The heart has contractile cells that produce powerful contractions to propel blood flow and a conducting system to control and coordinate the activity of the contractile cells. The conducting system is made up of nodal and conducting cells that initiate and distribute electrical impulses without contracting. It establishes the heart rate and ensures coordinated contraction of the atria and ventricles. The cardiac cycle involves systole where the heart contracts and diastole where it relaxes, moving blood from high to low pressure areas. Cardiac output, the amount of blood pumped per minute, is regulated by factors like blood volume, autonomic innervation, hormones, ion concentration and temperature.

1. The Heart

2. Overview Introduction The Heart and The Circulation System The Heartbeat Contractile cells The conducting system The cardiac cycle Heart Dynamics Factors controlling cardiac output

5. The Heartbeat Atria and ventricles contract in coordinated manner Ensures correct blood flow 2 types of cardiac muscle cells involved: Contractile cells Produce powerful contractions that propel blood Conducting system Control and coordinate activity of contractile cells

6. The Heartbeat: Contractile Cells Differs from skeletal muscle: Action potential Source of calcium ions Duration of resulting contraction

7. Cardiac vs. Skeletal Muscle Contraction Tetanus occurs because short refractory period Long refractory period which continues while relaxing = no tetanus! Type of contraction Initiate contraction Delay repolarization & initiate contraction Extracellular Calcium Ions Duration: 10 msec Duration: 250-300 msec Action Potential Skeletal Muscle Cardiac Muscle

9. The Conducting System Cardiac muscle tissue contracts on its own Does not need hormonal or neural stimulation These will change the force Called automaticity or autorhythmicity Atria contract followed by ventricles Coordinated by conducting system Network of specialized cardiac muscle cells Initiate and distribute electrical impulses Made up of two types of cells that do not contract: Nodal cells (responsible for establishing rate of contraction) Conducting cells (distribute the contractile stimulus to general myocardium)

10. Nodal Cells Cell membranes depolarize spontaneously and generate APs at regular intervals Electrically coupled to each other, conducting cells, and other cardiac cells Normal rate of contraction established by pacemaker cells Located in the SA (sinoatrial) node Depolarize rapidly and spontaneously Generate 70-80 APs/min = HR of 70-80 bpm

11. Conducting Cells Stimulus for contraction generated by SA node, but must be distributed so that: The atria contract together before the ventricles The ventricles contract together Wave must begin at apex and spread toward base This pushes blood toward base into the aorta and pulmonary trunk Now we will watch the CD!

12. The Cardiac Cycle Cardiac cycle: period between the start of one heartbeat and the beginning to the next Systole: contraction Diastole: relaxation Remember: Fluids move from high pressure to low pressure! Now we will watch the CD!

14. Heart Dynamics Refers to the movements and forces generated during cardiac contractions Each time the heart beats the 2 ventricles release = amounts of blood Stroke Volume (SV)—the amount of blood ejected by a ventricle during a single beat Can vary from beat to beat Cardiac Output (CO)—the amount of blood pumped by each ventricle in 1 min CO = SV x HR ml/min = ml x bpm

15. Factors Controlling Cardiac Output Highly regulated Why? Major factors: Blood volume reflexes Autonomic innervation Hormones Secondary factors: Extracellular ion concentration Body temperature

16. Blood Volume Reflexes Contraction active, relaxation passive 2 heart reflexes respond to changes in blood vol One occurs in R atrium and affects HR Atrial reflex One occurs in the ventricles and affects SV

17. Atrial Reflex Involves adjustments in HR Triggered by increase in venous return Walls of RA stretch  stimulate stretch receptors in wall  increase in sympathetic activity  cells of SA node depolarize faster  increase HR

18. Ventricular Reflex Amount of blood pumped out of each ventricle each beat depends on: Venous return Filling time Frank-Starling principle Major effect is that the output of blood from both ventricles is balanced under a variety of conditions

19. Factors Controlling CO: Autonomic Innervation ANS can modify HR Innervated by both parasympathetic and sympathetic divisions Innervate SA and AV nodes

21. Factors Affecting CO: Hormones Adrenal medulla NE and E Result? Thyroid hormones and glucagon Secreted by pancreas Produce similar effects to NE and E

22. Coordination of Autonomic Activity Cardiac centers in medulla Cardioacceleratory center Which neurons are activated? Cardioinhibitory center Which neurons are activated? Gets from heart to medulla through vagus nerve Both respond to changes in bp and arterial concentrations of oxygen and carbon dioxide Monitored by baroreceptors