Excretory Products and Their Elimination PDF

# Excretory Products and Their Elimination ## Introduction - Organisms carry out thousands of metabolic reactions for activities like growth, reproduction, movement, etc. - During these metabolic reactions, many toxic substances are produced and many substances are left unused. These are considere...

# Excretory Products and Their Elimination ## Introduction - Organisms carry out thousands of metabolic reactions for activities like growth, reproduction, movement, etc. - During these metabolic reactions, many toxic substances are produced and many substances are left unused. These are considered wastes and are passed into the blood to be eliminated from the body by the process of excretion. - Excretion differs from defecation, as defecation involves egestion of feces consisting of undigested food particles from the anus. - Excretion and osmoregulation are essential life processes for maintaining homeostasis. ## Types of Excretory Wastes ### Nitrogenous Wastes - Major nitrogenous metabolic wastes include **ammonia**, **urea**, and **uric acid.** - Other minor nitrogenous waste products excreted by the human body include **amino acids**, **creatine**, and **creatinine**, **hippuric acid**, **bilirubin**, and **biliverdin**. #### Ammonia - Out of major food components, the human body cannot store excess protein digestion products (amino acids). - Oxidative deamination of amino acids forms ammonia in the body, which is the most toxic form of nitrogenous waste products. - Ammonia is highly soluble in water and thus, requires a large amount of water for its elimination. #### Urea - Urea is comparatively less toxic than ammonia and less soluble in water. - It can be concentrated in the body for a longer time than ammonia and requires comparatively less water for its elimination. #### Uric Acid - Uric acid is formed in the liver and intestinal mucosa by the metabolism of excess proteins, ammonia, and purines in the diet. - Uric acid is the least toxic for the body and requires very less water for its elimination, as it is almost insoluble in water. - Uric acid is generally passed out as a paste or pellet along with feces and is often seen as a small white patch on one side of faecal matter. ### Non-Nitrogenous Wastes - Major non-nitrogenous wastes include **carbon dioxide**, **non-metabolised minerals and vitamins**, **excess of water**, **pigments**, **hormones**, and **drugs** in the body. - Carbon dioxide is eliminated through the lungs in the respiratory system. - Excess of water is removed by sweating, urination, and as moisture in the expired air. - Excess of minerals, vitamins, and pigments are excreted along with urine, sweat, or fecal matter. ## Modes of Excretion - Depending upon the main excretory products, there are three types of major nitrogenous excretions: **ammonotelism**, **ureotelism**, and **uricotelism**. ### Ammonotelism - Animals which excrete ammonia are called **ammonotelic**, and the phenomenon is called **ammonotelism**. - Ammonotelism is carried out by protozoan protists, poriferans, cnidarians, flatworms, roundworms, annelids like earthworm and leech, most aquatic crustaceans and aquatic insects, most aquatic molluscs, lungfishes, bony fishes, larvae of amphibians, aquatic tailed amphibians, crocodiles, etc. - Elimination of ammonia can be done by directly diffusing out of the whole surface as in soft-bodied invertebrates or as ammonium ions (NH₄⁺) from the gill epithelium of fish. - Kidneys do not perform any significant role in the removal of ammonia. ### Ureotelism - Animals which excrete urea are called **ureotelic**, and the phenomenon is called **ureotelism**. - Ureotelism is seen in Ascaris, earthworm, cartilaginous fishes (shark, stingrays), adult semi-aquatic amphibians like frogs and toads, many reptiles like turtles, terrapins and alligators, mammals including man, etc. - Since urea can be concentrated in the body without toxic effects, cartilaginous fishes maintain a high concentration of urea in their blood and body fluids for osmoregulation against seawater. ### Uricotelism - Animals which excrete uric acid are called **uricotelic**, and the phenomenon is called **uricotelism**. - Animals practicing uricotelism include terrestrial crustaceans, most insects like cockroaches, land