Control Of Circulation Flashcards
Why do we need to control our circulation?
Maintain blood flow
Maintain arterial pressure
Distribute blood flow
Auto-regulate/homeostasis
Function normally
Prevent catastrophe!
(maladapt in disease)
What are components of Circulation?
Anatomy
Blood
Pressure
Volume
Flow
Features of Arteries
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?
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
Features of 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
Features of Veins
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
Features of 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
Cardiac Output Equation
Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)
Blood Pressure Equation
CO x Total Peripheral Resistance (TPR)
(like Ohm’s law: V=IR)
Pulse Pressure (PP) Equation
Systolic – Diastolic Pressure
Mean Arterial Pressure Equation
Diastolic Pressure + 1/3 PP
What is 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 Pressure?
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
How is Blood Pressure measured?
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.
Blood Pressure Sounds
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 components of Blood pressure control?
Autoregulation
Local mediators
Humoral factors
Baroreceptors
Central (neural) control
Explain the balance of extrinsic and extrinsic 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
Give an example of a Vasoconstrictor (Local Humoral Factors)
Endothelin-1
Give examples of Vasodilators (Local Humoral Factors)
Hypoxia
Adenosine
Bradykinin
NO
K+, CO2, H+
Tissue breakdown products
What are control functions of the 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
Give examples of Vasoconstrictors (Circulating Hormonal Factors)
Epinephrine (skin)
Angiotensin II
Vasopressin
Give examples of Vasodilators (Circulating Hormonal Factors)
Epinephrine (muscle)
Atrial Natriuretic Peptide
What are Baroreceptors?
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?
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 baroreceptors?
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 are the main neural influences on the medulla?
Baroreceptors
Chemoreceptors
Hypothalamus
Cerebral cortex
Skin
Changes in blood [O2] and [CO2]
What are 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 are 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 orthostatic hypotension?
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
Common cause: BP drugs, B blockers, vasodilators
Lifestyle adaptation
What is 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
