Physiology of cardiac system | NPTE-PTA Cardiovascular system

Anatomy

The heart is a muscle that pumps blood throughout the body’s systems. The left side of the heart pumps oxygenated blood from the lungs to all body systems via the arterial supply. After exchanging oxygen for carbon dioxide and wastes in the tissue capillaries, blood returns to the right side of the heart through the veins. The right side then pumps deoxygenated blood into the lungs, where carbon dioxide (CO2) is released, and oxygen is absorbed. This cycle repeats with each heartbeat.

The heart consists of:

External walls
Blood-filled chambers
Valves that allow blood to flow through the chambers
Blood vessels that carry blood to the other structures
Electrical signaling conduction system

The three layers of the heart wall are:

Endocardium: innermost layer
Myocardium: muscular middle layer
Epicardium: outermost layer

Chambers of the heart:

Right atrium
Right ventricle
Left atrium
Left ventricle

Valves of the heart:

Atrioventricular valves:
- Tricuspid valve: lies between the right atrium and the right ventricle
- Mitral valve: lies between the left atrium and left ventricle
Semilunar valves:
- Aortic valve: oxygenated blood flows from the left ventricle to the aorta
- Pulmonary valve: deoxygenated blood flows from the right ventricle to the pulmonary artery

Blood vessels:

Arteries: carry blood away from the heart (oxygenated in the systemic circuit; the pulmonary arteries carry deoxygenated blood)
Veins: carry blood toward the heart (deoxygenated in the systemic circuit; the pulmonary veins carry oxygenated blood)
Capillaries: smallest blood vessels, where oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the surrounding tissues
Coronary arteries: branch off the base of the aorta and supply oxygenated blood to the heart muscle (myocardium) itself

Blood flow through the heart: step-by-step

Deoxygenated blood returns to the heart:
- Blood low in oxygen returns from the body to the heart through two major veins:
  - Superior vena cava (SVC): returns blood from the head, chest, arms, and neck
  - Inferior vena cava (IVC): returns blood from the legs, feet, and abdomen
- Both veins empty into the right atrium.
Right atrium
- Deoxygenated blood enters the right atrium, the upper right chamber of the heart.
- Blood then flows through the tricuspid valve into the right ventricle.
Right ventricle
- The right ventricle pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary arteries.
Pulmonary arteries to the lungs
- The pulmonary arteries carry deoxygenated blood from the heart to the lungs.
  - This is an exception to the general rule, since arteries usually carry oxygenated blood away from the heart in the systemic circuit.
Lungs oxygenate the blood
- In the lungs, carbon dioxide (CO₂) is removed and oxygen (O₂) is added to the blood through gas exchange at the alveoli.
- The blood is now oxygenated.
Left atrium
- Oxygen-rich blood returns to the heart through four pulmonary veins and enters the left atrium, the upper left chamber of the heart.
  - This is another exception to the general rule, since veins usually carry deoxygenated blood toward the heart in the systemic circuit.
- Blood then passes through the mitral (bicuspid) valve into the left ventricle.
Left ventricle
- The left ventricle pumps oxygenated blood through the aortic semilunar valve into the aorta.
- The left ventricle has the thickest myocardium because it must generate enough force to pump blood throughout the entire body.
Aorta to the body
- The aorta distributes oxygen-rich blood to the systemic circulation, supplying tissues and organs throughout the body.

Electrical signals in the heart

Electrical signals within the heart act as a pacemaker to the heart by assisting with controlling heart rate, coordinating the heart chambers (atria and ventricles), adapting to the changing needs of the body, and ensuring appropriate circulation. The normal conduction pathway follows this order: SA node → AV node → bundle of His → bundle branches → Purkinje fibers.

Definitions

Sinoatrial node: The sinoatrial (SA) node is the heart’s natural pacemaker. It initiates the electrical impulse that triggers atrial contraction, setting the normal heart rate and rhythm.
Atrioventricular node: The atrioventricular (AV) node receives the impulse from the SA node and briefly delays it, allowing the ventricles to fill with blood before passing the impulse on to the bundle of His.
Bundle of His: A specialized group of conductive cardiac fibers that carries electrical impulses from the AV node to the right and left bundle branches, helping coordinate ventricular depolarization and contraction.
Purkinje fibers: Specialized conductive fibers located within the ventricular walls that rapidly transmit electrical impulses throughout the ventricles, producing coordinated contraction of the right and left ventricles.

