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Overview of human excretory system, Nephron, Mechanism of excretion, Practice Problems and FAQs

Nowadays we all will have a water purifier system at home. Have you ever observed the working mechanism of a water purifier? Yes, you might have observed this when the company assigned person comes for the service. All the water purifiers contain filters that filter out the waste or unwanted products from the water and make it potable. Hence the water we consume contains all the essential nutrients that are not harmful to the body.

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Fig: Water purifier and filters

You know one such type of filtration system is present in our body too. Yes, the process of filtration occurs in the kidneys in our body. The kidneys filter the blood and separate the waste products from it. These waste products accumulate and form urine. Then this will get temporarily stored in the urinary bladder and from there it will get eliminated by the urethra through the urinary orifice.

Do you know in water purifiers, there are small mesh-like structures that mainly act as filters? Similarly, in the kidneys, nephrons are the structural and functional units that filter the blood. Let’s understand more about the excretion in humans in this article.

Table of contents

Human excretory system

Every living organism depends on various life-sustaining processes, such as digestion, absorption, circulation, and many more. Excretion is one of them. The modes of excretion vary in different organisms. In humans, which is a complex organism, a specialised system is used for excretion referred to as the human excretory system.

The source of nutrients varies in different organisms. These nutrients are metabolised in the body of an organism. In humans, the body starts to sort out useful and toxic substances after metabolic reactions. Excretion is the process through which the body removes all the metabolic wastes and toxins from the body. This is because the accumulation of toxins is harmful to the body. Elimination of waste materials occurs through kidneys, lungs, and skin mainly. It depends on the habitat and food habits. For example, aquatic animals excrete waste products in the form of ammonia, whereas birds and insects excrete waste products in the form of uric acid. On the other hand, humans excrete waste products in the form of urea.

Fig: Excretory products

Organs of the human excretory system

Organs in the human excretory system include the following:

  • A pair of kidneys
  • A pair of ureters
  • Urinary bladder
  • Urethra

Fig: Human excretory system

Kidneys

The kidneys are reddish-brown, bean-shaped organs that are situated close to the dorsal inner wall of the abdominal cavity between the levels of the third lumbar and last thoracic vertebrae. A mature human kidney is 10 to 12 cm long, 5 to 7 cm wide, and 2 to 3 cm thick. A single kidney typically weighs between 120 and 170 g. Due to the presence of the liver on the right side, the right kidney in humans is located slightly below the left one. The human kidney is metanephric. The inner side of the kidney is concave in shape. The ureter, blood vessels, and nerves enter the kidney through a point known as the hilum and are present at the inner concave surface of the kidney. There is a large funnel-shaped area called the renal pelvis present inner to the hilum. It is the area at the centre of the kidney and urine gets collected here. From here it is then funnelled into the ureter. The outer layer of the kidney is known as the renal capsule which is a tough capsule and it provides protection from infections and injuries.

Fig: Outer layer of the kidney

The inner part of the kidney is divided into two zones as follows:

  • Inner medulla
  • Outer cortex

Inner medulla

The inner lighter part of the kidney with striated appearance is known as the medulla. The medulla is further divided into conical masses which are referred to as medullary pyramids. They are 15 - 16 in number. The broad base of these pyramids is towards the cortex. Each pyramid terminates into a structure called the renal papilla. These pyramids extend to the next portion of the kidney called the minor calyx (7 - 13). Minor calyces lead to major calyces. This then opens into a funnel shaped structure called renal pelvis. This inturn leads to the ureter.

Fig: Inner medulla

Outer cortex

The outer darker region of the kidneys is known as the cortex. It extends as renal columns known as columns of Bertini in the space between the medullary pyramids.

Fig: Outer cortex

Ureters

Ureters are present in a pair and are long, thin, and whitish tubes. They are composed of smooth muscles. The length of ureters is about 25 - 30 cm and their diameter is about 3 mm. It originates from the kidney's hilum region, descends down the abdominal wall, and then makes an oblique bend inward and upward to open into the urinary bladder. Ureters are composed of transitional epithelium and their function is to transfer urine from the kidneys to the urinary bladder.

Fig: Ureter

Urinary bladder

It is a sac-like muscular organ that varies in shape, size, and position. The shape and size of the urinary bladder depend on the amount of urine present in it. The bladder has an oval shape when it is fully inflated. It is located in the pelvic cavity. It has a muscular, distensible wall made of thick muscles that are lined with transitional epithelium. This epithelium can grow to accommodate significant volumes of urine. The wall of the urinary bladder is composed of three layers, the inner and outer muscle layers are composed of longitudinal fibres and the middle muscle layer is composed of circular fibres.

Fig: Urinary bladder

Parts of the urinary bladder

The urinary bladder is divided into four parts as follows:

  • Apex is the anterosuperior part of the urinary bladder.
  • The fundus is the posteroinferior part of the bladder.
  • The body is present between the apex and the fundus.
  • The neck is the constricted part of the bladder and it leads to the urethra.
  • The triangular area between the three openings such as two openings of entry of ureters, and one opening of the exit of urethra is called trigone.

