· The organs involved in excretion are kidneys, liver, alimentary canal, skin and lungs.
· The kidneys are the chief excretory organs.
· The excretory system of humans consists of the kidney, ureter, urinary bladder and urethra.
· The excretory system of humans consists of the kidney, ureter, urinary bladder and urethra.
A. Kidney
· Kidney is a small, bean-shaped, reddish-brown organ.
· The outer side of the kidney is convex and the inner side is concave.
· It contains a notch, (concavity), the hilus (=hilum) on its inner side.
· Blood vessels, lymph vessels, nerves and ureters enter or leave the kidney through the hilus.
· Each kidney is about 11 cm long, 5 cm wide and 3cm thick.
· The weight of the kidney, in the adult male, on an average is about 150 g and in the adult female, it is about 135 g.
· Kidneys are located on either side of the vertebral column just beneath the diaphragm.
· The right kidney is slightly lower than the left and its space is occupied by membranous lobes of the liver (but in rabbits, the right kidney is upper than the left).
· Kidneys remain embedded in a mass of fat.
· They extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra.
· Last two pairs of ribs i.e. floating ribs protect the kidneys.
· Each kidney is covered by a tough fibrous structure called a renal capsule.
· The renal capsule is covered by a layer of adipose tissue called adipose capsule which, in turn, is covered by a fibrous membrane called renal fascia. They protect the kidneys from injuries.
· Kidney is commonly called retroperitoneal organ because it is surrounded by peritoneal membrane lying in the posterior abdominal wall in front of the kidney.
· The human kidney is metanephric that filters blood plasma only.
· Each kidney is about 11 cm long, 5 cm wide and 3cm thick.
· The weight of the kidney, in the adult male, on an average is about 150 g and in the adult female, it is about 135 g.
· Kidneys are located on either side of the vertebral column just beneath the diaphragm.
· The right kidney is slightly lower than the left and its space is occupied by membranous lobes of the liver (but in rabbits, the right kidney is upper than the left).
· Kidneys remain embedded in a mass of fat.
· They extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra.
· Last two pairs of ribs i.e. floating ribs protect the kidneys.
· Each kidney is covered by a tough fibrous structure called a renal capsule.
· The renal capsule is covered by a layer of adipose tissue called adipose capsule which, in turn, is covered by a fibrous membrane called renal fascia. They protect the kidneys from injuries.
· Kidney is commonly called retroperitoneal organ because it is surrounded by peritoneal membrane lying in the posterior abdominal wall in front of the kidney.
· The human kidney is metanephric that filters blood plasma only.
Internal structure (L.S.) of Human Kidney
· Each kidney is enclosed by a thin, tough, fibrous, whitish capsule called renal capsule which protects it from infection and injuries.
· Fat layer called adipose capsule lies around the renal capsule.
· Outermost fibrous membrane is called renal fascia.
· Outer surface of each kidney is convex and the inner is concave.
· In the concave depression, there is an opening called hilum where the renal blood, lymph vessels, the ureter and nerves enter.
· L.S. of kidney shows 2 distinct regions: outer cortex, a reddish-brown layer of tissue immediately below the capsule and outside the pyramids and inner medulla that consists of renal pyramids.
· Medulla is divided into 15-16 conical projections called renal or medullary pyramids.
· Each pyramid has a broad base towards the cortex and a narrow end called the renal papilla.
· Between pyramids, the cortex extends into the medulla as renal columns of Bertini.
· Medullary pyramids are connected with minor calyces (7–13). These minor calyces lead to major calyces (2–3).
· These major calyces further open into the funnel-shaped renal pelvis. It leads into the ureter.
· Histologically, the kidney consists of about 1 million nephrons, the structural and functional unit of the kidney, held together by the connective tissue.
B. Ureter
· Ureter is a long tubular structure extending from the kidney to the urinary bladder.
· It is about 25-30 cm long and 3 mm in diameter.
· The anterior part of the ureter is called the renal pelvis which is slightly expanded and the posterior tubular part is called ureter proper.
· It is composed of transitional epithelium.
· Ureters convey the urine from the kidney to the urinary bladder.
· Each kidney is enclosed by a thin, tough, fibrous, whitish capsule called renal capsule which protects it from infection and injuries.
· Fat layer called adipose capsule lies around the renal capsule.
· Outermost fibrous membrane is called renal fascia.
· Outer surface of each kidney is convex and the inner is concave.
· In the concave depression, there is an opening called hilum where the renal blood, lymph vessels, the ureter and nerves enter.
· L.S. of kidney shows 2 distinct regions: outer cortex, a reddish-brown layer of tissue immediately below the capsule and outside the pyramids and inner medulla that consists of renal pyramids.
· Medulla is divided into 15-16 conical projections called renal or medullary pyramids.
· Each pyramid has a broad base towards the cortex and a narrow end called the renal papilla.
· Between pyramids, the cortex extends into the medulla as renal columns of Bertini.
· Medullary pyramids are connected with minor calyces (7–13). These minor calyces lead to major calyces (2–3).
· These major calyces further open into the funnel-shaped renal pelvis. It leads into the ureter.
· Histologically, the kidney consists of about 1 million nephrons, the structural and functional unit of the kidney, held together by the connective tissue.
B. Ureter
· Ureter is a long tubular structure extending from the kidney to the urinary bladder.