Heart rate isn’t fixed - the autonomic nervous system continuously adjusts it. Sympathetic stimulation increases heart rate and contractility (the force of each contraction), while parasympathetic (vagal) stimulation slows the SA node and lowers heart rate. This balance is what allows cardiac output to rise during exercise, and it’s also why beta-blockers, which blunt sympathetic input, blunt the normal heart rate response. The Cardiac exercise testing chapter covers normal and abnormal heart rate responses to activity in more detail.

Crucial factors of cardiac function

The amount of blood pumped with each heartbeat is crucial to maintaining the vitality of the lungs and the peripheral system. The principles that are important in this process are preload and afterload. If preload or afterload is abnormal - for example, too little blood filling the ventricles or too much resistance to ejection - stroke volume and cardiac output fall; over time, chronic volume overload (excess preload) or pressure overload (excess afterload, as in longstanding hypertension) can lead to heart failure.

Definitions

Preload: The amount of ventricular stretch at the end of diastole, primarily determined by the volume of blood filling the ventricles (end-diastolic volume)
Afterload: The resistance or pressure the ventricles must overcome to eject blood during systole.
Stroke volume: Amount of blood ejected by a ventricle with each contraction.
Cardiac output: Heart rate multiplied by stroke volume to determine how much blood is pumped from the ventricles in liters per minute (normally about 4-8 L/min at rest in a healthy adult)
Diastole: Relaxation of the heart chambers, allowing them to fill with blood; the term usually refers to ventricular relaxation.
Systole: Contraction of the heart chambers; the term usually refers to ventricular contraction.

Example: Calculating cardiac output

A patient’s heart rate is $70$ bpm, and their stroke volume is $70$ mL. What is their cardiac output, in L/min?

Convert stroke volume from mL to L: $70 mL = 0.07 L$

Multiply heart rate by stroke volume: $70 \times 0.07 = 4.9$

Answer: Cardiac output ≈ $4.9$ L/min, which falls within the normal resting range of $4$ - $8$ L/min.

Clinical link: During activity, the PTA monitors heart rate, blood pressure, oxygen saturation (SpO2), and perceived exertion (RPE), since these reflect cardiac output, afterload, and how well the lungs are oxygenating the blood. A normal response is a rise in heart rate and systolic blood pressure as workload increases, with diastolic blood pressure and SpO2 staying fairly stable. An abnormal response - heart rate or systolic blood pressure that fails to rise or drops with increasing workload, a drop in SpO2, or new chest pain, dyspnea, or dizziness - means the PTA stops the activity and reports to the supervising PT rather than changing the plan of care. For example, a patient with heart failure has a reduced stroke volume, so their heart rate may rise disproportionately during gait training just to maintain cardiac output. Specific thresholds for normal and abnormal exercise responses are covered in the Cardiac exercise testing chapter.

Interventions for other systems that strain the heart and lungs

Interventions aimed at muscles, joints, nerves, or skin can still change preload, afterload, or breathing. Monitor vital signs during them, and stop and report an abnormal response to the supervising PT:

Breath-holding (Valsalva maneuver) during strengthening or lifting: raises intrathoracic pressure and exaggerates the blood pressure rise. Cue the patient to exhale during the exertion phase.
Aquatic immersion: water pressure shifts blood from the legs into the chest, raising preload and cardiac workload. Decompensated heart failure is a contraindication to immersion (see aquatic therapy).
Whole-body or large-area heat (e.g., a warm whirlpool): peripheral vasodilation can lower blood pressure and raise heart rate, so dizziness is most likely when the patient stands up afterward (see Modalities).
Head-down (Trendelenburg) positioning: increases venous return and pushes the abdominal contents up against the diaphragm, which a patient with heart failure or dyspnea may not tolerate (see the postural drainage precautions in Interventions for pulmonary system).
Intermittent pneumatic compression: shifts fluid toward the heart, so it is contraindicated in uncompensated heart failure and with an acute DVT.
Exercising or mobilizing a limb with a suspected, untreated DVT: part of the clot can break loose and travel to the lungs as a pulmonary embolism. Do not start activity; alert the medical team (see Cardiac pathologies).

