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NEET Biology
Urinogenital System

Frequently Asked Questions

The urogenital system (also called the genitourinary system) is the combined organ system of the urinary system and the reproductive system. It deals with the excretion of waste (urinary) and reproduction.

They are studied together because they share common structures (like the urethra in males) and are closely located anatomically. Both systems also develop from the same embryonic tissues.

The kidneys filter blood, remove nitrogenous wastes (like urea), maintain electrolyte balance, regulate blood pressure, and produce hormones (like erythropoietin).

Ureter: Tube that carries urine from kidneys to the bladder. Urethra: Tube that carries urine from the bladder to outside (and also semen in males).

The bladder stores urine temporarily before it is excreted.

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URINOGENITAL SYSTEM

The urinary system in vertebrates consists of the kidneys and their associated ducts, while the reproductive system comprises the male and female gonads, along with their respective ducts. However, these two systems are connected in the making of vertebrates. It is because male urinary ducts are also used to discharge gametes. Thus, these two systems are together described as the Urinogenital system.


1.0Origin and basic structure of vertebrate kidneys and ducts 

  • The vertebrate kidney is a paired organ. It has a basic structural and functional unit called the uriniferous tubules, also known as nephrons. 
  • These tubules arise in the embryo in a linear series from a part of mesoderm called the mesomere or nephrotome. 
  • It is a ribbon-like intermediate mesoderm that runs between the segmental mesoderm (epimere) and the lateral plate mesoderm (hypomere) on either side, along the entire trunk from the heart to the cloaca. 
  • A tubule is differentiated into three parts, i.e., the peritoneal funnel, the tubule and the Malpighian body.


A. Peritoneal funnel

  • It is a funnel-like ciliated structure near the free end of a nephron. 
  • It opens into the coelom via a nephrostome, a wide aperture, which drains waste from the coelomic fluid. Nephrostomes are usually confined to embryos and larvae. 
  • They are considered vestiges of a hypothetical primitive kidney. 

B. Malpighian body

  • The tubule begins as a blind, cup–like, hollow, double-walled Bowman`s capsule. 
  • This capsule encloses a tuft of blood capillaries, known as a glomerulus. 
  • Bowman`s capsule and enclosed glomerulus together form a renal corpuscle or Malpighian body. When glomeruli are encapsulated, it is called as internal glomeruli. 
  • It is common. Not encapsulated and suspended freely in the coelomic cavity it is called external glomeruli. 
  • It is found in larvae and embryos. Bowman`s capsule without glomeruli is termed aglomerular. These are found in embryos and larvae. 

C. Tubule

  • Malpighian bodies filter water, salts and waste from blood. 
  • This filtrate passes through convoluted ductules, known as tubules. In this passage, it undergoes reabsorption and secretion. 
  • The final filtrate is then drained into the longitudinal duct, which opens into the embryonic cloaca.

2.0Archinephros

  • It is the name given to the hypothetical, primitive kidney of ancestral vertebrates. 
  • It is regarded as a holonephros, or a complete kidney, because it runs the entire length of the coelom. 
  • Its tubules have one segment, with a single nephron arrangement in all body segments. 
  • Each tubule opens into the coelom through a nephrostome. The glomerulus is external and suspended in the coelom near each nephrostome. 
  • A common longitudinal Wolffian or archinephric duct drains all the tubules. It opens into the cloaca. 
  • The Archinephros arrangement is found in the larvae of some cyclostomes (e.g., Myxine) but not in adult vertebrates. 
  • Modern vertebrates exhibit three distinct types of adult kidneys: pronephros, mesonephros, and metanephros. 
  • These represent three successive stages of development from ancestral Archinephros. All three types are never functional at the same time. 


  1. Pronephros
  • It is the first to appear during embryonic development; hence, it is referred to as the pronephros. It is located just behind the head, on either side, dorsal to the anterior end of the coelom. 
  • A pronephros has tubules arranged opposite each of the anterior mesodermal somites. There are three such pronephric tubules in the frog embryo, seven in the human embryo and twelve in the chick embryo. Each tubule opens into the coelom by a nephrostome. 
  • External glomeruli project into the coelom near the nephrostome of each tubule. Sometimes, glomeruli unite to form a single compound glomerulus, known as a glomus. 
  • Glomus and tubules are surrounded by a large pronephric chamber derived from the pericardial or pleuroperitoneal cavity. 
  • All pronephric tubules open into the common pronephric duct that leads posteriorly into the embryonic cloaca. 
  • The pronephros is functional only in larval stages or the embryo. The mesonephros type soon replaces it. However, it serves a lymphoid function in adult cyclostomes and some teleost fish species.

