Excretory Products And Their Elimination

Uricotelism is the excretion of excess nitrogeneous waste in the form of uric acid. Uric acid is a compound like purines which are found in nucleic acids.

Whereas ammonia is highly toxic, and animals cannot survive when ammonia is present in body fluids. So, during the transition to land, the excretion is via a less toxic form of nitrogen like urea or uric acid favored by selection.

The Malpighian body has the glomerulus and Bowman's capsule. It is where ultrafiltration occurs. The glomerulus increases blood pressure by forming narrow branches of blood vessels that increases surface area for filtration to take place. The pressure is maintained by the efferent arteriole which restricts the flow of blood from the glomerulus to the latter part.

Bowman's capsule forms a cup shaped structure that encloses the glomerulus and performs the first step of blood filtration to later form urine.

ADH, also known as vasopressin is involved in facilitating water reabsorption from the later part of the tubule thereby preventing dieresis. It increases the permeability of the collecting duct for ions and water.

The following which is removed by our body by lungs is a) CO₂ only.

The main function of the lungs is to carry out gas exchange, where oxygen is provided and carbon dioxide is removed from the blood.

The pH of the human urine is approximately c) 6

Urine under pH 5.0 is acidic, and urine higher than pH 8.0 is alkaline, or basic. Urine is a by-product of metabolism in human beings.

A) Protonephridia - iv) Flatworms

A protonephridia is a network of dead-end tubules that lack any internal openings and are found in Platyhelminthes, Chordata etc. The ends described are called flame cells if they are ciliated and if not then solenocytes. The main function is osmoregulation.

B)Nephridia- iii)Earthworms

Nephridia are coiled tubular duct-like structures that are responsible to filter and remove wastes from the body of an earthworm. The nephridia are usually grouped in pairs throughout the different segments of the earthworm's body.

C)Maphigian tubules - ii) Cockroach

The Malpighian tubules have excretory and osmoregulatory function. The tubules are branched and extends from the alimentary canal absorbing solutes, water etc.

D) Green gland or Antennal Gland - i) Prawn

The glands that perform the excretory function in prawns is the antennal glands or the green glands. It is a pair of large glands present in some crustaceans like prawns which have an excretory function and would usually open at bases of the large antennae.

Birds and reptiles excrete nitrogenous waste in form of uric acid and not urea with minimum loss of water.

Most terrestrial insects excrete nitrogenous waste in form of uric acid and are known as uricotelic organisms.

Ureotelic organisms are those organisms that excrete nitrogenous wastes in form of urea which insects do not do.

The cortical regions extend in between medullary pyramids as renal columns which are called Columns of Bertini. It allows better anchorage of the cortex.

It is an excess of amino acid and protein metabolism end products accumulation like urea and creatinine in the blood that would be normally be excreted out in the urine.

The symptoms of uremia include fatigue, loss of appetite, nausea, etc.

Antidiuretic hormone (ADH) or vasopressin is released from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule thus preventing diuresis.

Diuresis is the excessive urine production.

Proximal convoluted tubule leads to reabsorption of 70-80% electrolyte. It also maintains the pH and the ionic balance of the body fluids by selective secretion of hydrogen ions, ammonia and potassium ions into the filtrate and by absorption of bicarbonate ions.

Distal convoluted tubule maintains the ionic balance. Conditional reabsorption of Na⁺ and water takes place in this part.

Henle's loop leads to formation of concentrated urine. Reabsorption is minimum in its ascending limb and the descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down the loop.

Counter current mechanism results in maintenance of concentration gradient in medulla. Presence of such gradient helps in passage of water from the collecting tubule thus concentrating the urine.

Renal corpuscle leads to filtration of blood.

Glycosuria is the presence of glucose in urine.

Gout is caused by accumulation of uric acid in joints. It is a form of arthritis.

Glomerulonephritis is inflammation of the tiny filters called glomeruli in your kidneys.

