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=related Watch the video and then draw a very simple diagram of the heart, labelling the key features and explaining the route of blood.

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

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

Exam Questions 1. Define the term bradycardia. (1 mark) A resting heart rate below 60 bpm. 2. Outline why the adaptations leading to bradycardia would be beneficial to trained athletes. (3 marks) Bradycardia likely to occur due to an increase in heart size. A larger heart will lead to increased stroke volume and cardiac output. This leads to increased performance in aerobic events.

Cardiac Cycle The cardiac cycle is the process by which blood is pumped through the heart. It involves the filling with blood (diastole) followed by emptying of blood from the heart into the arteries (systole). The are four phases to the cycle:

Phase 1 – Atrial diastole Phase 1 – Atrial diastole. The atria of the heart fill with blood Phase 2 – Ventricular diastole. The bicuspid and tricuspid valves (atrio-ventricular valves) open and blood passes from the atria into the ventricles. (semi-lunar vales remain closed) Phase 3 – Atrial systole. Both atria contract to ensure all blood is ejected from atria into the ventricles. Phase 4 – Ventricular systole. The semi-lunar valves open and the atrio-ventricular valves close. Both ventricles contract forcing blood into the aorta and the pulmonary artery.

a) Define bradycardia. (1) b) Outline why the adaptations leading to bradycardia might be beneficial to trained athletes. (3)

Outline the four phases of the cardiac cycle. (4)

Key Terms End Diastolic volume The volume of blood in the ventricles prior to contraction. End Systolic volume The volume of blood left in the ventricles after contraction. Your stroke volume can be calculated by: EDV-ESV=SV As aerobic fitness increases your SV will increase. This is due to your ESV decreasing as a result of the heart being stronger and therefore able to contract more powerfully and force more blood out. Also due to heart rate being lower the EDV will increase as there is more time for the ventricles to fill between beats.

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.

Regulating Heart Rate When exercising, the heart rate increases due to: 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. This alters the rate at which impulses are sent from the SAN

Heart Contraction and Regulation 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.