snails, land reptiles like lizards and snakes, and all birds. ### Minor Modes of Excretion - Minor modes of excretion include **aminotelism** and **guanotelism**. ## Different Excretory Structures in Animal Kingdom ### Protonephridia - A protonephridium consists of a network of closed tubules and capillaries, which end in **flame cells** having a tuft of cilia in their wide lumen. - Lumen of a flame cell opens into an excretory capillary. - Excretory capillaries form tubules which open to the outside through one to numerous excretory pores or nephridiopores. - Flame cells or protonephridia are found in platyhelminthes like Planaria, rotifers, larvae and adults of some annelids and cephalochordates like Amphioxus. - Protonephridia mainly performs **osmoregulation**. ### Renette or Giant Cell - Roundworms consist of an H-shaped excretory system of **renette** or **giant cell** consisting of four canals. - The middle point of all these canals have a bridge in the form of a network called **canaliculi** which lies exactly beneath the pharynx. ## Major Modes of Excretion - The following table summarizes the major modes of excretion: | Mode of Excretion | Excretory Product | Toxicity | Solubility | Water Requirement | Examples | |---|---|---|---|---|---| | Ammonotelism | Ammonia | Highly toxic | Highly soluble | Large amount of water | Protozoan protists like Amoeba, Paramoecium; Poriferans like Sycon; Cnidarians like Hydra; Flatworms like tapeworm (Taenia solium); Roundworms like Ascaris; Polychaete annelids like earthworm and leech; Aquatic crustaceans like prawn; Aquatic insects; Aquatic molluscs like Pila; Bony fishes; Larvae of amphibians (tadpole of frog); Crocodiles | | Ureotelism | Urea | Less toxic | Less soluble | Less amount of water | Ascaris; Earthworm; Cartilaginous fishes like shark and sting rays; Adult frogs; Adult toads; Turtles; Terrapins; Alligators; Mammals including man | | Uricotelism | Uric acid | Least toxic | Least soluble | Least amount of water | Most terrestrial insects like cockroach; Terrestrial crustaceans like Oniscus; Land snails like Helix; Land reptiles like lizards and snakes; Birds | ## Nephridia - Nephridia are the main excretory structures of annelids like earthworm. - A nephridium usually has a ciliated funnel-shaped opening called **nephrostome**. - The other end of a nephridium has an opening called **nephridiopore**. - Beating of cilia allows the coelomic fluid to enter in the tubule where salts are reabsorbed by the epithelium and later the fluid is transported to the blood from where it is drained out through a nephridiopore. - Nephridia carry out both **excretion** of nitrogenous waste and the process of **osmoregulation**. ## Malpighian Tubules - Malpighian tubules are the main excretory structures of most insects like cockroach. - They are a group of fine, unbranched, blind tubes which open into the alimentary canal at the juncture of midgut and hindgut. - Malpighian tubules take part in both **excretion** and **osmoregulation**. ## Antennal Glands - Antennal glands or green glands are paired glands present in crustaceans like prawns, crayfish, etc. - They perform the functions of **excretion** and **osmoregulation**. ## Human Excretory System - It consists of a pair of kidneys and their blood supply, a pair of ureters, a urinary bladder, and a urethra. ### Kidneys - Kidneys are a pair of dark brown or reddish brown, bean-shaped, metanephric structures originated from the embryonic **mesoderm**. - They are situated on either side of the body’s middle line, close to the dorsal inner wall of the abdominal cavity, below the diaphragm, between the levels of last (twelfth) thoracic and third lumbar vertebra. - Two pairs of floating ribs (11th and 12th) protect the kidneys from the front side, whereas thick abdominal muscles protect them from the backside. - Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness and weighs 120-170 g on an average (150-170 g in adult male and 120-145 g in adult female). - Left kidney is placed higher than the right kidney because of the presence of the liver on the right side. Left kidney is also slightly longer and narrower than the right one. - The outer side of each kidney is convex whereas the inner or medial side of each kidney is concave and bears a notch called the **hilum** or **hilus