Though there are many essential organs or parts of excretory system, the most basic ones are just three, namely, lungs, kidneys and skin. Some other accessory parts and component organs include gall bladder, liver, eccrine glands, urinary bladder, large intestine, urethra and ureter. Now let's proceed to the brief and comprehensive description of each of these excretory structures, so that you may develop a basic understanding of such body organs.

Lungs

In the posterior and innermost part of your mouth, you can find two openings—one leads to the stomach through esophagus, while the other one reaches the interior of lugs through a tubular cartilaginous structure, called trachea. There are two lobes of lungs, each of which is located under a protective set of ribs on either side of the body. Each lobe of the lungs undergoes branching again and again until a stage is reached when very small alveolar sacs are formed where the alveolar membrane serves as a place for gaseous exchange. At the terminal end of the respiratory tree, you can also find pulmonary alveoli that outcrop either from alveolar ducts or alveolar sacs and provide the total surface area of about one hundred cubic meter (100 m2).

The mechanism for the elimination of waste respiratory gases in the lungs is very simple in which the oxygen deficient or carbon dioxide rich blood is pumped from all parts of your body and poured into the alveolar blood vessels where, involving the mechanism of diffusion, oxygen is absorbed and excess CO2 (carbon dioxide) is released.

Kidney

Serving many regulatory roles of prime importance, kidney is one of the basic excretory system parts that is not only involved in the formation of urine, but also accomplishes certain homeostatic functions, such as regulation of electrolytes & blood pressure and the maintenance of acid-base balance. Besides being a natural filter of blood, the two kidneys in your body also produce hormones (such as erythropoietin and calcitriol) and an enzyme, called rennin.

Located in the abdominal cavity, each of the two kidneys is found on either side of the spine in humans, but the left kidney is usually larger than its counterpart on the right side. Here you also need to know that the weight of kidneys varies on the basis of gender differences as in the adult male it measures between 125 and 170 gram, while in females it may attain the mass of 115 to 155 gram.

Skin

Skin — the largest part of your body—not only functions as a protective structure but also serves as a primary excretory organ for the elimination of harmful and unnecessary substances. To perform its excretory function, your skin contain eccrine or sweat glands that produce sweat that contains oils, excess salts, water and other unnecessary substances. Finally, all these constituent parts of sweat are excreted out of the body through small pores in your skin. In addition to it, this protective covering also serves as an organ of thermoregulation because extra heat of the body is discharged through skin, and the process of sweating is, actually, an efficient cooling mechanism in the body.

Secondary/Accessory Excretory Organs

Liver

Though it is a secondary excretory organ, the vital role of this organ can be realized from the simple fact that harmful chemicals, poisons and other toxins, either produced inside the body or having entered from outside, are broken down and detoxified by the liver. For instance, ammonia—a poisonous substance produced as a byproduct of metabolism in your body—is transformed into a less harmful substance, urea, which is then filtered by the kidneys and is discharged out of the body in the form of urine.

Gall Bladder

Though the role of gall bladder in excretory system is considered to be insignificant, but it does something worthwhile to assist the overall process of excretion. Bile — a bitter-tasting fluid produced by the liver—is first stored in this structure and, at the time of need, it is discharged into the duodenum of small intestine where its function is to break down ethanol, fats and other acidic wastes, including ammonia, resulting in the formation of less harmful substances.

Urinary Bladder

The waste containing fluid, collected in the kidney, is transferred to the urinary bladder for temporary storage, which will be discharged out of the body as soon as the individual finds an appropriate place for urination. So, being an important excretory organ, your urinary bladder provides a short-term disposal for urine before it is, ultimately, discharged out of the body. Placed on the pelvic floor, this hollow muscular organ is also elastic or distensible and receives urine through ureters, which is then discharged via the urethra.

Ureters

Delivering the waste-containing fluid or urine from the kidneys to the bladder, the ureters are two tubes made up of smooth muscle fiber. For the assistance of peristaltic movements, this up to 30 cm long and 4 millimeter wide tube (in adult humans) contain traditional epithelium along with an additional layer of smooth muscle in its distal one-third. It is a painful fact to know that eighty percent (80%) failures of kidney transplant are owed to the issues arising from the ureters as it is extremely hard to work on them during surgical operations. The ureters contain check valves, termed as ureterovesical valves that serve to prevent the backflow of urine from the urinary bladder to the kidneys.

Urethra

Running through penis, in human males, it is not only longer than that of a female, but also has got dual functionality, that is, it serves as a carrier of semen as well as urine for their ultimate discharge out of the body. The shorter urethral tube, in human females, carries only urine which is discharged out of the body, just above the vaginal opening. The voluntary control that you have over urination is owed to the functioning of the external urethral sphincter that is composed of striated muscles.

Large Intestine

In the small intestine, the digested food particles are absorbed into the blood stream, while the undigested organic material is transferred to the large intestine that primarily serves as a storage organ for the excretory products. The ascending, transverse and descending colons also facilitate the absorption of leftover salts, water and vitamins, while the distal straight part (called rectum) is specifically designed for the temporary storage of the waste products before you discharge them out of the body with a voluntary action. The waste organic materials, called feces, are expelled out of your body with the help of internal and external sphincters but, meanwhile, feces has to pass through the anal canal and, finally, the anus.