S8: special circulations + peripheral arterial & venous disease Flashcards
State the major differences between the properties of the systemic and pulmonary circulation
Pulmonary circulation = low pressure, low resistance, must accept the entire cardiac output
Systemic circulation = high pressure, high resistance, delivers blood to all body tissues except lungs
State the normal pressures in the pulmonary artery, pulmonary capillaries and pulmonary veins
Pulmonary artery = 12-15mmHg
Pulmonary capillaries = 9-12mmHg
Pulmonary veins = 5mmHg
Explain the concept of ventilation perfusion matching in the pulmonary circulation
For efficient oxygenation = match ventilation of alveoli with perfusion of alveoli
Optimal V/Q ratio = 0.8
Alveolar hypoxia results in vasoconstriction of pulmonary vessels -> ensures that perfusion matches ventilation (helps to optimise gas exchange & opposite to that in systemic circulations)
Describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation
Systemic circulation = capillary hydrostatic pressure is more influenced by venous pressure, hypertension does not usually result in peripheral oedema
Pulmonary circulation = oncotic pressure > systemic, hydrostatic pressure < systemic capillaries -> minimises the formation of lung lymph
Low capillary pressure prevents pulmonary oedema (can get increase pressure from mitral valve stenosis, left ventricular failure)
Describe the relationship between the mechanical work and oxygen demand of the myocardium
Coronary blood flow increases with myocardial O2 demand
Small increase in amount of O2 extraction
Vasodilation due to metabolic hyperaemia & metabolites (adenosine, K+, decreased pH)
Describe the particular features of the coronary circulation
Higher capillary density, lower diffusion distance & continuous production of NO by coronary endothelium maintains a high basal flow
Coronary arteries are functional end arteries
Describe the consequences of partial or total occlusion of coronary arteries
Narrowed coronary arteries leads to angina on exercise
Sudden obstruction by thrombus causes MI
Describe the factors which influence blood flow through the brain
Anastomoses between basilar and internal carotid arteries
Myogenic autoregulation = increased BP causes vasoconstriction, decreased BP causes vasodilation, fails below 50mmHg
Metabolic regulation = very sensitive to changes in CO2 partial pressure; hypercapnia -> vasodilation, hypocapnia -> vasoconstriction
Cushing’s reflex = impaired blood flow to vasomotor control regions of the brainstem increases sympathetic vasomotor activity (increases arterial BP)
Describe the factors which influence blood flow through skeletal muscle
Very high vascular tone
Opening of precapillary sphincters allows more capillaries to be perfused
Various agents are vasodilators (K+, increased osmolarity, adenosine)
Adrenaline acts as a vasodilator through B2 receptors
Describe the factors which influence blood flow through skin
Arteriovenous anastomoses (AVAs) regulate heat loss from apical skin
Decreased core temperature increases sympathetic tone in AVAs
Increased core temperature opens AVAs
Reduced vasomotor drive to AVAs allows them to dilate -> diverts blood to veins near surface
Non-apical skin: sympathetic cholinergic fibres activate sweat glands that may in turn release vasodilators eg. bradykinin
Describe the venous drainage of the lower limbs
Deep veins are underneath the fascia with the major arteries, superficial veins are located in the subcutaneous tissue
Movement of blood is from superficial to deep veins
List some causes, symptoms and signs of peripheral venous disease
Causes: varicose veins (valves ineffective & blood movement is slow/reversed) due to walls weakening
Symptoms: heaviness, muscle cramps, ankle swelling, tenderness
Signs: varicose eczema, haemorrhage
Explain the role and functioning of the calf muscle pump
Soleus and gastrocnemius muscles contribute to pushing blood against gravity back towards the heart
Valves open -> blood pushed through to deep veins -> valves close to prevent retrograde movement
Perforating valves open again allowing filling from superficial veins
Venous pressure in foot reduced during exercise
Failure of calf muscle pump = not using calf muscles properly
List some causes, symptoms and signs of peripheral arterial disease, both acute and chronic
Acute limb ischaemia = trauma & embolism, 6 ‘P’ pain, pallor, perishingly cold, pulseless, paraesthesia, paralysis or reduced power, refer to vascular surgical unit immediately
Chronic peripheral arterial disease = intermittent claudication of the lower limb caused by atherosclerosis, exercise induced, pain goes away at rest, managed by stopping smoking, exercise, angioplasty, bypass graft
Leads to critical limb ischaemia -> rest pain and untreated leads to ulceration and gangrene, referral to vascular surgical unit
Describe where you would feel for the pulse of the femoral artery, popliteal artery, dorsalis pedis and posterior tibial pulse
Femoral pulse = felt at the mid inguinal point midway between the ASIS and pubic symphysis
Popliteal pulse = deep in the popliteal fossa
Dorsalis pedis pulse = just lateral to extensor hallucis longus tendon
Posterior tibial pulse = just behind the medial malleolus