Fig: Parts of the urinary bladder

Sphincters of the urinary bladder

The urinary bladder possess two sphincters as follows:

  • Involuntary internal sphincter - It is made up of circular smooth muscles.
  • Voluntary external sphincter - It is made up of skeletal muscles.

The sphincters typically relax during micturition, and this process is controlled by the cerebral cortex.

Fig: Urethral sphincters

Urethra

The urethra is described as a thin tube located at the lower end of the urinary bladder. It begins from the neck region of the urinary bladder and opens outside the body. It is short in females about 2 - 4 cm in length and long in males about 20 cm in length. Its function is to transfer urine as well as semen through the urogenital aperture in males. This aperture is situated at the tip of the penis in a man. This aperture passes urine in females.

Fig: Urethra

Nephron

Nephron is described as the structural and functional unit of the kidneys. Each kidney possesses approximately one million nephrons. They are long and coiled structures composed of two major parts, renal tubules, and the glomerulus. Connective tissues assist in holding the nephrons of the kidney together.

Fig: Nephron

Glomerulus

It is described as a tuft of capillaries that appears like the ball of thread and is formed by the afferent arteriole. The cup-shaped cavity of the Bowman’s capsule contains this structure. Ultrafiltration of the blood is the first step of urine formation that takes place here. Blood flowing through the glomerulus generates a significant amount of hydrostatic pressure because the afferent arteriole is wider than the efferent arteriole.

Fig: Glomerulus

Renal tubule

It starts with a Bowman's capsule, a double-walled cup-like structure, and its opposite end opens into the collecting duct. It consists of the distal convoluted tubule (DCT), Bowman's capsule, proximal convoluted tubule (PCT), and Henle's loop. Depending on the blood pressure and composition of the body, beneficial chemicals and water are reabsorbed from the renal tubule into the blood flowing through the capillaries surrounding the tubule when the glomerular filtrate passes through it. The name of this procedure is selective reabsorption.

Fig: Renal tubule

Bowman’s capsule

At the proximal end of the renal tubule, there is a double-walled cup-like structure called the bowman’s capsule present. In this cavity, the glomerulus is situated. The parietal layer of the bowman’s capsule is composed of flattened epithelial cells. The visceral layer is composed of special podocyte cells or foot cells that lie in close proximity to the glomerulus. These foot cells possess several feet-like processes known as pedicels. The glomerulus and bowman’s capsule collectively form the renal corpuscle, Malpighian body or Malpighian corpuscle. It is present in the cortex region of the kidney.

Fig: Malpighian body

Proximal convoluted tubule (PCT)

The bowman’s capsule leads to the proximal convoluted tubule, which is present in the cortex. It is described as a highly coiled structure. After ultrafiltration, the glomerular filtrate enters the first part of the renal tubule called PCT where it actively absorbs and secretes beneficial chemicals.

Fig: Proximal convoluted tubule

Loop of Henle

It is a tubular structure like a hairpin. It is a nephron segment that enters the renal medulla. It consists of ascending and descending parallel limbs. This structure is also responsible for selective reabsorption from the glomerular filtrate.

Fig: Loop of Henle

Distal convoluted tubule (DCT)

The ascending limb of the loop of Henle continues and leads to the distal convoluted tubule. It is characterised by the highly coiled tubular structure. It likewise resides in the renal cortex and exits into the collecting duct.

Fig: Distal convoluted tubule

Collecting duct

Many nephrons' collecting tubules open into an uncurved tube known as the collecting duct. It meets up with other collecting ducts to create the duct of Bellini. These run through the medullary pyramids and open into calyces which lead into the renal pelvis.

Fig: Collecting duct

Vasa recta

The peritubular capillaries are a small capillary network that surrounds the renal tubule and is created by the efferent arteriole leaving the glomerulus. A little vessel in this network creates a U-shaped vasa recta by running parallel to the Henle's loop.

Fig: Vasa recta

Excretion in humans

Excretion is the procedure through which the body gets rid itself of all metabolic wastes. Human excretion involves a number of procedures that pass through various body components and interior organs.

In lower organisms, the most common process of excretion is the diffusion. A human body is an exceptional example in which different life processes, such as respiration, circulation, digestion, etc. take place simultaneously. Therefore, different types of waste products are produced in the body that include carbon dioxide, water and nitrogenous products like urea, ammonia, and uric acid. Additionally, certain chemicals and other toxic compounds from medications and hormonal products are also formed. To get rid of these waste materials from the body, simple diffusion is not enough. Therefore, more complex and specific processes are required to eliminate these types of waste products. Both beneficial and toxic compounds can be found in the blood. Thus, our kidneys distinguish between beneficial nutrients through reabsorption and harmful substances through the production of urine.

A nephron is the structural and functional unit of the kidney where blood is filtered. One million nephrons can be found in each kidney. The blood passes via the kidneys' capillaries, where vital components including glucose, amino acids, salts, and the necessary amount of water are reabsorbed before the blood returns to its normal circulation.