renalis**, through which various structures like blood vessels, lymph vessels, nerves and ureter enter or leave the kidney. - Kidneys are covered by an outer layer of tough **renal capsule** made of white fibrous connective tissue with few yellow elastic fibres and few muscles, which protects them from injuries. - Outer to the renal capsule, a layer of fat or adipose tissue called the **adipose capsule** is present, which acts as a shock absorber. - The kidney is attached to the abdominal wall by an outermost fibrous covering called **renal fascia**. - Inside the kidney, hilum is connected with a broad funnel-shaped space called **renal pelvis** which projects into major and minor calyces. - The kidney tissue is differentiated into two functional zones: outer **renal cortex** and inner **renal medulla**. - **Renal Cortex**: It is the outer part which consists of glomeruli, Bowman’s capsule, proximal and distal convoluted tubules. - **Renal Medulla**: It is the inner part which consists of loops of Henle and collecting tubules of the nephrons. - Medulla is divided into 15-16 conical masses called as **medullary pyramids**. - A medullary pyramid has a broad base towards the cortical side and a pointed apex called the **renal papilla** towards the pelvis. - 1-3 renal papillae project into 7-13 **minor calyces**. Minor calyces join up and form 2-3 **major calyces** which further join to form the renal pelvis. It is lined by transitional epithelium and leads into ureter. - The cortex projects in between the medullary pyramids as renal columns called the **columns of Bertini**. - Structural and functional units of kidney are called **nephrons** or **uriniferous tubules**. - There are about 1 million nephrons in each kidney ### Ureters - Ureters are a pair of fine muscular, 25-30 cm long tubes with a diameter of about 3-4 mm. - Ureters leave from the renal pelvis from the hilum region and run along the abdominal wall to open into the urinary bladder in the region of **trigone** by oblique slits, one on each side. - There are three layers in the wall of ureter: **external adventitia**, **middle muscular layer**, and **inner mucosal layer**. - **External adventitia** is formed of connective tissue and consists of blood vessels, lymphatics, and nerve fibres. - **Muscular layer** has smooth muscle fibres. - **Mucosal layer** has connective tissue towards muscular coat and transitional epithelium towards the lumen. - Ureters carry urine from the kidneys to urinary bladder by peristalsis in their wall. ### Urinary Bladder - Urinary bladder is a muscular, sac-like, temporarily urine-storing structure which is present in the pelvic region and is composed of **transitional epithelium**. - It varies in shape and size according to the amount of urine contained in it. - An empty bladder is somewhat tetrahedral whereas a fully distended bladder becomes ovoid. - The wall of urinary bladder consists of a coat of smooth muscles called **detrusor muscle**. - It further consists of inner and outer layers of involuntary longitudinal muscle fibres and middle layer of circular muscle fibres. - Some of the involuntary circular muscles of the urinary bladder modify to form the **internal sphincter** in the region of urinary bladder and urethra. - Some voluntary muscles, inferior to the internal sphincter, also modify to form an **external sphincter**. - Both the sphincters undergo relaxation during the act of passing out urine. - The body of the urinary bladder has a triangular area called **trigone**. - The trigone consists of the two openings of ureters and an internal urethral orifice. - Normally urinary bladder holds 300-400 ml of urine. However, it can hold upto 700-800 ml of urine. - The wall of urinary bladder is innervated by both sympathetic and parasympathetic neural system. ### Urethra - Urethra is present only in mammals. - It extends from the neck of the urinary bladder and opens to the exterior through a urethral orifice. - Urethra is short in females (about 4 cm) and passes out urine through urethral orifice present in front of the vaginal aperture. - Urethra is longer in males (about 20 cm) and passes through the ejaculatory duct, prostate gland, Cowper’s glands and penis. - It brings out urine as well as semen through urinogenital aperture present at the tip of the penis. - Urethra