The human body is constantly at work. It is never completely at rest. For example, even when we are sleeping, our lungs are busy inhaling oxygen and exhaling carbon dioxide.
Carbon dioxide is a waste product generated by our body. Like carbon dioxide, various other waste products such as ammonia, urea, uric acid, water, and excess ions like phosphate, sulfate, sodium, potassium, and chloride, are produced in our body, as a result of different metabolic activities, like ingestion, digestion, and respiration. These waste or excretory products, if retained in the body, can cause the body to feel very may prove fatal and so must be totally or partially removed from our body.
The process of elimination of metabolic waste products from the animal body is called excretion. Among the different waste products ammonia, urea and uric acid are known as nitrogenous wastes. Of these, Ammonia is the most toxic and requires a lot of water for its elimination and is thus excreted only by those organisms that can compensate the loss of water. Generally, bony fishes, aquatic amphibians and aquatic insects excrete ammonia in the form of readily soluble ammonium ions through their gills or body surface.
by the process of diffusion. Animals that excrete ammonia are called a monotelic while the process is called a monotelism as excretion of ammonia requires a lot of water. Terrestrial animals have evolved a mechanism to convert ammonia into less toxic forms of urea and uric acid.
which help conserve water. Mammals including humans and many marine fish and terrestrial amphibians excrete urea. Excretion of nitrogenous wastes in the form of urea is called ureotelism and such animals are called ureotelic animals.
In these animals the liver converts ammonia into urea. by the ornithine cycle and releases it into the blood. The urea in the blood is then filtered by the kidneys and excreted. However, in some animals, a small amount of urea is retained in the matrix of the kidney to maintain the required osmolarity. The least toxic form of nitrogenous waste is uric acid.
Though excretion of uric acid involves a lot of energy, it helps conserve water considerably, as it is excreted in the form of pellets, or paste, involving very little water. Some animals excreting uric acid are reptiles, insects, land snails, and birds. Such animals that excrete nitrogenous waste in the form of uric acid, are called, Uricotelic animals and the process is called Uricotelism.
Did you know that in mammals uric acid is formed by the degradation of nucleic acids and is excreted as a part of urine? As animals excrete nitrogenous wastes in different forms, the structure of their excretory system also varies. The study of phylogeny of the excretory system, reveals that primitive animals such as invertebrates, possessed simple organs like, proteinifridia or flame cells, which in the course of time, evolved into complex organs such as the kidneys invertebrates.
Flame cells or proteinifridia, are the excretory structures in animals like, platyhelminthes, rotifers, cephalocordates, and certain annelids. Apart from performing the function of excretion, these cells help in osmoregulation, or the regulation of the fluid volume and ionic balance in the body. Another excretory structure, is the nephridia, found in many annelids like the earthworm.
Nephridia, are tubular structures that help remove nitrogenous waste, and maintain the fluid and ionic balance. Malfygian tubules are excretory structures found in arthropodons such as insects and arachnids. These tubules along with removing nitrogenous wastes perform the function of osmoregulation. The antennal gland or green gland is the excretory structure in crustaceans such as the crab and crayfish. This gland performs the function of excretion.
and Osmoregulation. The kidneys perform the main function of excretion in highly evolved mammals such as man. Along with the kidneys, the skin, liver and lungs also help in the removal of certain wastes from our body.
Our skin has sebaceous glands that secrete sebum through which substances like hydrocarbons, sterols and waxes are excreted. This secretion however, provides a protective oily covering for the body. Like the sebaceous glands, the sweat glands on our skin also help in the removal of excretory wastes. These glands produce sweat, a watery fluid containing sodium chloride and small amounts of substances such as urea and lactic acid.
Moreover, sweat also helps to cool the body's surface. Another organ that plays an important role in excretion is the liver, the largest gland in our body. It breaks down drugs and reduces their toxicity by drug metabolism. It also detoxifies ammonia and converts it into urea, which is eliminated by the kidneys. Moreover, sweat also helps to cool the body's surface.
