Urinary System and Excretion in Humans

Urinary System and Excretion in Humans



  • Animals accumulate various waste substances like ammonia, urea, and uric acid through metabolic activities or excess ingestion. 
  • These wastes need to be removed either totally or partially to maintain internal balance. 
  • Animals excrete mainly three types of nitrogenous wastes: ammonia, urea, and uric acid. 
  • Ammonia is the most toxic, requiring large amounts of water for elimination. 
  • Uric acid is the least toxic and can be removed with minimal water loss. 
  • Ammonotelism is the process of excreting ammonia. 
  • Many aquatic animals like bony fishes, aquatic amphibians, and aquatic insects are ammonotelic. 
  • Ammonia is excreted through body or gill surfaces via diffusion as ammonium ions, with minimal involvement of kidneys. 
  • Ureotelism is the process of excreting urea. 
  • Mammals, terrestrial amphibians, and marine fishes mainly excrete urea. 
  • Ammonia produced in metabolism is converted to urea in the liver, filtered by kidneys, and excreted through urine. 
  • Uricotelism is the process of excreting uric acid. 
  • Reptiles, birds, land snails, and insects excrete uric acid, usually in the form of pellets or paste, with minimal water loss. 
  • Invertebrates have simple tubular excretory structures, while vertebrates have complex kidney structures. 
  • Examples include protonephridia (flame cells), nephridia, Malpighian tubules, and antennal glands. 
  •  Protonephridia

- Found in flatworms, rotifers, some annelids, and cephalochordates (e.g., Amphioxus). 

- Primarily regulate ionic and fluid volume, aiding in osmoregulation. 

  • Nephridia

- Tubular excretory structures found in earthworms and other annelids.

- Help remove nitrogenous wastes and maintain fluid and ionic balance. 

  • Malpighian Tubules

- Found in most insects including cockroaches. 

- Aid in nitrogenous waste removal and osmoregulation. 

  • Antennal Glands

- Excretory structures in crustaceans like prawns. 

- Perform excretory functions.


Human Excretory System





  • The human excretory system comprises two kidneys, a pair of ureters, a urinary bladder, and a urethra 
  • Kidneys are reddish-brown, bean-shaped organs located between the last thoracic and third lumbar vertebrae near the dorsal inner wall of the abdominal cavity. 


Anatomy of the Kidney


  • Each kidney measures approximately 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness, with an average weight of 120-170 g. 
  • The kidney has a notch called the hilum through which the ureter, blood vessels, and nerves enter. 
  • Internally, the kidney consists of an outer cortex and an inner medulla, divided into conical masses called medullary pyramids. 
  • The cortex extends between the medullary pyramids as renal columns called Columns of Bertin.




Nephrons: Functional Units of the Kidney


  • Each kidney contains nearly one million nephrons, which are complex tubular structures. 
  • A nephron consists of two main parts: the glomerulus and the renal tubule. 
  • The glomerulus is a tuft of capillaries formed by the afferent arteriole, and blood is carried away by the efferent arteriole. 
  • The renal tubule begins with Bowman's capsule, encloses the glomerulus, and continues as the proximal convoluted tubule (PCT), Henle's loop, and the distal convoluted tubule (DCT).




Types of Nephrons


  • Majority of nephrons are cortical nephrons, where the loop of Henle extends only slightly into the medulla. 
  • Some nephrons are juxtamedullary nephrons, with a long loop of Henle that extends deep into the medulla.








Blood Supply to the Nephron

  • The efferent arteriole from the glomerulus forms peritubular capillaries around the renal tubule. 
  • In juxtamedullary nephrons, the peritubular capillaries form vasa recta, which run parallel to Henle's loop. 


Urine Formation

  • Filtration of blood occurs in the glomerulus, where small molecules are filtered into Bowman's capsule. 
  • Reabsorption of essential substances like glucose, ions, and water occurs in the renal tubule. 
  • Secretion of waste products like urea and drugs into the renal tubule also takes place. 
  •  Urine formed in the nephrons is collected in the collecting ducts and transported to the renal pelvis through the calyces. 


Functional Importance

  • The kidneys play a crucial role in maintaining the body's water and electrolyte balance, regulating blood pressure, and removing waste products from the blood.


Urine Formation 

  • Urine formation is a vital process carried out by the kidneys, ensuring the elimination of metabolic wastes and maintenance of fluid and electrolyte balance in the body.  
  • Through a series of intricate mechanisms, the kidneys filter blood, reabsorb essential substances, and excrete waste products, ultimately producing urine.


Glomerular Filtration

  • The first step in urine formation is glomerular filtration, occurring in the glomerulus. 
  • Glomerular capillary blood pressure causes blood to be filtered through three layers:

- endothelium of glomerular blood vessels,

- epithelium of Bowman’s capsule, and

- a basement membrane. 

  • Podocytes, specialized cells of Bowman’s capsule, have filtration slits that allow small molecules to pass into the Bowman’s capsule, forming the filtrate. 
  • Glomerular filtration rate (GFR) is the amount of filtrate formed per minute, typically around 125 ml/min or 180 liters/day in a healthy individual.


