control of the circulation

Question 1

blood flow (CO) through to organs

Answer 1

• Liver 27%
• Heart 4%
• Kidneys 22%
• Muscle 15%
• Brain 14% ect.

Question 2

arteries

Answer 2

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

Question 3

arterioles

Answer 3

• Principle 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

Question 4

what is TPR? (arteriolar resistance)

Answer 4

• 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

Question 5

capillaries

Answer 5

• 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

Question 6

veins

Answer 6

• 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

Question 7

lymphatics

Answer 7

• 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

Question 8

cardiac output (CO) =

Answer 8

Heart Rate (HR) x Stroke Volume (SV)

Question 9

blood pressure =

Answer 9

CO x Total Peripheral Resistance (TPR)

(like Ohm’s law: V=IR)

Question 10

pulse pressure (PP) =

Answer 10

Systolic – Diastolic Pressure

Question 11

mean arterial pressure (MAP) =

Answer 11

Diastolic Pressure + 1/3 PP

Question 12

Frank-Starling mechanism

Answer 12

• 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)

Question 13

blood volume (BV)

Answer 13

• 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

Question 14

blood pressure

Answer 14

• 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

Question 15

components of BP control

Answer 15

• Autoregulation
• Local mediators
• Humoral factors
• Baroreceptors
• Central (neural) control

Question 16

autoregulation varies

Answer 16

• Intrinsic ability of an organ
• Constant flow despite perfusion pressure changes
• Renal/Cerebral/Coronary = Excellent
• Skeletal Muscle/Splanchnic = Moderate
• Cutaneous = Poor

Question 17

balance of extrinsic and intrinsic control

Answer 17

• 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

Question 18

endothelium: control functions

Answer 18

• 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

Question 19

baroreceptors

Answer 19

• 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

Question 20

arterial baroreceptors - longer term

Answer 20

• 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

Question 21

cardiopulmonary baroreceptors

Answer 21

• 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

Question 22

main neural influences on medulla

Answer 22

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

Question 23

other higher centres

Answer 23

• 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

Question 24

central chemoreceptors

Answer 24

• 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

Question 25

Putting it together, standard BP control

Answer 25

Short term:
• Baroreceptors
• ↑BP ⇒ ↓Firing ⇒ ↑PNS/↓SNS ⇒ ↓CO/TPR = ↓BP
Long term
• Volume of blood
• Na+, H20, Renin-Angiotensin-Aldosterone and ADH

Question 26

key central effectors are peripheral

Answer 26

• Blood vessels (vasodilatation and vasoconstriction: affects TPR)
• Heart (rate and contractility: CO = HR x SV)
• Kidney (fluid balance: longer term control)