consists of very well-developed smooth muscle fibres. - Urethral sphincter keeps the urethra closed all the times except at the time of urination. ### Structure of Nephrons - Complex tubular structures called **nephrons** are the structural and functional units of kidney. - A nephron consists of two parts: **glomerulus** and **renal tubule**. #### Glomerulus - From the dorsal aorta, a renal artery enters each kidney and divides into many fine branches called the **afferent arterioles** which form a tuft of capillaries, the **glomerulus**, in each nephron. - Blood from the glomerulus is carried away by an **efferent arteriole**. - Efferent arterioles have narrower diameter than that of the afferent arterioles which aids in ultrafiltration of blood. - The blood vessels of glomerulus are covered by a single layer of endothelial cells. - They are about 100-500 times more permeable than blood capillaries. #### Renal Tubule - Renal tubule is about 3 cm long and 20-60 µm in diameter. - It begins with a double-walled structure called **Bowman's capsule** which continues further to form **proximal convoluted tubule** (PCT), **Henle's loop**, and **distal convoluted tubule** (DCT). - **Glomerulus** is enclosed within the Bowman's capsule to form **malpighian body** or **renal corpuscle**. - **Definition**: **Malpighian body** or **renal corpuscle**: Glomerulus along with the Bowman’s capsule is called as malpighian body or renal corpuscle. - The wall of the Bowman’s capsule consists of an inner visceral layer and outer parietal layer. - The visceral layer surrounds the glomerulus and consists of specialised cells called **podocytes** or **foot cells**. - The space between podocytes is called the **slit pores** or **filtration slits** of about 25 nm in diameter which help in passage of glomerular filtrate. - The parietal layer consists of flat squamous epithelium. - The space between the two layers of Bowman’s capsule is called **lumen** or **capsular space**. - Bowman’s capsule has a short narrow neck which continues to form a highly coiled network called the **proximal convoluted tubule** (PCT). - Active absorption and secretion occur in the PCT. - Thus, it is lined by cuboidal epithelial cells which possess brush borders with long microvilli to increase the surface area for maximum reabsorption of the glomerular filtrate. - PCT is surrounded by **peritubular blood capillaries** which branch from the efferent arteriole in which the reabsorbed materials are transported. - **Proximal convoluted tubule** continues to form a hairpin loop-like structure called the **Henle’s loop** or the **Loop of Henle** which has a proximal descending limb and a distal ascending limb. - The **descending limb of Henle’s loop** has a thick segment having a diameter equal to that of PCT and a later thin segment. - The thick segment is also lined by cuboidal epithelium but with less microvilli. - Thin segment is lined by squamous epithelial cells having sparse microvilli and few mitochondria. - Thin segment curves to form the **ascending limb** which also consists of a proximal thin segment lined by squamous cells for passive movement of some solutes and a distal thick segment lined by cuboidal cells having microvilli and mitochondria for active secretion of NaCl. - The ascending limb of Henle's loop extends to form the **distal convoluted tubule** (DCT) lined by cuboidal epithelial cells with sparse, irregular microvilli and deep mitochondria. - DCT lies close to the malpighian corpuscle in the cortical region of the kidney. - Certain modified columnar cells of the DCT near the malpighian corpuscle, which are placed near to the afferent arteriole and efferent arteriole contain dense cytoplasm and show sensitivity to NaCl and are collectively called **macula densa**. - The DCT of each nephron opens into **collecting ducts** which are lined by specialized cuboidal epithelium with very few microvilli. - Collecting ducts unite to form **ducts of Bellini**, which run through the renal papillae. #### Urine Formation - Urine formation occurs by three major processes: **glomerular filtration**, **tubular reabsorption**, and **tubular secretion**. ##### Glomerular Filtration (Ultra Filtration) - Filtration of blood occurring in the glomerulus is called **glomerular filtration**. - On an average, 1100-1200 ml