· It is about 25-30 cm long and 3 mm in diameter.
· The anterior part of the ureter is called the renal pelvis which is slightly expanded and the posterior tubular part is called ureter proper.
· It is composed of transitional epithelium.
· Ureters convey the urine from the kidney to the urinary bladder.
C. Urinary Bladder
· Itis a sac or bag-like structure situated in the lower part of the abdominal cavity.
· The inner lining is composed of transitional epithelium.
· A triangular structure seen in the bladder is called trigone or trigonum vesicae. It contains two openings of the ureter and a posterior opening of the urethra, which are 3 cm from each other.
· The opening of the bladder to the urethra is guarded by external and internal sphincters.
· Urinary bladder can store 300-800 ml of urine.
· Itis a sac or bag-like structure situated in the lower part of the abdominal cavity.
· The inner lining is composed of transitional epithelium.
· A triangular structure seen in the bladder is called trigone or trigonum vesicae. It contains two openings of the ureter and a posterior opening of the urethra, which are 3 cm from each other.
· The opening of the bladder to the urethra is guarded by external and internal sphincters.
· Urinary bladder can store 300-800 ml of urine.
D. Urethra
· Urethra is a tubular structure extending from the bladder to the outside.
· The size of the urethra is different in males and in females.
· It is about 4 cm long in males and 20 cm long in females.
· In females, it opens separately just above the vaginal opening called the urethral orifice.
· In males, it receives the male genital duct and opens outside at the tip of the penis. This common duct is called the urinogenital duct.
Functions of Human Kidney
The functions of the human kidney are:
i. Kidney removes metabolic nitrogenous waste products such as urea and uric acid from the blood.
ii. Kidney maintains salt and water concentration in the body i.e. osmoregulation.
iii. Kidney balances proper pH (about 7.4) of blood by removing excess acids and alkalis from the blood.
iv. As the kidney controls the fluid balance in the body, it maintains blood pressure.
v. Kidney maintains proper amount of mineral salts such as sodium, potassium in the body.
vi. Kidney eliminates toxic substances, drugs, pigments, excess vitamins from the blood.
vii. Kidney helps in the maintenance of a steady-state in the internal environment of the body i.e. homeostasis.
viii. Kidney produces erythropoietin (hormone) that stimulates the formation of erythrocytes (RBCs).
ix. Kidney secretes an enzyme, the renin which changes the plasma protein, the angiotensinogen into angiotensin II. The latter stimulates the adrenal cortex to secrete aldosterone which increases the rate of reabsorption of Na+ in the nephrons.
The functions of the human kidney are:
i. Kidney removes metabolic nitrogenous waste products such as urea and uric acid from the blood.
ii. Kidney maintains salt and water concentration in the body i.e. osmoregulation.
iii. Kidney balances proper pH (about 7.4) of blood by removing excess acids and alkalis from the blood.
iv. As the kidney controls the fluid balance in the body, it maintains blood pressure.
v. Kidney maintains proper amount of mineral salts such as sodium, potassium in the body.
vi. Kidney eliminates toxic substances, drugs, pigments, excess vitamins from the blood.
vii. Kidney helps in the maintenance of a steady-state in the internal environment of the body i.e. homeostasis.
viii. Kidney produces erythropoietin (hormone) that stimulates the formation of erythrocytes (RBCs).
ix. Kidney secretes an enzyme, the renin which changes the plasma protein, the angiotensinogen into angiotensin II. The latter stimulates the adrenal cortex to secrete aldosterone which increases the rate of reabsorption of Na+ in the nephrons.
Structure of Nephron (Uriniferous tubule)
· Nephron or uriniferous tubule is the structural and functional unit of the kidney where urine is formed.
· Each kidney has about 1 million nephrons.
· Each nephron is about 3 cm long and 20-25 mm in diameter.
· Human kidneys are metanephric that refers to long kidneys.
· Each nephron has a Malpighian corpuscle or renal corpuscle and renal tubule.
· The kidney can’t regenerate new nephrons.
1. Malpighian corpuscle or Renal corpuscle
· It consists of Bowman's capsule and glomerulus:
a. Bowman’s capsule
· It is a cup-shaped double-walled structure.
· It consists of Bowman's capsule and glomerulus:
a. Bowman’s capsule
· It is a cup-shaped double-walled structure.
· They are the outer parietal layer and inner visceral layers. The parietal layer consists of squamous epithelial tissue.
· The visceral layer has three layers. They are the outer epithelium, middle basement membrane and inner endothelial layer.
· The parietal layer of Bowman’s capsule is continuous with a renal tubule. Ultrafiltration of blood takes place in the Bowman’s capsule.
· The membrane through which filtration occurs is composed of capillaries, endothelium, basement membrane and podocytes.
· The podocytes are cells of the visceral layer of Bowman’s capsule which have foot processes towards the basement membrane.
· During filtration, fluid passes down through a foot process called slit pores.
· Mesangial cells are found in the region of the basement membrane on the surface of glomerular capillaries. They control glomerular filtration.
· Macula densa is also found in the inner part of Bowman’s capsule which also helps in the filtration of blood.
· The visceral layer has three layers. They are the outer epithelium, middle basement membrane and inner endothelial layer.
· The parietal layer of Bowman’s capsule is continuous with a renal tubule. Ultrafiltration of blood takes place in the Bowman’s capsule.
