Chapter 19 Cardiovascular Heart

Anatomy of the Heart

Coverings:

• Pericardium (pericardial sac)

• Serous Fluid

Layers:

• Epicardium (visceral pericardium)

• Myocardium

• Endocardium

Chambers:

• Right Atrium

• Right Ventricle

• Left Atrium

• Left Ventricle

Valves:

• Tricuspid Valve

• Biscupid (Mitral) Valve

• Pulmonary Semilunar Valve

• Aortic Semilunar Valve

• Chordae Tendonae & Papillary Muscles

Percadium (pericardial sac)

• A tough membrane surrounding the heart 

• Cavity between the epicardium and the pericardium is filled with lubricating serous fluid

Epicardium (visceral pericardium)

• It is the outer layer of the heart

• It is the connective tissue attached to the surface of the heart

Myocardium

• The middle, and thickest, layer of the heart composed of cardiac cells

• It is the contraction of the myocardium that pumps blood through the heart and into the major arteries

Endocardium

• The innermost layer of the heart made of simple squamous epithelium

• Lines the chambers where the blood circulates and covers the heart valves

Right Atrium

•  Receives unoxygenated blood via the precava and postcava from the body

• Upper heart chamber

  • Contracts to push blood into the right ventricle

Left Atrium

•  Receives oxygenated blood from the pulmonary veins

• Upper heart chamber

  • Contracts to push blood into the left ventricle

Right Ventricles

• Pumps unoxygenated blood to the lungs

• Lower heart chamber

Left ventricle

• Pumps oxygenated blood out to the body

• Lower heart chamber

Tricuspid Valve

Between right atrium and right ventricle

Bicuspid (mitral) Valve

Between left atrium and left ventricle

Pulmonary Semilunar Valve

Between right ventricle and pulmonary trunk

Aortic Semilunar Valve

Between left ventricle and the aorta

Papillary Muscles

Muscles which aid opening and closing the valves

Chordae Tendonae

• Heart strings which connect valves to papillary muscles 

• Prevent valves from inverting during ventricle contraction

Right Side Heart Blood Flow

• Oxygen-poor blood enters the heart from the body → oxygen-poor blood leaves the heart traveling to the lungs

  • Body →

  • Pre & Post Vena Cava →

  • Right Atrium →

  • Tricuspid Valve →

  • Right Ventricle →

  • Pulmonary Semilunar Valve →

  • Pulmonary arteries →

  • Lungs

Left Side Heart Blood Flow

• Blood is oxygenated → oxygen-rich blood returns to heart from lungs → Oxygen-rich blood leaves the heart traveling to the body

  • Lungs →

  • Pulmonary Veins →

  • Left Atrium →

  • Bicuspid Valve →

  • Left Ventricle →

  • Aortic Semilunar Valve →

  • Aorta →

  • Body

Pulmonary Circuit

• Transports blood to and from the lungs

• Muscle walls on right side of heart are relatively thin because blood is traveling a short distance to the lungs, so not much force is required

• Functions to pick up oxygen from the lungs and deliver carbon dioxide to exhaled

• Right side of heart → lungs → left side (Gas exchange (O₂ in, CO₂ out)

Systemic Circuit

• Transports oxygenated blood to virtually all of the tissues of the body 

• Muscle walls on left side of heart are much thicker because blood must travel through the entire body, so much more force is required

• Returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circuit 

• Left side of heart → body → right side. (Deliver oxygen/nutrients, remove waste)
  
  

Arteries vs. Veins

Arteries

•  blood vessels that carry blood AWAY from the heart

Veins

• blood vessels that carry blood TO the heart

Coronary Arteries vs. Coronary Veins

Coronary Arteries

•  deliver blood from the aorta to the myocardium

Coronary Veins

• drain blood into the right atrium

Cardiac Muscle Cell Characteristics for Coordinated Contraction

• Intercalated Discs: Join cardiac cells; contain:

  • Desmosomes: Prevent cells from separating during contraction.

