8.1 Kidney Structure and Nephron Function

The kidneys filter blood and produce urine, maintaining fluid balance, electrolyte concentrations, and waste removal. Understanding their layered anatomy and the nephron's step-by-step processing of filtrate is central to grasping how the urinary system works.

Kidney Anatomy and Function

Gross Anatomy

The kidney is a bean-shaped organ located in the retroperitoneal space, meaning it sits behind the peritoneum against the posterior abdominal wall. You have one on each side of the vertebral column, roughly at the level of T12–L3. The right kidney sits slightly lower than the left because the liver pushes it down.

• The renal hilum is the concave medial surface where the renal artery, renal vein, and ureter enter and exit. Think of it as the kidney's "doorway" for all major vessels and the ureter.
• The renal capsule is a thin, fibrous connective tissue layer that surrounds the entire kidney, providing physical protection and helping maintain its shape.

Internal Structures

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

• Renal cortex: The outer region, lighter in color. This is where you'll find the renal corpuscles and the convoluted tubules of nephrons. Most of the filtration and reabsorption activity happens here.
• Renal medulla: The inner region, made up of cone-shaped renal pyramids. These pyramids contain the loops of Henle and collecting ducts. The tip of each pyramid (the renal papilla) points inward and drains urine into a minor calyx.
• Renal pelvis: A funnel-shaped cavity that collects urine from the minor and major calyces and channels it into the ureter.

Blood supply: The renal artery delivers oxygenated blood to the kidney, branching progressively into segmental, interlobar, arcuate, and interlobular arteries before reaching the afferent arterioles that feed each nephron's glomerulus. After filtration, blood exits via efferent arterioles into peritubular capillaries (or vasa recta), then drains through a series of veins back to the renal vein. Don't confuse the afferent and efferent arterioles with the renal artery and vein themselves; the arterioles are tiny vessels at the nephron level, while the artery and vein are the major vessels at the hilum.

Nephron Structure and Function

Nephron Components

The nephron is the basic structural and functional unit of the kidney. Each kidney contains roughly 1 million nephrons. Every nephron has two main parts:

1. Renal corpuscle (glomerulus + Bowman's capsule): Where blood is filtered.
2. Renal tubule: Where the filtrate is modified. The tubule has four sequential segments:
   • Proximal convoluted tubule (PCT)
   • Loop of Henle (descending and ascending limbs)
   • Distal convoluted tubule (DCT)
   • Collecting duct (shared by multiple nephrons)

The nephron carries out three processes to form urine: filtration (at the renal corpuscle), reabsorption (moving useful substances from the tubule back into blood), and secretion (moving additional wastes from blood into the tubule).

The proximal convoluted tubule is the workhorse of reabsorption. About 65% of the filtered sodium and water is reclaimed here, along with virtually all filtered glucose and amino acids. This segment has a brush border of microvilli that dramatically increases surface area for transport.

Urine Concentration and Modification

• The loop of Henle builds the osmotic gradient in the medulla that makes concentrated urine possible. The descending limb is permeable to water but not to solutes, so water leaves the tubule as it descends into the increasingly salty medulla. The ascending limb is impermeable to water but actively pumps out sodium and chloride ions, diluting the tubular fluid while making the surrounding interstitium more concentrated.
• The distal convoluted tubule and collecting duct fine-tune the final urine composition. They selectively reabsorb or secrete potassium, hydrogen ions, and bicarbonate depending on what the body needs at that moment.
• Two hormones are especially important here:
  • ADH (antidiuretic hormone): Released from the posterior pituitary when you're dehydrated. It inserts aquaporin channels into the collecting duct walls, making them permeable to water so more water is reabsorbed and urine becomes concentrated.
  • Aldosterone: Released from the adrenal cortex in response to the renin-angiotensin-aldosterone system. It stimulates sodium reabsorption (and potassium secretion) in the DCT and collecting duct, which pulls water along with it.

Filtration in the Renal Corpuscle

Glomerular Filtration Barrier

The renal corpuscle consists of the glomerulus (a tuft of capillaries) surrounded by Bowman's capsule. Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. Because the efferent arteriole is narrower than the afferent, blood pressure stays high within the glomerulus, which is what drives filtration.

The filtration barrier has three layers that filtrate must pass through:

1. Fenestrated endothelium of the glomerular capillaries: Has small pores that allow fluid and small solutes through but block blood cells.
2. Basement membrane: A dense layer of extracellular matrix that blocks most proteins based on size and charge.
3. Podocytes (visceral layer of Bowman's capsule): These cells wrap around the capillaries with foot processes, and the narrow gaps between them (filtration slits) provide the final size-selective barrier.

Together, these layers allow water, ions, glucose, amino acids, urea, and creatinine to pass into Bowman's capsule while keeping larger molecules like albumin and all blood cells out of the filtrate. If you find protein or blood cells in the urine, that's a sign the filtration barrier is damaged.

Filtration Forces

Glomerular filtration is governed by Starling forces, the balance of pressures pushing fluid out of the capillary versus holding it in:

Net filtration pressure (NFP) is calculated as:

$NFP = P_{GC} - P_{BC} - \pi_{GC}$

$NFP = 60 - 18 - 32 = 10 \text{ mmHg}$

That ~10 mmHg of net pressure is enough to produce roughly 125 mL of filtrate per minute (the glomerular filtration rate, or GFR), which adds up to about 180 liters per day. Obviously you don't urinate 180 liters a day; over 99% of that filtrate gets reabsorbed by the tubules.

The ultrafiltrate in Bowman's capsule has a composition similar to plasma, minus proteins and cells.

Cortical vs Juxtamedullary Nephrons

Not all nephrons are identical. The two types differ in their location, the length of their loops of Henle, and their primary roles.

Cortical Nephrons

• Make up about 85% of all nephrons.
• Their renal corpuscles sit in the outer cortex, and their loops of Henle are relatively short, dipping only into the outer medulla.
• They are surrounded by peritubular capillaries and play a larger role in day-to-day reabsorption and secretion, including regulation of electrolyte and acid-base balance.
• Because their loops don't reach deep into the medulla, they contribute less to the medullary concentration gradient.

Juxtamedullary Nephrons

• Make up about 15% of all nephrons.
• Their renal corpuscles sit near the corticomedullary junction, and their loops of Henle extend deep into the inner medulla.
• These long loops are critical for building the steep osmotic gradient in the medulla (interstitial osmolarity can reach up to 1200 mOsm/L at the papilla), which is what allows the kidney to produce maximally concentrated urine.
• Instead of peritubular capillaries, juxtamedullary nephrons are served by the vasa recta, specialized hairpin-shaped capillaries that run parallel to the loops of Henle. The vasa recta use countercurrent exchange to remove reabsorbed water and solutes without washing out the medullary gradient. Blood flowing down into the medulla picks up solutes and loses water; blood flowing back up reverses the process. This preserves the gradient rather than dissipating it.
• Juxtamedullary nephrons become especially important during dehydration, when the body needs to conserve water by producing small volumes of highly concentrated urine.