Urinary System

The major organs involved in the urinary system are the kidneys, the ureter and the cloaca. This system functions to excrete water, remove nitrogenous wastes, and regulate the osmotic fluid, or acid-base balance of the bird's body. Each kidney has three distinct sections called cranial, middle and caudal lobes. They are found ventral to the lungs on either sides of the vertebral column. The circulation involved in the lobes are the renal arterial blood supply, the renal venous return and the renal portal system, which is unique to avian species. There is one renal artery per lobe. The renal portal system is made of cranial and caudal renal portal veins that deliver the blood to interlobular spaces.

Histology slide showing cross-section of the avian kidney.
1. Cortex
2. Medullary cone
3. Intralobular vein
4. Renal corpuscles of cortical nephrons
5. Renal corpuscles of medullary nephrons

Source: http://cal.vet.upenn.edu/projects/histo/Lab1718urinary/Lab1718gsc18avian4x.htm

Lobule of avian kidney. The renal medulla has a cone shape because the number of loops of Henle decreases when going from the upper portion of the lobule to the apex of the medullary cone.
Source: http://people.eku.edu/ritchisong/bird_excretion.htm

The nephrons are responsible for filtering the liquid waste left after digestion, reabsorbing any last useful substances and secreting the waste to dispose of it. Each nephron is composed of a renal corpuscle, or a network of capillaries enclosed in the Bowman's capsule, a proximal convoluted tubule leading away from the Bowman's capsule, a loop of Henle (if the nephron is mammalian-type), a distal convoluted tubule and a collecting tubule that leads to the ureter. 

Diagram showing structural difference between a
reptilian-type nephron and a mammalian-type nephron.
Source: http://people.eku.edu/ritchisong/bird_excretion.htm

Avian kidneys possess two different types of nephrons, one of which is the reptilian-type or cortical nephron named so because of it's similarities to nephrons found in reptiles. Like regular mammalian nephrons, the renal corpuscles of these nephrons are in the lobules of avian kidneys and are connected to a collecting duct by a renal tubule. They differ in the fact that they do not have a loop of Henle, are only found in the region of the cortex and have small renal corpuscles. A large percentage of avian nephrons are reptilian-type. 

Diagram showing structure of an individual mammalian nephron.
Source: http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect21.htm

The mammalian nephrons aid in the formation of an osmotic gradient in the interstitial fluids of the medullary cone. The proximal and distal tubules of both types of nephrons secrete and reabsorb filtrate that has passed through the glomeruli and has entered the Bowman's capsule. Secretion in the tubules is the addition of substances to the tubular fluid by cells found in the tubules while tubular reabsorption is the removal of substances by the cells.

The avian antidiuretic hormone arginine vasotocin helps control water permeability of collecting ducts. It is released at the posterior pituitary. Levels of secretion of this hormone are reduced when water is taken in excess, causing a dilute urine to be excreted. The opposite happens when too little water is taken in.

The glomerular filtration rate is affected even by a slight lack of water in the chicken's diet. The rate is greatly reduced, possibly because of vasoconstriction of vessels supplying the reptilian-type nephrons.

It is important to note that the chicken does not have a bladder. The waste passes from the kidney to the ureters, and then empties into the cloaca where it is passed back to the large intestine to allow additional re-absorption, then finally exits the body through the cloaca. The reason a chicken's urine is partly solid is because uric acid is not water soluble, as opposed to a mammals urine which is composed mostly of urea. Glomerular filtration and tubular secretion allow uric acid to enter tubule fluid in the avian kidneys. The renal portal system brings blood to a peritubular capillary network around the tubules and is a source of uric acid for secretion.