2. Mesonephros

  • It develops from the middle part of the intermediate mesoderm, just posterior to the pronephros, after the pronephros has degenerated. 
  • New mesonephric tubules join the pronephric duct and are segmentally disposed. These tubules further multiply to increase their number and disrupt segmental disposition. 
  • These tubules have internal glomeruli enclosed by Bowman`s capsule. Thus, the mesonephros removes waste directly from the blood in the glomerulus, unlike the pronephros, which eliminates waste indirectly from the coelom. 
  • The mesonephros is also known as the Wolffian body, and the old pronephric duct that it joins is referred to as the Wolffian duct. 

 3. Metanephros

  • The functional kidney of amniotes is called the metanephros. It is formed from the posterior end of nephrogenic mesoderm. 
  • When metanephric tubules develop, all mesonephric tubules disappear except those forming vasa efferentia in the male testis. The metanephros exhibits the following advancements over the mesonephros.  
  • Greater multiplication and thus, a large number of nephrons or tubules. This leads to more efficient waste removal from internal glomeruli.  
  • It develops a new urinary duct called the metanephric or Wolffian duct and grows anteriorly and dorsally. Its dilated distal tip forms a pelvis into which the metanephric tubules open, while its proximal end develops into the ureter. 
  • This ureter empties into the cloaca or urinary bladder in mammals.  Mammalian metanephros shows the highest organisation and has an additional feature of the loop of Henle between PCT and DCT in the tubule.

3.0Urinary bladder

  • The kidney ducts drain into the urinary bladder, where urine is stored before being excreted. The urinary bladder is found in most vertebrates. 
  • However, it is absent in cyclostomes, elasmobranchs, some reptiles and most birds. In most fish, it is a simple, terminal enlargement of the mesonephric duct, known as a tubal bladder. 
  • In dipnoi fishes, it originates from the dorsal wall of the cloaca. In tetrapods, it originates from the ventral wall of the cloaca. In amphibians, it is called the cloacal bladder. 
  • However, in amniotes, the adult bladder is derived from the proximal part of the embryonic allantois, hence called the allantoic bladder. 
  • Kidney ducts usually open into the cloaca dorsally. However, in mammals, except for monotremes, the ureters enter directly into the urinary bladder, and it empties outside via the urethra. 
  • Mammals also lack a cloaca because the dorsal part of the embryonic cloaca forms the rectum, and the ventral part becomes the urethra. 

4.0Interlinkage between the development of the kidneys and the gonaducts

  • Males: In male anamniotes, the mesonephros has two parts, viz., the anterior genital portion and posterior renal portion. In the genital portion in males, some nephrons lose their excretory function, form vasa efferentia, and become continuous with the seminiferous tubules of the adjacent testis. 
  • They transport sperm from the testes to the mesonephric duct or the Wolffian duct. 
  • Thus, the Wolffian duct forms a urinogenital duct in male anamniotes, serving as both the ureter for urine and the vas deferens for sperm. 
  • In male amniotes, the metanephric kidney has a separate duct for urine, so the Wolffian duct becomes solely a genital duct or vas deferens—the remnants of embryonic mesonephros and a coiled portion of mesonephric duct form the epididymis in the adult testis. 
  • The sperm carried by these modifications of the mesonephric duct, also known as the Wolffian duct, are ejaculated outside via the urethra, a modification of the ventral cloaca. 
  • Females: In all vertebrate embryos, except cyclostomes, the coelomic epithelium outside the mesonephric duct develops into the Müllerian duct. It is degenerated in males and functions in females as the oviducts. 
  • In female anamniotes, the Müllerian ducts exist as the gonoducts alone, while the Wolffian ducts exist as the ureters, the urinary ducts alone. 
  • This arrangement differs from that of male anamniotes. In female amniotes, the Wolffian duct degenerates as a result of the development of a separate ureter for the metanephric kidney. 
  • In viviparous mammals, the posterior ends of both Müllerian ducts fuse and are modified into a uterus that supports embryonic development.

On this page

  • 1.0Origin and basic structure of vertebrate kidneys and ducts 
  • 2.0Archinephros
  • 3.0Urinary bladder
  • 4.0Interlinkage between the development of the kidneys and the gonaducts