· The membrane through which filtration occurs is composed of capillaries, endothelium, basement membrane and podocytes.
· The podocytes are cells of the visceral layer of Bowman’s capsule which have foot processes towards the basement membrane.
· During filtration, fluid passes down through a foot process called slit pores.
· Mesangial cells are found in the region of the basement membrane on the surface of glomerular capillaries. They control glomerular filtration.
· Macula densa is also found in the inner part of Bowman’s capsule which also helps in the filtration of blood.
b. Glomerulus
· It is a tuft of capillaries formed from an afferent arteriole.
· It lies within the concavity of Bowman's capsule and is surrounded by the visceral layer of Bowman’s capsule.
· Blood enters the glomerulus through an afferent arteriole and releases out through an efferent arteriole. Glomerular filtration occurs in glomerulus.
2. Renal Tubule
· It arises from the neck of Bowman's capsule and is differentiated into- Proximal Convoluted Tubule (PCT), a loop of Henle and Distal Convoluted Tubule (DCT) and Collecting Tubule (CT).
a. Proximal convoluted tubule (PCT)
· It lies between Bowman’s capsule and descending limb of the loop of Henle.
· It is lined by single-layered ciliated cuboidal epithelial tissue.
· It is freely permeable to water.
· It lies between Bowman’s capsule and descending limb of the loop of Henle.
· It is lined by single-layered ciliated cuboidal epithelial tissue.
· It is freely permeable to water.
· A huge amount of solute absorption takes place in this region.
b. Loop of Henle
· It is a U shaped loop leading from PCT.
· It is formed of two limbs.
· They are thin descending limb and thick ascending limb in cortical nephron but in the juxtamedullary nephron, early part i.e. 2/3 part of ascending limb is only thin.
· The thin segment is lined by flat cells and maybe 2-14 mm in length while the thick segment is lined by cuboidal cells and is continuous with DCT at macula densa.
· Descending limb is freely permeable to water while ascending limb is highly permeable to solutes like sodium and chloride ions.
· Vasa recta lies in the middle region of the loop of Henle in blood capillaries.
· The thin segment is lined by flat cells and maybe 2-14 mm in length while the thick segment is lined by cuboidal cells and is continuous with DCT at macula densa.
· Descending limb is freely permeable to water while ascending limb is highly permeable to solutes like sodium and chloride ions.
· Vasa recta lies in the middle region of the loop of Henle in blood capillaries.
· The countercurrent mechanism takes place in the loop of Henle.
c. Distal convoluted tubule (DCT)
· It is a coiled and twisted tubule arising from the ascending limb of the loop of Henle.
· It is about 3 mm long and lined by cuboidal epithelial tissue.
· It is found in the cortex region.
· Posteriorly, it is connected with the collecting tubule.
· It is slightly permeable due to antidiuretic hormone (ADH).
d. Collecting tubule
· It is a straight tubule that continues from DCT lined by cuboidal epithelial tissue with a few microvilli.
· The collecting tubule is about 20 mm in length. It has two kinds of cells. They are p cells (principal cells) that respond to vasopressin and I cells (intercalated cells) which secrete acid. The I cells are also found in the other parts of renal tubules.
· The collecting tubule is highly permeable to antidiuretic hormone (ADH) or vasopressin which increases the permeability of water.
· The collecting tubule carries urine towards the pelvis.
Types of Nephrons
· According to the position, nephron can be divided into two groups.
· They are juxtamedullary nephrons and cortical nephrons.
a. Cortical nephrons:
a. Cortical nephrons:
· They are about 85% and have their glomeruli in the outer cortex and relatively short loops of Henle that extend a short distance into the medulla.
· It has no vasa recta.
b. Juxtamedullary nephrons:
· Their glomeruli are placed closer to the inner margin of the cortex and long loops of Henle are deep into the medulla which is associated with the vasa recta. The blood first passes through the capillaries of the glomerulus then flows through the hairpin loops of the vasa recta of the loop of Henle.
· The PCT runs posteriorly as the descending limb of the loop of Henle which becomes narrower posteriorly and makes U-turn and runs anteriorly as ascending limb of the loop of Henle. It runs anteriorly into the DCT. DCT, through collecting tubule, opens into collecting duct.
· The Bowman's capsule, glomerulus, PCT and anterior region of the loop of Henle lie towards the cortex and the narrow region of the loop of Henle lies towards the medullary region of the kidney.
· The collecting duct opens into the duct of Bellini in the medullary pyramid. From the medullary pyramid arise minor calyces which through major calyces open into the pelvis.
· The PCT runs posteriorly as the descending limb of the loop of Henle which becomes narrower posteriorly and makes U-turn and runs anteriorly as ascending limb of the loop of Henle. It runs anteriorly into the DCT. DCT, through collecting tubule, opens into collecting duct.
· The Bowman's capsule, glomerulus, PCT and anterior region of the loop of Henle lie towards the cortex and the narrow region of the loop of Henle lies towards the medullary region of the kidney.
· The collecting duct opens into the duct of Bellini in the medullary pyramid. From the medullary pyramid arise minor calyces which through major calyces open into the pelvis.
· Pelvis comes out of kidney through hilum and runs posteriorly as ureter containing urine.
Blood Supply
· Nephron receives blood supply from afferent arteriole, which is a branch of the renal artery.
· Afferent arteriole breaks down into capillaries which lie within the concavity of Bowman's capsule and is called the glomerulus.
· Ultrafiltration occurs in the Bowman's capsule and the remaining blood is carried by efferent arteriole that emerges out from the glomerulus.
· Efferent arteriole breaks down into capillaries and spreads over the renal tubule.
· Blood capillaries that surround the PCT, anterior region of the loop of Henle and DCT is called the peritubular capillary network and that surrounds the narrow region of the loop of Henle is called vasa recta.
· From these blood capillaries, excretory waste is absorbed by tubule and useful substances are diffused into surrounding blood capillaries.
· These blood capillaries make urine concentrated and regulate the volume of urine. Capillaries join to form venules, which finally fuse to form a renal vein that comes out of the kidney through the hilum and rejoins the main bloodstream.
Physiology of Excretion
· Being ureotelic, the excretory waste of man is urea.
· Being ureotelic, the excretory waste of man is urea.
· It is mainly synthesized in the liver, a part in the brain and in the kidney.
· Urea is drained into the bloodstream and carried to the kidney from where it is excreted as urine.
· It involves two steps:
(a) Urea formation
· Urea is formed in the liver by Kreb's cycle.
· It involves two steps:
(a) Urea formation
· Urea is formed in the liver by Kreb's cycle.
· The excess amino acid in the liver is converted into ammonia by Kreb's cycle which being toxic, changes into urea.
Ornithine cycle
· Urea formation occurs by the ornithine cycle in the liver.
· Urea formation occurs by the ornithine cycle in the liver.
· During catabolism, protein breaks up into amino acids.
· The amino acids are converted into keto acid with the removal of ammonia.
· Ornithine combines with one molecule of ammonia and carbon dioxide producing citrulline and water.
· Ornithine combines with one molecule of ammonia and carbon dioxide producing citrulline and water.
· Citrulline combines with another molecule of ammonia and arginine and water.
· Arginine is broken into urea and ornithine in the presence of an enzyme arginase and water.
· This cycle is repeated.
(b) Urine formation
· It occurs in nephrons of the kidney and consists of the following three steps:
(i) Ultrafiltration
(ii) Selective reabsorption
(iii) Tubular secretion
· It occurs in nephrons of the kidney and consists of the following three steps:
(i) Ultrafiltration
(ii) Selective reabsorption
(iii) Tubular secretion
Mechanism of Urine formation in Human being:
· Urine formation in nephron involves three basic steps as given below:
(i) Glomerular filtration or ultrafiltration.
(ii) Selective reabsorption of water and useful substances from the filtrate into blood capillaries.
(iii) Tubular secretion of substances not required by the body into the nephric filtrate.
(i) Glomerular filtration or Ultrafiltration
· The filtration under high pressure through the semipermeable wall of the Bowman's capsule is ultrafiltration.
· The Bowman’s capsule acts as an ultrafilter and lies in close contact with the glomerulus.
· The blood flowing through the afferent arteriole brings urea, water, several salts and blood proteins dissolved in the blood plasma.
· These substances are filtered via. the two membranes of the Bowman’s capsule i.e. an endothelial layer of the blood capillaries and epithelial layer of the Bowman’s capsule.
· These two layers are thin and are formed by a single layer and lie in close contact with each other.
· The diameter of the afferent arteriole is greater than (more than twice) that of the efferent arteriole.
· The diameter of the afferent arteriole is greater than (more than twice) that of the efferent arteriole.
· So, more blood comes to the glomerulus and less blood passes out. For this afferent arterioles form a capillary network around the renal tubule (Nephron).
· Due to the difference in diameter of the afferent and efferent arterioles, a capillary hydrostatic pressure of about 55 mm Hg is generated at the glomerulus.
· It is opposed by the osmotic pressure of blood, about 30 mm Hg and by filtrate hydrostatic pressure of about 15 mm Hg in a glomerular capsule.
· So, net filtration pressure = CHP – (OP + FHP) = 55 – (30 + 15)= 10 mm of Hg
Note: CHP = Capillary hydrostatic pressure, OP = Osmotic pressure, FHP = Filtrate hydrostatic pressure
Note: CHP = Capillary hydrostatic pressure, OP = Osmotic pressure, FHP = Filtrate hydrostatic pressure
· As a result, water and various solutes like glucose, amino acids, urea, uric acid, ammonia etc. are filtered through the Bowman’s capsule. This is called ultrafiltration.
· The filtered fluid is called ultrafiltrate of glomerular filtrate.
· The name ultrafiltration is given by Richards and his associates/friends in 1924 AD.
· The glomerular filtrate is similar to blood plasma in composition except for the plasma proteins and fats. Hence, it is isotonic to blood plasma.
· The volume of filtrate formed by both kidneys/minute is called glomerular filtration rate. It is 125 ml/minute = 7500 ml/hr = 180 litres/day.
· Out of 180 L of glomerular filtrate/day, only 1.5 – 2 L is excreted as urine and the rest 178 L is resorbed by kidneys.
· Out of 180 L of glomerular filtrate/day, only 1.5 – 2 L is excreted as urine and the rest 178 L is resorbed by kidneys.
(ii) Selective reabsorption
· Since the function of the nephron is to act as a filter not only urea but also the useful substances like glucose, amino acids, phosphate ions, Vitamin c etc. are also passed out through the nephron.
