What does a nephron consist of?
In a strict histological sense, a nephron consists of a capsule and a long, unbranched epithelial tubule divided into 5 segments. The capillary glomerulus formally belongs to the vascular system, and the collecting ducts to the excretory system; however, functionally they work in close coordination.
- Renal corpuscle (corpusculum renale) — includes Bowman's capsule and the capillary glomerulus. This is where plasma filtration and primary urine formation take place.
- Proximal convoluted tubule — the initial segment of the tubular system, about 2 cm long. Located in the cortex.
- Loop of Henle — consists of a descending (thin) limb and an ascending (thick) limb. Descends into the renal medulla.
- Distal convoluted tubule — continuation of the ascending limb. Returns to the cortex and invariably contacts the renal corpuscle of its own nephron at the vascular pole (strictly between the afferent and efferent arterioles).
- Collecting duct — an independent epithelial element into which the distal tubule empties. The ducts run perpendicular to the renal surface, merge into papillary ducts, and open into the renal calyces.
What are the types of nephrons?
The classification of nephrons is based on the depth of the renal corpuscle and the length of the loop of Henle. All renal corpuscles are located in the cortex, giving it a dark red color on a cross-section.
- Intermediate cortical nephrons (80%) — the main type. They have a medium-sized loop of Henle that descends only to the outer zone of the medullary pyramids.
- Juxtamedullary / long nephrons (about 20%) — renal corpuscles lie at the cortico-medullary junction (juxta — near). Their long loop of Henle penetrates almost the entire thickness of the pyramids.
- Short cortical nephrons (15–20% or 1% according to various sources) — possess a very short loop that does not descend deeply and remains within the medullary rays of the cortex.
Physiology: how is urine formed?
The process of urine formation is divided into basic stages taking place in different parts of the nephron:
- Filtration: occurs in the renal corpuscle, forming primary urine.
- Obligate (mandatory) reabsorption and secretion: take place in the proximal tubule. The process is genetically determined and stable. 100% of glucose, amino acids, and vitamins, as well as 70% of water and ions ($Na^+$, $K^+$, $Cl^-$) are reabsorbed. Ammonia, hydrogen protons, and p-aminohippuric acid (PAH) are secreted.
- Countercurrent mechanism (in the loop of Henle):
- Descending limb (thin segment made of squamous epithelium) is permeable to water, but not to salts. Water passively exits into the tissue 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 the osmolarity of the renal tissue and drives the function of the descending limb.
- Facultative reabsorption and secretion: occur in the distal tubules and collecting ducts. These processes are variable and depend on external regulatory factors.
Blood supply: the "wonderful network" of the kidney
The microvasculature of a cortical nephron is organized into two capillary networks:
- 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 leaves the renal corpuscle.
- Secondary (peritubular) capillary network: formed by the branching of the efferent arteriole. It entwines the nephron tubules and collecting ducts.
Evolution of the nephron (Phylogeny)
In the course of vertebrate phylogeny, the excretory system underwent three stages of complexification:
- Pronephros (head kidney): maintains a connection with the body cavity (coelom) via a ciliated funnel called the nephrostome. The capillary glomerulus lies externally in the coelom.
- Mesonephros (primary kidney): a renal corpuscle capsule forms around the capillaries. Connection with the coelom is sometimes retained, but primary filtration shifts to the capsule.
- Metanephros (secondary 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 degree of metanephros development.
Class and exam questions
Is the capillary glomerulus part of the nephron?
Histologically, the glomerulus belongs to the circulatory system, while the nephron includes only the capsule and the epithelial tubule. However, functionally they form a single renal corpuscle and are considered together.
How does reabsorption differ between proximal and distal tubules?
In the proximal tubules, reabsorption is obligate (mandatory, genetically determined, and stable). In the distal tubules, it is facultative—variable and subject to regulation.
Why does water exit from the descending limb of the loop of Henle?
Water exits passively along an osmotic gradient. This gradient is created by the ascending limb of the loop, which actively pumps sodium ions into the medullary interstitium of the kidney.
What else to study on this topic
- Renal corpuscle and filtration barrier
- Countercurrent multiplier system of the kidney
- Renal collecting ducts
- Phylogeny of the vertebrate urogenital system
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