Heart wall layers & chambers

Three wall layers: endocardium (inner), myocardium (muscular middle), epicardium (outer)
Four chambers: right atrium, right ventricle, left atrium, left ventricle
Left side pumps oxygenated blood to body; right side pumps deoxygenated blood to lungs

Valves

AV valves: tricuspid (right atrium→right ventricle), mitral/bicuspid (left atrium→left ventricle)
Semilunar valves: pulmonary (right ventricle→pulmonary artery), aortic (left ventricle→aorta)

Blood vessels

Arteries carry blood away from heart (usually oxygenated, except pulmonary arteries = deoxygenated)
Veins carry blood toward heart (usually deoxygenated, except pulmonary veins = oxygenated)
Capillaries: site of gas/nutrient/waste exchange
Coronary arteries branch off aorta to supply myocardium itself

Blood flow pathway

SVC/IVC → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary arteries → lungs (gas exchange) → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body
Left ventricle has thickest myocardium (pumps blood to entire body)

Electrical conduction system

Pathway: SA node → AV node → bundle of His → bundle branches → Purkinje fibers
SA node = natural pacemaker, sets heart rate/rhythm
AV node delays impulse to allow ventricular filling before contraction
Bundle of His/Purkinje fibers coordinate ventricular depolarization/contraction
Sympathetic stimulation ↑ heart rate/contractility; parasympathetic (vagal) ↓ heart rate; beta-blockers blunt sympathetic response

Cardiac function principles

Preload = ventricular stretch at end of diastole (end-diastolic volume)
Afterload = resistance ventricle must overcome during systole
Stroke volume = blood ejected per contraction; Cardiac output = heart rate × stroke volume (normal ~4-8 L/min at rest)
Diastole = chamber relaxation/filling; systole = chamber contraction
Abnormal preload/afterload → ↓ stroke volume/cardiac output → chronic overload can cause heart failure
Calculation example: HR 70 bpm × SV 0.07 L = CO ≈4.9 L/min (normal range)

Clinical monitoring (PTA)

Monitor HR, BP, SpO2, RPE during activity to assess cardiac output and oxygenation
Normal response: HR and systolic BP rise with workload; diastolic BP and SpO2 stable
Abnormal response (HR/systolic BP fails to rise or drops, SpO2 drops, chest pain/dyspnea/dizziness): stop activity, report to supervising PT
Heart failure patients may show disproportionate HR rise due to reduced stroke volume

Interventions for other systems that strain the heart and lungs

Valsalva during strengthening/lifting: exaggerated BP rise—cue exhaling on exertion
Aquatic immersion: shifts blood centrally (↑ preload/workload); decompensated heart failure contraindicates it
Whole-body heat (warm whirlpool): BP drop, HR rise, dizziness on standing
Trendelenburg: ↑ venous return, abdominal contents push on diaphragm—poorly tolerated in heart failure/dyspnea
Intermittent pneumatic compression: contraindicated in uncompensated heart failure and acute DVT
Suspected untreated DVT: do not mobilize (risk of pulmonary embolism); alert the medical team

Anatomy

The heart consists of:

External walls
Blood-filled chambers
Valves that allow blood to flow through the chambers
Blood vessels that carry blood to the other structures
Electrical signaling conduction system

The three layers of the heart wall are:

Endocardium: innermost layer
Myocardium: muscular middle layer
Epicardium: outermost layer

Chambers of the heart:

Right atrium
Right ventricle
Left atrium
Left ventricle

Valves of the heart:

Atrioventricular valves:
- Tricuspid valve: lies between the right atrium and the right ventricle
- Mitral valve: lies between the left atrium and left ventricle
Semilunar valves:
- Aortic valve: oxygenated blood flows from the left ventricle to the aorta
- Pulmonary valve: deoxygenated blood flows from the right ventricle to the pulmonary artery

