Process of osmoregulation
In humans, the kidney plays an important role in osmoregulation of body's internal environment. The body shows osmoregulation in two common ways or cases, dehydration and waterlogging. In case of dehydration, the hypothalamus gives the signal to the pituitary gland to secrete ADH (antidiuretic hormone). ADH increases the reabsorption of water in the renal tubules of the kidney and increases the blood volume and concentration of body fluids. In case of waterlogging, there is high level of fluids in the body. The hypothalamus signals the pituitary gland to stop the secretion of ADH and hence there is no reabsorption of water in the tubules of the kidney.
Creatine and creatinine
Creatine is a nitrogenous compound formed in the liver. Creatine is transported through the blood to organs, muscles etc. It is phosphorylated (addition of phosphate group) to form high energy compound creatine phosphate. Creatine phosphate restores ATP ( Adenosine triphosphate) after muscle contraction. Creatine phosphate in the muscles gets converted into creatinine. Creatinine is mainly filtered in the glomerulus and excreted through urine.
Ammonotelism
The mode of excretion in which elimination of nitrogenous waste (excretory product) is mainly in the form of ammonia is called ammonotelism. Ammonia is highly toxic and water soluble. It requires a large amount of water for elimination. The animals that follow this mode of excretion are known as ammonotelic animals. Example - Aquatic animals like bony fishes, aquatic amphibians etc.
Ureotelism
Ureotelism - The mode of excretion in which elimination of nitrogenous waste is in the form of urea is called as Ureotelism. Urea is comparatively less toxic to the body. Hence it can be stored in the body for a short period of time before it is excreted. It requires less amount of water for getting eliminated. The animals that follow this mode of excretion are known as ureotelic animals. Example - Humans, turtles, frogs, sharks etc.
Steps involved in Urea cycle
Urea cycle includes five steps involving five enzymes.
- Synthesis of carbamoyl phosphate: Condensation of ions and ,in presence of enzyme carbamoyl phosphate synthase forms carbamoyl phosphate. The formation of carbamoyl phosphate requires energy. This step consumes 2 ATPs.
- Formation of citrulline: Citrulline is synthesized from carbamoyl phosphate and ornithine with the help of enzyme transcarbamoylase.
- Synthesis of arginosuccinate: The enzyme arginosuccinate synthase condenses citrulline and aspartate to form arginosuccinate. This step requires ATP.
- Cleavage of arginosuccinate: Arginosuccinate lyase cleaves arginosuccinate into arginine and fumarate.
- Formation of Urea: Arginase is the fifth and final enzyme that cleaves arginine to form urea and ornithine. Ornithine enters the mitochondria where it is reused in the formation of citrulline (step 2).
Urea cycle
The liver converts ammonia to less toxic urea. Urea so formed is transported to kidneys and get excreted in urine. The process of production of urea is known as Urea cycle. It was discovered by Hans Kreb and Kurt Hanseleit in 1932, hence it is also known as Krebs - Hanseleit cycle. Urea has two amino group( -) in its structure, one derived from ammonia and the other from aspartate.
Uricotelism
The mode of excretion in which elimination of nitrogenous waste is in the form of uric acid is called as uricotelism.The animals that follow this mode of excretion are known as uricotelic animals. Most of these animals live in dry regions or do not consume plenty of water (eg. birds), hence they have to conserve water in their bodies. Uric acid is water insoluble crystals which require very less amount of water to get eliminated from the body. Example - Birds (class Aves), Helix (commonly known as land snails), cockroach, lizard, snakes etc.
External structure of kidney
- Kidneys are bean-shaped and dark-red in color. One side bulges outward (convex) and the other side is indented (concave).
- The indented or concave section is known as the hilum. This is where the renal artery, the renal vein and the ureter enter / exit the kidney.
- Each kidney is enclosed in a semi-transparent membrane called as the renal capsule. It is the container or sac in which the other components of the internal anatomy of the kidneys are stored.
- The renal capsule also protects the kidney against infections and trauma.
Internal structure of kidney.