Renal calculi are also known as kidney stones which occurs when urine contains crystal like substances like oxalates of calcium.

Vasa recta: These are capillaries in the that lie parallel to the loop of Henle.

Henle's loop: portion of a nephron that lies in between the proximal convoluted tubule and the distal convoluted tubule.

Counter current mechanism:

The transport of sodium chloride in the ascending limb of Henle's loop results in an osmolal gradient. This is passive in the thin region of the loop while active in the thick region. The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta in the following manner:
There is an active absorption of urea that occurs from the collecting tubule.

Passive absorption of water without NaCl takes place in the descending limb of the loop of Henle, which helps to concentrate NaCl as it enters the ascending limb.

In the vasa recta, absorption of water into the blood takes place and return of the absorbed solute to the interstitium follows. This is called the countercurrent exchange mechanism.

Dialysis unit's fluid: Plasma - Urea concentration

Dialysis is the process of removing excess water, solutes, and toxins from the blood in people whose kidneys are no longer able to perform the functions stated above i.e. the functions of excretion.

The selective re-absorption of glomerular filtrate takes place in the distal convolute tubule (DCT) .It is a portion of the functional unit of kidney called nephron which lies between the loop of Henle and the collecting tubule. Once the filtrate passes through the ascending limb of Henle, it then enters the distal convoluted tubule (duct of the renal tubule present in the cortex of kidney) that selectively reabsorbs minerals like calcium, sodium, and chloride thus regulating the pH of urine by secreting H+ ions and absorbing bicarbonate ions.

The excretory product from kidneys of reptiles is uric acid. Uric acid is a breakdown product of nitrogen, formed when protein is metabolized in the body. Uricotelic organisms excrete uric acid as a waste product.

The composition of sweat produced by sweat glands is lactic acid, salt, minerals and urea. The minerals may contain sodium, potassium, calcium, magnesium, etc.

Sweat glands are small tubular structures that produce sweat.

The glands that perform the excretory function in prawns is the antennal glands or the green glands. It is a pair of large glands present in some crustaceans like prawns which have an excretory function and would usually open at bases of the large antennae.

The excretory structure in amoeba is contractile vacuole. The function of the contractile vacuole is osmoregulation i.e. it regulates the water content of the cell. It excretes wastes out of the cell through the cell membrane via the process of diffusion.

The following abbreviations that are used in the context of excretory functions stand for:

a) ANF: Atrial Natriuretic Factor

Atrial natriuretic factor (ANF) is a hormone that acts on the kidney to increase the sodium excretion and glomerular filtration rate and it also antagonizes the renal vasoconstriction. In addition to that renin secretion is also inhibited.

b) ADH: Anti-Diuretic Hormone:

Antidiuretic hormone (ADH) or vasopressin is released from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule thus preventing diuresis.

Diuresis is the excessive urine production.

c) GFR: Glomerular Filtration Rate:

The rate of blood flow through the kidneys is known as the glomerular filtration rate, or GFR (The glomerulus or glomeruli are microscopic bundles of blood vessels present inside the nephrons).

d) DCT: Distal Convoluted Tubule

It is a portion of the functional unit of kidney called nephron which lies between the loop of Henle and the collecting tubule. Once the filtrate passes through the ascending limb of Henle, it then enters the distal convoluted tubule (duct of the renal tubule present in the cortex of kidney) that selectively reabsorbs minerals like calcium, sodium, and chloride thus regulating the pH of urine by secreting H+ ions and absorbing bicarbonate ions.

The differences between glycosuria and ketonuria are:

Sebaceous glands are responsible to eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin for protection.

Two actively transported substances in glomerular filtrate are: amino acids and glucose.

Once in the tubule wall, the glucose and amino acids of the glomerular filtrate diffuse directly into the blood capillaries along a concentration gradient. This is also known as renal glucose reabsorption.