  • Gap Junctions: Enable electrical continuity (action potential spread).

• Large Mitochondria: 25% cell volume for energy (vs. 2% in skeletal).

• Fewer, wider T-tubules: Efficient ion transmission.

• Aerobic respiration only: Requires constant oxygen.

• Autorhythmic Cells (Pacemakers): Generate spontaneous action potentials.

Autorhythmic (Pacemaker) Cells

• Fire spontaneous action potentials that trigger contractions at regular intervals

• Cells are self-exciting and establish the fundamental rhythm

• Coordinate the contraction of the heart

• The autonomic nervous system and endocrine system modify the heart beat

Desmosomes

• Mechanical couplings (like rivets) hold cells together, and prevents from separating during contraction

Gap Junctions

• Small hollow cylinders (opening) that connect the cytoplasm of two cells and permits the direct transfer of an action potential from one cell to another

• They help to synchronize contractions

After an action potential generates at the sinoatrial node, what steps does it take to spread through the rest of the heart?

1. Sinoatrial (SA) Node fires (70–80 APs/min) → sets sinus rhythm

2. Signal spreads through atria via internodal pathways.

3. Arrives at Atrioventricular (AV) Node (~150 ms delay).

4. Travels down Atrioventricular Bundle (Bundle of His).

5. Splits into Right & Left Bundle Branches.

6. Moves through Purkinje Fibers → triggers ventricular contraction (bottom-up).

First step in Cardiac Excitation

• Begins at the sinoatrial (sinus) node

  • A specialized clump of autorhythmic cells

  • Located in the superior and posterior walls of the right atrium close to the entrance of the pre cava

  • Generates 70-80 action potentials per minute

  • Sets the sinus rhythm (normal electrical pattern) of the heart

Second step in Cardiac Excitation

• The action potential spreads from the SA node throughout the atria, through specialized internodal pathways, to the atrioventricular (AV) node

  • AV node is located at base of the right atrium, near the junction of the atria and ventricles

  • This takes about 150 ms for this occur, during which time the atria begin contracting and blood begins flowing into ventricles

Third step in Cardiac Excitation

The nerve impulse then spreads from the atrioventricular node to atrioventricular bundle (Bundle of His)

What are the steps in Cardiac Action Potential?

• Initiation

• Depolarization

• Plateu Phase

• Repolarization

• Refractory Period

Initiation

• An action potential is generated by the sinoatrial node & travels down the conduction system

• Action potential spreads to the working muscle fibers

• Summary:

  • SA Node triggers AP, spreads through conduction system

Depolarization

• Resting membrane potential is -80 mV for cells in the atria and -90mV for cells in the ventricles

• Voltage-gated fast sodium Na+ channels open

• Results a rapid depolarization to approximately +30mV

• Within a few milliseconds the fast sodium Na+ channels close

• Summary:

  • Na⁺ channels open → rapid influx → membrane potential spikes to +30mV.

Plateau Phase

• Voltage-gated slow calcium Ca+ channels open in the cell membrane & sarcoplasmic reticulum

• Calcium Ca2+ flows into the cell’s cytoplasm and uncovers the myosin binding sites on actin 

• As Calcium Ca2+ enters, some potassium K+ channels open and potassium K+ leaves the cell

• A small potassium K+ out flow now balances calcium Ca+ inflow & the membrane potential plateaus

• The relatively long plate

• Summary:

  • Ca²⁺ channels open → Ca²⁺ enters → K⁺ starts to leave → charge stabilizes

Repolarization

• Voltage- gated potassium K+ channels open and potassium K+ flows rapidly out of the cell

• At the same time calcium Ca2+ channels close

• Potassium K+ leaving the cell restores the resting membrane potential

• This last approximately 75 ms

• Summary:

  • Ca²⁺ channels close, K⁺ exits quickly → restores resting potential

Refractory Period

• The absolute refractory period for cardiac is approximately 200 ms

• The relative refractory period last approximately 50 ms

• This extended period is critical, since the heart must fully contract to pump effectively

• Without extended refractory periods, premature contractions would occur and the heart would not be able to fill or effectively pump the blood

• Summary:

  • Absolute: 200ms, Relative: 50ms → prevents early contractions → allows refilling.

