During diastole the atria fills with blood.
If by the first heart beat you mean the "lub" in "lub dub" then it is the left and right atria that are contracting.
The left and right atrium contract at the same time
ventricles
The lamprey's heart consists of two chambers
A shark heart has two chambers.
the human heart contains four chambers and it is the ventricles (the two lower chambers) that do the pumping
There are four chambers in the human heart. There are two atria (the two upper chambers) and two ventricles (the two lower chambers).
The heart of fish have two chambers. The heart of reptiles and amphibians have three chambers. The heart of mammals have four chambers.
A perch has two chambers in it's heart.
The sharks heart has two chambers.
the upper chambers of the heart are called the right and left atrium and they are the receiving chambers of the heart
The cardiac ventricles are the two lower chambers of the heart's four chambers. They are larger and more narrowed than the atria. They function by collecting the blood that flows into them from the atria and then contract in order to expel blood into the peripheral tissues, using major vessels to carry it to the extremities and lungs.
The two top chambers of the heart are called the right and left auricle respectively
They have one chamber to their heart: a single tube connected to two valves at either end, and a muscle to help it contract.
The upper two chambers of the heart are the right atrium or auricle, and the left atrium or auricle.