Excretory System: Our bodies have efficient and complex systems that help us survive and keep us healthy. While we eat food daily with nutrients, it is the excretory system that is responsible for carefully sifting hazardous materials from the waste generated by the food and eliminating it regularly so that these harmful toxins do not stay in our bodies. Along with the respiratory, circulatory and digestive systems, the Excretory System is one of the most significant biological systems in our bodies. 

Students must understand the human body and the functions that it performs. In this article, we will discuss in detail about different excretory organs present in humans and their functions and how urine is formed inside our body. Continue reading to know more.

Excretory System: Definition

The Excretory System is defined as a biological system that helps in the elimination of the nitrogenous waste products of metabolism and also helps in maintaining homeostasis in our body.

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Excretory Organs in Different Organisms

1. Different groups of animals have a variety of excretory organs for excretion.
2. In most invertebrates, these structures are simple tubular forms, whereas vertebrates have a complex structured organ called a kidney.

Animal Group	Excretory Organs
Protozoans, Poriferans, Coelenterates	The plasma membrane, pellicle, and general body surface
Ctenophora	Anal pores
Platyhelminthes (flatworms)	Protonephridia with Flame cells
Nemathelminthes or Aschelminthes (Roundworms)	Protonephridia and canals and Renette cells
Annelida	Metanephridia and chloragogen cells
Arthropoda	Malpighian tubules, coxal gland, green glands, or antennary glands
Mollusca	Renal gland or organ of Bojanus and Keber’s organ
Echinodermata	Tube feet (podia) and dermal branchiae (thin walls of gills)
Chordata	Protonephridia, neural gland, pharyngeal nephridia, and Hatschek’s nephridium in cephalochordates, a pair of kidneys in vertebrates

Human Excretory System Diagram

The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.

Learn About Micturition Here

Human Excretory System Organs

The human excretory system includes the following organs:

Kidneys

• a. Kidneys are reddish-brown, bean-shaped structures present between the last thoracic and third lumbar vertebra.
• b. They lie close to the dorsal inner wall of the abdominal cavity.
• c. The right kidney is lower and smaller than the left kidney because of the presence of the liver on the right side.

• d. The outer surface of each kidney is convex and the inner side concave with a notch or hilum.
• e. Kidney is covered from outside to inside by three layers, i.e., renal fascia, adipose layer, and renal capsule. These coverings protect the kidneys from external shocks and injuries.
• f. The L.S. of mammalian kidneys show an outer cortex and an inner medulla.
• g. Inside the kidney, there are different structures like funnel shaped cavity or renal pelvis, cup-like renal calyces and renal pyramids.
• h. The cortex spreads in between medullary pyramids as renal columns called Columns of Bertini.
• i. Each kidney is composed of numerous complex tubular structures called nephrons (approx. one million).

Nephrons

1. Nephrons (or uriniferous tubules) are the basic structural and functional units of the kidney.
2. Each nephron consists of two parts, i.e., Malpighian body (or Renal corpuscle) and Renal tubule.

3. Malpighian body is formed of Bowman’s capsule and Glomerulus which filters out large solutes from the blood and delivers small solutes to the renal tubule for modification.

Name	Description	Function
Glomerulus	The tuft of thin-walled capillaries is formed by the branching of the afferent arteriole (a fine branch of the renal artery).	Involved in the ultrafiltration of blood.
Bowman’s capsule	The double-walled, cup-like structure surrounds the glomerulus.	Receives the glomerular filtrate and passes it to the PCT.

4. Renal tubule is a long, thin tubule attached to Bowman’s capsule with four distinct regions as follows:

Name of the parts	Description	Functions
Proximal Convoluted Tubule (PCT)	1. Located behind the neck of the Malpighian corpuscle making few coils. 2. Restricted to the cortical regions of the kidney.	1. Majority of the reabsorption takes place here only. 2. Absorption of nearly all essential nutrients, \(70-80\%\) of electrolytes and water, \(\mathrm{HCO}_{3}^{-}\)ions. 3. Selective secretion of \(\mathrm{H}^{+}\)and \(\mathrm{K}^{+}\)ions and ammonia. 4. Also maintains the pH and ionic balance of the body fluids
Henle’s Loop	1. Quite narrower and U-shaped tube. 2. Having a descending limb that ends into the medulla and an ascending limb that extends back from the medulla into the cortex.	1. Reabsorption of water, \(\mathrm{Na}^{+}, \mathrm{K}^{+}\),\(\mathrm{Mg}^{2+}\)and \(\mathrm{Cl}^{-}\). 2. The ascending limb is permeable to the urea. 3. Maintains the high osmolarity of medullary interstitial fluid.
Distal Convoluted Tubule (DCT)	Thicker and highly coiled structure situated in the cortex.	1. Active reabsorption of\(\mathrm{Na}^{+}\)ions and water. 2. With associated secretion of\(\mathrm{K}^{+}\), \(\mathrm{H}^{+},\) ammonia, some \(\mathrm{Cl}^{-}\)and \(\mathrm{HCO}_{3}^{-}\) ions are also reabsorbed. 3. Maintains the pH and \(\mathrm{Na}^{+}-\)\(\mathrm{K}^{+}\)balance in the blood.
Collecting tubule (or Straight tubule)	1. Last part of the nephron that combines and opens into a large collecting duct. 2. Extends from the cortex of the kidney to the inner parts of the medulla. 3. Many collecting tubules open into a large collecting duct.	1. Water is reabsorbed as it is highly permeable to water. 2. \(\mathrm{Na}^{+}\) ions are also reabsorbed along with small amounts of water to maintain the osmolarity. 3. Maintains the pH and ionic balance of the blood by selective secretion of \(\mathrm{H}^{+}\)and \(\mathrm{K}^{+}\)ions.