Humans produce urine from excess water and nitrogenous wastes. The urine generated is transported to the urinary bladder by the ureters. The sympathetic nervous system regulates the storage of urine in the urinary bladder. The parasympathetic nervous system controls the contractions of the urinary bladder and the passage of urine. We expel urine through the urethra, the opening to the urinary system that is controlled by the brain, which also causes the bladder to contract. The somatic efferent system permits the voluntary control over the external urethral sphincter.

Mechanism of excretion

In humans, the process of excretion takes place in the following steps:

  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
  • Micturition

GIF: Formation of urine

Glomerular filtration

It is the first step in the production of urine. In this procedure, the kidney's extra fluid and waste materials are filtered from the blood into the kidney's urine collection tubules and expelled from the body. Glomerular Filtration Rate refers to the volume of filtrate the kidneys generate in each minute (GFR).

GIF: Glomerular filtration

Tubular reabsorption

It involves the reabsorption of ions and compounds like water, glucose, amino acids, sodium ions etc. While sodium ions and glucose are absorbed actively, substances like water absorb passively.

Fig: Tubular reabsorption and secretion

Tubular secretion

Several substances like ammonia, potassium ions and hydrogen ions are secreted out in order to maintain the equilibrium between the body fluids. The functions of the various tubules in the process of tubular secretion are listed below:

  • Glomerulus filters the blood.
  • Water, ions, and nutrients are reabsorbed by proximal convoluted tubules (PCT). By secreting potassium, hydrogen, and ammonia into the filtrate and reabsorbing bicarbonate ions from the filtrate, they eliminate toxins and aid in regulating the ionic balance and pH of the body fluids.
  • Since the descending loop of Henle is impervious to electrolytes but permeable to water, the filtrate is concentrated.
  • Henle's ascending loop is permeable to electrolytes but impenetrable to water. Due to the movement of electrolytes from the filtrate to the medullary fluid, the filtrate becomes diluted.
  • Water and sodium ions can be reabsorbed by the distal convoluted tubule (DCT). By secreting and reabsorbing ions like PCT, it also contributes to the maintenance of pH and ionic balance.
  • The collecting duct reabsorbs a significant amount of water from the filtrate.

Micturition

Urine produced in the nephrons stretches and fills the urinary bladder. The receptors on the bladder's walls interact with the central nervous system to cause the sphincter muscles to relax and discharge urine. The process of discharging urine is known as micturition.

Fig: Micturition

Practice Problems

  1. Which of the following structure(s) is/are present in a pair in the human excretory system?
  1. Ureters
  2. Kidney
  3. Urinary bladder
  4. Both a and b

Solution: The human excretory system is made up of a pair of kidneys, a pair of ureters, urinary bladder and the urethra. Kidneys help in the formation of urine and two ureters transport urine from the kidneys to the urinary bladder. Urinary bladder serves as a temporary holding area for urine. Through urethra urine is expelled when the bladder contracts. Hence, the correct option is ‘d’.

Fig: Human excretory system

2. In the human excretory system, which of the following is an elastic and flexible organ?

  1. Kidney
  2. Urinary bladder
  3. Nephron
  4. Ureter

Solution: Urinary bladder is a sac-like muscular organ that varies in shape, size, and position. The shape and size of the urinary bladder depend on the amount of urine present in it. The bladder has an oval shape when it is fully inflated. It is located in the pelvic cavity. It has a muscular, distensible wall made of thick muscles that are lined with transitional epithelium. This epithelium can grow to accommodate the storage of significant volumes of urine. Hence, the correct option is ‘b’.

Fig: Urinary bladder

3. Which of the following human excretory system components does not have the same size in males and females?

  1. Ureters
  2. Urethra
  3. Urinary bladder
  4. Kidney

Solution: The urethra is described as a thin tube located at the lower end of the urinary bladder. It begins from the neck region of the urinary bladder and opens outside the body. It is short in females about 2 - 4 cm in length and long in males about 20 cm in length. Its function is to transfer urine as well as semen through the urogenital aperture in males. This aperture is situated at the tip of the penis in a man. This aperture passes urine in females. Hence, the correct option is ‘b’.

Fig: Urethra

4. List the major significance of the excretory system in humans?

Answer: The major significances of the human excretory system are as follows:

  • The excretory system’s function is to eliminate wastes from the body, including salts, carbon dioxide, nitrogen, and water.
  • It controls the body's fluid balance.
  • It also keeps the quantities of salt and water constant.

FAQs

  1. What is the maximum capacity of the human urinary bladder?

Answer: The human urinary bladder can stretch maximally to hold about 500 ml of urine in women and 700 ml in men.

  1. How many times does blood get filtered in a day?

Answer: Our kidneys filter all of the blood in our body 40 times each day.

  1. What is the maximum time a healthy bladder can hold urine?

Answer: If the urinary system is healthy, urine can remain in the bladder for up to 5 hours before being released, depending on the amount of liquid consumed.

  1. What is the colour of a normal urine?

Answer: The colour of the normal urine ranges from pale yellow to deep amber. This is because of the presence of pigment urochrome.

Fig: Urine colour

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