You have already learned about excretion. But did you know that in humans, skin is also an excretory organ? That’s right sweating is actually the excretion of our body. Let us study all about excretion and human excretory system.
Parts of the Human Excretory System
Every living organism generates waste in its body and has a mechanism to expel it. In humans, waste generation and disposal are taken care of by the human excretory system. The human excretory system comprises of the following structures:
- 2 Kidneys
- 2 Ureters
- 1 Urinary bladder
- 1 Urethra
Kidneys are the main organ of the human excretory system. The kidneys are paired organs in each individual. They are the primary excretory organ in humans and are located one on each side of the spine at the level of the liver. They are divided into three regions- the renal cortex which is the outer layer, the renal medulla which is the inner layer and the renal pelvis which is responsible for carrying the urine from the kidney to the ureter. The functional unit of a kidney is called the nephron.
There is one ureter that comes out of each kidney as an extension of the renal pelvis. The ureter is a thin muscular tube that carries urine from the kidneys to the bladder.
It is a sac-like structure that is lined with smooth muscle layer and is responsible for storage of urine till it is expelled from the body by micturition. Micturition is the act of expelling urine from the body. The bladder receives urine from the ureters, one from each kidney. The level of the urinary bladder placement in the body differs in men and women.
This is a tube that arises from the urinary bladder and functions to expel urine to the outside by micturition. The urethra is shorter in females and longer in the males. In males, the urethra functions as a common path for sperms and urine. The opening of the urethra is guarded by a sphincter that is autonomically controlled.
Other Excretory Organs
Apart from the above mentioned excretory organs, there are other organs that also perform some form of excretion.
The skin is the largest organ in the body. Its primary function is to protect the different organs of the body. However, the skin helps in excretion by the way of sweat. The skin eliminates compounds like NaCl, some amount of urea etc.
Lungs are the primary respiratory organs and they help take in oxygen and expel carbon dioxide. But, in this process, they also function to eliminate some amount of water in the form of vapour.
The liver has an important function in excretion. It is said to be the first line of defence when it comes to hormones, fats, alcohol, and drugs. Most drugs undergo a first pass metabolism which occurs in the liver. Few drugs are eliminated directly by the kidneys. The liver is said to play a role in the elimination of excess fats and cholesterol that is essential to the health of the body.
Structure of a Nephron
The structural and functional unit of the kidney is the nephrons. Each kidney consists of millions of nephrons that are all functioning together to filter urine and expel the waste products. Each kidney consists of the following parts:
- Bowman’s capsule– is the first part of the nephron which is a cup-shaped structure and receives the blood vessels. The glomerular filtration occurs here. The blood cells and proteins remain in the blood.
- Proximal Convoluted Tubule– The Bowman’s capsule extends downwards to form the proximal tubule. Water and reusable materials from the blood are now reabsorbed back into it.
- The loop of Henle– The proximal tubule leads to the formation of a u-shaped loop called the Loop of Henle. The Loop of Henle has three parts: The descending limb, the u-shaped bend, and the ascending limb. It is in this area that the urine becomes concentrated as water is reabsorbed. The descending limb is freely permeable to water whereas the ascending limb is impermeable to it.
- Distal Convoluted Tubule– The Loop of Henle leads into the distal convoluted tubule which is where the kidney hormones cause their effect. And the distal convoluted tubule leads to the collecting ducts.
- Collecting Duct– The distal convoluted tubule of each nephron leads to the collecting ducts. The collecting ducts together form the renal pelvis through which the urine passes into the ureter and then into the urinary bladder.
Functions of the Excretory System
The excretory system performs many functions such as:
- Helps eliminate waste products such as urea, uric acid ammonia, and other products via urine.
- It helps maintain the osmotic level of blood and plasma
- It helps maintain the electrolyte balance in the body
- And it also helps in the metabolism of those drugs that do not get metabolized in the liver.
Solved Example for You
Q: Which part of the nephron structure is completely impermeable to water?
(a) Proximal Convoluted Tubule (b) Ascending limb of the Loop of Henle
(c) Distal Convoluted Tubule (d) Descending limb of the Loop of Henle
Sol. (b). The ascending limb of the loop of Henle is completely impermeable to water.