· The removal of these useful substances from the body will be harmful. So, these substances are absorbed and reabsorbed at the various parts of a nephron.
· The selection of these substances also takes place through various parts of a nephron.
· Ultrafiltrate leaves Bowman's space via. the urinary pole to enter the PCT. Substances like proteins, glucose, amino acids, creatinine, bicarbonate ions, Na+ and Cl– ions, and water are reabsorbed in the proximal tubule.
· Ultrafiltrate leaves Bowman's space via. the urinary pole to enter the PCT. Substances like proteins, glucose, amino acids, creatinine, bicarbonate ions, Na+ and Cl– ions, and water are reabsorbed in the proximal tubule.
· The nephric filtrate is similar in composition to the blood plasma, so it is isotonic.
· Proximal tubule pumps Na+, Cl– and H2O into the renal interstitium.
· Proximal tubule pumps Na+, Cl– and H2O into the renal interstitium.
· The cells of the thin descending limb of Henle's loop are freely permeable to salt and water and the nephric filtrate is hypertonic to the blood plasma.
· The thin ascending limb is relatively impermeable to water, but salts can enter or leave the tubule, depending upon the conditions in the interstitium. The nephric filtrate remains here hypotonic.
· Urea enters the lumina of the thin limbs of Henle's loop.
· The thick ascending limb of Henle's loop is completely impermeable to water but the chloride pump actively removes Cl– ions from the lumen to the renal interstitium.
· Ultrafiltrate is isotonic with blood as it leaves the pars recta of the proximal tubule and hypotonic as it ascends the ascending thick limb.
· When cells of macula densa detect a low Na concentration in the ultrafiltrate, they stimulate the juxtaglomerular cells to release renin that converts angiotensinogen to angiotensin I.
· When cells of macula densa detect a low Na concentration in the ultrafiltrate, they stimulate the juxtaglomerular cells to release renin that converts angiotensinogen to angiotensin I.
· In the lungs, kidneys and other organs of the body, angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II constricts efferent arteriole, so increasing pressure within the glomerulus.
· Angiotensin II also stimulates the adrenal cortex to release aldosterone that increases the reabsorption of Na and Cl ions in the cells of DCT.
· The filtrate that leaves DCT to enter the collecting tubule is hypotonic. In the absence of ADH, cells of collecting tubule and DCT are completely impermeable to water and urine is hypotonic.
· Under the influence of ADH, the cells of the collecting tubule become freely permeable to water and urea, so water leaves the lumina of collecting tubules to enter the interstitium. So urine becomes concentrated and hypertonic.
· Selective reabsorption of sodium ions from the filtrate occurs under the influence of aldosterone (a hormone secreted by the adrenal cortex) in distal convoluted tubules. Here, some Cl– ions are also absorbed.
· Angiotensin II also stimulates the adrenal cortex to release aldosterone that increases the reabsorption of Na and Cl ions in the cells of DCT.
· The filtrate that leaves DCT to enter the collecting tubule is hypotonic. In the absence of ADH, cells of collecting tubule and DCT are completely impermeable to water and urine is hypotonic.
· Under the influence of ADH, the cells of the collecting tubule become freely permeable to water and urea, so water leaves the lumina of collecting tubules to enter the interstitium. So urine becomes concentrated and hypertonic.
· Selective reabsorption of sodium ions from the filtrate occurs under the influence of aldosterone (a hormone secreted by the adrenal cortex) in distal convoluted tubules. Here, some Cl– ions are also absorbed.
· The excretion of K+ and H+ ions also takes place. Water is absorbed under the influence of ADH. So, the nephric filtrate is hypertonic to the blood plasma.
· The filtrate is then known as urine which is excreted out in hypertonic conditions.
· Vasa recta and countercurrent exchange system
· The lumen of the arterial limbs of the vasa recta has a smaller diameter than that of the venous limb so when blood courses down the arterial limb, it loses water and gains salts as it returns via. the venous limb, it gains water and loses salts, maintaining a countercurrent exchange system.
· The lumen of the arterial limbs of the vasa recta has a smaller diameter than that of the venous limb so when blood courses down the arterial limb, it loses water and gains salts as it returns via. the venous limb, it gains water and loses salts, maintaining a countercurrent exchange system.
(iii) Tubular Secretion
· Most of the tubular secretion occurs in the distal convoluted tubule.
· Most of the tubular secretion occurs in the distal convoluted tubule.
· The nephric filtrate may contain unwanted substances like urea, uric acid, ammonia, creatinine, hippuric acids etc. and certain drugs.
· They are absorbed and removed out in the DCT. The cells of DCT remove them by diffusion. Therefore, tubular secretion is opposite to tubular reabsorption.
· The excretion of potassium and hydrogen ions may also take place under the influence of hormones.
· The excretion of potassium and hydrogen ions may also take place under the influence of hormones.
· These waste products are mixed with the nephric filtrate and form urine which is excreted out from the kidney in the form of hypertonic conditions.
· Urine is formed as a result of all three mechanisms– ultrafiltration, selective reabsorption and tubular secretion.
· Urine is formed as a result of all three mechanisms– ultrafiltration, selective reabsorption and tubular secretion.
Urine in Human
· The urine is a clear, yellowish fluid having an aromatic smell due to urinoid.