Regulation of GFR

  • Juxtaglomerular apparatus (JGA) helps regulate GFR. 
  • A decrease in GFR can activate JG cells to release renin, which stimulates glomerular blood flow and restores GFR to normal levels.



  • Around 99% of the filtrate must be reabsorbed by renal tubules. 
  • Tubular epithelial cells reabsorb substances like glucose, amino acids, and Na+ through active mechanisms. 
  • Nitrogenous wastes are absorbed passively, while water reabsorption occurs passively in initial nephron segments.


Tubular Secretion

  • Tubular cells secrete substances like H+, K+, and ammonia into the filtrate during urine formation. 
  • Tubular secretion helps maintain the ionic and acid-base balance of body fluids.



  • Glomerular filtration initiates urine formation by filtering blood through the glomerulus. 
  • Regulation of GFR ensures stable kidney function through mechanisms like JGA. 
  • Reabsorption reclaims essential substances from the filtrate, while tubular secretion eliminates excess ions and maintains balance. 
  • Understanding urine formation helps grasp the kidney's vital role in maintaining bodily homeostasis. 
  • It underscores the intricate processes involved in waste elimination and fluid balance regulation.


Function of the Tubules 

  • Renal tubules are essential components of the nephrons, the functional units of the kidneys.  
  • They play a crucial role in urine formation by facilitating the reabsorption of essential substances and the secretion of waste products.  
  • Understanding the functions of the renal tubules is essential for comprehending the intricate processes involved in maintaining fluid and electrolyte balance, as well as pH regulation in the body.  
  •  Let's delve into the specific functions of each segment of the renal tubules to grasp their significance in kidney function and overall physiological balance.


Proximal Convoluted Tubule (PCT):

  • Structure: Lined by simple cuboidal epithelium with brush border, increasing surface area for reabsorption. 
  • Function: Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water. Also involved in pH and ionic balance maintenance through selective secretion of hydrogen ions, ammonia, potassium ions, and absorption of bicarbonate ions.


Henle’s Loop:

  • Structure: Consists of descending and ascending limbs. 
  • Function: The descending limb is permeable to water but impermeable to electrolytes, concentrating the filtrate. The ascending limb is impermeable to water but allows electrolyte transport, diluting the filtrate.


Distal Convoluted Tubule (DCT):

  • Structure: Lined by simple cuboidal epithelium. 
  • Function: Conditionally reabsorbs sodium and water. Also involved in reabsorption of bicarbonate ions and selective secretion of hydrogen ions, potassium ions, and ammonia to maintain pH and sodium-potassium balance in blood.


Collecting Duct:

  • Structure: Long duct extending from the cortex to the inner medulla. 
  • Function: Allows extensive water reabsorption, producing concentrated urine. Permits passage of urea into the medullary interstitium to maintain osmolarity. Selectively secretes hydrogen and potassium ions to maintain pH and ionic balance of blood.




Understanding the functions of renal tubules elucidates the intricate mechanisms involved in urine concentration, electrolyte balance, and pH regulation.

It underscores the kidneys' vital role in maintaining homeostasis and overall bodily function.


Mechanism of concentration of the filtrate

  • Mammals, including humans, can produce concentrated urine, crucial for water conservation. 
  • The Henle’s loop and vasa recta play pivotal roles in this process, employing a mechanism known as the counter-current system.


Counter-Current System:

  • Henle’s Loop: Filtrate flows in opposite directions in its descending and ascending limbs, creating a counter-current. 
  • Vasa Recta: Blood flow in the vasa recta mirrors the counter-current pattern seen in Henle’s loop.


Formation of Osmolarity Gradient:

  • Proximity and counter-current flow in Henle’s loop and vasa recta lead to an increasing osmolarity gradient in the medullary interstitium. 
  • Osmolarity increases from about 300 mOsmol/L in the cortex to approximately 1200 mOsmol/L in the inner medulla.


Role of NaCl and Urea:

  • NaCl Transport: Ascending limb of Henle’s loop actively transports NaCl, exchanging it with the descending limb of vasa recta. 
  • Urea Transport: Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop, transported back to the interstitium by the collecting tubule.


Counter-Current Mechanism:

  • The coordinated transport of NaCl and urea facilitated by Henle’s loop and vasa recta is termed the counter-current mechanism. 
  • This mechanism maintains a concentration gradient in the medullary interstitium, crucial for urine concentration.


Filtrate Concentration:

  • The presence of an interstitial gradient allows for the easy passage of water from the collecting tubule, concentrating the filtrate (urine). 
  • Human kidneys can produce urine nearly four times more concentrated than the initial filtrate formed.





Regulation of Kidney Function 

  • The kidneys play a vital role in maintaining fluid balance, electrolyte levels, and blood pressure in the body.  
  • Their function is tightly regulated by hormonal feedback mechanisms involving various organs and systems, including the hypothalamus, juxtaglomerular apparatus (JGA), and the heart. 
  • These mechanisms ensure that the kidneys adapt to changes in blood volume, pressure, and electrolyte levels to maintain overall physiological balance.


