The stringy pectinate muscles are inside the heart chambers and help the heart walls expand and contract when the heart beats. This muscle group is located in the left and right auricles (atrial appendages) of the heart, as well as extending into the atria walls.
The heart circulates blood throughout the body. The heart must beat properly to help oxygenated blood move through the arteries, the lungs and all tissue throughout the body, and then return the deoxygenated blood to the heart through the veins. The heart walls must expand and contract with the blood circulation, pumping deoxygenated blood to the lungs, where the red bloods cells pick up oxygen, and then back out through the powerful heart ventricles to the rest of the body. The pectinate muscles help increase the amount of blood that flows through the heart.
The pectinate muscles are located more in the right atrium and auricle as compared to the left atrium. The right atrium receives deoxygenated blood from the veins, so it is vital that the right atrium can expand enough to contain a fairly large volume of blood before the blood goes to the lungs to be oxygenated. The muscles fold and unfold with blood flow through the heart.