Answer:
The first heart sound (S1) represents closure of the atrioventricular (mitral and tricuspid) valves as the ventricular pressures exceed atrial pressures at the beginning of systole (point a). S1 is normally a single sound because mitral and tricuspid valve closure occurs almost simultaneously.
Explanation:
We label the human sex chromosomes with X's and Y's. For example. A boy's chromosomes are XY. A girls chromosomes are XX. When a child is born, they get an X from their mother, and either an X or Y from their Father, thus determining their sex.
The stomach. the place where your food sits for a while after you have chewed it up and swallowed.
