Human Excretory System: Organs, Mechanism and Functions

Thе human еxcrеtory systеm, also known as thе urinary systеm, is responsible for rеmoving wastе products and еxcеss substancеs from thе body. It consists of sеvеral organs and structurеs that work together to maintain fluid and electrolyte balancе, regulate blood pressure, and еliminatе wastе through urinе production. 

• These harmful substances formed due to metabolism are called excretory products. 
• The process of elimination of the excretory product from the body is called excretion. 
• The organs concerned with the process of excretion are called excretory organs. 
• Kidney is concerned as the main excretory organ of humans. 
• Kidney has about 10 lakh structural and functional units called nephrons or urine ferrous abuse. 

Other than the Urinary System, the human excretory system possesses the skin and lungs. The skin eliminates excess water and salts through sweat, while carbon dioxide is excreted from the body through the lungs. 

Key Terms: Kidneys, Excretion, Waste Products, Nephron, Dialysis, Glomerular Filtration, Tubular Reabsorption, Tubular Secretion,

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Excretory System Organs 

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The system which is formed of excretory organs for the elimination of the metabolic excretory products from the body is called excretory system.

• The excretory system of man consists of two kidneys, two ureters, one urinary bladder and one urethra. 
• The kidneys are the prime excretory organs of the body which help to eliminate nitrogenous wastes as urine. 
• The liquid urine passes from each kidney through the ureter to the urinary bladder where urine is temporarily stored. 
• After a certain interval, the muscular wall of the bladder is contracted to force urine out through the single urethra. 
• A sphincter muscle is present at the junction of urinary bladder and urethra, by which we can evacuate urine as we desire.

Human Excretory System

Kidney 

Kidney is the chief excretory organ in man. There is a pair of kidneys situated in the abdominal cavity on either side of the vertebral column 

• From the lower border of thoracic vertebrae to the upper border of 2nd or 3rd lumbar vertebrae.
• The left kidney is narrower and longer. 
• The right kidney is situated at 1.5cm lower level than the kidney to accommodate space for the right lobe of the liver.

Ureter

The ureter of each kidney leaves from the renal pelvis in the hilum (hilus) region. 

• The ureter of each kidney runs backward along the abdominal wall to open into the urinary bladder. 
• It carries urine from the kidney to the urinary bladder

Urinary Bladder

It is a pear shaped muscular organ lying in the pelvic cavity. It is composed of transitional epithelium. The muscular layer of the bladder is called detrusor muscle, which consists of three layers of smooth muscle. It has two types of sphincters, namely-

•  The internal sphincter (part of the detrusor muscle) lies in the junction of bladder and urethra.
•  The external sphincter (part of skeletal muscle) located inferior to the internal sphincter. 
• Both the sympathetic and parasympathetic nerve supply the urinary bladder. 
• Internally the urinary bladder, a triangular area is present, which is called Trigone which is connected with the urethra.

Urethra

The Urethra is a tube -like structure which extends from the neck of the bladder and leads to the exterior.

 

• In male it is 20 cm in length and opens out at the tip of the penis through urogenital aperture. 
• In females, it is only 4 cm in length and opens by urethral orifice in front of vaginal aperture. 
• Female urethra carries only urine but in the case of male it carries both urine and semen.

Read More: Role of Other Organs in Excretion

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Structure and Parts of Kidney

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The structural setup of the kidneys and its sub-parts are discussed in brief here:-

• The kidneys are bean-shaped structures surrounded by a fibrous capsule. 
• Each kidney has a round lateral and a concave medial border. 
• In the medial border, there is a fissure called the hilum which gives passage for the entry of renal artery and lymphatics and for the exit of renal vein and ureter.