· The colour of the urine is due to the presence of a pigment called Urochrome (a breakdown product of Hb from worn-out RBCs/bile pigments).
· This colour also depends upon the quantity and concentration of the urine i.e. if the quantity is less and the concentration is high, then the urine excreted out is dark or dark-brown in colour and if the quantity is more and the concentration is less, then the urine excreted out is light-yellow in colour.
· The amount of urine formed in an adult human in a day is about 1500 ml and this amount also depends upon the fluid and food intake, muscular exercise, temperature and humidity of the atmosphere.
· This colour also depends upon the quantity and concentration of the urine i.e. if the quantity is less and the concentration is high, then the urine excreted out is dark or dark-brown in colour and if the quantity is more and the concentration is less, then the urine excreted out is light-yellow in colour.
· The amount of urine formed in an adult human in a day is about 1500 ml and this amount also depends upon the fluid and food intake, muscular exercise, temperature and humidity of the atmosphere.
Composition of Urine
· The urine of a normal person contains 95% water, 2% salts in ionic form, 2-2.6% urea, 0.3% uric acids, traces (0.1%) of ammonia, creatine, creatinine, hippuric acids etc. and it is important to note that no glucose should be detected or glucose=nil.
· If present, the condition is called “glycosuria”.
Micturition (Urination)
· Micturition is the expulsion of urine from the urinary bladder.
· It is a reflex process as it is controlled by the autonomic nervous system, but in grown-up children and in adults, it can be controlled voluntarily to some extent.
· When 300-400 ml of urine has accumulated in the urinary bladder, afferent autonomic nerve fibres in the bladder wall sensitive to stretch are stimulated. In infants, it initiates a spiral reflex and micturition occurs.
· Micturition occurs when autonomic efferent fibres convey impulses to the bladder, causing contraction of the detrusor muscle and relaxation of the internal urethral sphincter.
· The stimulation of the sympathetic nerve causes relaxation of the detrusor muscle of the urinary bladder and constriction of the internal sphincter, so it causes the filling of the urinary bladder and the sympathetic nerve is called the nerve of filling.
· The stimulation of the parasympathetic nerve causes contraction of the detrusor muscle of the urinary bladder and relaxation of the internal sphincter leading to the emptying of the urinary bladder, so parasympathetic nerve is called nerve of emptying or nerve of micturition.
· External sphincter is always constricted due to the action of the somatic nerve. During micturition, this nerve is inhibited, so the somatic or pudendal nerve is responsible for the voluntary control of micturition.
· The urine flows out from the urinary bladder through the urethra.
· The urine is light yellow coloured due to the presence of pigment urochrome.
· The pH of urine varies from 4.5 to 8 and the nature depends on the type of food consumed.
· The pH of urine varies from 4.5 to 8 and the nature depends on the type of food consumed.
Osmoregulation
· Osmoregulation is the phenomenon by which water content and salts/ionic concentration of the body cells are regulated.
· Kidney plays an important role in maintaining osmo-concentration of the blood.
· In aquatic animals such as fishes or in other cases where the possibilities of consumption of water are more, the urine excreted out is hypotonic i.e. the urine is more diluted or less concentrated than the blood plasma and this is to get rid of excess water in the body.
· In the case of land animals/terrestrial animals, the availability of water is less, so the urine excreted out is hypertonic i.e. the urine is more concentrated than the blood plasma and this is to prevent excess loss of water from the body.
· Nearly all the vertebrates including mammals can excrete hypertonic urine. In this condition, first, the isotonic nephric filtrate passes from Bowman’s capsule.
· When it passes through the renal tubules only solutes are absorbed and reabsorbed (no urine/water is absorbed) making the urine more dilute and in this condition, hypotonic urine is excreted out.
Osmoregulation in Human
· Human beings excrete hypertonic urine i.e. urine is more concentrated than the blood plasma and in this condition, first, the isotonic nephric filtrate passes from the Bowman’s capsule and when it passes via. renal tubules, water and some solutes are absorbed and reabsorbed. Then the hypertonic urine is collected in the collecting duct and the urine excreted out is hypertonic. So, this is to maintain/regulate the water balance in the body.
· Human beings excrete hypertonic urine i.e. urine is more concentrated than the blood plasma and in this condition, first, the isotonic nephric filtrate passes from the Bowman’s capsule and when it passes via. renal tubules, water and some solutes are absorbed and reabsorbed. Then the hypertonic urine is collected in the collecting duct and the urine excreted out is hypertonic. So, this is to maintain/regulate the water balance in the body.
· In warmer months or in summer seasons when excessive sweating occurs, a large amount of water is lost in the form of sweat. The urine excreted out during this time is hypertonic and this is the balanced water content in the body whereas in cold months or in winter seasons, the urine excreted out is hypotonic (more diluted) because during this no or less sweating occurs.
· A large amount of water is retained in the body or no water is lost along with the sweat. This concentration depends upon the length of the loop of the Henle i.e. when the loop of the Henle is longer, urine excreted out is always hypertonic. This depends upon the number of vasa recta.
· If there are more vasa recta, the loop of Henle is longer. If the loop of Henle is short i.e. there is less number of vasa recta, the urine excreted out is hypotonic. The vasa recta are the looped/loop-like vessels found around the loop of Henle. Since the vasa recta are in the form of the loops, the blood flowing in two limbs are in opposite directions i.e. the blood (nephric filtrate) flowing in the descending limb lies close to the blood outgoing in the ascending limb. This is called a countercurrent system/mechanism. This is the first countercurrent system.
