How the kidneys are structured and how they function in a vertebrate.
Comparative Kidney Structure And Function In Vertebrates.
The Kidney is the principal excretory organ in a vertebrate and they are two present. In general, each consists of a mass of coelomoducts opening into a collecting duct. Kidneys are short and posterior in all vertebrates but in Fishes and Salamanders, they extend along most of the body cavity. Indeed, comparatively there are three basic types of kidneys in vertebrates.
- Pronephros or Head kidney
- Mesonephros or Mid kidney
- Metanephros or Hind kidney
The kidneys of lower vertebrates from Cyclostomes to Amphibians and the embryonic kidneys of the higher groups develop segmentally, a pair per body somite (Pronephros, Mesonephros). Some of the tubules have Nephrostomes opening to the coelom, thus somewhat resembling the Nephridia of earthworms.
In Reptiles, Birds, and Mammals adult kidneys (Metanephros) are non segmental and drain wastes only from the blood (see below).
Table 1;Types of kidneys in vertebrates
|Pronephros or head kidney||Mesonephros or mid-kidney||Metanephros or hind kidney|
|Embryonic history and adult structure||First to appear in embryo; develops segmentally, far forward in body cavity; each unit with a nephrostome opening from the coelom; no glomeruli||Develops segmentally in middle part of body cavity; some nephrostomes open coelom, but chiefly by glomeruli||Last to develop; not segmentally; posterior in body cavity; no nephrostomes; many glomeruli; all excretion from blood stream|
|FISHES AND AMPHIBIANS||Functions in larva; disappears in adult||Becomes functional kidney of adult||(Not developed)|
|REPTILES, BIRDS, AND MAMMALS||Appears transiently in embryo and soon disappears||Appears after pronephros; functions during embryonic life, disappearing before hatching or birth; duct persists as vas deferens in male||Last to appear; becomes functional kidney after hatching or birth|
Generally, from each kidney of whatever type, a common collecting duct, the Ureter, carries the waste posteriorly. In Amphibians, Reptiles and Birds the Cloaca, to which Urinary bladder connects in Amphibians and some Reptiles. Normally, the waste or Urine is always fluid except in Reptiles and Birds, where the semisolid excretions (Uric acid) are voided as a white paste (Guano) with the faeces.
In most mammals the Ureters connect directly to the bladder, while a median duct, the Urethra discharges to the exterior, passing through the Penis in the males or Vulva in females. Hence, the interrelated excretory and reproductive systems of vertebrates are commonly termed the Urogenital system.
The kidney in health elaborates a urine appropriate in volume and composition to the needs of the body at all times. Physiologic anatomy of the kidney shows that each human kidney for instance, consists of an inner Medulla and an outer Cortex, the latter containing about a million minute excretory units, or Nephrons. A Nephron is made up of: