Cardio physiology 2 - Heart valves, conduction

How many heart valves are there

4 - 2 AV valves + 2 Semilunar valves

Bicuspid valve

valve between the left atrium and the left ventricle.

Tricuspid valve

valve between the right atrium and the right ventricle

Semilunar (arterial) valves

found between the ventricles and the arteries into which the ventricles pump their blood

Aortic valve

Semilunar valve separating the left ventricle and the aorta

Pulmonary valve

Semilunar valve separating the right ventricle and the pulmonary trunk

What are the valves made of

Collagen covered by endothelium

Valve rings

Cartilage that valves attach to

Blood flow through the valves

· Poorly oxygenated blood returns to right atrium.
· Right AV valvue - blood flows to the atrium
· Blood moved from right ventricle to pulmonary trunk via pulmonary valve
· Oxygenated blood returns to the left atrium - blood pushed through tricuspid valve to left ventricle
· Blood pushed from left ventricle to aorta via aortic valve

What is the function of the heart valves

Ensure unidirectional blood flow to the heart, preventing blood flowing backwards

How do valves open and close

Valves open/close passively due to difference in pressure

Is energy expended to open valves

No

How does a heart valve open

When the pressure is greater behind the valve, the valve opens

How does a valve close

When the pressure is greater in front of the valve, it closes

How does the atrioventricular valve open

When pressure in the atrium exceeds ventricle pressure, valve opens so blood flows into the ventricle

How does the atrioventricular valve close

The ventricle contracts, achieving a pressure greater than the pressure of the atrium, closing the valve

Why is it called the tricuspid valve

3 cusps/leaflets attached to the valve rings.

Why is it called the bicuspid valve

2 cusps/leaflets attached to the valve rings

AV valve apparatus

Consists of the cusps/leaflets, chordae tendineae, papillary muscles

Chordae tendineae

thin bands of fibrous tissue that attach to the valves in the heart and the papillary muscles

Papillary muscles

Cone shaped muscles that contract, pulling the chordae tendineae tight to keep the AV valve closed

AV valve apparatus function with a relaxed ventricle

AV valve is open, and the semilunar valve is closed. Papillary muscles/chordae tendineae are slack. Blood flows from the atrium to the ventricle

AV valve apparatus function with a contracted ventricle

Ventricle squeezes, increasing pressure. Blood is pushed back onto AV valve, closing it. Papillary muscle contract, pulling chordae tendineae tight. Ventricular pressure will increase, causing the semilunar valve to open

How many leaflets do the semilunar valves have

3

Difference between the AV valves and the semilunar valves

No chordae tendineae or papillary muscles - pressure pushing back is not enough to force the valve open

Cardiac skeleton

Dense connective tissue that lies between the heart valves and is makes up the heart valve rings

Functions of the cardiac skeleton

- Physically separates the atria from the ventricles
- Electrically inactive - blocks direct spread of electrical impulses from the atria to the ventricles
- Supports the heart - attachment for the valves, leaflets, cardiac muscle

Coronary circulation

Systemic circulatory system that supplies blood to and provides drainage from heart tissues

Coronary arteries

Arteries that supply the heart

Aortic sinus

Outpocket of the aorta - site where left and right coronary arteries separate

Cardiac veins

Collect poorly oxygenated blood which empties into the coronary sinus, returning blood to the right atrium

Coronary sinus

Collection of veins that collect from the myocardium, returning the poorly oxygenated blood back tot he right side of the heart

Systole

Represents the time during which the left/right ventricles contract, ejecting blood into their respective artery

Diastole

represents the time when the ventricles are contacted

Is myocardial blood flow steady?