· The second countercurrent system is maintained by the loop of Henle itself. The blood flowing in two limbs of the loop of Henle lie in the opposite directions i.e. the blood flowing through the descending limb come/lie to that of the blood outgoing through the ascending limb.
· During this, the salts i.e. (Na, Cl) are reabsorbed by the ascending limb and then pass to the surrounding medullary tissue and retained by the vasa recta. This limb is impermeable to water i.e. no absorption of water takes place. It makes urine more dilute than the blood plasma and the hypertonic urine is excreted out.
· In the descending limb, the water is reabsorbed and is then passed into the surrounding capillary network. This makes the nephric filtrate more concentrated than the blood plasma and the hypertonic urine is passed out.
· The overall function of the countercurrent system is to make the salt/ions more concentrated in the medullary tissue than the osmotic pressure of the interstitial fluid is increased.
· Hereby passing out the water from the ascending limb and then to the renal tubules.
Role of hormone (ADH) in the absorption of water i.e. the formation of hypertonic urine
· The absorption of water by the kidney tubules or the formation of the volume of urine is under the influence of Antidiuretic hormone (ADH) or vasopressin released in blood by the posterior lobe of the pituitary gland.
· It increases the reabsorption of water by the kidney tubules i.e. DCT, CT and CD to increase their permeability.
· Sensory nerve cells in the hypothalamus (osmoreceptors, supraoptic nucleus) detect changes in osmotic pressure of blood. Nerve impulses from the osmoreceptors stimulate the posterior pituitary to release ADH.
· In dehydration and following haemorrhage, osmotic pressure rises and more ADH is secreted and more water is therefore reabsorbed and urine output is reduced, i.e., the body retains more water and osmotic pressure is decreased.
· Sensory nerve cells in the hypothalamus (osmoreceptors, supraoptic nucleus) detect changes in osmotic pressure of blood. Nerve impulses from the osmoreceptors stimulate the posterior pituitary to release ADH.
· In dehydration and following haemorrhage, osmotic pressure rises and more ADH is secreted and more water is therefore reabsorbed and urine output is reduced, i.e., the body retains more water and osmotic pressure is decreased.
· Urine is hypertonic.
· On the contrary, after a large fluid intake, ADH secretion is reduced, less water is reabsorbed and there is the production of more dilute urine.
· Alcohol and certain drugs also suppress the secretion of ADH and hypotonic urine is excreted out. The ADH increases the reabsorption of water making the urine hypertonic.
· On the contrary, after a large fluid intake, ADH secretion is reduced, less water is reabsorbed and there is the production of more dilute urine.
· Alcohol and certain drugs also suppress the secretion of ADH and hypotonic urine is excreted out. The ADH increases the reabsorption of water making the urine hypertonic.
· Any variation in the osmotic concentration in the blood due to less or excess consumption of water by the body is detected by the osmoregulatory cells of the hypothalamus and then the feedback mechanism is operated and the regulation/absorption of water by ADH is an example of homeostatic feedback mechanism.
· The hyposecretion of ADH creates a large volume of urine formation and a large amount of water is lost from the body. This condition is pathologically known as Diuresis (a large volume of urine is formed and excreted out and the person suffers from diabetes insipidus).
· But the function of ADH is to make the anti-diuresis-less volume of urine is formed making the urine hypertonic.
· Hence, Vasopressin is also called the anti-diuresis hormone.
· The filtrate that leaves DCT to enter the collecting tubule is hypotonic.
· The filtrate that leaves DCT to enter the collecting tubule is hypotonic.
· In absence of ADH, the cells of DCT are completely impermeable to water and urea, so urine becomes hypertonic.
Role of Aldosterone in the absorption of salts
· The salts like Na, phosphates, HCO3– etc. can be absorbed in the PCT.
· This salt is secreted in the remaining parts of the renal tubules. The secretion of these salts in the tubules is under the influence of the Aldosterone hormone secreted by the adrenal cortex of the adrenal gland.
· The secretion of aldosterone is under the influence of angiotensin. The formation of angiotensin is influenced by the angiotensinogen (plasma protein) formed in the liver.
· The formation of angiotensinogen is influenced by the secretion of renin by Juxtaglomerular cells of the kidney.
· Then only the aldosterone is secreted which increases the reabsorption of the sodium ions and the secretion of K+ ions in the kidney tubules.
Homeostasis
· It is a phenomenon or process by which an animal is regulating itself and maintaining the internal environmental/physiological state of the body.
· The term homeostasis was coined by American Physiologist Walter Cannon in 1932 AD.
· In the case of aquatic animals, ammonia is excreted out in the surrounding environment water by the process of diffusion through the integument or skin whereas, in the land or terrestrial animals, the integument or the skin is made impervious/barrier which prevents an excess of loss of water from the body through the skin by evaporation.
Homeostasis in Human
· In humans, the homeostatic organs are the skin, kidneys and liver.
Role of Skin in Homeostasis
· Skin is the outermost covering of the body.
· It contains two types of glands: sudoriferous (sweat) glands and sebaceous (oil) glands.
· The Sudoriferous glands are tubular glands pointed at both the ends and are derived from stratum Malpighi/germinativum.
