Circulation Flashcards
Why do we need to control circulation
In order to
–Maintain blood flow
–Maintain arterial pressure
–Distribute blood flow
–Auto-regulate/homeostasis
–Function normally
–Prevent catastrophe!
–(maladapt in disease)
What are the components of circulation?
Anatomy
•Blood
•Pressure
•Volume
•Flow
What is the blood flow (CO) through 11 organs?
Liver 27%
•Kidneys 22%
•Muscle 15%
•Brain 14%
•Skin 6%
•Bone 5%
Heart 4%
•Other 3.5%
•Bronchi 2%
•Thyroid 1%
•Adrenal 0.5%
What are arteries like?
Low resistance conduits
•Elastic
•Cushion systole
•Maintain blood flow to organs during diastole
What are arterioles?
•Principal site of resistance to vascular flow
•Therefore, TPR = Total Arteriolar Resistance
•Determined by local, neural and hormonal factors
•Major role in determining arterial pressure
•Major role in distributing flow to tissue/organs
What is TPR? (Arteriolar resistance)
• Vascular smooth muscle (VSM) determines radius
• VSM Contracts = ↓Radius = ↑Resistance ↓Flow
• VSM Relaxes = ↑Radius = ↓Resistance ↑Flow
• Or Vasoconstriction and Vasodilatation
• VSM never completely relaxed = myogenic tone
What are capillaries?
40,000km and large area = slow flow
•Allows time for nutrient/waste exchange
•Plasma or interstitial fluid flow determines the distribution of ECF between these compartments
•Flow also determined by
–Arteriolar resistance
–No. of open pre-capillary sphincters
What are veins like?
Compliant
•Low resistance conduits
•Capacitance vessels
•Up to 70% of blood volume but only 10mmHg
•Valves aid venous return (VR) against gravity
•Skeletal muscle/Respiratory pump aids return
•SNS mediated vasoconstriction maintains VR/VP
What are lymphatics?
•Fluid/protein excess filtered from capillaries
•Return of this interstitial fluid to CV system
–Thoracic duct; left subclavian vein
•Uni-directional flow aided by
–Smooth muscle in lymphatic vessels
–Skeletal muscle pump
–Respiratory pump
What is CO, BP, PP, MAP equations?
Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)
Blood pressure = CO x Total Peripheral Resistance (TPR)
(like Ohm’s law: V=IR)
Pulse pressure (PP) = Systolic – Diastolic Pressure
Mean Arterial Pressure (MAP)= Diastolic Pressure + 1/3 PP
What governs flow?
- Ohm’s law
F = ∆P/R Or
Flow = Pressure Gradient
Resistance
- Poiseuille’s equation
Flow = radius to the power of 4
How does the heart respond to volume?
Frank-starling mechanism?
SV increases as End-Diastolic Volume increases
•Due to Length-Tension (L-T) relationship of muscle
•↑EDV = ↑Stretch = ↑Force of contraction
•Cardiac muscle at rest is NOT at its optimum length
•↑VR = ↑EDV = ↑SV = ↑CO (even if HR constant)
What is blood volume?
•Venous return important beat to beat (FS mechanism)
•Blood volume is an important long term moderator
•BV = Na+, H20
•Renin-Angiotensin-Aldosterone system
•ADH
•Adrenals and kidneys
•But that’s another lecture!
What is the goal of controlling circulation?
Maintain blood flow!
CO = SV x HR
This needs pressure to push blood through peripheral resistance
MAP = CO x TPR
What is BP like?
BP = Pressure of blood within and against the arteries
•Systolic = Highest, when ventricles contract (100-150mmHg)
•Diastolic = Lowest, when ventricles relax (not zero, due to aortic valve and aortic elasticity .. 60-90mmHg)
•Mean arterial pressure = D + 1/3(S-D)
•Measured using a sphygmomanometer
•Using brachial artery
–Convenient to compress
–Level of heart
What is BP like?
BP = Pressure of blood within and against the arteries
•Systolic = Highest, when ventricles contract (100-150mmHg)
•Diastolic = Lowest, when ventricles relax (not zero, due to aortic valve and aortic elasticity .. 60-90mmHg)
•Mean arterial pressure = D + 1/3(S-D)
•Measured using a sphygmomanometer
•Using brachial artery
–Convenient to compress
–Level of heart
What is used to measure BP?
