25.3 Gross Anatomy of the Kidney

Gross Anatomy of the Kidney

The kidneys filter blood, produce urine, and regulate fluid balance, electrolyte levels, and blood pressure. Understanding their gross anatomy gives you the structural framework you need before diving into how nephrons actually work at the microscopic level.

External Anatomy of the Kidneys

The kidneys are bean-shaped organs located in the retroperitoneal space (behind the peritoneum, against the posterior abdominal wall) between the T12 and L3 vertebrae. The right kidney sits slightly lower than the left because the liver takes up space above it.

Each kidney measures roughly 11 cm long, 6 cm wide, and 3 cm thick. The superior pole is the upper end and the inferior pole is the lower end.

On the medial surface, you'll find the hilum, a concave indentation where the renal artery, renal vein, ureter, lymphatics, and nerves enter and exit the kidney. Think of it as the kidney's "doorway" for all its plumbing and wiring.

Three layers of tissue support and protect each kidney, from outermost to innermost:

• Renal fascia anchors the kidney to the surrounding abdominal wall and adjacent structures
• Perirenal fat capsule (adipose capsule) cushions the kidney against physical trauma
• Renal capsule is a thin, fibrous layer that directly covers the kidney surface, maintaining its shape and providing a barrier against infection

Internal Structures of the Kidneys

If you slice a kidney in half (coronal section), you'll see distinct regions from outside to inside:

• Renal cortex: The outer region, lighter in color. It contains the renal corpuscles (each made of a glomerulus wrapped in a Bowman's capsule) and the proximal and distal convoluted tubules.
• Renal medulla: The inner region, made up of cone-shaped renal pyramids. These pyramids contain the straight portions of the nephron tubules (loops of Henle) and collecting ducts. The tip of each pyramid is called a renal papilla, which points inward toward the renal pelvis.
• Renal columns: Extensions of cortical tissue that dip down between adjacent pyramids. They carry blood vessels and connective tissue between the cortex and medulla.
• Renal sinus: The internal cavity of the kidney that houses the renal pelvis, calyces, and branches of the renal blood vessels and nerves, all surrounded by fat.

Urine drains through a series of progressively larger collecting structures:

1. Urine exits the renal papillae at the tips of the pyramids
2. Minor calyces (cup-shaped structures) collect urine from the papillae
3. Minor calyces merge into major calyces
4. Major calyces drain into the renal pelvis, a funnel-shaped chamber continuous with the ureter

A renal lobe consists of one renal pyramid plus the cortical tissue surrounding it. These lobes are the structural subunits of the kidney.

The nephron is the functional unit of the kidney. Each one consists of a renal corpuscle (Bowman's capsule + glomerulus) and a renal tubule (proximal convoluted tubule → loop of Henle → distal convoluted tubule), which drains into a collecting duct.

Blood Flow Through the Kidneys

Renal blood flow follows a very specific path. This sequence matters because it explains how blood gets from a large artery all the way down to the microscopic capillaries where filtration happens, and then back out again.

Arterial pathway (toward the nephron):

1. Renal artery branches off the abdominal aorta and enters the kidney at the hilum
2. Segmental arteries branch from the renal artery, each supplying a different segment of the kidney
3. Interlobar arteries travel between renal pyramids (through the renal columns)
4. Arcuate arteries branch off at the corticomedullary junction and arc along the base of the pyramids
5. Cortical radiate arteries (also called interlobular arteries) branch upward from the arcuate arteries into the cortex
6. Afferent arterioles branch from cortical radiate arteries and deliver blood to the glomeruli

Capillary networks (where the work happens):

1. Glomerulus: A ball of capillaries inside Bowman's capsule where filtration of blood occurs
2. Efferent arteriole: Carries blood away from the glomerulus (note: this is arteriole-to-capillary-to-arteriole, which is unusual and called a portal system)
3. Peritubular capillaries: Surround the convoluted tubules in the cortex, where reabsorption and secretion occur
4. Vasa recta: Straight capillaries that descend into and ascend from the medulla alongside the loops of Henle; these participate in the countercurrent exchange that helps concentrate urine

Venous pathway (back to the heart):

1. Blood drains through cortical radiate (interlobular) veins → arcuate veins → interlobar veins → renal vein
2. The renal vein exits at the hilum and empties into the inferior vena cava

Cortical vs. Juxtamedullary Nephrons

Not all nephrons are identical. The two types differ in location and function:

• Cortical nephrons (~85% of all nephrons) have their renal corpuscles in the outer cortex and short loops of Henle that dip only into the outer medulla. Their efferent arterioles feed into peritubular capillaries. These nephrons handle most of the routine filtration and reabsorption.
• Juxtamedullary nephrons (~15%) have their renal corpuscles near the corticomedullary junction and long loops of Henle that plunge deep into the medulla. Their efferent arterioles give rise to vasa recta instead of peritubular capillaries. These nephrons are critical for producing concentrated urine because their long loops create and maintain the osmotic gradient in the medulla.

Key Structures by Renal Region

Renal cortex contains:

• Renal corpuscles (glomeruli + Bowman's capsules)
• Proximal convoluted tubules (PCT)
• Distal convoluted tubules (DCT)
• Cortical portions of collecting ducts

Renal medulla contains:

• Renal pyramids
• Loops of Henle (descending and ascending limbs)
• Medullary collecting ducts
• Vasa recta

The juxtaglomerular apparatus (JGA) sits where the afferent arteriole contacts the distal convoluted tubule. It contains specialized cells that monitor blood pressure and filtrate composition, playing a key role in regulating blood pressure and glomerular filtration rate (GFR) through the renin-angiotensin-aldosterone system.

Functions of the Renal Regions

The cortex and medulla each handle different parts of urine formation:

Renal cortex functions:

• Filtration occurs in the renal corpuscles, where blood pressure forces fluid and solutes from the glomerular capillaries into Bowman's capsule
• Reabsorption of water, glucose, amino acids, ions, and other useful substances from the tubular fluid back into the blood occurs in the PCT and DCT
• Secretion of wastes, drugs, and excess ions from the blood into the tubular fluid also occurs in the PCT and DCT

Renal medulla functions:

• Urine concentration happens in the loops of Henle and collecting ducts through countercurrent multiplication and exchange
• The medulla maintains an osmotic gradient (increasing solute concentration from the outer to inner medulla) that allows water to be reabsorbed from the collecting ducts when antidiuretic hormone (ADH) is present
• Urea recycling between the collecting ducts and the loop of Henle contributes to maintaining this osmotic gradient

The Kidney's Role in the Urinary System

The kidneys are one part of a larger system. The complete urinary system includes:

• Kidneys (produce urine)
• Ureters (transport urine from the renal pelvis to the bladder)
• Urinary bladder (stores urine)
• Urethra (expels urine from the body)

Urine formation involves three processes: filtration at the glomerulus, selective reabsorption along the tubules, and secretion of additional wastes into the tubular fluid. The finished urine collects in the renal pelvis, flows down the ureters by peristalsis, and is stored in the bladder until urination occurs.