The avian cardiovascular system is able to quickly respond to changes in levels of activity (e.g., resting vs. flying) via changes in heart rate, cardiac output, & blood flow (by vasocontriction and vasodilation of vessels).
A square is a quadrilateral with all right angles and all equal sides. A square is also a parallelogram, a rectangle, and a rhombus and has all the properties of all these special quadrilaterals. Figure 3 shows a square. Figure 3 A square has four right angles and four equal sides. Figure 4 summarizes the relationships of these quadrilaterals to one another.
So a square IS still a rectangle (still has four right angles!). Both are also parallelograms. One way to see that this is useful, is to realize that any special properties from the larger class: e.g. diagonals bisect in a parallelogram) still apply to the special cases (e.g. a square or a rectangle).