· The Sudoriferous glands are tubular glands pointed at both the ends and are derived from stratum Malpighi/germinativum.
· They secrete sweat which is salty in taste.
· It is an aqueous fluid and contains 99.5% water, NaCl, urea, lactic acid, amino acid and glucose. The normal pH of sweat is 4.5.
· The volume of sweat varies depending upon the activity and temperature. When sweat evaporates, it provides cooling to our body.
· The Sebaceous or oil glands are out-pushing of the wall of hair follicles and each gland consists of branching alveoli which secrete an oily or wax-like secretion called sebum.
· The Sebaceous or oil glands are out-pushing of the wall of hair follicles and each gland consists of branching alveoli which secrete an oily or wax-like secretion called sebum.
· It keeps the skin oily, waterproof and removes wax, sterols, other hydrocarbons and fatty acids from the body. It also lubricates the hair.
Regulation of body temperature
· Human body temperature remains fairly constant at about 36.8°C or 98.4°F with a slight variation of 0.50 to 0.750C. during exercise, the body temperature is increased.
· When simultaneous heat loss is not done, the body temperature is increased continuously. The body temperature is increased slightly in the evenings and in the case of women, it is increased before ovulation.
· To ensure the constant body temperature, there is a slight balance between heat gain and heat loss from the body i.e. when heat is gained by the body (increased body temperature), the heat loss by the body should be done at the same time.
· When the heat is gained by a body, metabolic rate is increased and when heat is lost, metabolic rate is decreased.
a. Heat gain (warmness of the body)
· Heat is gained by a body in the form of energy during carbohydrates, fats and deaminated amino acids are metabolized and the heat-producing organs include muscles, liver and the digestive organs.
· During the contraction of the muscles/skeletal muscles, heat is produced i.e. during the exercise in the body.
· Liver is an important homeostatic organ for heat production. It produces heat in the form of chemical energy as a by-product.
· During the contraction of the smooth muscles of the alimentary canal as well as during chemical reactions, heat is produced in the body.
b. Heat loss (Loss of the body temperature)
· Heat is lost by the body through the skin by producing sweat. Some amount of heat is lost via. expired air, urine, faeces.
· But the heat loss from a body through the skin in the form of sweating is to maintain constant body temperature.
· There is no control over heat loss from other routes.
· The heat loss from the body also depends upon the following factors:
i. Difference between body and environmental temperature
ii. Amount of (time period) body surface exposed out
iii. Types of clothes worn
iv. Amount of sweating discharge from the body is to maintain the constant body temperature and during this, a large amount of water, as well as salts, are lost from the body so, human beings have to drink a lot of water and consume salts through their diet depending upon the sweating.
i. Difference between body and environmental temperature
ii. Amount of (time period) body surface exposed out
iii. Types of clothes worn
iv. Amount of sweating discharge from the body is to maintain the constant body temperature and during this, a large amount of water, as well as salts, are lost from the body so, human beings have to drink a lot of water and consume salts through their diet depending upon the sweating.
Role of Liver in Homeostasis
a) Fat metabolism
· Liver helps to convert stored fats (Chylomicrons) into such a form that provides energy to the tissue cells.
a) Fat metabolism
· Liver helps to convert stored fats (Chylomicrons) into such a form that provides energy to the tissue cells.
b) Deamination of proteins
· In the liver, the harmful toxic substance i.e. ammonia is converted into urea which is eliminated through the kidneys.
c) Detoxification
· Liver detoxifies the toxic substances such as drugs and the other substances which are produced by the microbes or microorganisms.
d) Secretion of bile
· Liver cells secrete bile which emulsifies the fats. This process is called emulsification.
e) Secretion of heparin
·Liver secretes an enzyme called heparin which prevents the clotting of blood inside the blood vessels.
f) Absorption of vitamins
· Liver helps absorb fat-soluble vitamins i.e. vitamin A, D, E and K as well as some of the water-soluble vitamins such as vitamin B2, B3, B6, B9 (Vitamin-M) and B12 (Cyanocobalamin).
g) Breakdown of erythrocytes and defence
· Liver cells also degrade the haemoglobin of worn-out RBCs into the bile pigments (bilirubin and biliverdin). This is done by the Kupffer cells.
h) Production of heat
· Liver produces heat in the form of chemical heat energy as a by-product. It helps to regulate the body temperature by producing heat.
i) Inactivation of hormones
· Liver inactivates certain hormones such as insulin, glucagon, cortisol, aldosterone, thyroxine and sex hormones.
j) Production of blood plasma
· Liver produces the blood plasma such as fibrinogen, angiotensinogen etc.
· Liver cells also excrete cholesterol, certain products of steroid hormones, some vitamins and many drugs. Actually, the liver secretes these substances in the bile. The bile carries these substances to the intestine and is passed out with the faeces.
· Liver is also involved in glycogenesis and glycogenolysis.
· By performing these functions, the liver maintains the internal physiological phenomenon of the body, aiding as the homeostatic organ.
Functions of skin
The various functions of the skin are as follows:
1. It acts as a physical barrier against different microorganisms e.g. bacteria, fungi, protozoa, viruses and physical and chemical agents like dust, smoke etc.
2. It is associated with the production of vitamin D.
3. It protects the body from Ultraviolet radiation.
4. It prevents the loss of water from the body.
5. It has an immunogenic role.
6. It has a cosmetic role.