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CVS > Control Of The Circulation > Flashcards

Control Of The Circulation Flashcards

1
Q

Blood flow

A

Liver - 27%
Kidney - 22%
Muscle - 15%
Brain - 14%
Skin - 6%
Bone - 5%
Heart 4%
Bronchi - 2%

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2
Q

Blood volume

A

Small veins and venules - 43%
Large systematic veins - 20%
Pulmonary circulation - 12%
Heart - 10%
Systemic arteries - 10%
Capillaries - 5%

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3
Q

Arteries

A

Low resistance conduits.
Elastic.
Cushion systole.
Maintain blood flow to organs during diastole.

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4
Q

Arterioles

A

Principle site of resistance to vascular flow.
TPR = total arteriolar resistance.
Major role in arterial pressure and distributing floe to tissue/organs.

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5
Q

TPR

A

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.

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6
Q

Capillaries

A

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.

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7
Q

Veins

A

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.

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8
Q

Lymphatics

A

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.

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9
Q

Cardiac output

A

Heart rate x stroke volume

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10
Q

Blood pressure

A

Cardiac output X total peripheral resistance

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11
Q

Pulse pressure

A

Systolic – Diastolic Pressure

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12
Q

Mean Arterial Pressure

A

Diastolic Pressure + 1/3 PP

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13
Q

What governs flow?

A

Ohm’s law
Poiseuille’s equation

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14
Q

Frank-Starling Mechanism

A

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
↑Venous return = ↑EDV = ↑SV = ↑CO (even if HR constant)

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15
Q

Blood pressure

A

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

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16
Q

Components of BP control

A

Autoregulation
Local mediators
Humoral factors
Baroreceptors
Central (neural) control

17
Q

Myogenic autoregulation

A

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.

18
Q

Local humoral factors

A

Vasoconstrictors: Endothelin-1, Internal Blood Pressure (myogenic contraction).
Vasodilators: Hypoxia, Adenosine, Bradykinin, NO
K+, CO2, H+, Tissue breakdown products.

19
Q

Endothelium control functions

A

Essential for control of the circulation

Nitric Oxide (NO) = potent vasodilator
Prostacyclin = potent vasodilator
Endothelin = potent vasoconstrictor

20
Q

8) Circulating (hormonal) factors

A

Vasoconstrictor: Epinephrine (skin), Angiotensin II, Vasopressin.
Vasodilators: Epinephrine (muscle), Atrial Natriuretic Peptide.

21
Q

Baroreceptors

A

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.

22
Q

Arterial baroreceptors

A

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 (Na+, H20).

23
Q

Cardiopulmonary baroreceptors

A

Atria, ventricles, PA stretch:
Secretion of ANP.
↓ vasoconstrictor centre in medulla, ↓ BP; and ↓release angiotensin, aldosterone & vasopressin (ADH), fluid loss.
Blood volume regulation

24
Q

Central neural control loop

A

Baroreceptor - brain stem - (increases or decreases sympathetic NS or vasoconstrictors) - heart rate/stroke volume/vessel diameter - BP increases or decreases.

25
Q

Main neural influences on medulla

A

Baroreceptors
Chemoreceptors
Hypothalamus
Cerebral cortex
Skin
Changes in blood [O2] and [CO2]

26
Q

Hypothalamus

A

Stimulation of anterior hypothalamus ↓ BP and HR;
The reverse with posterolateral hypothalamus.

27
Q

Cerebral cortex

A

Cerebral cortex can affect blood flow & pressure.
Stimulation usually ↑ vasoconstriction.
Emotion can ↑ vasodilatation and depressor responses eg. blushing, fainting.
Effects mediated via medulla but some directly.

28
Q

Central chemoreceptors

A

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.

29
Q

BP control

A

Short term:
Baroreceptors
↑BP ⇒ ↑Firing ⇒ ↑PNS/↓SNS ⇒ ↓CO/TPR = ↓BP.

Long term
Volume of blood
Na+, H20, Renin-Angiotensin-Aldosterone and ADH.

30
Q

Fainting

A

Fall in HR and venous pooling.
CO falls.
Treatment - lay supine.

31
Q

Blood loss

A

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 EARLY.

32
Q

Orthostatic hypotension

A

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.
Investigate: Lying/ standing BP; tilt test.
Common cause: BP drugs, B blockers, vasodilators
Lifestyle adaptation.

33
Q

Heart failure

A

Usually due to myocardial infarction, valve disease or dilated cardiomyopathy
Reduced BP stimulates Na+, H20 retention by kidney
Leads to swelling, pulmonary congestion, excess tachycardia
Stretch of atria leads to atrial fibrillation
Treat with ACE inhibitor, diuretic, cautious B blocker
Revascularisation, anticoagulation, resynchronisation (device).

CVS (23 decks)

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