Session 8 Flashcards
What are the 2 circulations of the lungs?
- Bronchial circulation
- Pulmonary circulation
What is the function of the pulmonary circulation?
- Directs output of right heart
- Must accommodate entire cardiac output
- Needs gas exchange
What is the function of the bronchial circulation?
- Infuses parts of the lungs that are too far from the alveoli with O2
- Meets metabolic requirements of lungs
What kind of pressures and resistance does the pulmonary circulation work at?
Low pressure and low resistance
- Pressure in the arteries and ventricles are very similar in systole
Why are the atria at a much lower pressure than the ventricles?*
- Atria do not undergo the same level of systole as ventricles
- RA: 0-8mmHg
- LA: 1-10mmHg
Why is the diastolic pressure in the aorta so high (60-90mmHg)?
There is much more elastic recoil in the aorta which allows the maintenance of a higher pressure.
Why is the pressure in the pulmonary circulation so low?
Low resistance - ready flow is needed to deliver all the blood to the lungs for oxygenation
What contributes to the low resistance?
- Short, wide vessels
- Lots of capillaries
- Less smooth muscle in the arterioles which keeps lumen open and allows flow (no constriction)
What promotes efficient gas exchange in the lungs?*
- High density of capillaries in the alveolar wall (high SA)
- Short diffusion distance as endothelium + epithelium thickness is around 0.3 micrometres
- High transport capacity
What is the meaning of perfusion and ventilation?
Perfusion: blood flow
Ventilation: air flow
What is needed for efficient oxygenation?
- Ventilation and perfusion of alveoli must be matched
- Alveoli that are not being ventilated must have blood flow diverted from them
- V/Q ration = 0.8
- Mismatch will result in hypoxia as blood leaving the lungs will have less oxygen
What ensures the optimal ventilation/perfusion ratio?
Hypoxic pulmonary vasoconstriction that regulates pulmonary vascular tone (degree of vessel constriction)
What will alveolar hypoxia result in?
- Vasoconstriction of pulmonary vessels
- Narrows lumen of vessels that lead to poorly ventilated alveoli to ensure that perfusion = ventilation
Why is blood flow diverted from poorly ventilated alveoli?
Optimising gas exchange
What is the issue with chronic hypoxic vasoconstriction?
- Can occur at altitude or because of COPD/emphysema
- Chronic increase in vascular resistance contributes to chronic pulmonary hypertension
- Right ventricle pumps blood at a higher pressure
- High afterload
- Right ventricular heart failure
What is the influence of gravity on low pressure vessels?*
- Greater hydrostatic pressure on vessels in lower lung during orthostasis
- Those vessels are distended
- Vessels at the level of the heart are continuously open
- Vessels at the apex of the lungs have a lower hydrostatic pressure and therefore are only open during systole and collapse during diastole
What is the effect of exercise on pulmonary blood flow?
- Increased CO
- Increase in pulmonary arterial pressure opens apical capillaries that usually collapse in diastole
- More O2 taken up by lungs
- Capillary transient time reduced but gas exchange not compromised
How does tissue fluid form?
STARLING FORCES
- Hydrostatic pressure within capillary pushes fluid out into the tissues
- Oncotic/colloid osmotic pressure that is exerted by large plasma proteins draws fluid back into the capillary
What influences hydrostatic capillary pressure?
Venous pressure in the systemic circulation (5x greater effect)
How can you tell that hydrostatic pressure is not greatly influenced by arterial pressure?
- Hypertension is high arterial blood pressure
- Patients with hypertension do not present with peripheral oedema
- If it was affected, they would have it
How is the formation of lung lymph minimised?*
Filtration = reabsorption, so most forces are equivalent
- OP lungs > OP periphery
- CHP lungs < CHP systemic
- Plasma oncotic pressure same
What is the reason for formation of oedema?*
- Filtration rate exceeds reabsorption
- Increased capillary pressure causes more fluid to be filtered out
Why is the pulmonary capillary pressure between 9-12 mmHg?
Prevents formation of pulmonary oedema so that only small amounts of lung lymph leave the capillary
When can you get pulmonary oedema?
- Mitral valve stenosis: buildup of pressure in LA
- Left ventricular failure: more blood left over at the end of systole means that you need more diastolic pressure to compensate
- Left atrial pressure rising to 20-25 mmHg
What problems does pulmonary oedema cause?
Impairment of gas exchange and fluid making it very difficult to breathe when patients sleep/lay down as the lymph then forms throughout the lung rather than mainly at bases
How is pulmonary oedema treated?
- Diuretics (symptom relief)
- Underlying cause
Why do individuals who experience a reduced blood supply experience syncope?
- Brain has a high O2 demand
- Neurones are active all the time
- Shortness of O2 for 3/4 minutes will lead to permanent brain damage and death
How does the cerebral circulation meet the high demand for O2?
- High capillary density that provides a large SA for gas exchange and a short diffusion distance
- High basal flow rate
- High O2 extraction due to continuous O2 use so therefore a good diffusion gradient
How is a secure blood supply ensured structurally?