It degrades hemoglobin, hormones, drugs and vitamins, and adds it to bile, as bile pigments, bilirubin, and biliverdin, which are then excreted through the faeces. The lungs are also excretory organs that help in the removal of large amounts of carbon dioxide, a by-product of cellular respiration, from our body. In addition, the lungs also help in the removal of water.
Besides the kidneys, the skin, liver and lungs, saliva too helps in the removal of small amounts of nitrogenous wastes from our body. Therefore, excretion is an important process, which is vital for all living organisms. The excretory organelles may differ from animal to animal, and have evolved from simple to complex structures based on the needs of the organism. The excretory system is a biological system that helps in the removal of metabolic waste products from an organism.
Much of the excretion in man is executed by the urinary system, which constitutes a pair of kidneys, a pair of ureters, a urinary bladder, and the urethra. Of these, the kidneys play of a very vital role. They filter blood by removing nitrogenous wastes, regulate blood pressure, and assist in the production of red blood cells. The kidneys are compact, bean-shaped organs on either side of the spinal cord in the lower back.
More precisely, they are situated between the last thoracic and third lumbar vertebrae, in close proximity to the dorsal inner wall of the abdominal cavity. The eleventh and twelfth ribs partially protect the upper part of the kidneys. The right kidney, which is near the liver, is slightly lower than the left one, which is adjacent to the spleen. The kidneys are reddish brown in color.
In an adult, each kidney measures about 10 to 12 centimeters in length, 5 to 7 centimeters in breadth, and 2 to 3 centimeters in thickness. Each kidney weighs about 120 to 170 grams. The kidneys have an outer convex surface, and an indentation or a notch on the inner concave surface. called the hilum, through which blood vessels, nerves, and the ureters pass.
The kidney has a tough, fibrous outer layer called a capsule, and its longitudinal section shows that its inside is differentiated into two main regions, an outer cortex and an inner medulla. There are about 8 to 18 cone-shaped structures in the medulla, called, medullary pyramids, which project into the calluses. The tip of each medullary pyramid is called, a papilla. The cortex protrudes or extends between the pyramids, as renal columns, called, the columns of Bertini.
The medullary pyramids project into the calluses. which lead to a wide funnel shaped space called the renal pelvis. Each kidney contains about a million nephrons in the cortical region, which are the basic functional units of the kidney. The nephrons filter nitrogenous wastes and excess salts from the blood, and help in body fluid regulation and urine formation. Each nephron has two parts.
the glomerulus and the renal tubule. The glomerulus is a ball-shaped network of capillaries formed by the afferent arteriole, which is a fine branch of the renal artery. It performs the first step in filtering blood.
The filtered blood from the glomerulus exits through the efferent arteriole. The other part of the nephron, the renal tubule, helps in the reabsorption of salts and water as the urine passes through it, thereby concentrating the urine. The renal tubule begins with the Bowman's capsule and consists of proximal convoluted tubule PCT, Henle's loop and distal convoluted tubule or DCT. The Bowman's capsule is a double walled funnel shaped structure that encloses the glomerulus. It collects the glomerular filtrate.
The Bowman's capsule, together with the glomerulus, is called the Malfygian body or renal corpuscle. The Bowman's capsule opens into the proximal convoluted tubule, a highly coiled structure, and is followed by a hairpin shaped loop called Henle's loop. Henle's loop has a descending limb and an ascending limb.
The ascending limb opens into the distal convoluted tubule, which is also a highly coiled tubular structure. The distal convoluted tubules of several nephrons open into a collecting duct. Several collecting ducts converge and pass through the medulla into the renal pelvis. which leads to the ureter. The collecting ducts along with Hemli's loop lie in the medulla, while the Melfygian body, proximal convoluted tubule, and distal convoluted tubule of a nephron, lie in the cortex of the kidney.
The nephrons in the kidney are also distinguished as cortical nephrons and juxtamedullary nephrons. based on the length of Henle's loop. Cortical nephrons constitute the majority of nephrons and have a very short Henle's loop that just about extends into the medulla, while the juxtamedullary nephrons have a very long Henle's loop that runs deep into the medulla.
