sentence: Electricity going through the heart makes the cells contract.
sentences_before: When they get smaller we also say they 'contract'.
sentences_after: The SA Node is a group of cells in the right atria. These cells start an 'electrical impulse'. This electrical impulse travels through the atria making them contract. This motion is called 'atrial systole'. The AV Node makes the impulse slow down.
concept_name: electricity
headings: Heart Heart's Pacemaker
categories: Basic English 850 words Anatomy of the cardiovascular system