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