Each kidney is made up of three layers renal cortex, renal medulla and renal pelvis. The renal medulla has 15 to 16 conical structure called medullary pyramids. The renal medullary pyramid ends into a structure called renal papilla. Columns of Bertini is present between the medullary pyramids. The medullary pyramids are connected to minor calyces. the minor calyces lead to major calyces. The major calyces open into the renal pelvis. The renal pelvis leads to the ureter. A kidney has about 10 lakh structural and functional units called nephrons.
Role of liver in excretion
The role of liver in the process of excretion:
- Conversion of ammonia (highly toxic nitrogenous waste) to less toxic urea by a process known as urea cycle.
- Production of bile juices that help in the breakdown of lipids (lipid metabolism).
- Breakdown of haemoglobin from worn out RBCs which leads to formation of bilirubin and biliverdin. Bilirubin and biliverdin enter the intestine along with bile juice and are excreted through faeces.
Malphigian capsule
Malpighian corpuscle is a part of a nephron. The Bowman's capsule and the glomerulus collectively form the Malpighian capsule or renal corpuscle.
Glomerulus - It is a network of capillaries. Blood enters the glomerulus through an afferent arteriole and leaves it through efferent arteriole. Glomerular filtration takes place in the glomerulus.
Bowman's capsule - It is a double layered cup-shaped structure. Its lumen is continuous with the lumen of the renal tubule. It has two layers:
Glomerulus - It is a network of capillaries. Blood enters the glomerulus through an afferent arteriole and leaves it through efferent arteriole. Glomerular filtration takes place in the glomerulus.
Bowman's capsule - It is a double layered cup-shaped structure. Its lumen is continuous with the lumen of the renal tubule. It has two layers:
- The outer parietal layer - made of squamous cells.
- The inner visceral layer - surrounds the glomerulus and is composed of a special type of cells called podocytes.
Uriniferous tubules
The uriniferous tubule (also referred as nephron) is a microscopic structural and functional unit of the kidney. It is made of a renal corpuscle and a renal tubule. The renal corpuscle consists of a network of capillaries called glomerulus and Bowman's capsule. The corpuscle and tubule both are connected. They are made of epithelial cells. The tubule has five parts, namely
- Proximal convoluted tubule which is connected to the Bowman's capsule
- The loop of Henle which has two parts, ascending loop of Henle and descending loop of Henle.
- Distal convoluted tubule.
- The collecting tubule and
- Collecting ducts.
Blood supply to the kidney tubules
- Renal arteries branch off from the dorsal aorta to enter the kidneys. Each renal artery gives rise to afferent arterioles which form the glomerulus inside the Bowman's capsule.
- Efferent arterioles arise from the glomerulus and have narrow lumen than that of afferent arteriole.
- The efferent arteriole divides to form the peritubular capillary network around the proximal and distal convoluted tubules.
- The capillaries of vasa recta arise from the peritubular capillary network. these capillaries extend parallel to the loops of Henle and the collecting ducts in the medulla.
- All the capillary networks join to form renal venules which further join to form a renal vein.
- Renal vein opens into inferior vena cava.
Juxtaglomerular apparatus
The smooth muscle cells of both afferent and efferent arterioles are swollen and contain dark granules. These cells are called juxtaglomerular cells. The granules contain inactive renin. In case of low blood pressure, renin is secreted from these cells. Renin converts angiotensinogen in the blood to angiotensin. Angiotensin stimulates the secretion of aldosterone by the adrenal cortex. Aldosterone increases the blood pressure through vasoconstriction, reabsorption of sodium ions by distal convoluted tubule and water by collecting duct.
Formation of urine - Ultrafiltration
The formation of urine occurs in two major steps; ultrafiltration and reabsorption.
Ultrafiltration
Ultrafiltration
- The high hydrostatic pressure forces passes small molecules, such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule. This process is called as ultrafiltration.
Formation of urine
The urine formation in the nephron takes involves three steps:
- Glomerular filtration - The semi-permeable glomerular capillaries act as filters. It allows water, glucose, salts, amino acids and other nitrogenous waste materials to pass through it and enter the Bowman's capsule. The filtrate so formed is called glomerular filtrate.
