Heart FunctionLearning 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 contractionIncrease in size of heart as a result of training

Route of Blood through the Heart

• http://www.youtube.com/watch?v=VUtehbgbpRk&feature=related

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 Contractshow 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 RateWhen 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.