13.2: The Kidneys — Exterior and Interior Structures

The kidneys’ location, protective layers, and internal organization into cortex and medulla provides the foundation for learning how they carry out filtration and maintain homeostasis. 

Master this section and you'll be able to:

• Identify the major external and internal structures of the kidney, including its protective layers, cortical and medullary regions, hilum, and renal pelvis.

The Kidneys — External Gross Anatomy

Anatomically, the kidneys lie on either side of the spine in the retroperitoneal space, not within the abdominal cavity. The kidneys are the primary organs of the renal system and — as you saw in the previous section — carry out essential homeostatic functions. They are a pair of bean-shaped, brown organs about the size of your fist.

Each kidney is wrapped in three protective layers that help cushion and stabilize it in the body:

1. Closest to the surface of the kidney is the renal (fibrous) capsule, a thin but tough layer that protects against injury and infection.
2. Surrounding this is a thick layer of perirenal fat that acts like a shock-absorbing cushion and helps keep the kidney in place.
3. The outermost layer is the renal fascia, a strong sheet of connective tissue that anchors the kidney and its surrounding fat to nearby structures in the posterior abdominal wall. 

The kidneys are highly vascular organs, receiving about one quarter (25%) of the resting cardiac output, which supports their continuous role in filtering and regulating the composition of the blood.

The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right. The right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen.

Figure \(\PageIndex{1}\): Kidney Position. The kidneys are located deep to the floating ribs, which provide protection for the kidneys. (Image credit: "Kidney Position Relative to the Thoracic Cage" by Jennifer Lange is licensed under  CC BY-NC-SA 4.0; modification of "Slagter - Drawing and Chest X-ray of the ribs and clavicula - no labels" by Ron Slagter, and "Slagter - Drawing Anatomy of the kidney, adrenal glands and bladder - no labels" by Ron Slagter.)

Resting on top of each kidney is an adrenal gland (adrenal meaning on top of renal), primarily endocrine organ. The upper parts of the kidneys are partially protected by lower ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia.
 

Internal Gross Anatomy of the Kidneys

The renal hilum (see image below) is the medial-facing, concave entry and exit site for structures servicing the kidneys: blood vessels, nerves, lymphatics, and ureters.  The renal arteries branch directly from the abdominal aorta, whereas the renal veins return blood directly to the inferior vena cava. The vein, artery,  and ureter are arranged in an anterior-to-posterior order.

The ureters are structures that come out of the kidneys, bringing urine downward into the bladder.

   

Figure \(\PageIndex{2}\): Internal Anatomy of the Kidney. This coronal section of a kidney shows the major regions and structures involved in filtering the blood and producing urine. The cortex, the outer region, contains most of the glomeruli. The medulla forms the deeper, triangular renal pyramids, which contain the loops and collecting ducts that help concentrate urine. Between the pyramids are renal columns, extensions of cortical tissue that carry blood vessels into the medulla. Filtered urine drains from the pyramids into the renal pelvis, which funnels it into the ureter. Shown also are branches of the renal artery and renal vein, along with arterioles, venules, and capillaries that support the high rates of filtration and reabsorption within the nephron.

Renal Cortex & Medulla

As you saw above, the kidneys are enclosed by three protective layers: the renal capsule (a tough fibrous covering), the adipose capsule (a layer of fat that cushions the organ), and the renal fascia (connective tissue that anchors the kidney in place).

Beneath these coverings lies the renal cortex, the outer region of the kidney that appears granular because it contains the superficial portions of the nephrons (more details below), the kidney’s functional units.
The renal cortex extends deep into the medulla in the form of renal columns. These  columns appear as vertical “bands” of cortical tissue that separate the cone-shaped renal pyramids (which are part of the medulla). Their function is to provide an anchor for the cortex and to provide a corridor for the kidney's blood vessels and nerves. 

Special cells in the renal cortex also produce the hormone erythropoietin.  

The renal medulla is the inner region of the kidney’s parenchyma (functional tissue). It contains several triangular structures called renal pyramids. Each pyramid is packed with parts of nephrons and associated blood vessels that help concentrate the urine.

A nephron begins in the cortex with a cup-shaped structure called the Bowman’s capsule. This capsule surrounds a tuft of capillaries known as the glomerulus. (See image on the right.) Blood arrives in the glomerulus from branches of the renal arteries, and plasma is filtered across the capillary walls into the Bowman’s capsule.

Once the filtrate has entered the capsule, it flows through the nephron in a specific sequence: first the proximal convoluted tubule, then the loop of Henle, then the distal convoluted tubule, and finally the collecting ducts. The collecting ducts carry the processed filtrate toward the renal pelvis and into the ureter. Each segment of the nephron has unique permeability properties that allow the kidneys to regulate water balance, electrolyte levels, and overall homeostasis in the body. For more details see the next page\
 

Renal Pelvis

The renal pelvis is a funnel-shaped cavity in the center of the kidney that collects urine from the major calyces and represents the expanded upper end of the ureter. It sits within the renal sinus at the hilum and channels urine out of the kidney into the ureter for transport to the bladder.