Blood vessels:

Arteries: carry blood away from the heart (oxygenated in the systemic circuit; the pulmonary arteries carry deoxygenated blood)
Veins: carry blood toward the heart (deoxygenated in the systemic circuit; the pulmonary veins carry oxygenated blood)
Capillaries: smallest blood vessels, where oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the surrounding tissues
Coronary arteries: branch off the base of the aorta and supply oxygenated blood to the heart muscle (myocardium) itself

Blood flow through the heart: step-by-step

Deoxygenated blood returns to the heart:
- Blood low in oxygen returns from the body to the heart through two major veins:
  - Superior vena cava (SVC): returns blood from the head, chest, arms, and neck
  - Inferior vena cava (IVC): returns blood from the legs, feet, and abdomen
- Both veins empty into the right atrium.
Right atrium
- Deoxygenated blood enters the right atrium, the upper right chamber of the heart.
- Blood then flows through the tricuspid valve into the right ventricle.
Right ventricle
- The right ventricle pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary arteries.
Pulmonary arteries to the lungs
- The pulmonary arteries carry deoxygenated blood from the heart to the lungs.
  - This is an exception to the general rule, since arteries usually carry oxygenated blood away from the heart in the systemic circuit.
Lungs oxygenate the blood
- In the lungs, carbon dioxide (CO₂) is removed and oxygen (O₂) is added to the blood through gas exchange at the alveoli.
- The blood is now oxygenated.
Left atrium
- Oxygen-rich blood returns to the heart through four pulmonary veins and enters the left atrium, the upper left chamber of the heart.
  - This is another exception to the general rule, since veins usually carry deoxygenated blood toward the heart in the systemic circuit.
- Blood then passes through the mitral (bicuspid) valve into the left ventricle.
Left ventricle
- The left ventricle pumps oxygenated blood through the aortic semilunar valve into the aorta.
- The left ventricle has the thickest myocardium because it must generate enough force to pump blood throughout the entire body.
Aorta to the body
- The aorta distributes oxygen-rich blood to the systemic circulation, supplying tissues and organs throughout the body.

Electrical signals in the heart

Definitions

Sinoatrial node: The sinoatrial (SA) node is the heart’s natural pacemaker. It initiates the electrical impulse that triggers atrial contraction, setting the normal heart rate and rhythm.
Atrioventricular node: The atrioventricular (AV) node receives the impulse from the SA node and briefly delays it, allowing the ventricles to fill with blood before passing the impulse on to the bundle of His.
Bundle of His: A specialized group of conductive cardiac fibers that carries electrical impulses from the AV node to the right and left bundle branches, helping coordinate ventricular depolarization and contraction.
Purkinje fibers: Specialized conductive fibers located within the ventricular walls that rapidly transmit electrical impulses throughout the ventricles, producing coordinated contraction of the right and left ventricles.

Crucial factors of cardiac function

Definitions

Preload: The amount of ventricular stretch at the end of diastole, primarily determined by the volume of blood filling the ventricles (end-diastolic volume)
Afterload: The resistance or pressure the ventricles must overcome to eject blood during systole.
Stroke volume: Amount of blood ejected by a ventricle with each contraction.
Cardiac output: Heart rate multiplied by stroke volume to determine how much blood is pumped from the ventricles in liters per minute (normally about 4-8 L/min at rest in a healthy adult)
Diastole: Relaxation of the heart chambers, allowing them to fill with blood; the term usually refers to ventricular relaxation.
Systole: Contraction of the heart chambers; the term usually refers to ventricular contraction.

Example: Calculating cardiac output

A patient’s heart rate is $70$ bpm, and their stroke volume is $70$ mL. What is their cardiac output, in L/min?

Convert stroke volume from mL to L: $70 mL = 0.07 L$

Multiply heart rate by stroke volume: $70 \times 0.07 = 4.9$

Answer: Cardiac output ≈ $4.9$ L/min, which falls within the normal resting range of $4$ - $8$ L/min.

