Lobes of the Lungs

Right Lung
There are three lobes in the right lung: 1) right upper lobe, (2) right middle lobe, and (3) right lower lobe. The right upper lobe occupies the upper 1/3 of the right lung. The right middle lobe is typically the smallest of the three, and appears triangular in shape, being narrowest near the hilum.   The right lower lobe is the largest of all three lobes, separated from the others by the major fissure.  Grossly, these lobes can be separated from one another by two fissures which anatomically correspond to the visceral pleural surfaces of those lobes from which they are formed.  The horizontal fissure separates the upper lobe from the middle lobe.  The right oblique fissure is more expansive in size than the horizontal fissure, separating the right upper and middle lobes from the larger right lower lobe. 

Left Lung
The lobar architecture of the left lung is slightly different than the right. Because there is no defined left horizontal fissure, the left lung has only two lobes; the left upper and left lower lobes.  These two lobes are separated by the oblique fissure, identical to that seen on the right side, although often slightly more inferior in location.  The portion of the left lung that corresponds anatomically to the right middle lobe is incorporated into the left upper lobe as the lingula lobe.
 

It is important to understand that in most individuals, interlobar fissures are usually not completely formed; in some individuals there may be complete absence of a fissure thus losing the demarcation between lobes on gross examination.