Electrical Control Cardiac muscles are specialized muscle tissue When it contracts, it does so in waves The heart also coordinates all of its contractions into a steady rhythm of both atria contracting & then both ventricles contracting

The contractions begin at specific places in the heart The sinoatrial node (SA node) is a group of specialized cardiac muscle cells in the right atrium The SA node generates an electrical pulse that moves outward throughout the rest of the atrial cells (like a wave in a pond)

When the electrical pulse reaches the atrioventricular node (AV node) located between the atria & ventricles, the pulse is relayed to the ventricular cells causing them to contract The ventricular contraction is a fraction of a second after the atrial contraction

A heartbeat has two phases: 1. Systole: occurs when the ventricles contract closing the AV valves & opening the SL valves (lub) 2. Diastole: occurs when the ventricles relax closing the SL valves & opening the AV valves (dub)

This creates the characteristic ‘lub-dub’ sound of a human heartbeat If one of the valves does not close completely, then some blood is allowed to move backwards in the heart - this is called a murmur

Blood Vessels A persons pulse is a series of pressure waves within an artery caused by the contraction of the left ventricle When the blood surges through the arteries, the elastic walls of the vessels expand and stretch The average pulse rate is 70-90 beats per minute (bpm)

The Arteries The large muscular vessels that carry blood away from the heart are called arteries The walls of the arteries have three layers: 1. A smooth endothelial layer 2. A middle layer of smooth muscle 3. Outer layer of connective tissue for strength

As the arteries move away from the heart, they split into smaller & smaller vessels First arteries become arterioles, then capillaries A capillary is so tiny that blood cells must move through it in single file!

The Veins After gas exchange occurs between the cells & the capillaries it is time to begin the journey back to the heart The capillaries, now carrying O2 poor blood turn into veinules Several veinules will merge to become larger veins

Veins continue to merge to form the very large vein, the Vena Cava Veins & the Vena Cava have small valves inside to prevent the blood from moving backwards

THE END!!