Ingredients:
Arm, Sphygmomanometer, Stethoscope, 2 Ears
Directions:
Inflate cuff to above systolic BP, until pulse
impalpable or Korotkoff sounds absent.
Serving suggestion:
Slowly deflate cuff, listening all the time.
What are BP sounds? (Korotkoff)
0) > Systolic Pressure = no flow, no sounds
1) Systolic pressure = high velocity = tap
2-4) Between S and D = thud
5) Diastolic pressure = sounds disappear
What are the components of BP control?
Autoregulation
Local mediators
Humoral factors
Baroreceptors
Central (neural) control
What is myogenic auto regulation like?
•Intrinsic ability of an organ
•Constant flow despite perfusion pressure changes
•Renal/Cerebral/Coronary = Excellent
•Skeletal Muscle/Splanchnic = Moderate
•Cutaneous = Poor
What is the balance of extrinsic and intrinsic control?
Brain & heart: intrinsic control dominates to maintain BF to vital organs
•Skin: BF is important in general vasoconstrictor response and also in responses to temperature (extrinsic) via hypothalamus
•Skeletal muscle: dual effects:- at rest, vasoconstrictor (extrinsic) tone is dominant; upon exercise, intrinsic mechanisms predominate
What are the local humoral factors?
Vasoconstrictors
•Endothelin-1
•Internal Blood Pressure
(myogenic contraction)
Vasodilators
•Hypoxia
•Adenosine
•Bradykinin
•NO
•K+, CO2, H+
•Tissue breakdown products
What are the control functions of endothelium
•Essential for control of the circulation
•EDRF = Nitric Oxide (NO) = potent vasodilator
•L-Arg is converted into NO by NO synthetase
•Prostacyclin = potent vasodilator
•Endothelin = potent vasoconstrictor
What is Endothelium like in local control
(Furchgott’s Experiment)
Vasodilatation
(Endothelium
Derived
Relaxation
Factor) (NO)
Ach
Endothelium intact/rubbed of
Vasodilation/vasoconstriction
How is NO produced?
In endothelial cell:
Endotoxin cytokines
Ach bradykinin substance-P insulin -> R
Shearing forces
Citrulline?
How is NO produced?
In endothelial cell:
Endotoxin cytokines
Ach bradykinin substance-P insulin -> R
Shearing forces
Citrulline?
How is endothelin produced?
+
Angiotensin 2
Vasopressin
Thrombin
Oxygen free radicals
Shearing forces
- NO, postacyclin, ANP
ECE, G-protein - smooth muscle contraction Ca2+
How is PG12 prostacyclin produced?
Membrane phospholipids
Aspirin NSAIDS
5-HPETE
Cycle-endoperoxides
What are circulating (hormonal) factors?
Vasocontrictors
•Epinephrine (skin)
•Angiotensin II
•Vasopressin
Vasodilators
•Epinephrine (muscle)
•Atrial Natriuretic Peptide
What are baroreceptors like?
Pressure sensing
•Primary (Arterial) =carotid sinus & aortic arch
•Secondary = veins, myocardium, pulmonary vessels
•Afferent Glossopharyngeal (IX);
•Efferents sympathetic and Vagus (X)
•Firing rate proportional to MAP and PP, integrated in the medulla
•↑BP ⇒ ↑Firing ⇒ ↑PNS/↓SNS ⇒ ↓CO/TPR = ↓BP
•and vice versa
What are arterial baroreceptors like?
•Key role in short-term regulation of BP; minute to minute control, response to exercise, haemorrhage
•If arterial pressure deviates from ‘norm’ for more than a few days they ‘adapt’/’reset’ to new baseline pressure eg. in hypertension
•The major factor in long-term BP control is blood volume
What are cardiopulmonary baroceptors like?
Atria, ventricles, PA
•Stimulation = ↓vasoconstrictor
centre in medulla, = ↓ BP
•Also ↓release angiotensin, aldosterone & vasopressin (ADH), leading to fluid loss
•Play an important role in blood volume regulation
What is the central neural control loop?
Baroreceptors -> brainstem -> heart rest, stroke, vessel diameter -> BP (+/-) - back to baroreceptors
+/- sympathetic or vasoconstrictors
What is the neural pathway like?
