Feb 12 - Cardiac Output and Its Control Flashcards
What is cardiac output (CO)?
It’s the volume of blood pumped by each ventricle per minute. It is determined by two factors: heart rate and stroke volume
What is heart rate?
The number of times the heart beats per minute
The average resting heart rate is 70 beats per minute
What is stroke volume?
The volume of blood that is pumped per beat. The average resting stroke volume is 70 ml of blood per beat
How much is average cardiac output?
CO = HR x SV, therefore it equals approximately 5 L/minute
What is cardiac reserve?
It is the difference between the cardiac output at rest and at its peak (the maximum volume of blood that can be pumped by the heart per minute). During exercise the cardiac output increases to 20-25 L/min
How does the heart increase its output when demands on the body increase?
By increasing heart rate and increasing stroke volume
How does the autonomic nervous system modulate the heart rate
Sympathetic drive increases heart rate, while parasympathetic drive slows it down. In both individuals, both systems are typically active at any given time
What is the situation in which there is no direct autonomic control of the heart rate?
Heart transplant
Describe the SA node below threshold
Below threshold, SA node pacemaker cells high high potassium concentration but low sodium concentration intracellularly; a slow constant influx of sodium leads to progressive depolarization of the cell interior until threshold is hit; meanwhile, there is a low level of potassium leakage via potassium channels
What happens as resting membrane potential passes threshold?
As the resting membrane potential passes threshold, voltage-gated calcium channels open to permit the rapid influx of calcium concentration and the firing of the action potential
What happens as the cell repolarizes?
As the cell repolarizes, calcium is both sequestered internally and pumped out and potassium rapidly exits the cell to restore RMP; the Na/K ATPase restores intracellular sodium and potassium concentrations
How does sympathetic stimulation affect the heart rate?
Norepinephrine, released from sympathetic nerve endings, inactivates the potassium ion channels responsible for the outward leak, resulting in build-up of potassium (and thus positive charge) within the cell; there is also increased inward of the sodium and calcium, adding to this effect
How does sympathetic stimulation affect the AV node?
Sympathetic stimulation also reduces AV node delay, and speeds up the spread of the action potential throughout the specialized conduction pathway
How does sympathetic stimulation affect the force of contraction?
Sympathetic stimulation increases the strength of contraction of the atrial and ventricular cells since calcium influx has increased. Higher intracellular calcium concentration results in more forceful (and more rapid) contraction, i.e. there is an increase in cardiac contractility
How does parasympathetic stimulation affect the SA node?
Acetylcholine released from the parasympathetic nerve endings reduces SA node depolarization. This is achieved by slowing the closure of K ion channels, thereby enhancing K ion permeability, facilitating export of positive charge out of the cell and making the interior more negative (i.e. increasing polarity)
How does parasympathetic stimulation affect the AV node?
It increases AV node delay, and slows the spread of the action potential throughout the specialized conduction pathway
How does parasympathetic stimulation affect contractility?
Parasympathetic stimulation decreases the strength of contraction of the atrial cells by reducing calcium influx and intracellular calcium cycling. It has minimal effect on ventricles
The sympathetic and parasympathetic systems antagonize one another to control heart rate. Explain
The sympathetic nervous system controls the heart rate in emergency situations or when challenged, such as during exercise. The parasympathetic system controls the heart in relaxed situations. The activities of these two systems on the heart are in turn synchronized by the cardiovascular control centre situated in the brain stem.
What would happen if the there was no sympathetic or parasympathetic control of the heart?
In their absence, the heart rate would be around 100 bpm, indicating that at normal resting heart rate (70 bpm), parasympathetic influence prevails
How does epinephrine affect the heart rate?
The hormone epinephrine (adrenalin) is secreted into the blood by the adrenal medulla (in the adrenal glands) upon sympathetic stimulation and has similar actions to norepinephrine: an increase in heart rate and contractility
Mathematically, what is cardiac output?
CO = HR x SV
What determines stroke volume?
Venous return (intrinsic control) Sympathetic nervous system (extrinsic control)
What happens when blood returned to the heart from the venous system increases?
The more blood is returned to the heart from the venous system, more blood must be pumped out of the heart. This occurs via an increase in the end-diastolic volume (EDV), which in turn translates into a larger stroke volume (SV)
Why is EDV and SV imporant?
The relationship between the EDV and SV comprises an intrinsic control - it is inherent in the properties of cardiac muscle. This ability of the heart to adjust SV is dependent on the length-tension relationship of cardiac muscle