Human Kidney Anatomy

Parts of a Kidney

The parts of a kidney are mentioned below: 

• Renal Cortex- The kidney is found to be made of an outer reddish brown part called the cortex.
• Renal Medulla- Each kidney has a paler inner part called the medulla. 
• Renal pyramids- In the medulla, there are 6-15 longitudinally arranged conical structures called renal pyramids.
• Renal columns (columns of Bertin)- The renal pyramids are separated from each other by renal columns (columns of Bertin). 
• Papillae- The apices of the pyramids are called papillae.
• Renal pelvis- The kidney has a central cavity near the hilum. This is called sinus which contains the renal pelvis. The pelvis is actually expanded to the upper part of the ureter. 
• Major calyx- Within the kidney, the pelvis breaks up into 2 or 3 main divisions called the major calyx. 
• Minor calyx- Each major calyx further breaks up into about 8 divisions called the minor calyx. These connect the collecting tubules of nephron.

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Structure of a Nephron

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Microscopically, the cortex and medulla of the kidneys are composed of blood vessels, nephrons, lymphatics and nerves. Nephron is the structural and functional unit of the kidney. 

• Each kidney has about 1 million nephrons. 
• So, the total number of nephrons in both the kidneys is 2 millions.
• Structurally, each nephron is made of two parts:

1. Malpighian Corpuscle
2. Renal Tubule

Nephron

Malpighian Corpuscle

Here, the Filtration of blood occurs and the non-protein part of blood is separated from the protein part. The Malphigian corpuscle is divided into two parts. 

(A) Bowman's Capsule: It is the cup-like structure that surrounds the glomerulus. It is made of two layers :

• Visceral layer: It is the inner layer in contact with the basement membrane of the glomerulus 

1. It is made of one layer of specialised cells called the Podocytes. 
2. These cells have tentacle-like cytoplasmic processes called pedicels which remain attached to the basement membrane 
3. In between two pedicels there are slit-like pores (filtration slits) through which glomerular filtrate enters the Bowman's capsule.

• Parietal layer: It is the outer layer connected to the renal tubule. It is made of a single layer of squamous epithelial cells.

(B) Glomerulus: Glomerulus is a small cluster of blood capillaries. It is surrounded by the Bowman's capsule. The afferent arteriole from the renal artery enters the Bowman's capsule and divides into about loops of blood capillaries which then reunite to form efferent arteriole which comes out of the glomerulus.

• This different arteriole divides multiple times and forms the second set of capillaries called peritubular capillaries. 
• These capillaries wrap the entire length of the tubule.
• The glomerular blood capillary is made of a single layer of flat endothelial cells. 
• The cell layer of glomerular blood capillaries, visceral layer of Bowman's capsule and basal lamina together form the filtration membrane. 

Renal Tubule

It arises from the Bowman's capsule. The tubule is divided into the following parts:

1. Proximal convoluted tubule: It is a tortuous tubule which measures 14 mm in length. It is made of a single layer of cuboidal epithelial cells with brush border microvilli. 

• The cells contain large spherical nucleus and granular cytoplasm with rod shaped mitochondria, golgi bodies, vesicles and pinocytic vacuoles. 
• It ends in Henle's loop.

2. Henle's loop: It is a U shaped loop which consists of 4 segments, namely- 

• Descending thick segment. It is straight and made up of cubical epithelial cells with brush border microvilli.
• Descending thin segment: It is made of flat squamous epithelial cells with irregularly oriented microvilli.
• Ascending thin segment: It is made of squamous epithelial cells with flattened nucleus, mitochondria etc. 
• Ascending thick segment: It is straight and made of cubical epithelial cells without microvilli. The Henle's loop joins the distal convoluted tubule.

100. Distal convoluted tubule: It is present in the cortex.

•  It is lined by cuboidal epithelial cells with granular cytoplasm having few mitochondria and few microvilli.
• The distal convoluted tubule is joined with the collecting tubule.

