cardiac mechanics
TRANSCRIPT
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Cardiac Mechanics
Bioengineering/Physiology 6000
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
The Heart
• Structure– Macro and micro
• Function• Cells
– Pacemaker– Conduction system– Contractile myocytes
• Electrophysiology– Action potentials– Cell to cell coupling
• Mechanics– EC coupling– Cardiac cycle
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Average Hemodynamic ValuesVariable Cow Man Dog Rabbit Rat
Weight (kg)
414 70 20 4 0.6
Cardiac Output (ml/sec)
680 110 42 5.2 1.2
Heart rate (min-1)
71 76 99 288 349
Stroke Volume (ml)
570 87 25 1.1 0.21
Velocity in ascending aorta
16 18 32 22
Cardiovascular PhysiologyBy William R. Milnor
690
560
.2 (5)
2700
.7 (1.4)
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Hummingbird Physiology(What we need to explain)
• Heart is 20% of body volume (largest of any animal), 2-3% of body mass
• Heart rate varies from 30 (deep rest), 500 (while perching), to 1200 (during high speed chases)
• Highest known mass-specific metabolic rates among vertebrate homeotherms (100 times greater than elephant)
• Eats .5-8 times body weight per day• Respiratory rate of 250 at rest• Dives produce 7-10 G at speeds of
up to 100 kph.
R.K. Suarez, Hummingbird flight: Sustaining the highest mass-specific metabolic rates among vertebrates. Cellular and Molecular Life Sciences (CMLS) 48(6), 1992.
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Electrophysiology Review
• Pacemaker cells –Neurogenic vs.
myogenic–SA Node–AV Node–Purkinje Fibers
• Conduction system• Ventricular myocytes• The Electrocardiogram
(ECG)
Cardiac Mechanics Bioengineering 6000 CV Physiology
Excitation-Contraction Coupling(ECC)
Bioengineering 6000 CV PhysiologyCardiac Mechanics
Structure-Function Relationship
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
EC Coupling
• Action potential causes influx of Ca2+
• In mammals and birds, this causes release of more Ca2+
• Ca2+ interacts with actin/myosin to cause contraction
• Pumps gather up Ca2+ or remove it from cell
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
D.M.Bers Nature VOL 415 10 January 2002
Calcium-induce Calcium Release
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Calcium Measurement
• Calcium-sensitive dye• Optical recording system• Stimulate cell and synchronize with
optics
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
diastole
systole
+40 mV
-80 mV50 ms
EC Coupling and Ca2+
1) Control
2) SR disabled (caffeine)
3) No Ca2+ entry (Ba2+)
Bioengineering 6900 B-TACCardiovascular Review
Ca+2 Measurements
http://www.youtube.com/watch?v=x3s2L2rvNLM
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
CICR Summary• Calcium causes contraction• Calcium is stored in the sarcoplasmic reticulum (SR)• A small increase of Ca in the vicinity of the SR causes a much
larger release of Ca from the SR.• Contraction can be graded
mV
t
Action Potential
Twitch (tension)
Cai transient
[Ca++]
gramCai from ICa
10-7 10-5
100
Tens
ion
[% m
ax]
Rest
Cai [M]
0
Range
ove
r AP
Contraction
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Contractile Proteins
Cardiac Mechanics Bioengineering 6000 CV Physiology
Pretension, Frank-Starling, and Control of Contraction
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Pretension of Muscle (Explain?)
