Download Feb 17 CV IV

Cardiac Output = Heart Rate X Stroke Volume
• What controls Stroke Volume?
CV IV
• SV = End Diastolic Volume – End Systolic Volume
EDV
ESV
Effectiveness of heart pump
Blood flowing back to heart
(venous return)
• How does SNS affect muscle contractility?
Contractility
• Sympathetic nervous system control
L-type Ca++
Channel
(dihdryopyride receptor)
– ↑ SNS activity → ↑ force production at any EDV
Stroke Volume (ml)
↑ SNS activity
200
β
Adeylate
cyclase
Gs
Protein kinase A
rest
ATP
100
cAMP
Ca++
0
100
200
300
400
Ventricular end diastolic volume (ml)
Ca++ pump
SR
Ryanodine receptor
1
Summary - Cardiac Output
In myocytes cAMP/Protein kinase A:
1. Increase L-type Ca++ channel opening
2. Increase RyR opening
Both serve to increase cytoplasmic Ca++ and
increase contraction
End Diastolic Volume
(Frank-Starling Mech)
↑ Sympathetic activity
↑ Blood Epinephrine
↓ Parasympathetic activity
baroreceptors
3. Increase Ca++ pump activity
Serves to increase Ca++ clearance and
increase relaxation
Relative Contributions of SNS and
PNS to Heart Function
PNS primarily to SA node, AV
node and Atria
→ Mainly effect HR, smaller
effect on atrial contractility
SNS to all areas of heart
→ effects HR and contractility
Heart Rate
Stroke volume
Cardiac Output
Blood flow
Flow (Q) = Δ pressure / resistance
Generated by the heart
Function of volume & compliance
Blood vessels
Flow must = cardiac output
Q= CO = ΔP/R
2
Aortic blood pressure
In CV system:
P1 = mean arterial pressure (MAP)
P2 = right aterial pressure ≈ 0
MAP = DP + 1/3 (SP – DP)
= 75 + 1/3 (125 – 75)
= 92 mmHg
In response to the pulsatile contraction of the heart:
pulses of pressure move throughout the vasculature,
decreasing in amplitude with distance
Aorta
The blood contained in a
single heart contraction
(the stroke volume)
stretches out the arteries,
so that their elasticity
continues to “squeeze”
on the blood, keeping it
moving during diastole.
3
length
Blood flow
Resistance =
Flow (Q) = Δ pressure / resistance
viscosity
8 ⎛ Lη ⎞
π ⎜⎝ r 4 ⎟⎠
• Length of blood vessels usually constant
• Viscosity usually constant over short term
– Exceptions
MAP-RAP
Generated by the heart
Function of volume & compliance
Flow must = cardiac output
Q= CO = ΔP/R
Blood vessels
• Dehydration ↓ blood volume ↑ blood cell concentration
• Change in Blood cell production
η
normal
45
hematocrit
55
Importance of vessel radius:
Plasma includes
water, ions, proteins,
nutrients, hormones,
wastes, etc.
The hematocrit is a
rapid assessment
of blood composition.
It is the percent of the
blood volume that is
composed of RBCs
(red blood cells).
Resistance ∝
1
radius 4
ra is 2 times rb;
Flow in A is 16 times flow in B
4
Factors affecting vessel radius
Resistance =
8 ⎛ Lη ⎞
π ⎜⎝ r 4 ⎟⎠
1. Transmural pressure
•
Pressure difference across wall of vessel
Poutside
• Small change in radius → big change in
resistance
Pinside
Eg during inspiration Po decrease therefore radius ↑
during valsalva maneuver Po increase therefore radius ↓
Factors affecting arteriole radius
Factors affecting arteriole radius
2a Local Controls of radius
• Depends on metabolic state of tissue
2b. Autoregulation
• without any neural or endocrine signals
vessels can control flow in response to
pressure change
Max Vessel Dilation
– Active tissue
•
•
•
•
↑ CO2
↓pH
↑ adenosine
↑ K+
Normal
Q
(ml/min)
– all can lead to dilation of vessel and ↑ flow
Max Vessel Contraction
Autoregulatory range
70
150
Press (mm Hg)
5
Factors affecting arteriole radius
• Autoregulation is a myogenic response
– As flow increases, smooth muscle stretches
• Opens stretch activated calcium channels
• Smooth muscle contracts
3. Sympathetic nervous system
•
most arterioles receive sympathetic
postganglionic nerves
vasoconstriction
norepinephrine
Smooth muscle contraction
α1 Adrenergic receptors
↑ Intracellular Ca++
Activation of phospholipase C
Production of DAG & IP3
Enhance v-gated Ca++
channels
Activate Protein Kinase C
Factors affecting arteriole radius
Notes on SNS activity and vessel radius
1. Sympathetic tone
•
•
↑ SNS → vasoconstriction
↓ SNS → vasodilation
2. Whole body control rather than local
control
3. Recall arterioles don’t receive
parasympathetic
4a. Hormones
• Circulating epinephrine from adrenal
medulla
– most vessels also have β-adrenergic
receptors
• These cause vasodilation
– But, for most vessels,
• the number of α-receptors >> β-receptors
• Therefore SNS activity is dominant
6
Factors affecting arteriole radius
4b. Other Hormones
• Exception
– Blood vessels in skeletal muscle
– the number of β receptors >> α receptors
– Therefore when blood epinephrine is high
vessels in skeletal muscle dilate
1.
2.
Angiotensin II released from kidney
Anti-diuretic hormone (vasopressin) from posterior
pituitary
•
3.
These two are vasoconstrictors
Atrial naturetic peptide from the atria
•
This is a vasodilator
We’ll come back to these 3 later renal physiology section
Factors affecting arteriole radius
Summary
Neural Controls
Sympathetic Nervous System
Hormonal Controls
Local Controls
Vasoconstrictors
Epinephrine
Angiotensin II
Vasopressin
Vasoconstrictors
autoregulation
Vasodilators
Epinephrine
Atrail Naturetic Peptide
Arteriole smooth muscle
↓
Arteriole radius
Vasodilators
↓ oxygen
↓ pH
↑ K+
↑ CO2
↑ adenosine
• Different vascular beds have different
importance of controls
– Coronary & cerebral – local (metabolic)
– Skin – neural
– Muscle – neural, metabolic, hormonal
• SNS can be used to regulate flow to different
tissues
– i.e. increased SNS activity in one tissue and reduced
activity in another will redirect blood flow
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Venous Return
•
Muscle pumps and venous return
Factors Affecting Venous pressure
1. Transmural pressure
1. Muscle pumps
2. Respiratory pumps
2. SNS activity
– ↑ SNS activity → venoconstriction →
↓ blood volume stored in veins → ↑ venous return
Venous Return
∵
Blood flow
↑ Sympathetic activity
respiratory pump
↑ blood volume
Skeletal muscle pumps
↑ venous pressure
↑ venous return
↑ EDV
↑ stroke volume
Flow (Q) = CO = Δ pressure / resistance
= MAP - RAP / TPR
∵ RAP ≈ 0
CO = MAP / TPR
MAP = CO x TPR
MAP = HR x SV x TPR
MAP = HR x (EDV-ESV) x TPR
8
Aortic pressure
viscosity
MAP = HR x (EDV-ESV) x TPR
VR
Venous
pressure
SNS
radius
PNS
Blood volume
Muscle pumps
Respiration pump
Hormones
autoregulation
Metabolic / local
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