1. 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.

2. 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

5. Route of Blood through the Heart http://www.youtube.com/watch?v=VUtehbgbpRk&feature=re lated 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.

6. 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.

7. 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.

8. 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)

9. 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

10. 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.

11. Effects of Training on the Heart Stroke volume increases; The heart experiences hypertrophy (athlete’s heart); Resting heart rate decreases (bradycardia)

12. 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.