of blood is filtered by both the kidneys in one minute (i.e., about 1/5th of the blood coming out by the right and left ventricle of the heart in one minute). - **Definition**: **Ultra filtration**: The process of filtration of almost all the constituents of the blood plasma except proteins into the lumen of the Bowman's capsule. - Blood flows through glomerular capillaries under a pressure. This pressure is partly because of the difference in the diameter of glomerular capillaries and afferent renal arterioles, the latter being wider than the glomerular capillaries and partly by the natural arterial pressure caused by pumping activity of heart. - Blood pressure in glomerular blood (**Glomerular Blood Hydrostatic Pressure**, GBHP) is about 60 mm Hg. - Osmotic concentration of proteinaceous content of glomerular blood (**Blood Colloidal Osmotic Pressure**, BCOP) is equivalent to 32 mm Hg. - The pressure exerted by the filtrate in Bowman’s capsule against the filtration is 18 mm Hg. - The pressure being exerted on glomerular blood for undergoing filtration or the pressure that promotes filtration is called **glomerular filtration pressure** or **Effective Filtration Pressure** (EFP). - **EFP = GBHP – BCOP – CHP = (60 – 32 -18) mm Hg = 10 mm Hg** - The blood flowing in the glomerular blood capillaries is separated from the capsular space of Bowman’s capsule by **four layers**, i.e., endothelial covering of glomerular blood vessels, basement membrane of blood vessels, basement membrane of visceral layer of Bowman’s capsule, and the inner wall or visceral layer (epithelium) of Bowman's capsule. - **Endothelial covering of blood vessels** contains fenestrations of 50-100 nm. - The epithelium of Bowman’s capsule has cells called **podocytes** with **filtration slits** or **slit pores**. - Therefore, the actual barrier between blood and capsular space consists of two basement membranes which are, however, permeable to small-sized molecules. ##### Tubular Reabsorption - A normal, healthy individual releases only 1.5 litres of urine in a day. - Considering the amount of total glomerular filtrate produced in a day, i.e., 180 litres, it is evident that nearly 99% of the glomerular filtrate is taken back (reabsorbed) into the body. - Reabsorption occurs in all parts of renal tubule: **proximal convoluted tubule**, **loop of Henle**, **distal convoluted tubule**, **collecting tubule**. - **Definition**: **Reabsorption**: The absorption of useful essential substances from glomerular filtrate by epithelial cells of renal tubules either by active or passive mechanisms is called reabsorption. - **Reabsorption in Proximal Convoluted Tubule (PCT)** - PCT is the major site of reabsorption. - Almost all the essential nutrients, and 70%-80% of electrolytes and water are reabsorbed by the PCT. - This is due to the presence of simple cuboidal brush-border epithelial cells with abundant mitochondria and microvilli, which greatly increase the absorptive surface of PCT. - Glucose, amino acids, Na⁺, K⁺, and Ca²⁺ are reabsorbed through to active transport. - Cl⁻ and other anions are reabsorbed through diffusion. - Water moves out by the process of osmosis. - There is a selective secretion of hydrogen ions, potassium ions and ammonia into the filtrate and absorption of bicarbonate ions from the filtrate which maintains the pH and ionic balance of the body fluids. - The filtrate, however, remains isotonic to blood. - **Reabsorption in Loop of Henle** - **Descending Limb**: Thick segment is nearly impermeable. - Thin segment is permeable to water but almost impermeable to electrolytes. - Thus, it loses a lot of water due to osmosis. - It makes the filtrate hypertonic as the concentration of NaCl becomes high. - **Ascending Limb**: It is impermeable to water but allows transport of electrolytes actively or passively. - The thin segment loses NaCl to interstitial fluid through diffusion. - The thick segment actively transports NaCl into the outer interstitial fluid. - This causes high osmolarity of medullary interstitial fluid. - Due to loss of NaCl, the filtrate becomes hypotonic to blood plasma in the ascending limb of loop of Henle. - **Reabsorption in Distal Convoluted Tubule (DCT):** - DCT performs conditional reabsorption of sodium ions and water, along with reabsorption of bicarbonate ions and