Anastomoses between basilar and internal carotid arteries that form a ‘circle’ so even when the blood supply is cut off from one side perfusion still occurs
How is a secure blood supply ensured functionally?
- Myogenic autoregulation (perfusion during hypertension)
- Metabolic factors
- Brainstem regulates other circulations
What is myogenic autoregulation?
- Smooth muscle cells in the cerebral resistance vessels responds to changes in transmural pressure
- Maintains cerebral blood flow when BP changes
How does myogenic autoregulation work?*
- Increased blood pressure = vasoconstriction
- Decreased blood pressure = vasodilation
Why does hypercapnia cause vasodilation while hypocapnia causes vasoconstriction?
High partial pressures of CO2 indicate that the neurones are very active, meaning that the blood flow must increase to provide oxygen
Why does panic hyperventilation cause syncope/dizziness?
- Causes hypocapnia
- This causes vasoconstriction
- Blood supply to the brain becomes reduced
What causes vasodilation?
+ Adenosine
+ PCO2
+ K+ conc
- PO2
What is the function of the rigid cranium?
Protection
What happens when there is an increase in intercranial pressure and what causes the increase?
- Can be caused by tumour/haemorrhage
- Impairs cerebral blood flow as the blood vessels may become compressed and the blood will not flow
What happens if blood flow to vasomotor control regions is reduced?
- Increased sympathetic vasomotor activity
- Powerful vasoconstriction occurs and BP rises to help maintain the cerebral blood flow
- Increase in vagus activity due to baroreceptors sensing the pressure
What do patients present with that usually indicates the presence of a space-occupying lesion?
Bradycardia and hypertension
What is the function of the coronary circulation?*
- Delivery of O2 at a high basal rate
- Meeting increased demands (eg. exercise)
When does flow to the coronary arteries (LEFT) mostly occur and why can this be a problem when a person has narrowing of the coronary arteries?*
- Coronary arteries mostly fill with blood during diastole
- Narrowing of arteries already compromises and prolongs their filling
- During exercise, diastole is cut short and so the artery cannot receive as much blood
What determines the oxygen demand of the myocardium?
- Metabolic work
- External work
Why can coronary arteries not fill in systole?
- Tension in walls compresses coronary vessels in ventricles
- Reduces blood flow
What are the features of the coronary circulation?
- High capillary density
- Short diffusion distance
- Continuous NO production by endothelium to maintain high basal flow
What is metabolic hyperaemia?
An increase in blood flow that occurs when the tissue is metabolically active to provide more O2 via vasodilation
Why are coronary arteries so prone to atheromas?
They have few anastomoses as they are end arteries
Why can narrow coronary arteries lead to angina?
- Diastole cut short during exercise
- Hypoxia
- Stress and cold causing sympathetic coronary vasoconstriction
Why do skeletal muscles have a high vascular tone?
- Permit lots of dilation
- Flow can increase over 20x in active muscle
- Increased recruitment allowed
What are pre-capillary sphincters and why do they open?
- Smooth muscles that adjust blood flow through capillaries
- Allow more capillaries to be perfused when needed
- Reduce diffusion distance
What acts to promote vasodilation?
- Inc. K+ conc.
- High osmolarity
- Adenosine
- Low pH
- Adrenaline (smooth muscle arterioles via B2 receptors)
What is the role of the cutaneous circulation?
Maintaining a constant body temperature (37 degrees Celsius)
- Also maintains blood pressure
How does blood flow through the skin?*
Arterio-venous anastomoses (precapillary communications between arteries and veins) in acral surfaces
- Heat lost as they’re close to the surface
How are AVAs controlled?
Sympathetic vasoconstrictor fibres
- Decreasing core temp. will increase sympathetic tone and reduce blood flow to apical skin
- Reduced vasomotor drive allows dilation and diverting blood to veins near surface
Where does most of the venous return come from?
Superficial veins
How does blood move in the lower limb?
Superficial -> deep veins
Where are deep and superficial veins found?
Deep: deep fascia
Superficial: subcutaneous tissue
All originate from INFERIOR VENA CAVA
What are some deep veins in the lower limb?*
- Popliteal vein
- Femoral/deep femoral vein
- Peroneal
- Anterior/posterior tibial
What are the superficial veins in the lower limb?*
- Short saphenous vein
- Long saphenous vein (medial)
Where does the long saphenous vein run down?
- Medially
- In front of medial malleolus (anatomical landmark)
What muscles push the blood back towards the heart and against gravity and how do they do that?*
- Soleus
- Gastrocnemius
Muscles are constricted within the fascia and therefore pump blood
How do valves allow the blood pumping?
- Valves open to push blood through to deep vein and prevent retrograde movement
- Perforating valves allow filling from superficial veins
What is peripheral venous disease?
Varicose veins (common in saphenous veins) forming due to valves being ineffective and blood movement slowing or reversing
Why do the varicosities develop?*
Vein walls weaken and the valve cusps separate, so blood pools in the veins
Does peripheral venous disease cause problems?
- Can present symptomatically
- Usually only removed due to aesthetic reasons
What are the symptomatic developments in patients with peripheral venous disease?
