Structure of Kidney

The human kidney is a marvel of biological engineering, exquisitely designed to perform its dual roles of excretion and osmoregulation. A deep understanding of its structure is paramount to grasping the complex physiological processes it orchestrates.

I. Gross Anatomy of the Kidney:

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Location and Shape: — The kidneys are a pair of reddish-brown, bean-shaped organs, situated retroperitoneally (behind the peritoneum) in the abdominal cavity, one on each side of the vertebral column. They extend from the level of the T12 vertebra to the L3 vertebra. The right kidney is typically slightly lower than the left due to the presence of the liver.
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Size and Weight: — Each kidney measures approximately 10-12 cm in length, 5-7 cm in width, and 3-4 cm in thickness, weighing about 120-170 grams in adults.
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Coverings: — The kidney is encased by three layers of connective tissue:

* Renal Capsule: The innermost layer, a tough, fibrous transparent membrane that directly adheres to the kidney surface, providing protection against trauma and infection. * Adipose Capsule (Perirenal Fat): A layer of fatty tissue surrounding the renal capsule, offering cushioning and insulation. * Renal Fascia (Gerota's Fascia): The outermost layer, a thin, dense connective tissue that anchors the kidney to the posterior abdominal wall and surrounding structures.

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Hilum: — The medial concave border of each kidney features a vertical cleft called the hilum. This serves as the entry and exit point for the renal artery (entering), renal vein (exiting), ureter (exiting), nerves, and lymphatic vessels. The structures passing through the hilum collectively form the renal pedicle.
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Renal Sinus: — The hilum leads into a cavity within the kidney known as the renal sinus. This space is filled with adipose tissue, blood vessels, nerves, and the renal pelvis.
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Internal Structure (Macroscopic): — Upon sagittal section, two distinct regions are visible:

* Renal Cortex: The outer, reddish-brown granular region, extending from the renal capsule to the bases of the renal pyramids. It contains the renal corpuscles, proximal convoluted tubules (PCTs), and distal convoluted tubules (DCTs) of the nephrons.

Extensions of the cortex, called renal columns of Bertin, project into the medulla, separating the renal pyramids. * Renal Medulla: The inner, darker region, composed primarily of 8-18 cone-shaped structures called renal pyramids.

The broad base of each pyramid faces the cortex, while its apex, known as the renal papilla, points towards the renal sinus. The medulla contains the loops of Henle and collecting ducts of the nephrons.

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Renal Lobes: — Each renal pyramid, along with the overlying cortical tissue and half of each adjacent renal column, constitutes a renal lobe. There are typically 8-18 renal lobes per kidney.
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Collecting System: — Urine formed in the nephrons drains into collecting ducts, which then empty into papillary ducts at the renal papillae. From there, urine flows into minor calyces (cup-shaped structures, one for each papilla), which merge to form 2-3 major calyces. The major calyces, in turn, unite to form the large, funnel-shaped renal pelvis, located within the renal sinus. The renal pelvis narrows inferiorly to become the ureter, which transports urine to the urinary bladder.

II. Microscopic Anatomy: The Nephron – Functional Unit of the Kidney:

Each kidney contains approximately 1-1.2 million nephrons, which are the fundamental structural and functional units. A nephron consists of two main parts: the renal corpuscle and the renal tubule.

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Renal Corpuscle (Malpighian Corpuscle): — Located in the renal cortex, it is responsible for the initial filtration of blood. It comprises two components:

* Glomerulus: A tuft of highly permeable capillaries formed by the afferent arteriole (bringing blood in) and drained by the efferent arteriole (carrying blood out). The glomerular capillaries are fenestrated, allowing for efficient filtration.

* Bowman's Capsule: A double-walled, cup-shaped structure that surrounds the glomerulus. It has two layers: * Parietal layer: The outer layer, composed of simple squamous epithelium. * Visceral layer: The inner layer, intimately associated with the glomerular capillaries, composed of specialized epithelial cells called podocytes.

