Excretory System

The human excretory system is one of the most well-developed systems in the body. It is concerned with the excretion of the wastes found in the blood. The main functional organ here is the kidney which performs two key functions, filtering the blood and maintaining its osmolarity. The detailed aspects of the excretory system are covered below. 

Table of Contents:

• Excretion
• Modes of Excretion
• Human Excretory System
• Kidneys
• Nephron
• Urinary Bladder
• Processes Of Urine Formation

Excretion

Excretion refers to the procedure occurring in the body that removes metabolic wastes from organisms. It is essential for the body as it eliminates the toxic products whose accumulation can compromise health. These toxic products are generated via different biochemical processes that involve metabolism. 

There are different organs responsible for excretion, such as the skin that releases sweat, the liver to filter foreign components such as drugs, the lungs to remove carbon dioxide and the large intestines to process the food and excrete the stool along with the kidney that produces urine as the excretory product. 

Modes of Excretion

The excretion product varies according to the habitat of organisms. While some live in water, others reside in hot deserts and lands. The concentration of toxic products and method of excretion varies to avoid harming the body. The different modes of excretion are: 

• Ammonotelism: It is seen in aquatic organisms where excretion is of ammonia. The organisms are referred to as Ammonotelic. The secretion of ammonia is done due to easy diffusion of the molecule, high solubility in water, lack of water conservation requirement and occurrence of process at a lower energy cost. Common examples include sponges, fish with bones, protozoans, earthworms and others. 
• Ureotelism: The ureotelic animals spend comparatively less time in the water; hence, their mode of excretion requires the conservation of relatively more water. Thus, their excretory product is urea. Though it requires higher energy compared to ammonia, the energy expenditure here is low compared to uric acid. Further, urea is the lesser toxic form and hence can be stored at higher concentrations in the body, contributing to osmoregulation. Examples of organisms include terrestrial animals, mammals and marine fish. 
• Uricotelism: The uricotelic animals have the least contact with water, and hence their excretion method is in relation to the conservation of a higher amount of water. Uric acid is the least toxic and hence exhibits efficient storage. The common organisms excreting uric acid are land snails, birds, insects and reptiles.

Human Excretory System

The human excretory system is a well-developed organ system comprising of the following:

• A pair of bean-shaped kidneys
• 2 ureters connect each kidney with the bladder
• A single urinary bladder
• A urethra extends from the bladder to excrete the urine. 

Kidneys

The kidneys serve the major roles in the body primarily associated with the functionality of the human excretory system. They are present on both sides of the spine above the abdomen and liver. These comprise nephrons as their functional unit that receives the blood. The kidney has a high amount of blood vessels, with intertubular capillaries surrounding the nephrons and blood vessels extending between the lobes. 

The received blood is further filtered and regulated for concentration. The excretory waste obtained from blood is poured into the bladder for storage through the ureter. Once the bladder is full or more than partially full, the brain gets the signal to cause the urge to pass the urine. The urination occurs through the urethra. 

The waste product of kidneys in humans is urine salt, an aqueous solution of waste products, urea, and uric acid. The last two are the nitrogenous compounds. Uric acid forms by the breakdown of nucleic acids, while urea is formed by amino acid breakdown.

Nephron

The nephron is present in around a quantity of 1 million per kidney. They comprise Bowman’s capsule and long tubes or loops. The Bowman’s capsule is in the cortex part of the kidney, while the other long tubular part is in the renal medulla. The length of the medulla depends on the need for water conservation and varies among organisms situated in different habitats. 

The nephrons comprise a Malphigian body, further comprising three parts, glomerulus, renal tubule and loop of Henle.

• Glomerulus

Associated with the filtration system, it has capillaries surrounded by a glomerular basement membrane that acts as a filter barrier. The high pressure in the capillaries allows filtration of water, small waste product molecules, dissolved substances, ions and nutrients. The formed composition is known as glomerular filtrate.

• Renal Tubule (PCT)

Also known as PCT or Proximal Convoluted Tubule, it is the first long tubular structure of the nephron. The glomerular filtrate passes through it for reabsorption of ⅔ rd of its quantity, including removal of glucose and organic solutes. It also adds up with a few acids, thus detoxifying the body of drugs. 