time
tens
ion
Isometric Contractionwith Pretension
activ
e te
nsio
n
resting tension
isotonic tension
Bioengineering 6900 B-TACCardiovascular Review
Isotonic Contraction and Preload
1g1
1
1g3
31
2
3
4
tens
ion
[g]
muscle length
pretension
1g2
2
2g1
1
22g
2
32g
3
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Intrinsic Regulation of Cellular Contraction
• Spacing in contractile elements– Determines contractility– Adjusted passively
0
20
40
60
80
100
120
1 1.5 2 2.5 3 3.5 4
Tens
ion
[% o
f max
imum
]
Striation Spacing [µm]
Total tension
Passive (rest) tension
Twitch tension
mV
t
Action Potential
Twitch (tension)
Cai transient
[Ca++]
gramCai from ICa
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Total tension
Passive (rest) tension
Twitch tension
Frank Starling Mechanism
• Increased pretension can increase contraction• Cell: striation spacing• Muscle: pretension • Heart: Increased filling produces increased output• Does not require neural input
Muscle Level Whole Heart Level
0
20
40
60
80
100
120
1 1.5 2 2.5 3 3.5 4Tens
ion
[% o
f max
imum
]
Striation Spacing [µm]
Cell Level
Bioengineering 6000 CV PhysiologyCardiac Mechanics
Extrinsic Regulation of Contractility
• Free [Ca]i is key component• Positive inotropic agents:
– epinephrine: stimulate β receptors and increase Ca influx and uptake (load SR)
• Negative inotropic agents:– ACh: acts mostly on atria to shorten AP and
reduce [Ca]i – Acidosis
Mus
cle
tens
ion
Passive (rest) tension
Total tension
Twitch tension
Muscle length
Effect of NE/E
Cell
Muscle
Whole Heart
10-7 10-5
100
Tens
ion
[% m
ax]
Rest
Cai [M]
0
Range
ove
r AP
Contraction
Cardiac Mechanics Bioengineering 6000 CV Physiology
The Cardiac Cycle and Afterload
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Cardiac Cycle
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Cardiac Cycle
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Work of the Heart
• Work = Pressure * Volume(force * distance)
• Cardiac cycle as PV diagram• Efficiency
– Convert work to O2 consumption– Compare to total O2
consumption– 10--15% efficient
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Effect of E/NE on Contraction
• constant pretension• reduced end-systolic
volume• increased stroke volume
1
2
3
4
tens
ion
[g]
muscle length
maximum isotonic tension
Pres
sure
Volume
Left Ventricle
strokevolume
Bioengineering 6900 B-TACCardiovascular Review
Isotonic Contraction and Afterload
1g1
1g
1
11
2
3
4
tens
ion
[g]
muscle length
2g1g
4
4
2g 1g
5
5
2g
pretension
isotonic tension
3g2
2
3g3
3
Afterload reducesshortening
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Contraction at the Whole Heart
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Whole Heart Simulation
Cardiac Mechanics Bioengineering 6000 CV Physiology
Animal Hearts
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Animal Hearts• Air-breathing vs. water breathing vs. fetal• Open Systems• Separate left and right hearts
– Higher pressure good for rapid transport but require lymphatic system
– Lower pressure (e.g., pulmonary) does not require lymphatics and stays drier
– Both sides must have equal flow• Shared ventricles
– Shunts from pulmonary to systemic (P->S)– Allows adjustment of flow through lungs/gills– Flows to both parts of circulation are not equal but
pressures are
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Open Systems
• Blood empties into body space• Bathes tissues directly, blood in
small chambers• Low pressure system (4-10 mm
Hg)• Typically limited regulation and
low oxygen transport (with exceptions)
• Insects bypass lungs and transport oxygen directly so open circulation does not carry oxygen
B
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Rigid Pericardium
• Compliant– Minimal constraint– Lubrication
• Non-compliant– contraction of one chamber
assists filling the other
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Water Breathing Fishes
• 4-chambered, sequential heart with valves• Gills perform gas exchange and also ion balance (like kidneys in
mammals)• Gill circulation under higher pressure than systemic circulation
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Air Breathing Fishes
• Gills and vascularized air sac both provide O2; gills used for CO2 and ion balance
• Blood directed to different parts of system by the heart, also between gills and air-breathing organs
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Amphibians (e.g., Frog)
• Shared ventricle but blood separated– Spiral fold– Initial flow through pulmonary
because of reduced pressure– Resistance to flow varies with
breathing (inhale reduces resistance)
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Frog II
• Partial mixing of systemic blood flow– Cerebral flow oxygenated, systemic mixed
• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Reptiles (non crocodilian)
• Partial separation of single ventricle by 2 septa
• In ventricular systole, flow determined by relative resistances of pulmonary and system, e.g., breathing, diving
• Pressure differences in arteries also directs flow: lower pressure value opens first
•
Bioengineering 6000 -- SystemsPhysiology ICardiac Mechanics
Fetal Heart• Lungs collapsed so minimal
pulmonary flow• Oxygenated blood comes from
placenta and some passes through foramen ovale to left atrium
• Rest of returning blood goes from RV and most through Ductus ateriosus to aorta
• At birth, lungs inflate, flow to them increases, placenta flow gone so systemic resistance increases
• Left side pressure increases, D.A. and F.O close
Numbers are percentage of ventricular output