selective secretion of hydrogen and potassium ions and ammonia. - This maintains the pH and sodium-potassium balance in blood. - This conditional reabsorption depends on the production of aldosterone and antidiuretic hormone (vasopressin). - Under the influence of aldosterone, Na⁺ is actively reabsorbed from the filtrate, whereas Cl⁻ accompanies it passively. - HCO₂⁻ also passes out. Vasopressin helps in reabsorption of water. - **Reabsorption in Collecting Ducts**: - Collecting ducts extend from the cortex to the inner medullary region of the kidney. - Their walls become permeable and allow reabsorption of water under the influence of ADH. - Large amounts of water could be reabsorbed from the filtrate to produce concentrated urine. - A part of urea diffuses out of the lower parts of collecting ducts into medullary interstitium, which keep up the osmolarity. - It makes the medulla hyperosmotic. - It also maintains pH and ionic balance of the blood by selective secretion of hydrogen ions and potassium ions. ##### Tubular Secretion - Tubular secretion is an important process in urine formation as it maintains ionic and acid base balance of the body fluids. - **Secretion in Proximal Convoluted Tubule:** - Active secretion of hippuric acid, creatinine, pigments, drugs like penicillin occurs in the PCT. - Ammonia, hydrogen ions, bile salts, oxalates, catecholamines, toxins, etc. are also secreted in the PCT. - **Secretion in Loop of Henle:** - There is no secretion in the descending limb of loop of Henle. - Diffusion of urea occurs in the filtrate in the thin segment of ascending limb of Henle’s loop. - **Secretion in Distal Convoluted Tubule:** - K⁺, H⁺, NH3, ammonia and bicarbonate ions are mainly actively secreted into the filtrate in the DCT along with smaller quantities of uric acid, creatinine, and extra salts. #### Mechanism of Concentration of the Filtrate - The Henle’s loop and vasa recta play a significant role in producing concentrated urine by a counter current mechanism. - The length and depth of Henle’s loop indicate the urine concentrating ability of the kidneys. - NaCl is reabsorbed from the ascending limb of Henle’s loop into the descending limb of vasa recta and is then returned to the interstitium by the ascending limb of vasa recta. - Again, some amount of urea which enters the thin segment of ascending limb of Henle’s loop is transported back to the interstitium by the collecting tubule. - This counter current mechanism maintains a concentration gradient in the medullary interstitium which helps in unhindered movement of water from the collecting tubule, thereby, allowing the concentration of the filtrate in the collecting tubule (urine). - Human kidneys have an ability to produce urine nearly four times concentrated than the initial filtrate formed. ## Regulation of Kidney Function - The functions of the kidneys are efficiently controlled and regulated by hormonal negative feedback mechanisms involving the hypothalamus (Antidiuretic hormone), Juxtaglomerular apparatus (Renin-Angiotensin-Aldosterone-System) and heart (Atrial Natriuretic Factor). ### Regulation by Antidiuretic Hormone (ADH) - An increase in blood osmolarity above 300 mOsmolL⁻¹ due to excessive loss of fluid from the body (i.e., decrease in body fluid volume or decrease in blood pressure) causes activation of osmoreceptors in the body which sends stimulating signals to the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the posterior pituitary (neurohypophysis) and to promote thirst. - ADH causes water reabsorption in the distal convoluted tubule (DCT), thereby preventing loss of water in the urine (diuresis). - This increases the body fluid volume (blood pressure increased). - ADH also acts as a **vasoconstrictor** by decreasing the diameter of the blood vessels. - This constrictory effect of ADH on the blood vessels also increases the blood pressure, thereby increasing the glomerular blood flow followed by increase in glomerular filtration rate (GFR). - Increase in body fluid volume produces a negative feedback and switch off the osmoregulators, thereby suppressing the release of ADH from the posterior pituitary. ### Regulation by Juxta glomerular Apparatus (JGA) or Renin-Angiotensin-Aldosterone-System (RAAS) - A decrease in the blood volume (decrease in blood pressure) lowers