Important facts

1. Several adjacent collecting tubules combine to open into a short and thick duct of Bellini. Then all of the ducts open into the renal pelvis through medullary pyramids in the calyces.
2. Peritubular Capillary Network (PTCN) is formed when a minute vessel of the peritubular capillaries runs parallel to the Henle’s loop forming a U-shaped vasa recta. All these capillaries join to form renal venules.

Functions of Kidney

Kidney serves the following functions:

1. It helps in the regulation of water and electrolyte balance. 
2. It helps to regulate arterial pressure.
3. It helps in the excretion of metabolic waste and foreign chemicals. 
4. It also helps in the secretion of hormones like renin.

Ureters

The ureter is a long, muscular tube that continues with the pelvis and emerges at the hilum, and opens into the urinary bladder.

Urinary bladder

It is a thin-walled, pear-shaped, white transparent sac present in the pelvic cavity and temporarily stores urine.

Urethra

1. It is a membranous tube, which conducts urine to the exterior.
2. The urethral sphincters keep the urethra closed except during excretion of urine.

Physiology of Urine Formation

The formation of urine by the kidney takes place in the following three steps:

1. Glomerular filtration or Ultrafiltration
2. Selective reabsorption
3. Tubular reabsorption

Micturition

1. Urine is produced and drained continuously by the nephron into the renal pelvis, from where it is carried to the urinary bladder.
2. The urine is stored in the bladder temporarily till a voluntary signal is given by the CNS (Central Nervous System).
3. The process of release of urine through the urethra is called micturition and the neural mechanism causing it is called the micturition reflex.

Excretory System Functions

Following are the functions of the excretory system:

1. It helps in the elimination of toxic nitrogenous wastes from our body.
2. It also helps in the filtration of blood by ultrafiltration.
3. It helps in osmoregulation (i.e., maintaining salt and water balance in our body).

Disorders of Excretory System

Following are the disorders of the excretory system:

1. Uremia: Presence of excessive amount of urea in the blood.
2. Kidney failure (or renal failure): Partial or total inability of kidneys to carry out the excretion and osmoregulation.
3. Renal Calculi: Formation of stone or insoluble mass of crystallised salts (calcium, magnesium, phosphates, oxalates, etc.)
4. Glomerulonephritis: Inflammation of glomerulus of kidney.
5. Diabetes Insipidus (DI): Condition characterised by excessive thirst and frequent dilute, urination.

Artificial Kidney (Haemodialyser)

1. Artificial kidney is a machine that is used to filter the blood to remove urea and other nitrogenous wastes of a person, whose kidneys are damaged by the process called haemodialysis.
2. It works on the principle of dialysis (i.e., the diffusion of small solute molecules through a semipermeable membrane like cellophane).
3. Hemodialyzer is a cellophane tube that is suspended in a dialysing fluid (salt-water solution) of the same composition as that of plasma except the nitrogenous wastes (urea).
4. Pores of the cellophane tube allow the passage of molecules based on concentration gradient.

Summary

Excretion is one of the most important physiological processes accomplished by the excretory system. This system helps in expelling the toxic nitrogenous wastes from the body and osmoregulation. It also helps in the filtration of blood by ultrafiltration. Humans excrete urea which is formed in the liver. Besides kidney, skin, liver, lungs, intestine and salivary glands also help in the excretion of certain substances.

Frequently Asked Questions About Human Excretory System

Let’s look at some of the commonly asked questions about Human Excretory System:

Q.1: Draw a diagram of the excretory system?
Ans:

 

Q.2: How does the excretory system work?
Ans: The excretory system works by the following three steps:

1. Ultrafiltration
2. Selective reabsorption
3. Tubular secretion

Q.3: What is an excretory system?
Ans: Excretory system is one of the biological systems in our body that helps in the elimination of unwanted and toxic nitrogenous wastes as well as helps in maintaining the salt-water balance in the body.

Q.4: Name the instrument which is used in dialysis.
Ans: The name of the instrument which is used in dialysis is called Haemodialyser.

Q.5: What are the four parts of the renal tubule?
Ans: The four parts of the renal tubule are:

1. Proximal Convoluted Tubule (PCT)
2. Loop of Henle
3. Distal Convoluted Tubule (DCT)
4. Collecting tubule

Q.6: Name the functional unit of the human kidney?
Ans: Nephrons are the structural and functional unit of our kidneys.

Q.7: What are the accessory excretory organs in our body?
Ans: Besides kidneys, skin, and lungs helps in the excretion of some amount of salt and carbon dioxide from our body. They are the accessory excretory organs.

Q.8: Name the major product that we excrete.
Ans: Human beings are ureotelic animals, as urea is the major excretory product.

Learn About Excretion in Animals Here

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