500. Collecting tubule: It is a broad tubule in which the distal part of the distal convoluted tubule opens.

•  It comprises cuboidal epithelial cells.
•  A number of collecting tubules unite to form the Duct of Bellini which joins the ureter in the pelvis of the kidney

Read More: Difference Between Mitochondria and Chloroplast

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Accessory Excretory Organs

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The two kidneys are called the primary excretory organs, because 70% excretory products are eliminated by them. The other organs which help to eliminate 30% excretory products are called accessory excretory organs. These are skin, liver, lungs, salivary glands and large intestine.

Skin

Human skin possesses sweat glands and sebaceous glands, basically developed for the regulation of body temperature by lubrication of skin.

(1) Sweat glands: These produce aqueous fluid called sweat. It excretes water, NaCl. urea, amino acids, CO, traces of lactic acid, etc.

(2) Sebaceous glands: These secrete oily substances called sebum. It excretes wax, sterols, fatty acids, and certain hydrocarbons.

Liver

Liver plays a major role in the excretion of cholesterol, bile pigments (bilirubin and biliverdin), steroid hormones, drugs, excess of vitamins and inactivated products. 

• These substances are carried by the bile to the intestine and are eliminated with faeces. 
• Ammonia, the most toxic metabolic waste product, is formed during breakdown of amino acids in the liver. 
• Being highly soluble, it can kill cells when present in blood. 
• The liver readily converts ammonia into a less toxic substance called urea. 
• The urea is released into blood and then excreted through the kidney..

Lungs 

Thе lungs arе thе primary organs of thе rеspiratory systеm in humans and othеr mammals.

• Thеir main function is to facilitatе thе exchange of oxygеn and carbon dioxidе bеtwееn thе atmosphеrе and thе bloodstream.
• Oxygеn is required by thе body's cеlls for еnеrgy production, whilе carbon dioxidе is a wastе product that nееds to bе еliminatеd. 
• Lungs help in elimination of CO, produced in the body. 
• Some moisture and some volatile materials are also lost in the expired air.

Read More: Kidney Function Test

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Mechanism of Excretion in Kidneys

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As already mentioned, kidneys cause the removal of nitrogenous wastes as urine. Urine is formed in the nephron of kidney by the following processes:-

Glomerular Filtration

The first step in the formation of urine is the production of an ultrafiltrate of the plasma by the glomerulus. 

• The ultrafiltrate lacks cellular elements and is protein free. 
• The concentration of salts and organic molecules such as glucose and amino acids are similar in the plasma and ultrafiltrate.
• Ultrafiltration is driven by Starling forces across the glomerular capillaries 
• Changes in these forces alter the glomerular filtration rate [GFR) GFR and renal blood flow (RBF) are normally held within narrow ranges by Autoregulation.

Tubular Reabsorption

When glomerular filtrate passes through the renal tubule, various threshold substances like glucose, amino acids, water, NaCl etc are reabsorbed at its various parts and brought back to blood. The reabsorption takes place at various parts as follows-

(a) Proximal convoluted tubule 

Active Reabsorption: NaCl, glucose, amino acids, phosphate, sulphate etc are reabsorbed in this process. Passive Reabsorption: About 67% of the filtered water is reabsorbed passively with NaCl. 

• This reabsorption of water with Na is called obligatory water reabsorption
• Urea is also reabsorbed by passive processes here. 
• The key element in proximal tubular reabsorption is the Na-K -ATPase pump in the basolateral membrane. 
• The reabsorption of every substance including water is influenced by the Na-K-ATPase pump.

(b) Henle's loop

 

The descending segments reabsorb 15% of the filtered water.The ascending segments (thick ascending limb) reabsorb rest of glucose, sodium, approximately 25% of the filtered NaCl, bicarbonate, urea etc. 

• However, it is impermeable to water. 
• The key element in solute reabsorption by adsorption in a thick ascending limb is the Na-K-ATPase pump in the basolateral membrane. 

