What are the parts of the nephron?
In a strict histological sense, a nephron consists of a capsule and a long, unbranched epithelial tubule divided into 5 distinct segments. The glomerular capillary tuft formally belongs to the vascular system, and the collecting ducts belong to the excretory system, but functionally they operate in close coordination.
- Renal corpuscle (corpusculum renale) — includes Bowman's capsule (glomerular capsule) and the capillary glomerulus. Plasma filtration and primary urine formation occur here.
- Proximal convoluted tubule — the initial segment of the tubular system, approximately 2 cm long. Located in the renal cortex.
- Loop of Henle — consists of a descending (thin) limb and an ascending (thick) limb. Descends into the renal medulla.
- Distal convoluted tubule — the continuation of the ascending limb. It returns to the renal cortex and establishes contact with the renal corpuscle of its own nephron at the vascular pole (strictly positioned between the afferent and efferent arterioles).
- Collecting duct — an independent epithelial structure where distal convoluted tubules drain. These ducts run perpendicular to the renal surface, merge into papillary ducts, and open into the renal calyces.
What are the types of nephrons?
Nephron classification is based on the depth of the renal corpuscle's location and the length of the loop of Henle. All renal corpuscles are located in the cortex, giving it a dark red color on cross-section.
- Cortical (mid-cortical) nephrons (80%) — the predominant type. They possess a medium-sized loop of Henle that descends only into the outer zone of the medullary pyramids.
- Juxtamedullary / long-looped nephrons (about 20%) — renal corpuscles lie at the corticomedullary junction (juxta — near). Their long loops of Henle penetrate almost the entire thickness of the pyramids.
- Superficial (short) cortical nephrons (15–20% or 1% depending on sources) — possess very short loops of Henle that do not descend deeply, remaining within the cortical medullary rays.
Physiology: How is urine formed?
Urine formation involves several key physiological processes occurring in different parts of the nephron:
- Filtration: occurs in the renal corpuscle, resulting in primary urine.
- Obligate (mandatory) reabsorption and secretion: takes place in the proximal convoluted tubule. This process is genetically determined and stable. 100% of glucose, amino acids, and vitamins are reabsorbed, along with 70% of water and ions ($Na^+$, $K^+$, $Cl^-$). Ammonia, hydrogen ions, and p-aminohippuric acid (PAH) are secreted.
- Countercurrent mechanism (in the Loop of Henle):
- Descending limb (thin segment lined by simple squamous epithelium) is permeable to water but impermeable to solutes. Water moves passively into the interstitium along an osmotic gradient.
- Ascending limb (thick segment) is impermeable to water. Its cells are rich in mitochondria and actively pump $Na^+$ into the interstitium. This increases medullary hyperosmolality, driving the function of the descending limb.
- Facultative reabsorption and secretion: occurs in the distal tubules and collecting ducts. These processes are variable and depend on systemic regulatory factors.
Blood Supply: The Renal Portal System
The microvasculature of a cortical nephron features a unique portal system comprising two capillary beds:
- Primary capillary network (glomerulus): formed by the branching of the afferent arteriole (vas afferens). Located inside the renal corpuscle and responsible for plasma filtration.
- Efferent arteriole (vas efferens): collects blood from the glomerulus and exits the renal corpuscle.
- Secondary (peritubular) capillary network: formed by the branching of the efferent arteriole. It surrounds the nephron tubules and collecting ducts.
Evolution of the Nephron (Phylogeny)
During vertebrate phylogeny, the excretory system underwent three stages of progressive development:
- Pronephros (head kidney): maintains connection with the body cavity (coelom) via a ciliated funnel called a nephrostome. The capillary glomerulus lies externally within the coelom.
- Mesonephros (body kidney): a Bowman's capsule forms around the capillaries. Connection with the coelom is occasionally retained, but primary filtration shifts to the capsule.
- Metanephros (permanent kidney): complete loss of the nephrostome. Filtration occurs exclusively from blood into the capsular space, and the tubule lengthens significantly. The human kidney represents the highest evolutionary development of the metanephros.