L4.3.3: Cardiac Output

• CARDIAC OUTPUT

  ● Cardiac Output (CO)

  ○ The amount of blood pumped by each side of ventricle of the heart in 1 minute

  ● Stroke Volume (SV)

  ○ Volume of blood pumped by each ventricle in one contraction (each heartbeat)

  ○ About 70 ml of blood is pumped out of the left ventricle with each heartbeat

  ● Heart Rate (HR)

  ○ Typically 75 beats per minute
• CARDIAC OUTPUT

  ● Relationship of the 3:

  ○ CO = HR x SV

  ○ CO = HR (75 bpm) x SV (70 ml/beat)

  ○ CO = 5250 ml/min = 5.25 L/min
• REGULATION OF STROKE VOLUME

  ● 60% of blood in ventricles (about 70 ml) is pumped with each heartbeat

  ● FRANK-STARLING LAW OF THE HEART — 3 factors influence stroke volume: preload, contractility, and afterload
• FRANK-STARLING LAW OF THE HEART

  ○ Preload

  ■ The critical factor controlling stroke volume is how much the cardiac muscle is stretched

  ■ The more the cardiac muscle is stretched, the stronger the contraction and the higher the stroke volume

  ■ Influenced by venous return (blood returning to the heart) and length of time the ventricles are relaxed and filling (determined by heart rate)
• FRANK-STARLING LAW OF THE HEART

  ○ Contractility

  ■ Ability of cardiac muscle to generate tension

  ■ Influenced by factors other than stretch; amount of calcium ions present, sympathetic nervous stimulation, and hormones like epinephrine and thyroxine

  ■ As contractility increases, stroke volume increases
• FRANK-STARLING LAW OF THE HEART

  ○ Afterload

  ■ Amount of pressure the ventricles must overcome to eject blood

  ■ Determined by the blood pressure in the pulmonary trunk and aorta pressing back on the semilunar valves

  ■ If afterload increases, stroke volume decreases because it gets difficult for the ventricles to pump blood
• FACTORS MODIFYING BASIC HEART RATE

  ● Neural (ANS) controls

  ● Hormones and ions

  ● Physical factors
• ● Neural (ANS) controls

  ○ Sympathetic nervous system speeds heart rate (during times of physical or emotional stress)

  ○ Parasympathetic nervous system, primarily vagus nerve fibers, slow and steady the heart rate
• ● Hormones and ions

  ○ Epinephrine and thyroxine speed heart rate

  ○ Excess or lack of calcium, sodium, and potassium ions also modify heart activity
• ● Physical factors

  ○ Age, gender, exercise, body temperature