Two metabolic disorders which can be diagnosed by analysis of urine are:

a) Ketonuria (presence of excess ketone bodies in the urine). Symptoms may include frequent urination, thirst, dilated pupils etc.

b)Glycosuria (presence of excess glucose in the urine). It is usually asymptomatic but can result in excessive urination.

The main processes of urine formation are: Glomerular Filtration, Reabsorption and tubular secretion.

Glomerular filtration removes solutes from the blood which is the first step of urine formation. The force of hydrostatic pressure in the glomerulus is the driving force that pushes the filtrate out of the capillaries.

Tubular reabsorption is the process by which the nephron removes water and reabsorbs solutes from the tubular fluid and returns them to the blood. Reabsorption of water also occurs passively in the initial segments of the nephron. For example, substances like glucose, amino acids, etc., are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport in the tubules.

Tubular secretion occurs in the proximal convoluted tubule of the nephron and is responsible for the transport of specific molecules out of the blood and then into the urine. It helps in the maintenance of ionic and acid base balance of body fluids.

Actively transported: Glucose, amino acids, Na⁺

Passively transported: Nitrogenous wastes and water

Reabsorption of water also occurs passively in the initial segments of the nephron. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively.

a. urinary excretion = tubular reabsorption + tubular secretion – Glomerular filtration.

Tubular reabsorption: Tubular reabsorption is the process by which the nephron removes water and solutes from the fluid from the tubules (pre-urine) and returns the same to the blood which is circulating.

Tubular secretion: It is secretion of H⁺ and NH₄⁺ from the blood into the tubular fluid that helps to keep blood pH and maintain osmolarity.

b. Dialysis fluid = Plasma– nitrogenous wastes

Nitrogenous wastes: Any metabolic waste product that contains nitrogen is called nitrogenous wastes.

The substances that exit from the tubules in order to maintain a concentration gradient in the medullary interstitium are urea and sodium chloride.

Urea is a breakdown protein of amino acid metabolism which is freely filtered at the glomerulus. The high osmolarity of the renal medullary interstitium provides a concentration gradient needed for water reabsorption to occur.

a) Kidneys - Urea, uric acid, ammonia (based on organisms but usually urea like in human beings)

b) Lungs - CO₂ and H₂O

c) Liver- Bilirubin, Biliverdin, cholesterol, vitamins and other drugs

d) Skin - Water, Sweat ( salt, urea, lactic acid) , Sebum (sterols, hydrocarbons etc.)

A renal corpuscle is the part of the functional unit of kidney called nephron which is responsible for filtration of blood. It is having the glomerulus which is a tuft of capillaries together and a glomerular capsule which is known as Bowman's capsule. Each nephron has a renal corpuscle.

The Juxtaglomerular (JG) apparatus has a regulatory role to play.

When there is a fall in glomerular filtration rate (GFR), they activate the JG cells to release renin which gets converted to angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, is a powerful vasoconstrictor that increases the GFR.

Angiotensin II is also responsible for activating the adrenal cortex to release Aldosterone which causes reabsorption of Na+ and water from the distal parts of the tubule leading to an increase in blood pressure and GFR.

This mechanism is known as the Renin-Angiotensin mechanism that regulates the kidney function.

The juxtaglomerular apparatus is a structure formed by the distal convoluted tubule and the glomerular afferent arteriole.

Organisms which generally excrete ammonia as nitrogenous wastes is said to be ammonotelic in nature which aquatic animals are generally.

Ammonia is a compound which is highly toxic and is soluble in water as the nitrogen has lone pair of electrons which makes it susceptible to form hydrogen bonds with the oxygen lone pair of water.

Excretion of urea requires a large amount of water which can only be provided by aquatic animals while on the other hand terrestrial animals cannot afford to lose so much of water and get dehydrated so they convert their ammonia to products like urea or uric acid which are comparatively less toxic.

The terrestrial mammals or even amphibians mainly excrete nitrogenous wastes in form of urea and hence are known as ureotelic organisms while reptiles and etc. excrete nitrogenous wastes in form of uric acid and hence are called uricotelic organisms.