What is the neural pathway like?
What are the main neural influences on medulla
•Baroreceptors
•Chemoreceptors
•Hypothalamus
•Cerebral cortex
•Skin
•Changes in blood [O2] and [CO2]
How do the arterial baroreceptors affect central control?
Arterial pressure -> Arterial Baroreceptors -> decreased Sympathetic Outflow to Heart, Arterioles & Veins
/ Parasympathetic Outflow To Heart
What occurs in the Autonomic NS?
What do other neural centres do?
CV reflexes require hypothalamus and pons
•Stimulation of anterior hypothalamus ↓ BP and HR; reverse with posterolateral hypothalamus
•Hypothalamus also important in regulation of skin blood flow in response to temperature
•Cerebral cortex can affect blood flow & pressure. Stimulation usually ↑ vasoconstriction, but emotion can ↑ vasodilatation and depressor responses eg. blushing, fainting. Effects mediated via medulla but some directly
What are the central chemoreceptors?
Chemosensitive regions in medulla
•↑PaCO2 = vasoconstriction, ↑peripheral resistance, ↑BP
•↓PaCO2 = ↓medullary tonic activity, ↓BP
•Similar changes with ↑ and ↓ pH
•PaO2 less effect on medulla; Moderate ↓ = vasoconstriction; Severe ↓ = general depression
•Effects of PaO2 mainly via peripheral chemoreceptors
What is the standard BP control?
Short term:
–Baroreceptors
–↑BP ⇒ ↑Firing ⇒ ↑PNS/↓SNS ⇒ ↓CO/TPR = ↓BP
•Long term
–Volume of blood
–Na+, H20, Renin-Angiotensin-Aldosterone and ADH
What are the key central effectors
Peripheral
• Blood vessels (vasodilatation and vasoconstriction: affects TPR)
• Heart (rate and contractility: CO = HR x SV)
• Kidney (fluid balance: longer term control)
What is the feedback loop in circulation?
Blood pressure -> baroreceptor discharge -> sympathetic and parasympathetic outflow -> vasomotor tone and CO ->
What is the CR of circulation ?
Physiological
•Cold
•Standing up
•Running
•Lifting
•Injury
•Blood loss
Pathological
•Fainting
•Orthostatic hypotension
•POTS
•Heart failure
•Hypovolaemic shock
•Cardiogenic shock
•Heart block
•Cushing’s syndrome
•Respiratory failure
•General anaesthetic
What is fainting like?
‘Neuro-cardiogenic syncope’
•Aetiology = emotion, heat, standing, dehydration
•Symptoms = nausea, air hunger, sweating
•Physiology = Fall in HR and Venous Pooling (X nerve)
•Signs = Collapse due to ↓ CO
•HR falls, CO falls, BP falls, perfusion to brain reduced
•‘Neuro-cardiogenic syncope’ = Faint!
•Treatment = lay supine and elevate limbs to ↑VR
•Frank-Starling leads to improved SV and CO
•Long term: fluids, salt .. Midodrine (α agonist)
How does blood loss affect circulation?
Blood loss
•Perfusion to brain must be maintained
•Local vasoconstriction
•Maintain CO/BP by ↑HR
•Sympathetic outflow
•Widespread cutaneous vasoconstriction
•Eventually .. SHOCK (BP↓, Pulse↑, organ hypoperfusion) and death
•Treat: rapid volume replacement
What is orthodontic hypertension?
Aetiology = standing quickly, too long, dehydration, hot room
•Symptoms = lightheaded, sweating, syncope
•Physiology = Fall in BP and Venous Pooling (X nerve)
•Failure to reflexly maintain BP and HR
•Perfusion to brain reduced
•Treatment = lay supine and elevate limbs to ↑VR
•Frank-Starling leads to improved SV and CO
•Investigate: Lying/ standing BP; tilt test
What is POTS?
POTS
‘Postural orthostatic tachycardia syndrome’
•Standing
•Palpitation, dizzy, near syncope, sweating, debilitating
•Physiology = Excess tachycardia response
•Investigate = Tilt test
•HR↑ >40bpm; BP usually OK
•Not well understood
What are the key controls of circulation?
•Autoregulation
•Peripheral humoral
•Circulating humoral
•Baroreceptors
•Central neural
