control of the circulation Flashcards
1
Q
blood flow (CO) through to organs
A
- Liver 27%
- Heart 4%
- Kidneys 22%
- Muscle 15%
- Brain 14% ect.
2
Q
arteries
A
- Low resistance conduits
- Elastic
- Cushion systole
- Maintain blood flow to organs during diastole
3
Q
arterioles
A
- 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
4
Q
what is TPR? (arteriolar resistance)
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
5
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
6
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
7
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
8
Q
cardiac output (CO) =
A
Heart Rate (HR) x Stroke Volume (SV)
9
Q
blood pressure =
A
CO x Total Peripheral Resistance (TPR)
(like Ohm’s law: V=IR)
10
Q
pulse pressure (PP) =
A
Systolic – Diastolic Pressure
11
Q
mean arterial pressure (MAP) =
A
Diastolic Pressure + 1/3 PP
12
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
- ↑VR = ↑EDV = ↑SV = ↑CO (even if HR constant)
13
Q
blood volume (BV)
A
- 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
14
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
15
Q
components of BP control
A
- Autoregulation
- Local mediators
- Humoral factors
- Baroreceptors
- Central (neural) control
16
Q
autoregulation varies
A
- Intrinsic ability of an organ
- Constant flow despite perfusion pressure changes
- Renal/Cerebral/Coronary = Excellent
- Skeletal Muscle/Splanchnic = Moderate
- Cutaneous = Poor
17
Q
balance of extrinsic and intrinsic control
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
endothelium: control functions
A
- 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
19
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
20
Q
arterial baroreceptors - longer term
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
21
Q
cardiopulmonary baroreceptors
A
- 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
22
Q
main neural influences on medulla
A
- Baroreceptors
- Chemoreceptors
- Hypothalamus
- Cerebral cortex
- Skin
- Changes in blood [O2] and [CO2]
23
Q
other higher centres
A
- 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
24
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
25
Q
Putting it together, standard 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
26
Q
key central effectors are peripheral
A
- Blood vessels (vasodilatation and vasoconstriction: affects TPR)
- Heart (rate and contractility: CO = HR x SV)
- Kidney (fluid balance: longer term control)