Relaxation Period

• Diastole

• End of a heartbeat when the ventricles start to relax, atria are starting to contract

• As ventricles relax, pressure lessens & blood back flows from the pulmonary trunk & aorta

• Aortic & pulmonary semilunar valves close to prevent backflow of blood into the heart

• Ventricles continue to relax & pressure continues to drop 

• When pressure in the ventricles drop below the atrial pressure, the atrioventricular valves open & ventricles begin to fill

• Summary:

  • Atria contract → ventricles fill.

  • AV valves open, semilunar valves closed.
    

Ventricular Contraction

• Systole

• Near the end of atrial systole, the AP from the atrioventricular node causes depolarization in the ventricles

• Begins form the bottom up

• Blood is pushed against the atrioventricular valves closing them 

• For about 50 milliseconds, all the heart valves are closed 

• Muscles continue to contract increasing the blood pressure inside the ventricles

• Semilunar valves open when the blood pressure in ventricles exceeds the blood pressure in the pulmonary trunk & aortic arch 

• Blood is pushed out of the heart & continues until the ventricles start to relax

• Summary:

  • Ventricles contract from base upward.

  • AV valves close, semilunar valves open.

  • Blood ejected into pulmonary trunk & aorta.
    

Cardiac Output

• Total amount of blood moved among a period of time, depends upon heart rate & stroke volume

• Will initially stabilize with the increasing heart rate & compenstates for decreasing stroke volume

• At very high rates, the output will decrease as increasing heart rate is no longer able to compensate for the decreasing stroke volume 

Stroke Volume

• how much blood is actually pumped

• Initially, physiological conditions that cause heart rate to increase also triggers an increase in stroke volume

• As heart rate rises, less time is spent in diastole & so there is less time for the ventricles to fill

• Stroke volume initially remains high, but as heart rate increases stroke volume gradually decreases due to decreased filing time

Factors that Affect Heart Rate

• Hormones (Epinephrine, norepinephrine, & thyroid hormone)

• Calcium ions

• Caffeine

• Nicotine

• Age

• Gender

• Physical condition

• Bodhy Temperature

Hormones

• Epinephrine, norepinephrine, & thyroid hormone)

• These hormones increase cardiac rate & contractility

• ↑ HR & contractility (via sympathetic nerves).

Calcium Ions

• as ion levels increases, so do heart rate & contractility

• ↑ HR and contraction strength

Caffeine

• works by increasing the rates of depolarization at the SA node

• ↑ HR by increasing SA node depolarization and sympathetic activity

Nicotine

• stimulates the activity of the sympathetic neurons that deliver impulses to the heart

• ↑ HR by increasing SA node depolarization and sympathetic activity

Age

• Heart rate increases w/ age

• The ability to generate maximum heart rates decreases w/ age

• ↑ HR in infants, ↓ max HR as age increases.

Gender

• Average adult male heart rate is between 70-72 bpm

• Average adult female is between 78-82 bpm

• Difference is largely due to size of the heart

• Females heart is smaller than males

• Females: slightly higher resting HR due to smaller heart size.

Physical Condition (health)

• The heart is like a muscle, it becomes stronger as a result of exercise

• Resting heart rate of those in good physical condition is slower, because less effort is needed to pump blood

• ↓ Resting HR; more efficient stroke volume.

Body Temperature

• Increased body temp increases heart rate

• Decreased body temp decreases heart rate

• ↑ temp = ↑ HR; ↓ temp = ↓ HR.

Chemical recations that decrease Heart Rate

• Thyroid Hormones (lower than average)

• Estrogen (higher than average)

• Alcohol (don't mix w/ medications, which both slow down heart rate)