In aortic dissection, blood enters in -
Aortic dissection 

Aortic dissection is a catastrophic illness characterized by dissection of blood between and along the laminar planes of aortic media.
Blood enters inside the arterial wall (in the media layer) and leads to the formation of blood-filled channels within the aortic wall, which often ruptures outward causing massive haemorrhage.
The most frequent pre-existing histologically detectable lesion is cystic medial degeneration.
In aortic dissection, there is an intimial tear through which the blood enters into the media.
Usually, the tear is within 10 cms of the aortic valve.
The dissecting hematoma spreads characteristically along the laminar planes of the aorta, usually between the middle and outer third.
Some times, blood ruptures into the lumen distally, i.e. there is the second tear distally. So that a complete new vascular channel is formed inside the media of aortic wall. Blood enters from proximal tear into the media and comes out into the lumen from the distal tear.
This is called double - barrel aorta with a false-channel in the media.
Later on, this false channel becomes endothelized → chronic dissection.

  Classification of Aortic dissection

The risk and nature of serious complications of dissection depend strongly on the level of aorta affected, with the most serious complications occurring from the aortic valve to the arch.
Thus aortic dissections are generally classified into two types -

1. Proximal lesion (Type A)

More common and more dangerous.
Involve either the ascending portion only or both ascending and the descending portion of aorta.

2.   Distal lesion (Type B)

Involve only descending part distal to the subclavian artery.
Intima
Media
Adventitia
Any of the above layer