The Heartbeat

Question 1

Cardiac Cycle

Answer 1

• cardiac cycle refers to a complete heartbeat - a contraction and relaxation of each chamber of the heart
• a cardiac cycle takes about 0.8s
• the cardiac cycle is usually divided into 2 main phases:
1. Diastole - the period of relaxation and filling of the heart with blood
2. Systole - the period of contraction and emptying of the heart

Question 2

5 Stages of the Cardiac Cycle

Answer 2

1. Heart is fuller relaxed; atria begin to fill with blood; atrioventricular (AV) and semilunar (SL) valves are closed.
2. Blood fills atria and pushes AV valves open; ventricles begin to fill.
3. Atria contract, filling ventricles completely.
4. Ventricles begin to contract forcing AV valves closed; SL valves remain closed.
5. Ventricles contract fully; forcing SL valves open and ejecting blood into arteries.

Diastole begins when the relaxed atria begin to fill with blood (Stage 1).

Systole begins when the ventricles begin to contract (Stage 4).

Question 3

Conducting System of the Heart

Answer 3

Main “Players”
• SA nose
• AV node
• Bundle of His
• Left/Right Bundle Branch
• Purkinje Fibers

Question 4

Ventricular Contraction: Atrioventricular Node (AV Node)

Answer 4

• catches electrical signal from a trial muscles
• sends the signal to the bundle of His
• electrical pathway is divided into left and right bundle branches
• signal will travel down until it reaches the Purkinje fiber
• Purkinje fibers make contact with muscles in ventricle
• causes contraction of ventricles from the bottom up (like a tube of toothpaste)

Question 5

The Heart

Answer 5

• muscular organ located in the middle of the chest
• size of a clenched fist
• beats over 100 000 times per day, 37 million times per year, and more than 2.5 billion times in lifetime

Question 6

Pericardium

Answer 6

• two-layered connective tissue
• surrounds the heart
• has fluid between the layers
• protects the heart from friction with other tissues and organs in the thoracic (chest cavity) as the heart beats

Question 7

Heart Sounds

Answer 7

• with each heartbeat, the valves open or close to direct blood flow
• heart sounds can be heard using a stethiscope
• the “lubb-dubb” sound of the heartbeat is caused by the closing of the heart valves
- “Lubb” is the atrioventricular valves closing as the ventricles begin to contract
- “Dubb” is the semilunar valves snapping shut

Question 8

Control of Heart Rate

Answer 8

• heart muscle is myogenic - has a natural rate of contraction
• this rate changes in response to signals from the medulla oblongata (brainstem)
• the send signals through autonomous nervous system
- sympathetic nerves: increase HR, breathing, etc.
- parasympathetic nerves: decrease HR, breathing, etc.

Question 9

Atrial Contraction: Sinoatrial Node (SA Node)

Answer 9

• the pacemaker
• the beginning of the conducting system
• receives nervous signals from brain (sympathetic/parasympathetic nervous system)
- only to control rate of contraction (BPM)
• depolarizes atrial muscles
• the depolarization sends waves of electrical signals to all atrial muscles (L and R)
- causes contraction to both atria

Question 10

ECG (Electrocardiogram)

Answer 10

• abnormalities can be identified by comparing with a normal ECG pattern
• common heart patterns:
- tachycardia 
- brachycardia
- irregular heartbeat 
- ventrical fibrilation