• Renal Tubule (Loop of Henle)

It is the U-shaped structure of the nephron that extends to the medullary region of the kidney. It has ascending and descending limbs that serve different functions. It is connected to PCT, and the filtrate from them reaches the descending loop of Henle. It is water permeable but ion impermeable. The medulla region has higher osmolarity that causes urine concentration and hence water conservation by the body. 

Further moving into the thin ascending loop, the filtrate experiences ion permeability and water impermeability. Here the action of electrolyte balancing occurs by reabsorption of sodium ions, eventually lowering the urine concentration. 

• Renal Tubule (DCT)

It is the hub for the hormonal regulation of urine concentration. The ADH or antidiuretic hormone and angiotensin II are commonly seen. This region concerns ion and pH balance, thus changing urine concentration. The urine further travels to the collecting duct, finally reaching the bladder through the ureter. 

Urinary Bladder

It refers to the sac-like organ with muscular walls. It functions to store urine for further excretion from the body. It is lined on the inside with epithelial cells, thus forming transitional epithelium. The stratified cells are associated with shape-changing based on urine content and allow elasticity. The capacity of the urinary bladder is around 0.5 litres. 

Processes Of Urine Formation

The complete process involved in urine formation within the kidneys is called renal or kidney function. The following are the steps involved:

Glomerular Filtration

This initial step in urine formation occurs in the glomerulus, a network of capillaries within the Bowman's capsule. A fluid known as "glomerular filtrate" is generated when blood passes through the glomerular membrane. Water, electrolytes, waste products, and other tiny molecules are present in this filtrate.

Tubular Reabsorption

The glomerular filtrate is subsequently reabsorbed into the bloodstream by the renal tubules, where important chemicals such as water, glucose, amino acids, and electrolytes are reabsorbed. This process aids in maintaining the body's balance and preventing the loss of important components.

Tubular Secretion

Additional waste materials, poisons, and excess substances that were not filtered originally are actively released into the renal tubules from the bloodstream during this stage. This adds to the production of urine.

Concentration and Dilution

The renal tubules are important in altering urine concentration dependent on the body's requirements. When the body is appropriately hydrated, this process assists in preserving water and excreting surplus water.

Urine Storage

Once the processes mentioned above are completed, urine is produced and gathered in the renal pelvis. It is then transported to the urinary bladder for temporary storage.

Urination

The last stage includes the voluntary relaxation of the bladder sphincters and the synchronised contraction of the bladder wall muscles, which results in the ejection of urine from the body via the urethra.

Practice Problems

Q1. What type of nervous system controls the human excretory system?

a. Peripheral Nervous system
b. Enteric Nervous system
c. Central Nervous System
d. Autonomic Nervous System

Ans. d. Autonomic Nervous System

The sympathetic and parasympathetic nervous system is associated with increased or decreased urine production. 

Q2. What happens in dehydration?

a. Water conservation
b. Regulation through ADH 
c. Increased electrolyte retention
d. All of the above

Ans. d. All of the above

Kidneys function to maintain osmoregularity through water conservation, regulation through ADH, and increased electrolyte retention.

Q3. What is the GFR value?

a. 60 - 80 millilitres per minute
b. 100 millilitres per minute
c. 90 - 120 millilitres per minute
d. 140 millilitres per minute

Ans. c. 90 - 120 millilitres per minute

GFR or glomerular filtration rate varies with different factors, but the general range for adults is 90 to 120 millilitres per minute.

Frequently Asked Questions

Q1. Why is increased sodium intake expected to harm the kidneys?
Answer: The kidneys function to maintain electrolyte balance. The excess sodium intake must also be balanced with the excretion of its ion. To maintain the osmolarity, the excess excretion of ions is compensated with increased water retention leading to swelling and increased blood pressure allowing for the increased functionality of kidneys.

Q2. What are the disorders associated with the excretory system?
Answer: The common disorders of the excretory system include kidney stones, bladder infections, urinary tract infections, and kidney failure. 

Q3. How does alcohol consumption affect urine production?
Answer: Alcohol acts diuretic, which means it increases urine production. It also inhibits the release of ADH, which is concerned with water reabsorption. Hence, alcohol consumption is associated with dehydration and increased urine production.