Download Heart Function

Heart Function
Learning Objectives:
• To understand the structure of the heart;
• To understand the relationship between cardiac output, stroke volume
and heart rate;
• To understand the short and long term effects of exercise on the heart.
Key Terms:
• Heart Rate
• Stroke Volume
• Cardiac Output
• Pulmonary
circulation
• Systemic
circulation
• Venous Return
• Starling’s Law
• Hypertrophy
Number of beats per minute
Volume of blood leaving left ventricle per beat
Volume of blood leaving left ventricle per minute
Blood circulation from heart to lungs and back to
heart
Blood circulation from heart to body and back to
heart
The volume of blood returned back to the heart
The greater the venous return, the greater the
strength of contraction
Increase in size of heart as a result of training
Route of Blood through the
Heart
• http://www.youtube.com/watch?v=VUtehbgbpRk&feature=re
lated
Watch the video and then draw a
very simple diagram of the heart,
labelling the key features and
explaining the route of blood.
How the Heart Contracts
how electrical signals initiate heart contraction
• The impulse for contraction of the heart is generated in the sino-atrial
node (SAN).
• This causes the atria to contract forcing blood down to the ventricles.
• The impulse travels to the atrio-ventricular node (AVN).
• From here it travels down the septum (through the Bundle of His) to
the tip of the ventricles.
• The impulse is carried via the Purkinje fibres to into the walls of the
ventricles.
• This causes the ventricles to contract.
Heart Contraction and Regulation
• The heartbeat is said to be ‘intrinsic’ because it comes from within
itself, and ‘myogenic’ because it occurs without nervous stimulation.
• The time in which the heart is contracting is called systole, and
relaxation is called diastole.
• Heart rate can be altered by the autonomic nervous system sending
signals to the SAN.
• The parasympathetic branch causes HR to become slower (via the
vagus nerve).
• The sympathetic branch causes HR to speed up (via the sympathetic
nerve).
• The hormone adrenaline can also cause HR to rise.
Measuring Cardiac Output
Cardiac output = heart rate x stroke volume
Average for adult male when resting is:
70bpm (HR) x 70ml (SV) = 5L (cardiac output)
During strenuous exercise this can rise to:
200bpm (HR) x 180ml (SV) = 36L (cardiac output)
Short Term Effects of Exercise on
the Heart
• Heart Rate increases;
• Venous return increases;
• Stroke Volume increases…..;
• …..because of Starling’s Law;
• Cardiac output increases…..;
• ….due to the fact that Cardiac output = HR x SV
Regulating Heart Rate
When exercising, the heart rate increases due to:
• The action of the sympathetic nerves on the SAN
• Increased carbon dioxide levels in the blood;
• Increased acidity in the blood;
• These changes are detected by chemoreceptors;
• These send nerve impulses to the medulla of the brain;
• This causes a decrease in (parasympathetic) vagus
stimulation;
• And an increase in sympathetic stimulation.
Effects of Training on the Heart
• Stroke volume increases;
• The heart experiences hypertrophy
(athlete’s heart);
• Resting heart rate decreases
(bradycardia)
Cardiovascular Drift
• An increase in heart rate that occurs during prolonged
exercise that compensates for a decrease in stroke
volume in an attempt to maintain cardiac output.
• This is caused by a reduction in fluid in the blood (due to
sweating and heat generated by muscle contraction)
• This decreases venous return (as there is less blood)
• Stroke volume therefore decreases (due to Starling’s
Law)
• Cardiac output must increase to assist against the
problems caused by sweating and overheating.