The renal capsule is a tough fibrous layer surrounding the kidney

Kidney

The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 and covered in a thick layer of perinephric adipose tissue

Adipose tissue

In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...

. It provides some protection from trauma and damage.

The renal capsule relates to the other layers in the following order (moving from innermost to outermost):

1. renal medulla
   Renal medulla
   The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the arcuate arterioles. The arcuate arterioles each in turn branch into...
   
   
2. renal cortex
   Renal cortex
   The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections that extend down between the pyramids. It contains the renal corpuscles and the renal tubules except for parts of...
   
   
3. renal capsule
4. perinephric fat (or "perirenal fat")
5. renal fascia
   Renal fascia
   The renal fascia is a layer of connective tissue encapsulating the kidneys. The deeper layers below the renal fascia are, in order, the perinephric fat , renal capsule, and finally the parenchyma of the renal cortex...
   
   
6. paranephric fat (or "pararenal fat")
7. peritoneum
   Peritoneum
   The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
   
    (anteriorly), and transversalis fascia
   Transversalis fascia
   The transversalis fascia is a thin aponeurotic membrane which lies between the inner surface of the Transversus abdominis and the extraperitoneal fascia....
   
    (posteriorly).

Adipose capsule

The perinephric fat may be regarded as a part of the renal capsule, called the adipose capsule of kidney

Adipose capsule of kidney

The adipose capsule of kidney is a structure between the renal fascia and renal capsule, and may be regarded as a part of the latter...

.