There is a difference in the content of glomerular filtrate and urine.

Glomerular filtrate contains all contents of blood plasma except the protein part. After that reabsorption of the filtrate to obtain amino acids, ions like Na+, nutrients and water occur in the proximal and distal convoluted tubules. So this composition of urine (filtrate) is different from the originally obtained glomerular filtrate. Some ions are constantly added to this fluid to maintain the pH and osmotic balance.

The glomerular filtrate is formed at the first step of the urine formation in the Bowman's capsule while urine is formed at the end of the nephron.

The procedure advised for the correction of extreme renal failure is kidney transplantation.

A functioning kidney is used in transplantation from a living donor or can also be from a deceased donor ( only if the deceased has just died can their kidneys be used for the transplantation) . The kidney should be preferably from a close relative so that the immune system does not reject it. Donor compatibility is evaluated via blood tests that matches the blood types and antigens.

Organisms which generally excrete ammonia as nitrogenous wastes is said to be ammonotelic in nature which aquatic animals are generally.

Ammonia is a compound which is highly toxic and is soluble in water as the nitrogen has lone pair of electrons which makes it susceptible to form hydrogen bonds with the oxygen lone pair of water.

Excretion of urea requires a large amount of water which can only be provided by aquatic animals while on the other hand terrestrial animals cannot afford to lose so much of water and get dehydrated so they convert their ammonia to products like urea or uric acid which are comparatively less toxic.

The terrestrial mammals or even amphibians mainly excrete nitrogenous wastes in form of urea and hence are known as ureotelic organisms while reptiles and etc. excrete nitrogenous wastes in form of uric acid and hence are called uricotelic organisms.

Thus this is how the conversion of ammonia to urea or uric acid have been the adaptation that the terrestrial organisms have gone through for conservation of water.

Afferent arteriole: The afferent arteriole is the arteriole that brings blood to the glomerulus.

Efferent arteriole: The efferent arteriole is the arteriole that carries blood away from the glomerulus.

Bowman's capsule: It performs the first step in the filtration of blood to form urine. It is a sac like structure that encloses the glomerulus which is a bundle of small capillaries.

Glomerulus: The glomerulus increases blood pressure by forming narrow branches of blood vessels that increases surface area for filtration to take place.

A haemodialysing unit is also called artificial kidney because it is responsible for performing the same functions that a kidney functions.

It helps to remove toxic nitrogenous wastes from the blood without the loss of plasma proteins.

This unit is provided to those patients whose kidneys have failed or are malfunctioning so that the toxic excretory products can be removed from their body which otherwise would have resulted in accumulation of urea in blood which is lethal due to its toxicity.

^A haemodialysis unit

Selective reabsorption is the process in which some molecules like glucose, ions and certain amino acids get reabsorbed from the filtrate as they pass through the neuron. Sodium reabsorption in the late distal tubule and collecting duct is regulated by hormones, which stimulate or inhibit sodium reabsorption as necessary.

Kidney function is monitored by hormones like: ADH i.e. Antidiuretic hormone and ANF i.e. Atrial Natriuretic Factor.

ADH facilitates reabsorption of water from the DCT and collecting duct by increasing the water and salt permeability as well as by accelerating ion and water transfer by osmotic gradient creation.

The adrenal cortex is activated by Angiotensin II as a result of which aldosterone is released.

Aldosterone in turn induces the DCT to absorb more ions and water.

ANF regulates blood flow by causing vasodilation and increasing excretion of sodium.

Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa recta play an important role in the same.

The transport of sodium chloride in the ascending limb of Henle's loop results in an osmolal gradient. This is passive in the thin region of the loop while active in the thick region. The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta in the following manner:
There is an active absorption of urea that occurs from the collecting tubule.

Passive absorption of water without NaCl takes place in the descending limb of the loop of Henle, which helps to concentrate NaCl as it enters the ascending limb.

