Excretory Products and Their Elimination 

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water, and ions like Na+, K +, Cl–, phosphate, sulfate, etc., either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially. Ammonia, urea, and uric acid are the major forms of nitrogenous waste excreted by animals.
In this chapter, you will learn the mechanisms of elimination of these substances with special emphasis on common nitrogenous wastes. Ammonia is the most toxic form and requires a large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water. 




Homeostasis

Maintenance of steady-state (Walter Cannon). Homeostatic mechanisms are important for a normal life as they maintain conditions within a range in which, the animal's metabolic processes can occur.

Osmoregulation

The regulation of solute movement and hence water movement (which follows solutes by osmosis) is called osmoregulation. Maintenance of saltwater concentration in a steady state. Based on osmoregulation, animals are either osmoconformers or osmoregulatory.

Osmoconformers

These animals can not actively control the osmotic condition of their body fluids. Instead of this, they change or adapt the osmolarity of body fluids according to the osmolarity of the surrounding medium. 
Example: All marine invertebrates and some freshwater invertebrates

Osmoregulator

These animals maintain an internal osmolarity different from the surrounding medium in which they inhabit. Osmoregulator animals must either eliminate excess water if they are in a hypotonic medium or they should continuously take in water to compensate for water loss if they are in a hypertonic medium. 
Example: most vertebrates (except hagfish)

HUMAN EXCRETORY SYSTEM 


  1. A pair of kidneys 
  2. A pair of ureters
  3. A urinary bladder
  4. And a urethra

KIDNEYS

Kidneys are mesodermal in origin. Mammalian kidneys are bean-shaped, reddish-brown colored with a tough fibrous connective tissue covering called renal capsules.  Kidneys are located laterally on either side of vertebral column levels between the last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. In humans, the right kidney is at a slightly lower level than the left kidney.





The dorsal surface of the kidney is attached to the dorsal abdominal wall, so only its ventral surface is covered by the peritoneum. This type of kidney is called retroperitoneal kidney or extraperitoneal kidney.

Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness with an average weight of 120-170 gm. Towards the center of the inner concave surface of the kidney is a notch called hilum(Hilus renal) through which blood vessels(renal artery) and nerves enter. Whereas the ureter and renal vein leave the kidney. Inner to the hilum is a broad funnel-shaped space called the renal pelvis with projections called calyces.  
The outer layer of the kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex, and an inner medulla. The medulla is divided into a few conical masses (medullary pyramids) (8 to 12 in number) projecting into the calyces (sing.: calyx). The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.

COMPENSATE HYPERTROPHY 

If one kidney is removed from the body of a human being then the other one increases in size and tries to perform the function of the removed kidney also. It is an example of a regeneration method called compensate hypertrophy. 






Post renal urinary tract

Urine passes from the pelvis into the ureter. Both the ureter open through separate oblique openings into the urinary bladder. The oblique openings prevent the backflow of urine. Externally, the bladder is lined by detrusor muscle, it is involuntary in nature while internally the bladder is lined by transitional epithelium or urothelium. This epithelium(transitional epi./urothelium) has a great capacity to expand so that a large volume of urine can be stored if required.
The opening of the urinary bladder is controlled by sphincters made of circular muscles. In humans two sphincters are present. Inner Internal sphincter (made up of involuntary muscle) Outer External sphincter (Voluntary muscle). These normally remain contracted and during micturition(voiding of urine) these relax to release urine.

During the act of micturition, urine leaves the urinary bladder and enters the membranous duct called Urethra.

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