(c) Distal convoluted tubule and collecting tubule

These two parts are impermeable to water molecules. But about 8% to 18% water is reabsorbed there under the influence of ADH and aldosterone hormone. 

• This type of water reabsorption is called facultative water reabsorption. 
• The distal convoluted tubule reabsorbs Na". K. CI. urea, 8% of filtered NaCl and bicarbonate.

Tubular Secretion

During reabsorption different new substances formed in various parts are secreted to urine: 

(a) Proximal convoluted tubule 

These parts secrete hippuric acid, sulphur compounds, creatinine, penicillin etc.

(b) Distal convoluted and collecting tubule

These parts secrete urea, quinine, etc. The distal convoluted tubule secretes K⁺, H⁺ in exchange for the reabsorption of Na⁺ ion. 

• The secretion of hippuric acid, H+ ions, etc make normal urine acidic. 
• Thus about 180 litres of glomerular filtrate while passing through the renal tubules undergo concentration, change in composition and are converted into about 1.5 litres of urine per day.

Read More: Pituitary Glands

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Functions of Kidney

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The kidneys perform the primary role of excretion of nitrogenous wastes products from the body. Apart from this, it also performs several other functions such as:-

• Kidney mainly excretes the waste products, especially the nitrogenous and sulphur containing end products of protein metabolism.
• It helps to maintain the water balance of the body.
• It helps in the regulation of acid-base balance of blood.
• It controls osmotic relations between blood and tissue cells.
• It helps to maintain electrolyte balance of the body and normal hydrogen ion concentration.
• It excretes toxic metabolic products through urine.
• It synthesises NH₃, hippuric acid and other substances. 

Read More: Difference between Left and Right ventricle

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Dialysis

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Physiologically, the term dialysis means the diffusion of solutes from an area of higher concentration to the area of lower concentration through a semipermeable membrane. 

• This principle has been used to dialyse the blood of patients with renal failure particularly those developing uraemia. 
• Two types of dialysis process are in practice: 

1. Haemodialysis or Artificial Kidney

The process can save life in many types of acute renal failure [produced due to circulatory stock, etc.] and in patients with chronic renal failure, may prolong the life of many patients.

• The Haemodialysis machine is also called an artificial kidney. 
• Haemodialysis is done on a hospitalised patient through an intravenous route for 3-5 hours. 
• During this process the patient's radial artery is connected to the haemodialysis machine.

2. Peritoneal Dialysis

Here the peritoneum is used as a semipermeable membrane. 

• The process is useful for young children, old patients with cardiovascular disorders and can be done at home or at work without hospitalisation. 
• It enhances survival in patients with chronic renal failure for many years.

Disadvantages of Dialysis

The disadvantages are:

• It is an expensive process and needs to be repeated almost every week. Therefore in people with irreversible terminal renal failure, it cannot act as a remedy. 
• The process cannot maintain completely normal body fluid composition 
• It cannot replace all the multiple functions (for eg, endocrine and metabolic functions) of the kidney.
•  So the health of patients maintained on artificial kidneys usually remain impaired significantly. 
• Therefore renal transplantation is the final way out to all the problems associated with chronic renal failure.

Also Read:

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Things to Remember

• Excretion is the physiological process for the safe removal of water soluble and toxic waste products produced due to cellular metabolism.
• Human excretory system comprises kidneys, a pair of ureters, a urinary bladder and a urethra. 
• Kidney is divided into three main areas-cortex, medulla and pelvis.
• Nephron is the structural and functional unit of kidney 
• More than one million nephrons present in each kidney, Nephron tubule is closed at the beginning but open at distal end. 
• A network of capillaries, known as glomerulus, is present within the Bowman's capsule. 
• Nephron can be divided into two types according to the position-juxtaglomerular and cortical. 
• Juxtaglomerular nephrons consist about 15% and cortical nephrons consist about 85% of the nephrons.
• Kidney also helps to maintain water and sodium content of blood called osmoregulation.

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