Each nephron also has a fine network of capillaries called peritubular capillaries. which is formed from the efferent arteriole leaving the glomerulus. A minute vessel from this network of capillaries forms a U-shaped structure called the vasorector, which lies parallel to the loop of Henle. They are greatly reduced or absent in cortical nephrons. The capillaries of many such efferent arterioles unite to form the renal vein, which joins the inferior vena cava.
Let's now study the other parts of the urinary system. The kidney opens into the ureters, through the renal pelvis. Each ureter is about 25 cm long, and carries urine from the pelvis of the kidneys, to the urinary bladder. The urinary bladder is a hollow, muscular and elastic organ, that temporarily stores urine, till it is released from the body.
Urine leaves the urinary bladder via the urethra, a tube through which urine is discharged. The urethra is enveloped by the urethral sphincter, which controls the flow of urine from the bladder to the outside. Hence the urethra along with the urinary bladder, ureters and kidneys, constitute the human excretory system.
The kidney is the major excretory organ in humans, and one of its primary functions is urine formation. Each kidney contains about a million nephrons, which are the basic functional units of the kidney, and help in urine formation. Urine formation involves three major processes, namely, glomerular filtration or ultrafiltration, reabsorption, and secretion.
The first step in urine formation is the filtration of blood, which takes place in the glomerulus, and hence the name, glomerular filtration. The pressure of blood in the glomerular capillaries forces the blood to pass through three layers, namely the endothelium of the blood vessel, or the capillary, the basement membrane of the glomerulus, and the epithelium of Bowman's capsule. The epithelial cells in Bowman's capsule, also known as, podocytes, are intricately arranged, leaving a few minute openings called slit pores, or filtration slits.
These slits or membranes, help in the filtration of blood, and almost all the constituents of the plasma except the proteins, pass into the lumen of Bowman's capsule. A reason why glomerular filtration is known as ultrafiltration. Did you know that, on average, the human kidney filters about 1,100 to 1,200 milliliters of blood per minute, which is about one fifth of the blood pumped out by each ventricle of the heart in a minute?
The amount of filtrate formed in the Bowman's capsule of a nephron, due to glomerular filtration in the kidneys every minute, is called the glomerular filtration rate or GFR, which in a healthy individual, is about 125 milliliters per minute, or 180 liters per day. The blood pressure in the capillaries acts as a major force in glomerular filtration. Besides, There are certain intrinsic mechanisms in the kidneys that auto-regulate the GFR.
The juxtaglomerular apparatus or JGA is one such microscopic structure that regulates the GFR. The JGA is formed by cellular modifications between the afferent and to some extent the efferent arteriole and the distal convoluted tubule or DCT of the same nephron at their point of contact. The juxtaglomerular cells release renin whenever there is a drop in the GFR, which stimulates glomerular blood flow and normalizes the GFR.
Did you know that of the 180 liters of glomerular filtrate formed per day, the amount of urine released is just 1.5 liters. This is because about 99% of the filtrate is reabsorbed in the renal tubules during the process of reabsorption. Reabsorption is performed by the tubular epithelial cells present in the different parts of the renal tubule through active or passive mechanisms.
While substances in the filtrate such as sodium, amino acids and glucose are absorbed by active transport, nitrogenous wastes are absorbed by passive transport. Water is also reabsorbed passively in the initial segments of the nephron. Apart from absorption, the cells in the renal tubule also selectively secrete substances such as potassium and hydrogen ions and ammonia into the filtrate to maintain the pH and ionic balance in body fluids. This process of reabsorption and secretion occurs in different parts of the renal tubule, namely the proximal convoluted tubule or PCT, Henle's loop, and distal convoluted tubule.
The collecting duct also takes part in the process. The proximal convoluted tubule is the section of the nephron situated between Bowman's capsule and Henleys loop. The glomerular filtrate from Bowman's capsule enters this tubule which is lined by cuboidal brush border epithelial cells that help to increase the surface area for reabsorption. The proximal convoluted tubule reabsorbs about 70 to 80% of the electrolytes and water, and almost all the essential nutrients and vitamins.
