25.Mechanical properties of the heart; Starling's law

Question 1

What should be mentioned?

Answer 1

• Anatomy of the heart
• Histology of the heart
• Elements of contraction
• Cardiac Muscle
• Major Characteristics of Cardiac Muscle
• Properties of a single working fiber
• SEC, PEC
• Types of contraction
• Contraction
• Collagen
• (Frank-)Starling experiments
• Volume fractions
• Starling’s heart-lung preparation
• Starling’s experiments
• Role of Starling’s law
• ”Heterometric autoregulation”

Question 2

Anatomy of the heart

(picture)

Answer 2

Question 3

Histology of the heart

Answer 3

Heart of the mammalian animals consist of 4 layers (from inside out):

• Endocardium (endothelial layer) = heart valves
• Myocardium =heart muscle cells
• Epicardium (directly on the outer side of the heart)
• Pericardium
• a sac containing the heart, that is filled with pericardial fluid.

Question 4

Myocardium

Answer 4

Contractile components (CC)
-Heart muscle fibers (working fibers)
The working fibers are able to stretch, under normal physical activity, they only work at their minimum contraction capacity.

Non-contractile components
− Serially attached elastic elements (SEC)
− Parallelly attached elastic elements (PEC) − Collagen

Question 5

Elements of contraction

Answer 5

1. Mechanical activity of the heart need the contractile components and the elastic components together.
   - working fibers (”single cells” syntitium) and the Serial Elastic (SEC) and Parallel Elastic Components (PEC).
2. The elastic elements passively store energy while stretched, which will be used as energy during the next contraction.
   - SEC is stretched during systole, while PEC is during diastole.
3. A collagen fiber system prevents from overexpansion and rupture.
   - Does not exist in skeletal muscle since it has only SEC, PEC

Question 6

Systole and diastole:

Answer 6

Systole occurs when the heart contracts to pump blood out, and diastole occurs when the heart relaxes after contraction.

Question 7

Major Characteristics of Cardiac Muscle

Answer 7

Question 8

Properties of a single working fiber

Answer 8

Skeletal muscle

within a broad range of sarcomeric length (1.9-2.6 μm), similar and maximal contraction can be found. On the other hand, contraction of cardiac muscle fibers depend on the actual length of the sarcomeres.

Heart muscle
the availability of Ca++ is depend upon the length of the fiber (sarcomere). Heart muscle shows maximal tension only at increased sarcomeric length. Working fibers of the heart possess a stretch dependent reserve.

At very short sarcomeric lengths both types of muscles perform less, since the optimum actin/myosin constellation is distorted.

At very large sarcomeric lengths only few (or no) myosin heads are opposed by active actin binding sites: therefore the performance is small in both types of muscles.

Question 9

SEC,PEC

(picture)

Answer 9

• Serially attached elastic fibers (SEC)
• parallelly attached elastic fibers (PEC)
• passively supporting the filling of the heart

Question 10

Major Characteristics of Cardiac Muscle

Answer 10

Cardiac Muscle (Contrasts with Skeletal Muscle):

• Striated organized by sarcomeres
• Cells shorter than skeletal muscle .
• Very red
• More mitochondria
• Intercellular collagen network holds the cells together (mechanical protection, and may store energy as well)

-Cells continue to divide after actin and myosin synthesized; but cell division stops at or about birth; no myotubes formed.

• No attachment to bone, tendons, etc.
• -except to connective tissue rings at the four valves and to the serially and parallelly attached fibers.

Question 11

Types of contraction

Answer 11

-Isotonic
contraction with constant tension

-Isometric
Tension increases without any change in length

-Auxotonic
tension and length increase simultaneously

-Preload
Muscle length is adjusted with (pre)load = starts with isometric contraction until equilibrium is reached with load, then isotonic contraction.

-Afterload
contraction begins with isotonic contraction (lifting a weight), then blocking of contraction with a load (no shortening, but isometric contraction.

Question 12

Types of contraction (picture)

Answer 12

Question 13

Contraction of heart (picture)

Answer 13

Isometric contraction (1st phase):

-At the beginning of the contraction, the weight stretches

the SEC elements only. The weight has not moved yet. Stretch is present, but no shortening is seen.

Isotonic contraction (2nd phase):

-The stretch in the SEC increases and when it gets into balance with the weight, the weight begins to move.