Interventions for other systems that strain the heart and lungs

Breath-holding (Valsalva maneuver) during strengthening or lifting: raises intrathoracic pressure and exaggerates the blood pressure rise. Cue the patient to exhale during the exertion phase.
Aquatic immersion: water pressure shifts blood from the legs into the chest, raising preload and cardiac workload. Decompensated heart failure is a contraindication to immersion (see aquatic therapy).
Whole-body or large-area heat (e.g., a warm whirlpool): peripheral vasodilation can lower blood pressure and raise heart rate, so dizziness is most likely when the patient stands up afterward (see Modalities).
Head-down (Trendelenburg) positioning: increases venous return and pushes the abdominal contents up against the diaphragm, which a patient with heart failure or dyspnea may not tolerate (see the postural drainage precautions in Interventions for pulmonary system).
Intermittent pneumatic compression: shifts fluid toward the heart, so it is contraindicated in uncompensated heart failure and with an acute DVT.
Exercising or mobilizing a limb with a suspected, untreated DVT: part of the clot can break loose and travel to the lungs as a pulmonary embolism. Do not start activity; alert the medical team (see Cardiac pathologies).

Heart wall layers & chambers

Three wall layers: endocardium (inner), myocardium (muscular middle), epicardium (outer)
Four chambers: right atrium, right ventricle, left atrium, left ventricle
Left side pumps oxygenated blood to body; right side pumps deoxygenated blood to lungs

Valves

AV valves: tricuspid (right atrium→right ventricle), mitral/bicuspid (left atrium→left ventricle)
Semilunar valves: pulmonary (right ventricle→pulmonary artery), aortic (left ventricle→aorta)

Blood vessels

Arteries carry blood away from heart (usually oxygenated, except pulmonary arteries = deoxygenated)
Veins carry blood toward heart (usually deoxygenated, except pulmonary veins = oxygenated)
Capillaries: site of gas/nutrient/waste exchange
Coronary arteries branch off aorta to supply myocardium itself

Blood flow pathway

SVC/IVC → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary arteries → lungs (gas exchange) → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body
Left ventricle has thickest myocardium (pumps blood to entire body)

Electrical conduction system

Pathway: SA node → AV node → bundle of His → bundle branches → Purkinje fibers
SA node = natural pacemaker, sets heart rate/rhythm
AV node delays impulse to allow ventricular filling before contraction
Bundle of His/Purkinje fibers coordinate ventricular depolarization/contraction
Sympathetic stimulation ↑ heart rate/contractility; parasympathetic (vagal) ↓ heart rate; beta-blockers blunt sympathetic response

Cardiac function principles

Preload = ventricular stretch at end of diastole (end-diastolic volume)
Afterload = resistance ventricle must overcome during systole
Stroke volume = blood ejected per contraction; Cardiac output = heart rate × stroke volume (normal ~4-8 L/min at rest)
Diastole = chamber relaxation/filling; systole = chamber contraction
Abnormal preload/afterload → ↓ stroke volume/cardiac output → chronic overload can cause heart failure
Calculation example: HR 70 bpm × SV 0.07 L = CO ≈4.9 L/min (normal range)

Clinical monitoring (PTA)

Monitor HR, BP, SpO2, RPE during activity to assess cardiac output and oxygenation
Normal response: HR and systolic BP rise with workload; diastolic BP and SpO2 stable
Abnormal response (HR/systolic BP fails to rise or drops, SpO2 drops, chest pain/dyspnea/dizziness): stop activity, report to supervising PT
Heart failure patients may show disproportionate HR rise due to reduced stroke volume

Interventions for other systems that strain the heart and lungs

Valsalva during strengthening/lifting: exaggerated BP rise—cue exhaling on exertion
Aquatic immersion: shifts blood centrally (↑ preload/workload); decompensated heart failure contraindicates it
Whole-body heat (warm whirlpool): BP drop, HR rise, dizziness on standing
Trendelenburg: ↑ venous return, abdominal contents push on diaphragm—poorly tolerated in heart failure/dyspnea
Intermittent pneumatic compression: contraindicated in uncompensated heart failure and acute DVT
Suspected untreated DVT: do not mobilize (risk of pulmonary embolism); alert the medical team