Topic 2:Intro to CV system & Cardiac Muscle Mech.
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CV System: Hemodynamics
- refers to three inter-related parameters:
- - Pressure
- - Blood Flow
- - Vascular Resistance
Is the pressure constant throughout the system of blood flow of the heart?
- No, because the blood is forced to flow
- across different resistances along the system.
Pressure are variable depending on where you are in the system
Equation for Pressure of systemic blood flow
analogous to Ohm's Law:
P = Q * R
- P: pressure
- Q: flow
- R: resistance
muscle fiber orientation and its effects
- 1. muscle layers are wrapped in such
- a way that small amounts of sarcomere shortening can elicit large changes in
- chamber volume
2. orientation of muscle layers force the heart chamber to twist and rotate
to squeeze the blood out during contraction.
This improves the efficiency of the pumping
Features of cardiac myocytes (4)
- 1. arranged in syncytium
- 2. Intercalated disks w/ gap junctions join cell ends together
- 3. Extracellular collagen matrix btwn cells
- 4. Highly enriched in capillaries and mitochondria (33% cell volume) for steady energy source
blood to the heart is supplied by???
% of the blood supply to heart?
% of energy supplied to heart?
- - blood supplied by coronary arteries
- - heart uses ~ 4% of the blood supply
- - has ~ 10% of the energy demand in the body
Sources of ATP production in cardiac muscles (2)
- 1. glycolysis
- 2. Ox. Phos.
3 Cardiac Cell Types
- 1. Working Cells:
- majority of all cells in the heart which contract in the ventricles and atria.
- 2. Pacemaker Cells:
- Spontaneously depolarizing cells that can initiate APs in SA & AV nodes
- 3. Conducting Cells:
- Specialized cells (e.g.: Purkinje Cells,
- Bundle of His) that conduct APs at specific rates to specified regions of the working cells
Mech. Parameters of muscles
- - Force, tension, or mass (F=ma)
- - Length (muscle,cell or sarcomere length)
- - time
- - Cross sectional Area
- - Stress (force/area)
- - Velocity (length/time)
- - Work (force x length)
force vs. time @ a constant length.
length vs. time @ a constant force
force, length & time all change.
3 types of forces in an isometric contraction
1. Total force: force measured at the peak of an isometric contraction
2. Active force: force generated by crossbridges
Active force = total force - passive forces.
3. Passive force:resting force = force measured when muscle is @ rest
- passive force is more significant in cardiac vs skeletal muscle
3 Parts of Action/Tension curve in Striated Muscles
1-3 descending limb
: no to some overlap between thin and thick filament
- 2-3 plateau: thin and thick filament
- completely overlaps = max force 2.0-2.2 μm sarcomere lengths
- 3-6 ascending limb: double overlap of
- thick and thin filament = steric hindrance = less binding of crossbridges
Can cardiac muscles be extended
to a length beyond the plateau of the active force curve?
Role of Titin in the Passive Force-Length Relationship
@ short/relaxed sarcomere lengths, titin is bundled and folded
@ long/stretched sarcomere lengths, the elastic part of titin is stretched
characteristics of Titin and diff. btwn cardiac and skeletal
- primary source of resting tension
- - largest known single protein
- - Cardiac titin is shorter than skeletal muscle titin which explains the diff. in their resting tensions and the inability of cardiac muscle to be extended to long lengths
- Cardiac muscle >> stiffer than Skeletal muscle
distance vs time graphs of isotonic contraction
- slope = shortening velocity
- 0 load = max shortening velocity
the greater the load, the slower shortening velocity
real cardiac muscle uses isometric or isotonic contraction?
it uses NEITHER
the load (mass) that stretches a resting muscle before it is stimulated
can be viewed as an unsupported weight that stretches a resting muscle before it is stimulated
load encountered after the muscle is stimulated and starts to contract
preload and afterload as it relates to the heart
- - Venous return to the heart during diastole dilates the ventricles
- - causes the passive elastic components of the muscle (titin) to stretch
- - This passive force upon the muscle fibers bf ventricular contraction is the preload
- During systole
, the arterial and pulmonary blood pressures determine the afterload
upon the ventricular wall tension
Afterloaded Isotonic Contractions
contractions with time varying combination of both isometric and isotonic contractions
what can be measured in the heart as an organ?
- 1. BP = Blood Pressure
- - determines afterload
- - Systolic & Diastolic values [~120/80 mmHg]
- 2. HR = Heart Rate
- [60-80 bpm]
- 3. Left Ventricular Chamber Volumes
- - EDV = End Diastolic Volume [125-150 ml]
- (determines preload)
–ESV = End Systolic Volume [65-80 ml]
what component of the heart regulates
pre- and afterloads during the cardiac cycle?
heart valves which open and close
Law of LaPlace
- correlates the mech. info. from the heart cells to chamber pressures and
- The Law of LaPlace correlates the information that the whole heart and cell loop plots are related, but not equal due to the complicated and time varying
- geometry of the heart
Law of LaPlace Equation
T = Pr/2h or P = 2Th/r
- T = Wall Tension
- P = Ventricular Pressure
- r = radius of chamber
- h = wall thickness
Why is LV wall of heart thicker than RV wall?
Pressure in the left ventricle is higher than right.
the thicker walls helps to compensate for this increase in pressure @ the LV
application of LaPlace Law
Law of LaPlace as it relates to Heart Plasticity
Persistent higher P (pressure) leads to higher T (wall tension) that is first normalized by increases mostly in h (wall thickness).
Note: The heart will always try to keep wall tension (T) normalized.
This is compensated pathologic hypertrophy
Eventually, the myocytes can’t meet the demand and the heart fails.
- Some myocytes are replaced by fibrosis reducing T, P & h while r increases. This is
- decompensated, dilated hypertrophy & heart failure.
Muscle cells/strips and the whole
heart have analogous , but unequal mechanical parameters
Length-Tension relationships ≈ Pressure-Volume Loops
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