the glomerular blood flow, thereby decreasing the glomerular filtration rate. - This results in activation of Juxtaglomerular (JG) cells. - The activated JG cells secrete **renin**. - **Angiotensinogen** is a protein produced in the liver and released into the blood. - **Renin** converts angiotensinogen into **angiotensin I**. - **Angiotensin I** is converted into **angiotensin II** by angiotensin-converting enzyme (ACE) produced by the vascular endothelial cells in the lungs. - **Angiotensin II** increases the blood volume by three ways: - Angiotensin II acts as a **vasoconstrictor** by decreasing the diameter of the blood vessels, which in turn increases the glomerular blood pressure (increase in glomerular filtration rate). - Angiotensin II stimulates adrenal cortex to secrete **aldosterone**. - Aldosterone causes reabsorption of sodium ions and water from distal convoluted tubule, thus increasing the blood volume. - Angiotensin II also stimulates proximal convoluted tubule (PCT) to reabsorb more NaCl and water resulting in an increase in blood volume. - As the system operates through renin-angiotensin-aldosterone, it is called RAAS or renin-angiotensin-aldosterone system. ### Regulation by Atrial Natriuretic Factor (ANF) - Increase in blood volume and blood pressure stimulates the release of **Atrial Natriuretic Factor** (ANF) from the walls of the atria of heart. - ANF brings the blood volume back to its normal state by: - Acting as a **vasodilator** which increases the diameter of the blood vessels and results in decrease in the blood flow, i.e., blood pressure. - Inhibiting the release of renin from JGA. - This further inhibits NaCl and water reabsorption by DCT and collecting tubule, thereby increasing the urinary excretion. - Reduces aldosterone release from the adrenal gland resulting in decreased reabsorption of salt and water, thus decreasing the blood volume and blood pressure. - ANF mechanism, therefore, acts to check on the renin-angiotensin-aldosterone-system (RAAS). ## Micturition - Human kidneys continuously filter the body's blood and produce urine. - This urine is poured into the ureters which deliver the urine to the urinary bladder. - Urinary bladder temporarily stores the urine until it receives a voluntary signal of urine expulsion from the central nervous system. - The process of release of urine from the urinary bladder is called **micturition** or **urination**. - Micturition is regulated by nerves, both from CNS and ANS (sympathetic and parasympathetic). - The detrusor muscle and the internal sphincter are both under the autonomic control whereas the external sphincter is a voluntary muscle under the control of the CNS. - Urine is filled in the urinary bladder when the sympathetic nerve of the autonomic nervous system stimulates the relaxation of the detrusor muscle of the urinary bladder and constriction of the internal sphincter. This causes stretching of the wall of the urinary bladder. - The capacity of urinary bladder is over 800 ml. - However, after a filling of around 500 ml (in some persons 300-400 ml) the sensory stretch receptors present in the wall of urinary bladder sends signals to the central nervous system (CNS). - The parasympathetic nerve of the autonomic nervous system stimulates the contraction of detrusor muscle of the urinary bladder and relaxes the internal sphincter which results in the release of urine. - The external sphincter is always constricted due to tonic contraction of its skeletal muscle fibers under the control of the somatic nerve (voluntary control). - During micturition, the CNS passes on motor signals which initiate the inhibition of the pudendal nerve that causes relaxation of the external urethral sphincter along with contraction of the detrusor muscle. - This mechanism starting from sensory stretch receptors of the distended urinary bladder to CNS and back to urinary bladder for its emptying is called the **micturition reflex**. ## Urine - Normally, human beings excrete out transparent, pale yellow, acidic, hypertonic urine which has a specific gravity of 1.015-1.025 and a characteristic unpleasant smell. ## Definitions - **Urochrome**: The pigment produced by the breakdown of haemoglobin from the worn-out RBCs. - **Diuretics**: Substances that increase urine formation and thereby reduces water