- Aching, cramping and throbbing along affected veins
- Haemorrhage which can be reduced by placing legs above the level of the heart
- Varicose eczema
- Superficial vein thrombophlebitis
- Chronic venous insufficiency
- Haemosiderin staining
- Lipodermatosis
- Venous ulceration
What is superficial vein thrombophlebitis?
- Inflammatory process that causes clots in the veins
- Increases risk of DVT
What is haemosiderin staining?*
- Red blood cells are broken down
- Haemoglobin stored as haemosiderin
- ‘Rusty’ colour along affected vein
What is lipodermatosclerosis?*
Inflammation and thickening of the fat layer under skin
What is venous eczema/ulceration?*
- Itchy, red, tight swelling
- Hard to touch
- Often around hard nodular areas
Which patients commonly suffer from chronic venous insufficiency?
Patients who have had a DVT
Why do calf muscle pumps fail?
Plantarflexion of the ankle joint during walking allows the muscles to be used properly
Who is at risk of calf muscle failure?
- Elderly people
- Obese people
- Injured people
- People with Parkinson’s disease (shuffling gait means that there is no plantarflexion)
What is the result of deep vein incompetence?
Flow backwards so the vessel gets overwhelmed.
What is the result of superficial vein incompetence?*
Blood flow goes the wrong way, from deep to superficial vein and that makes the
What treatment improves ulceration?
Ligation and vein stripping
What is arterial thrombosis?*
- Most commonly caused by atheromas
- Platelet rich and pale
- Activated
- Aggregate
- ‘Plug hole’
What is venous thrombosis?*
- Caused by stasis
- Usually another factor too (trauma, chemo, dehydration, pregnancy)
- Low flow
- Little platelet
- Fibrin rich
- Dark red
What is the Virchow’s triad?
Any of the three can contribute to a thrombotic state:
- Stasis
- Vessel wall damage
- Hypercoagulability
What is deep vein thrombosis (DVT)?
Clotting of blood in a deep vein (commonly calf) which causes impaired venous return and hypercoagulability
What are the features of DVT?*
- Inflammatory response following thrombosis
- Pain, swelling, redness
- Signs vary in severity and frequency
- Calf tenderness
- Warmth
- Visible superficial veins
- Oedema
- Pyrexia with no obvious cause
- Asymmetry in leg size
What are the differential diagnoses for DVTs?
- Soft tissue trauma
- Cellulitis
- Lymphatic obstruction
How is DVT prevented after surgery?
- Promote mobility soon after surgery
- Prophylaxis to lower DVT
- Anticoagulant agents
What is a fatal consequence of DVT?*
Pulmonary embolism
How does the body adapt to stenosis?*
Occlusion in the main vessel promotes collateral arteries forming to provide an alternative route around the occlusion to ensure that blood flow is maintained.
What is acute limb ischaemia?
- Occlusion that develops acutely over minutes/days
- No chance to develop collaterals
What can cause acute limb ischaemia?
- Popliteal artery aneurysm
- AF
- Sudden atherosclerotic plaque rupture
What are the 6 Ps of leg ischaemia?
- Pain
- Pallor
- Perishing with cold
- Pulseless
- Paraesthesia
- Paralysis/reduced power
What is the treatment of acute limb ischaemia?
- Vascular surgery unit referral
- Extent of threat assessed
- Imaging done
- Decides if angioplasty/thrombectomy/thrombolysis/amputation
What is chronic peripheral arterial disease similar to?
Coronary artery disease and stable angina, as intermittent claudication only happens on exertion (same as stable angina)
What is intermittent claudication?
Cramping pain in the leg that is induced by exercise, usually intermittent (comes and goes), and goes away upon rest
What causes intermittent claudication?
Atherosclerosis
How is chronic peripheral arterial disease treated?
Exercise, smoking cessation, antiplatelet drugs, angioplasty and bypass graft (saphenous vein)
What is critical ischaemia and what is it similar to?
- Similar to STEMI/unstable angina
- Blood supply so poor that pain is felt even at rest
What does untreated critical ischaemia lead to?
Ulceration and gangrene
How is critical ischaemia treated?
- Assess extent of threat of limb survival
- Imaging needed
- Angioplasty/thrombectomy/intra-arterial thrombolysis/amputation
What is the pathology of claudication?*
- Atheroma most common
- Becomes ischaemia if untreated
- Stenosis will dictate where claudication presents
Where can the femoral pulse be palpated?*
- Mid inguinal point
- Midway between anterior superior iliac spine and pubic symphysis
Where can the popliteal pulse be felt?*
Deep in popliteal fossa
Where can the dorsalis pedis pulse be felt?*
Lateral to extensor hallucis longus tendon
Where can the posterior tibial pulse be felt?*
Behind the medial malleolus?
What is Doppler sonography?*
Sonogram using ultrasound and Doppler effect to measure real-time flow and velocity
- Flow and velocity change with atheromas and stenosis
What is the ankle-brachial pressure index (ABPI)?*
The systolic pressure recorded in brachial artery of arm and systolic pressure in the arteries of the ankle
How is ABPI calculated?
- Ankle systolic pressure divided by brachial systolic pressure
- If <0.9 = peripheral artery disease