- Tubular reabsorption - The filtrate in the Bowman's capsule enters the Proximal convoluted tubule (PCT). About 65 % of glomerular filtrate is reabsorbed in PCT. Glucose, amino acid, vitamins, hormones, various salts, water and some urea from the filtrate is absorbed.The filtrate reaches Loop of Henle which consist of descending and ascending limb. As the filtrate flows through the descending loop of Henle only water is reabsorbed. Sodium and other solutes are not absorbed here. Further, the filtrate flows through the ascending loop of Henle where reabsorption of sodium, potassium, calcium, magnesium, and chloride are reabsorbed. The ascending loop of Henle is impermeable to water, hence no water is reabsorbed in this region. The filtrate then flows through the Distal convoluted tubule(DCT). In DCT, active reabsorption of sodium takes place under the influence of aldosterone. Chloride ions are also reabsorbed. Water is reabsorbed under the influence of ADH(antidiuretic hormone). Passing through the DCT, the filtrate enters the collecting ducts. Further reabsorption of water takes place in the collecting ducts.
- Tubular secretion - Secretion is the final step in the formation of urine. Creatinine, hippuric acid, drugs etc are actively secreted into the filtrate In the proximal convoluted tubule. Urea enters the filtrate by diffusion in the thin segment of ascending loop of Henle. Potassium, hydrogen, and bicarbonate ions are secreted into the filtrate in the DCT. Removal of hydrogen ions and ammonia from the blood in PCT and DCT helps to maintain the pH of blood.
- Thus the urine created by this process then passes to the central part of the kidney called the pelvis. The urine then passes from pelvis to urinary bladder through ureters.
Control by antidiuretic hormone (ADH)
- ADH produced in the hypothalamus of the brain and released into the blood stream from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more water.
- The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mos mL.
Control by juxtaglomerular apparatus (JGA)
- JGA operates a multihormonal Renin-Angiotensin-Aldosterone system (RAAS).
- The JGA responds to a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin, into the blood stream.
- In the blood, renin initiates chemical reactions that convert a plasma protein, called angiotensinogen, to a peptide, called angiotensin II, which works as a hormone.
- Angiotensin II increases blood pressure by causing arterioles to constrict. It also increases blood volume in two ways: firstly, by signalling the proximal convoluted tubules to reabsorb more NaCl and water, and secondly, by aldosterone, a hormone that induces the distal convoluted tubule to reabsorb more Na and water.
Regulation of micturition
Micturition is regulated by the following:
Control by antidiuretic hormone (ADH)
- ADH produced in the hypothalamus of the brain and released into the blood stream from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more water.
- The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mos mL.
- JGA operates a multihormonal Renin-Angiotensin-Aldosterone system (RAAS).
- The JGA responds to a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin, into the blood stream.
- In the blood, renin initiates chemical reactions that convert a plasma protein, called angiotensinogen, to a peptide, called angiotensin II, which works as a hormone.
- Angiotensin II increases blood pressure by causing arterioles to constrict. It also increases blood volume in two ways: firstly, by signalling the proximal convoluted tubules to reabsorb more NaCl and water, and secondly, by aldosterone, a hormone that induces the distal convoluted tubule to reabsorb more and water.
- A peptide called Atrial Natriuretic Factor (ANF), opposes the regulation by RAAS.
- The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure.
- ANF inhibits the release of renin from the JGA, and thereby inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from adrenal gland.
- Thus ADH, RAAS and ANF provide an elaborate system of checks and balance that regulate the kidney functioning, to control body osmolarity, salt concentrations, blood pressure and blood volume.
Excretion of urine
- The urine passes into collecting ducts to the pelvis and then through the ureter it passes into urinary bladder.
- Then, the muscles in the bladder wall contract and the sphincter muscles relax, forcing urine out of the bladder and down the urethra. This process is known as micturation.
Process of kidney transplant
- Kidney transplantation or renal transplantation is the organ transplant of a kidney into a patient with end-stage renal disease.
- A kidney transplant operation is done under general anaesthetic and usually takes between three and five hours.
- In a kidney transplant, old kidney i left in place and the new kidney is placed lower down.
- The new kidney's blood vessels are joined to the blood vessels that supply the leg and its ureter is attached to the bladder.
Comments
Post a Comment