Excitation of the Heart

Intro Muscle cells of the myocardium are excitable: with electrical stimulation they will contract Leads to contraction of heart Leads to pumping of blood Does not require stimulation from CNS

Sinoatrial Node SA node aka “the pacemaker” Found within the wall of the right atrium Where electrical signals are initiated Sets HR Controlled by the autonomic nervous system

STOP! I know what you're going to say: “If the heart stimulates itself, why is it controlled by the autonomic nervous system?” Because we don't need to tell our heart to beat it is an automatic process therefore it is grouped under the autonomic nervous system All process in the body are either autonomic or somatic

Internodal Pathways The electrical signal spreads through both atria via the internodal pathways Causes the atria to contract from the top down Forces blood into ventricle

Atrioventricular Node AV node Located at the bottom of the right atria Passes the electrical signal from the atria to the ventricles Also passes signal into a region of specialized tissue that runs down the ventricular septum: the bundle of His  Splits to form the right and left bundle branches

Purkinje Fibres From the bundle of His the branches pass the signal on to the Purkinje fibres Purkinje fibres pass the electrical signal to the ventricles

Coronary Circulation Remember that the heart is a working muscle that needs a constant supply of oxygen as well as fuel and nutrients Blood is supplied to the heart through two main arteries: the right and left coronary arteries Branch off of the aorta and divide multiple times, supplying all regions of the myocardium with oxygenated blood

Cardiac Cycle Defined as the series of events that occurs through one heart beat Diastole: phase of relaxation  Heart fills with blood Systole: phase of contraction  Heart contracts and ejects blood

Pressure During the cardiac cycle there are dramatic changes in pressure  Pressure propels the blood through the circulation Systolic blood pressure: pressure observed in the arteries during the contraction phase Diastolic blood pressure: pressure observed in the arteries during relaxation of heart Normal bp is 120/80

The Vascular System and Blood Vascular system is formed by a network of vessels that transport blood throughout the body As you follow the path of blood through the body away from the heart, the vessels branch out and get smaller Main categories of vessels:  Arteries, arterioles, capillaries, venules, and veins

Arteries Carry blood away from heart Thick, muscular walls that are very elastic Ability to stretch and recoil is important in assisting the movement of blood during diastole

Arterioles Smaller than arteries Surrounded by rings of smooth muscle that can contract or relax  Controlled by the nervous system Nervous system can control the distribution of blood flow to different organs using arterioles

Capillaries Smallest vessel Walls are very thin – one cell thick Location of exchange of gases and nutrients Interesting fact: if you were to line up all of the capillaries from one person, they would form a line of more than 40,000 km long.

Veins Return blood to the heart Become larger as they move away from the capillaries Venules --> veins --> vena cava Carry deoxygenated blood (except the pulmonary veins)‏

Blood Main role is to transport oxygen, carbon dioxide and nutrients Two main components: plasma and blood cells Plasma: fluid component  Composed mostly of water  Makes up about 55% of blood  Within you will find nutrients, proteins, ions, and gases

Blood Cells Red blood cells – most abundant blood cell  Transport O2 and CO2  Contain a specialized protein called hemoglobin which can bind O2 and CO2 White blood cells – less than 1% of blood  Play an important role in protecting the body from disease

Platelets Incomplete cells – fragments Important in the regulation of blood clotting

In your notes... Refer to pages 115 and 116 to explain the skeletal muscle pump and the thoracic pump