Shortening occurs and the stretching force remains unchanged (isotonic contraction).

Question 14

Collagen

Answer 14

At maximal stretch:
-collagen fibers are expanded and display maximal resistance and prevent rupture.

The cardiac muscle can be easily stretched until a limit, when the collagen fibers are stretched, however, a sudden resistance of the collagen will occur: no rupture.

Question 15

Role of collagen

(picture)

Answer 15

Question 16

(Frank-)Starling experiments

Answer 16

Starling, the English physiologist, confirmed that the heart can (mechanically) adapt itself to increased load even if it is fra from its neural connections.

-The cardiac muscle automatically adapts to the increased mechanical load. „law of the heart” (Starling): increased stretch results in increased contraction.

Question 17

Volume fractions

Answer 17

EDV (End Diastolic Volume)
At the end of diastole (maximal relaxation, right before the next heart contraction), the ventricles are maximally filled.

ESV (end systolic volume)
When the ventricles are maximally emptied (end of systole, right before relaxation), there is still some blood remaining in them.

Stroke Volume (SV)
This volume fraction passes into the aorta at each cycle. − EDV-ESV

Cardiac output (CO)
The volume of blood pumped into the circulation by the heart in one minute.

Heart rate times stroke volume:

-(EDV – ESV) x Frequency = CO = SV x Frequency

• Cardiac output is one of the most important physiological and clinical parameter describing the performance of the heart. It equals the amount of blood forwarded into the aorta from the left ventricle per unit time
• At the end of systole certain amount of blood still remains in the heart. That plays an important role in the adaptation of the heart to the increased stretch.

Question 18

Volume fractions (Picture)

Answer 18

Question 19

Starling’s heart-lung preparation

Answer 19

Heart can adapt to the increased load – due to mechanical reasons.

-This adaptation is observed in isolated heart as well − not related to neural and hormonal functions.

Starling’s heart-lung preparation

• The arterial side of the systemic circulation was replaced by an artificial “peripheral resistance” segment.
• The venous side was replaced by a “reservoir”.
• Neural connections are separated (=heart is denerved)

Question 20

Starling’s experiments

Answer 20

Starling performed two basic experiments in order to prove, the heart can adapt to the increased load:

1. First experiment:
   - he changed the level of the reservoir and thus simulated the changes of the venous return
2. Second experiment
   - he changed the peripheral resistance by narrowing and dilating the diameter of the rubber tube.

Question 21

Experiment 1: increasing the venous return

Answer 21

Immediate reaction: Increased EDV

Later, after a few cycle: ESV increases (but little less than EDV) → SV increases

• Finally SV (and CO) will increase − In order to deal with the extra load
• Extra load led to stronger contraction (heart rate is unchanged due to the denervation of the heart)

Question 22

Experiment 2: increased peripheral resistance

Answer 22

Immediate reaction: residual volume increases (ESV↑; SV↓)

• Later, after a few cycle: ESV and EDV ↑ proportionally = SV ↑ to the same level. The increased residuum further dilates the heart, generating increased contraction (performance). ESV and EDV proportionally increase: therefore the SV remains constant, so same amount of blood/minute can be pumped against increased peripheral resistance,
• Finally SV (and CO) will be set as it was before
• (heart is denervated)

Cardiac Output (CO) will not change! (i.e. it is set to the original level)!!

(more blood remains in the heart), so SV temporarily decreased (the heart gets enlarged)

Question 23

Properties of a single working fiber (picture 1)

Answer 23

Question 24

Properties of a single working fiber (picture 2)

Answer 24

Question 25

Properties of a single working fiber (picture 3)

Answer 25

Question 26

Role of Starling’s law

Answer 26

The heart can increase the diastolic reserve, resulting in increased stretch and consequently increased performance.

• *Role:**
• *1. Posture changes**

laying down from standing up:

• more blood gets into the ventricle, leading to dilation, resulting in increased performance (immediately).
• (later on, the nervous system sets the heart rate, so the heart is adapted to the resting conditions)

2. Heterometric autoregulation

The volume flow through the Right and Left heart should be ± the same. (If not – circulation collapses)

• Slight differences however exist, called heterometry.
• But the increased amount of blood leaving the right compartment for example, will stretch, dilate the other (left) side, therefore the Starling mechanism is activated leading to the automatic compensation between the two parts (heterometric autoregulation)