No - blood flow ceases when ventricles are contracted, and peaks while the heart is relaxed

Coronary artery disease

Caused by atherosclerosis of the coronary arteries supplying blood to the heart tissues

Atherosclerosis

Condition which the arteries are hardened and narrowed due to excessive plaque accumulation in vessel wall

What are atherosclerotic plaques made of

Fat, cholesterol, calcium

Symptoms of coronary artery disease

- Angina (chest pain due to restricted blood flow)
-Myocardial infarction (heart attack - caused by complete lack of blood flow, heart muscle dies due to loss of blood supply)

Cardiac muscle connection

Connected by desmosomes/gap junctions. So tightly connected that when one myocyte is excited, all myocytes are excited via action potential spread through gap junctions

Syncytium

A set of cells that act together

How many syncytia does heart muscle have

2 - left/right atria and left/right ventricle

Role of the the heart being a function synctium

Allows for all-or-none response

Order of heart contraction

Left and right atria contract together, then the left and right ventricles contract

What causes heart muscle to contract

Action potentials in myocytes - cause ejection of blood

Feature of heart action potentials

Myogenic (muscular) in origin - not caused by nervous/hormonal stimulation

2 types of specialized cardiac muscle cells

Contractile and Conducting cells

Contractile cells

Perform the mechanical work of pumping/contracting to propel blood forward. Do not initiate their own action potentials, but are stimulated when passed an AP via gap junctions

Conducting cells

Autorhythmic cells that initiate/conduct APs - responsible for contractile cell pumping.

Myofibrils

Protein filaments that are needed for contraction - contractile cells have lots, conducting cells have none

Distribution of heart cells

99% = contracting cells
1% = conducting cells

How are heart cells electrically contact each other

Through gap junctions

Location of the conducting myocytes

Sinoatrial node (SAN), Internodal pathways, atrioventricular node (AVN), bundle of His (AV bundle), left/right bundle branches, Purkinje fibers

SAN location

Wall of the right atrium

AVN location

Base of the right atrium

Location of the internodal pathways

Extend from the SAN to the AVN, can cross the ineterarterial septum to the left atrium

Location of the bundle of His

Passes through the cardiac skeleton

Location of the left and right bundle branches

Travel along the intraventricular septum, making contact with purkinje fibers

Location of Purkinje fibers

Extend into the myocardium of ventricles

How are the atrium and ventricles electrically connected

Via the atrioventricular node and the bundle of His

Why can't the atria stimulate the ventricles

Physically separated by the cardiac skeleton

Role of the sinoatrial node

Pacemaker of the heart - generates action potentials very quickly, which travel through gap junctions to other conducting myocytes

Role of the atrioventricular node

Pass stimuli from the atrium to the ventricle, cause AV nodal delay (slows down propagation of action potentials - takes 100 ms to take signal from AVN to bundle of His)

What is the use of the AV nodal delay

Ensures that the atria depolarize and contract before the ventricles do, allowing the ventricles to fill with blood before they contract

Left and right bundle branches

Carry the signal from the AVN down the intraventricular septum to the Purkinje fibers

Purkinje fibers

Nerve fibers throughout the ventricles that quickly conduct signals to the contracting myocytes - causes simultaneous contraction of both ventricles

Sequence of heart excitation

1. Sinoatrial node myocytes initiate action potential
2. AP passes through the conducting myocytes in the internodal pathways to the atrial contracting myocytes
3. Left and right atria depolarize and contract
4. Action potential passes through the internodal pathways to the conducting myocytes of the atrioventricular node
5. Takes 100 ms for AP to pass through the AVN
6. Stimulus travels through the AV node to the bundle of His
7. Bundle of His splits into left and right bundle branches, carries AP
8. Bundle branches carry AP to the ventricles, making contract with Purkinje fibers
9. Purkinje fibers spread depolarizing stimulus to the ventricles
10. Ventricular contracting myocytes are depolarized, ventricle contracts

Wolff-Parkinson-White Syndrome

Abnormal piece of muscle that directly connects the atria and the ventricles, causing the electrical signal to bypass the atrioventricular node (and no slowed response)

Symptoms of Wolff-Parkinson White Syndrome

Disrupts coordinated movement of electrical signals of the heart, leading to tachycardia (fast heartbeat) and other arrythmias