EXCRETORY SYSTEM OF BIRDS: STRUCTURE AND ELEMENTS - BIOLOGY - 2025

The excretory system of birds is made up of kidneys, ureters and cloaca. All three are responsible for eliminating waste from the blood of these animals. The kidneys are responsible for filtering nitrogen and uric acid waste from the blood. These are sent through the ureters to one of the chambers of the cloaca, from which they are expelled abroad (EncyclopediaBritannica, 2013).

If one of these three organs fails, the bird quickly dies from blood poisoning from high levels of urea (MelissaBielawski, 2017).

Excretory system of a bird. Photo retrieved from: people.eku.edu

The main functions of the excretory system of birds are: to maintain electrolyte balance, maintain water balance and eliminate waste from the metabolic process, in particular nitrogenous products such as uric acid.

Structure of the excretory system of birds

The kidneys

The most important organs of the excretory system of birds are the kidneys. These are two reddish-brown organs, each generally made up of three lobes.

They are found behind the lungs and on either side of the spine of birds. The kidneys have two thin, straight tubes connected in their midlateral part known as ureters (PoultryHub, 2017).

A kidney is made up of the renal cortex and the renal medulla. A microscopic examination of a dissected kidney shows how it is made up of a large number of renal tubules or nephrons, each one divided into cortical and medullary parts.

Birds have two types of nephrons, some similar to those found in mammals with a Loop of Henle (used to help concentrate urine) found in the kidney medulla, and other reptile-like nephrons located in the cortex. renal.

The nephrons have a duty to extract the components of urine from the blood that flows through the kidneys.

A nephron is made up of a complex network of capillaries contained by a capsule, called Bowman's capsule, into which blood is directly filtered. It also has a spiral segment that runs from Bowman's Capsule to Henlen's Loop (in mammalian nephrons) and finally has a distal tubule that directs urine to the ureters for subsequent elimination from the body.

Ureters

The ureters open and connect to the cloaca, located adjacent to the vas deferens of the male or the oviduct of the female. The ureters are connected internally to the kidneys through funnel-shaped structures in each of the kidney lobes.

They are tubes that are used to transport urine directly into the cloaca. As birds do not have a bladder, the ureters must deposit the material filtered by the kidneys in the cloaca chamber destined for their storage (Kalhagen, 2017).

The cloaca

The cloaca is an organ located in the lower part of the digestive, excretory and reproductive systems of birds. It is used to expel feces and lay eggs. It is located on the back of the body, under the base of the tail of birds and is covered by feathers at the lower end of the abdomen.

Birds have a single hole to expel feces, urine, and lay eggs. The cloaca is the organ that allows the execution of all these functions to the extent that the bird needs it. Within it are multiple skin and muscle folds that subdivide it into chambers suitable for different uses (Lovette & Fitzpatrick, 2016).

Bird faeces are usually stored in one or more chambers of the cloaca. Within it, the continuous absorption of nutrients and solid and liquid waste are mixed and excreted simultaneously once the digestion of the bird concludes (MAYNTZ, 2017).

Urine

Unlike mammals and amphibians, birds generally do not have a bladder. Urine passes directly from the kidneys to the cloaca through the ureters, from where it is transported by a peristaltic movement to the intestine. There the excess water is reabsorbed before waste disposal.

This process of reabsorption of water in birds is similar to that which takes place in mammals. However, birds lack the ability to concentrate urine as efficiently as mammals can.

The urine of the birds is a thick paste with a low water content and a high uric acid content, a product of nitrogen metabolism. After mixing with solid waste in the cloaca, it is expelled from the bird's body as a white or creamy paste on solid feces.

When the kidneys do not function efficiently or normally, and even when the bird has eaten protein-rich food, uric acid can become concentrated in the blood in such a way that the excretory system is unable to eliminate it.

In these cases, the nephrons tend to become inflamed with high concentrations of urea deposits and white lines appear on the surface of the kidneys. The accumulation of urea can lead to kidney cell damage and the eventual development of nephritis..

Similarly, the high concentration of uric acid in the blood can result in the filtration of acid through the capillary walls, which triggers a disease known as visceral gout, characterized by presenting whitish deposits on the surface of the viscera.

Comparison with the excretory system of other animals

The excretory system of birds retains some similarities to that of reptiles, in that both have a cloaca and urine is deposited in a creamy semi-solid state. However, the location, shape, and color of the organs that make up both systems differ widely.

Apart from mammals, birds are the only vertebrate animals that can retain water in their bodies through an osmotic process of urine production. However, the ability of birds to concentrate urine is limited compared to that of mammals.

References

1. (2013). Internal Organs. In EncyclopediaBritannica, The Nature of Birds (p. 15). Sun 90.
2. Kalhagen, A. (February 22, 2017). The Spruce. Retrieved from Avian Anatomy 101: thespruce.com.
3. Lovette, IJ, & Fitzpatrick, JW (2016). Urogenital System. In IJ Lovette, & JW Fitzpatrick, Handbook of Bird Biology (p. 196). Oxford: Wiley.
4. MAYNTZ, M. (February 22, 2017). The Spruce. Retrieved from What Is a Bird's Cloaca ?: thespruce.com.
5. (2017). Cuteness. Retrieved from Excretory System of Birds & Reptiles: cuteness.com.
6. (February 1, 2017). Poultry Hub. Retrieved from Excretory system: poultryhub.org.