Which of the following vertebrate organs receives only oxygenated blood?

Intracellular fluid and extracellular fluid constitute what percentage of total body weight,respectively?

Select the correct option:
$(1)$ $AB / O$ blood group is the universal recipient.
$(2)$ Mitral valve / Semilunar valve is located between the atrium and the ventricle.

$A$: Elevated levels of $Na^+$ decrease the heart rate and contractility.
$R$: Elevated $Na^+$ levels increase the excitability of the $SA$ node.

The given figure shows the schematic plan of blood circulation in humans with labels $A$ to $D$. Identify the label and give its function/functions.

$A$: 'Dub' is a long and sharp sound.
$R$: It is caused by the closing of atrioventricular valves.