Excretory Products and their Elimination

Introduction

Excretion refers to elimination of waste products from the body. It is a physiological process that occurs in every living being. The eliminated bodily wastes include amino acids, ammonia, urea, uric acid, water, and carbon dioxide. Though animal excretion is different from human beings, the process occurs in both. This study guide based on excretory products & their elimination aims to educate the process of excretion in human and animal bodies.

Animal Excretion 

The process of elimination of metabolic waste products from the animal body to regulate the composition of body fluids and tissues is called excretion.

In animals, the process of excretion differs as there are different excretory structures. For example:

Invertebrates have excretory structures that look like simple tubular forms. On the other hand, vertebrates have complex tubular organs called kidneys. 

  • Platyhelminthes, rotifers, some annelids and the cephalochordate amphioxus have excretory structures in the form of protonephridia or flame cells. They are primarily involved with ionic and fluid volume regulation called osmoregulation 
  • Earthworms and other annelids have tubular excretory structures called nephridia. These tubules help get rid of nitrogenous wastes while maintaining a fluid and ionic balance 
  • When it comes to insects including cockroaches, myriapods and tardigrades, the excretory structures are in malpighian tubules. These tubules also help in the removal of nitrogenous wastes similar to nephridia and further help in osmoregulation
  • In crustaceans like prawns, crabs, lobsters, barnacles, crawfishes, etc., antennal glands (green glands) perform the excretory function

Human Excretory System

In humans, the excretory system consists of a pair of kidneys, ureters, a urinary bladder and a urethra. 

Each kidney of an adult mortal measures:

  • 10-12 cm in length
  • 5-7 cm in range
  • 2-3 cm in consistency 
  • 120-170g in weight

Structure of the Kidney

  • In the centre part of the kidney, towards the hollow inner face, there is a notch called hilum. It is through this, the blood vessels, ureters and jitters enter
  • Inner to the hilum, there is the renal pelvis (a broad channel-shaped space) with protrusions called calyces 
  • The external layer of the kidney is a tough capsule
  • Inside the kidney, there is an outer cortex and an inner medulla. The medulla has 8 to 18 conical masses known as the renal pyramids

A normal kidney consists of nearly one million complex tubular structures (functional units) called nephrons. Each nephron has two divisions: 

  1. The glomerulus

  2. The renal tubule

The glomerulus is a stack of capillaries formed by the sensational arteriole, a fine branch of the renal artery. An efferent arteriole carries down blood from the glomerulus. 

Malpighian Corpuscle and Henle

The Malpighian corpuscle, PCT and DCT of the nephron are located in the cortical region of the kidney.

The Henle is situated in a way that dips into the medulla. Also, in most nephrons, there is a presence of cortical nephrons (the small circle of Henle extending little into the medulla). In contrast, some nephrons have a longer circle of Henle (known as juxtamedullary nephrons) that easily runs deep into the medulla. 

Afferent Arteriole

The arteriole arising from the glomerulus forms a fine capillary network around the renal tubule. These are called the peritubular capillaries. A nanosecond vessel of this network runs resemblant to the Henle’s circle forming a ‘U’ shaped vasa recta. Vasa recta is absent or largely reduced in cortical nephrons.

Part of Other Organs in Excretion

Other than the kidney, many other organs are present in the human body that are responsible for excretion.

  • Lungs

Carbon dioxide (about 18L per hour)and water (about 400 ml) are the 2 main waste products that lungs tend to excrete. The excretion occurs in the form of vapours.

  • Liver

Liver is responsible for breaking down the toxins (chemicals that enter our body) through a process called detoxification. Let us take amino acids for example. The liver breaks down amino acids that are found in protein and converts it into ammonia. Ammonia is further converted into urea, which is excreted with urine.

  • Skin

Human skin is said to excrete water, heat, and urea (to some extent). It occurs in the form of sweat. Though the primary function of sweat is to regulate the body and keep it cool, waste excretion is its main motive. Apart from sweat, the sebaceous glands also perform certain excretion by letting out excess sebum. 

Disorders of the  Excretory System

Malfunctioning of kidneys can lead to accumulation of urea in blood; a condition called uremia, which is dangerous and may lead to kidney failure. 

In case of acute renal failure (kidney failure), kidney transplantation is the only option. A performing kidney is used in transplantation from a donor rather than a close relative to minimise its chances of rejection by the vulnerable system of the host. However, it is said that modern clinical procedures have increased the success rate of such a complicated procedure. 

Conclusion 

Numerous nitrogen-containing substances, ions, CO2, water, etc. that accumulate in the body have to be excluded. The nature of nitrogenous wastes formed and their excretion vary among creatures, substantially depending on their habitat. Kidneys play a major function in the elimination process of human beings.

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