Hormonal Regulation:

  • Osmoreceptors in the body monitor changes in blood and body fluid volume, as well as ionic concentration. 
  • Excessive fluid loss activates osmoreceptors, triggering the release of antidiuretic hormone (ADH) from the hypothalamus. 
  • ADH promotes water reabsorption in the renal tubules, preventing excessive urine production (diuresis). 
  • Increased body fluid volume inhibits ADH release, completing the feedback loop. 
  • ADH also constricts blood vessels, raising blood pressure, which in turn can increase glomerular blood flow and glomerular filtration rate (GFR).


Role of Juxtaglomerular Apparatus (JGA):

  • JGA responds to changes in glomerular blood flow, pressure, and GFR. 
  • Decreased blood flow/pressure/GFR stimulates JG cells to release renin. 
  • Renin converts angiotensinogen to angiotensin I and then to angiotensin II. 
  • Angiotensin II is a potent vasoconstrictor, increasing glomerular blood pressure and GFR. 
  • Angiotensin II also stimulates the release of aldosterone from the adrenal cortex. 
  • Aldosterone promotes reabsorption of sodium and water in the distal tubules, further raising blood pressure and GFR. 
  • This regulatory mechanism is known as the Renin-Angiotensin mechanism.


Atrial Natriuretic Factor (ANF):

  • Increased blood flow to the heart's atria triggers the release of ANF. 
  • ANF causes vasodilation, reducing blood pressure. 
  • This mechanism acts as a counterbalance to the renin-angiotensin system, helping regulate blood pressure and fluid balance.


Micturition Reflex




Storage and Release of Urine:

  • Urine produced by the nephrons is stored in the urinary bladder until signaled for release. 
  • Stretch receptors in the bladder walls detect the filling of urine and send signals to the central nervous system (CNS). 
  • In response, the CNS initiates motor messages to contract the bladder's smooth muscles and relax the urethral sphincter, allowing urine release.


Micturition Reflex:

  • The coordinated neural mechanism responsible for urine release is called the micturition reflex. 
  • Stretch receptors detecting bladder fullness initiate the reflex, leading to bladder contraction and urethral sphincter relaxation.


Urine Characteristics:

  • Urine is a light yellow-colored, watery fluid with a slightly acidic pH (around 6.0) and a characteristic odor. 
  • On average, an adult excretes 1 to 1.5 liters of urine per day, containing approximately 25-30 grams of urea. 
  • Changes in urine characteristics can indicate various metabolic disorders or kidney malfunctions.


Clinical Importance of Urine Analysis:

  • Analysis of urine aids in diagnosing metabolic disorders and kidney dysfunction. 
  • Presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine may indicate conditions like diabetes mellitus.


Role of Other Organs in Excretion

  • Lungs:

- Lungs eliminate large amounts of carbon dioxide (CO2) and significant quantities of water daily, aiding in waste removal. 

- CO2 is expelled during exhalation, while water is lost through respiratory processes.


  • Liver:

- The liver, the largest gland in the body, secretes bile containing various waste products such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. 

- These substances are ultimately eliminated along with digestive wastes through feces.


  • Skin:

- Sweat glands in the skin produce sweat, a watery fluid containing sodium chloride (NaCl), small amounts of urea, and lactic acid. 

- While the primary function of sweat is to regulate body temperature by cooling the skin surface, it also aids in the removal of certain waste products. 

- Sebaceous glands in the skin secrete sebum, eliminating substances like sterols, hydrocarbons, and waxes. Sebum forms a protective oily layer on the skin.


  • Saliva:

- Interestingly, small amounts of nitrogenous wastes can be eliminated through saliva. 

- While saliva primarily aids in digestion and lubrication of food, it also plays a minor role in waste elimination.


Disorders of the Excretory System 

  • The excretory system plays a crucial role in maintaining the body's internal environment by eliminating metabolic wastes.  
  • However, malfunctions in this system can lead to various disorders that can significantly impact overall health.


Uremia and Hemodialysis:

  • Uremia is a condition characterized by the accumulation of urea in the blood due to kidney malfunction. 
  • Hemodialysis is a lifesaving procedure used to remove urea and other waste products from the blood in patients with kidney failure. 
  • During hemodialysis, blood is pumped into a dialysis machine containing a dialyzing fluid that mimics plasma composition. The dialyzer removes waste products by diffusion across a semipermeable membrane.


Kidney Transplantation:

  • Kidney transplantation is the preferred treatment for acute renal failure (kidney failure). 
  • A functioning kidney from a donor, preferably a close relative, is transplanted into the recipient to restore kidney function. 
  • Modern clinical procedures have improved the success rate of kidney transplantation, offering hope to patients with renal failure.


Renal Calculi (Kidney Stones):

  • Renal calculi are stone-like formations composed of crystallized salts such as oxalates, formed within the kidney. 
  • These stones can cause severe pain and may block the urinary tract, leading to complications.



  • Glomerulonephritis is the inflammation of the glomeruli, the filtering units of the kidney. 
  • This condition can impair kidney function, leading to proteinuria, hematuria, and decreased urine output.