Q:

Why does the rbx register not affect the value of the al register?

I am trying to teach my self x86-84 assembly language programming, and am using a pdf called "x86-84 Assembly Language Programming with Ubuntu". There is one part that is talking about widening conversions and it gives this example: 
mov al, 50
mov rbx, 0
mov bl, al

According to the author. the 64-bit rbx register now has a value of 50, but how come the instruction "mov rbx, 0" does not set the value of the whole register to 0?

A:

first line: al=50
second line: rbx=0
third line: set the lowest byte of rbx register to 50
result: rbx=50, as well as bx=50 and bl=50

I found nice picture that explains how the registers are shared:

By altering ah, or al registers you alter also AX, EAX and RAX registers

