineffective veins, slow blood movement or even reversed
- Walls of superficial veins weaken, valve cusps separate and incompetent
Symptoms/signs:
- Leg cramps, ankle swelling, haemorrhage from veins, ankle swelling
Chronic venous insufficiency can occur as varicose veins progresses (venous hypertension)
- Can get venous eczema or ulceration
Deep vein thrombosis
Cause: - Mainly occurs due to blood stasis and another factor (medication, pregnancy, immobility etc.)
- Low flow so little platelet component = fibrin rich
Symptoms/signs:
- Calf tenderness, swelling, warmth, pyrexia with no other cause, asymmetry
Explain the role and functioning of the calf muscle pump
The calf muscles (soleus and gastrocnemius muscles) contribute to pushing blood against gravity - alongside valves
Venous pressure in feet is reduced during exercise (with exercise, less blood pools)
Interpret some causes, symptoms and signs of peripheral arterial disease, both acute and chronic
Acute limb ischaemia:
Cause:
- Acute occlusion (collateral circulation can’t develop)
Symptoms/signs (6 P’s):
- Pain, pallor, perishing with cold, pulseless, paraesthesia, paralysis
Chronic peripheral arterial disease: Cause: - Atherosclerosis (exercise induced) Symptoms/signs (6 P's): - Intermittent claudication in lower limb (exercise induced but goes away with rest)
Critical limb ischaemia:
- Development of chronic peripheral vascular disease
- Pain at rest (like unstable angina)
- If untreated, can lead to ulceration and gangrene
Describe were you would feel for the pulse of the fermoral artery, popliteal artery, dorsalis pedis and posterior tibial pulse
Fermoral artery:
- Mid-inguinal point (between anterior superior iliac crest and pubic symphis)
Popliteal artery:
- Deep in popliteal fossa
Dorsalis pedis:
- Lateral to extensor hallucis longus tendon (ask to raise big toe)
Posterior tibial pulse:
- Behind medial malleolus
Explain the basic principles behind the use of Doppler ultrasound to measure velocity and direction of blood flow in arterial and venous systems
Doppler can estimate flow and velocity of blood
- Detects turbulence (from atheroma and stenosis)
- Echos produced by moving blood are detected on flow direction and velocity (Doppler effect is the apparent change in frequency of waves)
- Velocity increases due to stenosis