Podocytes have foot-like processes (pedicels) that interdigitate, forming filtration slits (slit pores) through which the filtrate passes. The filtration membrane (or blood-glomerular barrier) consists of the fenestrated endothelium of the glomerulus, the glomerular basement membrane, and the filtration slits of the podocytes.

This membrane allows water and small solutes to pass but restricts blood cells and large proteins.

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Renal Tubule: — A long, convoluted tube extending from Bowman's capsule, responsible for modifying the glomerular filtrate through reabsorption and secretion.

* Proximal Convoluted Tubule (PCT): Originates from Bowman's capsule, highly coiled, and located in the renal cortex. Its cells have numerous microvilli (brush border) and abundant mitochondria, indicating its primary role in extensive reabsorption (e.

g., 65% of water, Na+, Cl-, K+, 100% of glucose and amino acids) and secretion (e.g., H+, ammonia, drugs). * Loop of Henle: A U-shaped segment that dips into the renal medulla. It consists of: * Descending limb: Thin-walled, highly permeable to water but relatively impermeable to solutes.

It extends deep into the medulla. * Ascending limb: Has a thin segment (in the inner medulla) and a thick segment (in the outer medulla). It is impermeable to water but actively transports solutes (Na+, Cl-, K+) out of the tubule into the interstitial fluid, contributing to the medullary osmotic gradient.

* Distal Convoluted Tubule (DCT): A coiled segment located in the renal cortex, extending from the ascending limb of the loop of Henle. Its cells have fewer microvilli than PCT cells. It is involved in regulated reabsorption of water and solutes (e.

g., Na+, Cl-) and secretion (e.g., K+, H+), primarily under hormonal control (aldosterone, ADH). * Collecting Duct: Several DCTs open into a common straight tube called the collecting duct. These ducts extend through the renal cortex and medulla, eventually merging to form larger papillary ducts that open at the renal papillae.

Collecting ducts play a crucial role in final water reabsorption (under ADH control) and urea recycling, contributing significantly to urine concentration.

III. Types of Nephrons:

Based on the location of their renal corpuscles and the length of their loops of Henle, nephrons are classified into two types:

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Cortical Nephrons (85%): — Their renal corpuscles are located in the outer part of the renal cortex, and their loops of Henle are short, barely penetrating the renal medulla. They are primarily involved in excretory and regulatory functions.
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Juxtamedullary Nephrons (15%): — Their renal corpuscles are located deep in the renal cortex, close to the medulla, and they possess very long loops of Henle that extend deep into the renal medulla. These nephrons are critical for establishing and maintaining the medullary osmotic gradient, which is essential for concentrating urine.

IV. Blood Supply to the Nephron:

Blood supply to the nephron is unique and crucial for its function:

The afferent arteriole branches from the interlobular artery and supplies blood to the glomerulus.
The efferent arteriole drains blood from the glomerulus. Unlike other capillary beds, the efferent arteriole does not immediately merge into a venule. Instead, it branches into a second capillary network:

* Peritubular Capillaries: Surround the PCT and DCT in the renal cortex, involved in reabsorption and secretion. * Vasa Recta: Long, straight capillaries that run parallel to the loops of Henle of juxtamedullary nephrons in the renal medulla. They are essential for maintaining the medullary osmotic gradient and facilitating countercurrent exchange.

V. Juxtaglomerular Apparatus (JGA):

This specialized structure is located at the point where the distal convoluted tubule makes contact with the afferent arteriole of the same nephron. It consists of three main components:

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Macula Densa: — Specialized chemoreceptor cells in the wall of the DCT, sensitive to the concentration of NaCl in the tubular fluid.
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Juxtaglomerular (JG) Cells: — Modified smooth muscle cells in the wall of the afferent arteriole, containing secretory granules of renin.
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Extraglomerular Mesangial Cells (Lacis Cells): — Located in the space between the afferent and efferent arterioles and the macula densa, their exact function is still debated but they are thought to transmit signals between macula densa and JG cells.

The JGA plays a critical role in regulating glomerular filtration rate (GFR) and systemic blood pressure through the Renin-Angiotensin-Aldosterone System (RAAS).