In the vasa recta, absorption of water into the blood takes place and return of the absorbed solute to the interstitium follows. This is called the countercurrent exchange mechanism.

Human kidneys can produce urine nearly four times concentrated than the initial filtrate which is initially formed.

When we compare volume of the filtrate formed per day with that of the urine released per day, we find that nearly 99 per cent of the filtrate has been reabsorbed by the renal tubules. This is called reabsorption. During urine formation, the tubular cells also secrete certain into the filtrate. Tubular secretion is crucial as it helps in the maintenance of ionic balance in the body fluids.

Proximal Convoluted Tubule has simple cuboidal epithelium that increases the surface area for reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment. It also maintains the pH and ionic balance of the body fluids by selective secretion.

Henle’s Loop: The process of reabsorption is very little in its ascending limb. The descending limb is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate or urine.

Distal Convoluted Tubule is where conditional reabsorption of sodium ions and water takes place. It is also responsible for reabsorption of bicarbonate ions and selectively secreting hydrogen and potassium ion to maintain the ionic balance and pH.

Collecting Duct: From this long duct, large amounts of water could be reabsorbed to produce concentrated urine. It plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of ions as well as maintain osmolarity.

Micturition:

Urine is formed by the nephrons which are the functional unit of the kidneys which is ultimately carried to the urinary bladder. It is stored there till a voluntary signal is given by the central nervous system (CNS) for the release of the urine. This signal stretched the urinary bladder as it gets filled with urine. In response, the stretch receptors on the bladder walls would now send signals to the CNS that passes on motor messages that would lead to contraction of the muscles of the bladder leading to relaxation of the sphincter. This releases the urine and this process is called micrturition.

Disorders of the excretory system:

Glomerulonephritis: It is the inflammation of the glomeruli of kidney. If this inflammation pursues, the kidneys may stop working completely, resulting in kidney failure. There are two types of glomerulonephritis known—acute and chronic.

Uremia: Malfunctioning of kidneys would lead to accumulation of toxic urea in blood. This condition is called uremia, which is highly harmful and can lead to failure of the kidney.

Renal calculi: Stone or insoluble mass of crystallised salts like those of calcium oxalates, etc. are formed within the kidney resulting in kidney stones.

Tubular secretion is crucial as it helps in the maintenance of ionic balance in the body fluids.

Proximal Convoluted Tubule has simple cuboidal epithelium that increases the surface area for reabsorption. It also maintains the pH and ionic balance of the body fluids by selective secretion.

Distal Convoluted Tubule is where conditional reabsorption of sodium ions and water takes place. It is also responsible for selectively secreting hydrogen and potassium ion to maintain the ionic balance and pH.

Collecting Duct: From this long duct, large amounts of water could be reabsorbed to produce concentrated urine. It plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of ions as well as maintain osmolarity.

Reabsorption is minimum in its ascending limb which plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.

The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes so water can flow out. This concentrates the filtrate as it moves down.

The ascending limb is impermeable to water but allows transport of electrolytes. Therefore, as the concentrated filtrate passes through the ascending limb, it gets diluted due to the passage of electrolytes to the medullary fluid.

Kidneys are reddish brown in colour, bean shaped, situated between the last thoracic and third lumbar vertebra level close to the dorsal inner wall of the abdominal cavity.

Towards the centre of the inner side of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter.

Inside the kidney, there are an outer cortex that extends in between the renal columns called Columns of Bertini and an inner medulla divided into medullary pyramids projecting into the calyx.

Each kidney has structures called nephrons which are the structural and functional units having two parts – the glomerulus and the renal tubule.

The renal tubule starts with a cup-like structure called Bowman’s capsule, by the glomerulus is enclosed and together they are called the renal corpuscle.

The tubule continues to form proximal convoluted tubule (PCT) followed by a hairpin shaped Henle’s loop having a descending and an ascending limb.

The ascending limb continues as another highly coiled tubular structure called distal convoluted tubule (DCT). The DCTs open into a tube called collecting duct.