content from the body. ## Composition of Urine | Constituent | Category | Percentage | |---|---|---| | Water | | 96% | | Organic substances | Nitrogenous substances | 2.5% | | | Non-nitrogenous substances | 1.5% | | **Nitrogenous Substances** | Urea | | | | Uric acid| | | | Creatine| | | | Creatinine | | | | Ammonia | | | | Amino acids| | | | Hippuric acid | | | **Non-nitrogenous substances** | Oxalic acid | | | | Phenolic acid| | | | Water-soluble vitamins| | | | Hormones| | | **Inorganic Substances** | Chloride | | | | Sulphate | | | | Phosphate | | | | Sodium | | | | Calcium | | | | Magnesium | | | | Iodine | | | | Arsenic | | | | Lead | | ## Abnormal Constituents in Urine - The abnormal constituents of urine are formed due to metabolic disorders and kidney malfunctioning. - The important abnormal constituents are glucose, albumin, ketone bodies, blood cells, pus cells, etc. ## Kidney Functions - Kidneys primarily function to filter out and remove metabolic nitrogenous and non-nitrogenous wastes from the blood. - **Regulation of salt and water balance** (osmoregulation): - Kidneys maintain a balance between ions and water between the blood and interstitial fluid by selective reabsorption or secretion. - Hypotonic urine is excreted if the body has excess water whereas the urine is hypertonic if the body is water deficit. - **Regulation of salt balance**: Kidneys maintain a proper sodium-potassium ion balance which is important for the proper functioning of nerves, muscles and other cells. - **Conservation of water**: - Kidneys help in conservation of water by removing water from the urine because of the presence of a very high osmotic concentration in interstitial fluid of the renal medulla which is about 1200 mOsmol/L. - **Regulation of blood pressure**: - Blood pressure is regulated through secretion or non-secretion of renin from the juxtaglomerular apparatus. - **Regulation of pH**: - Kidneys regulate pH of the body fluids by removing excess of H⁺ ions or HCO₃⁻ ions. - **Elimination of extra materials**: - Extra vitamins, drugs, pigments, salts and toxic chemicals are eliminated from the body by the kidneys. - **Erythropoiesis**: - In response to decreased RBC count, the **juxta glomerular cells** produce a hormone called **erythropoietin** which stimulates the bone marrow to increase the rate of formation of red blood corpuscles. ## Role of Other Organs in Excretion - In humans, accessory or additional excretory organs include lungs, liver, skin, large intestine and salivary glands, which help in the elimination of excretory wastes. ### Lungs - Lungs remove approximately 200 ml of CO₂ per minute and 16.5 ml of water per hour in a normal resting condition. ### Liver - Liver breaks down the haemoglobin of aged RBCs into **bilirubin** and **biliverdin** (bile pigments) which are released into duodenum along with bile and eliminated along with the faecal matter. - Liver also excretes inactivated derivatives of steroid hormones, cholesterol, few vitamins, and drugs. - Deamination of excess of amino acids occurs in the liver resulting in the production of ammonia which combines with CO₂ to form urea. - The urea is filtered by the nephrons for removal along with urine. ### Skin - In human beings, skin has a very less contribution in excretion. - Sweat produced by the **sweat (sudoriferous) glands** present in skin helps in the removal of NaCl, lactic acid, urea and amino acids. - **Sebaceous glands** (oil glands) which are usually associated with hair, produce an oily secretion called sebum. - These glands eliminate substances like oils, fatty acids, waxes, sterols and some hydrocarbons through sebum. ### Large Intestine - Epithelial cells of large intestine excrete calcium, magnesium and iron into the lumen of intestine along with faecal matter. ### Salivary glands - Salivary glands secrete substances like potassium iodide, mercury, lead and thiocyanate. ## Disorders of the Excretory System ### Uremia - It is the accumulation of urea and other nitrogenous end products like uric acid, creatinine, etc. in the blood due to malfunctioning of kidneys which is accompanied by vomiting, oedema, hypertension, pain and twitching of muscles, etc. - It is corrected by the removal of urea by the process of haemodialysis ### Renal Failure - Failure to eliminate all the nitrogenous