It also selectively secretes ammonia, hydrogen ions and potassium ions into the filtrate, and absorbs bicarbonate to maintain the pH and ionic balance of body fluids. The proximal convoluted tubule is followed by Henle's loop, where minimal reabsorption takes place. However, the region plays a vital role in maintaining the high osmolarity of medullary interstitial fluid. The descending limb of Henle's loop is permeable to water and almost impermeable to electrolytes, which helps concentrate the filtrate as it moves down.
The ascending limb is permeable to electrolytes actively or passively and is impermeable to water. Therefore, electrolytes pass into the medullary fluid and dilute the concentrated filtrate as it passes upwards. Henle's loop is followed by the distal convoluted tubule where conditional reabsorption of water and sodium occurs.
This region helps maintain the sodium potassium balance and pH level in the blood, by reabsorbing bicarbonate and selectively secreting potassium and hydrogen ions and ammonia. The distal convoluted tubules of several nephrons open into a straight tube called the collecting duct, which extends from the cortex of the kidney to the inner parts of the medulla. The duct helps to reabsorb water.
thereby increasing the concentration of urine according to the body's state of hydration. It also maintains osmolarity by enabling small amounts of urea to pass into the medullary interstitium. Further, it selectively secretes hydrogen and potassium ions and maintains the ionic and pH balance of blood. Therefore, the nephron filters blood by reabsorbing substances that are needed and excreting the rest as urine.
The urine produced by our body is around four times more concentrated than the initial filtrate, which reflects the conservation of water in the nephrons. Henle's loop and the vassar recta are the parts of the kidney that help concentrate the urine. A counter current is created in both these parts. This is because the direction of the flow of filtrate in the two limbs of Henle's loop and the flow of blood in the two limbs of the vasorector are opposite.
The counter current along with the proximity of Henle's loop to the vasorector maintains an increasing osmolarity in the inner medullary interstitium. The difference in osmolarity between the cortex and the medulla is pronounced as we can see that the osmolarity in the cortex is 300 milliosmoles per liter while it is as high as 1200 milliosmoles per liter in the inner medulla. This difference in osmolarity is caused by the difference in the concentration of urea and sodium chloride. The sodium chloride that is transported through the ascending limb of Henle's loop is exchanged with the descending limb of the vasorector. Then the ascending limb of the vasorector returns the sodium chloride to the interstitium.
In the same way, small amounts of urea entering the thin segment of the ascending limb of Henle's loop are transported back to the interstitium by the collecting tubule. This unique arrangement of the Henle's loop and the vasorector together with the counter current mechanism help maintain the concentration gradient in the medulla which aids the easy passage of water from the collecting duct into the medulla due to osmosis thus concentrating the urine. Urine thus helps to eliminate the numerous waste compounds generated by our body.
Urine formation is vital for human health, and it is difficult to survive without producing and eliminating it. Kidney functions are monitored and regulated by the hormonal feedback mechanisms of the hypothalamus. Juxtaglomerular apparatus or JGA and heart. Our body has several osmoreceptors that are activated by changes in the volume of body fluids, volume of blood and ionic concentration. An osmoreceptor is a sensory receptor that detects changes in osmotic pressure.
For example, a decrease in the level of body fluids activates the osmoreceptors, which stimulate the hypothalamus, which in turn, neurally stimulates the neurohypophysis of the pituitary to release the antidiuretic hormone or ADH, or vasopressin. ADH prompts the distal convoluted tubules. or DCTs, and the collecting ducts, to reabsorb more water, thereby preventing diuresis.
On the other hand, if the fluid volume in the body increases, the osmoreceptors suppress the release of ADH, leading to increased excretion of water in urine. ADH also has the ability to to constrict the blood vessels. This increases blood pressure, thereby increasing the glomerular blood flow in the kidney, and consequently, the glomerular filtration rate, or GFR.
Like the hypothalamus, the JGA also plays a vital role, in regulating kidney functions. The juxtaglomerular cells detect a fall in the glomerular blood pressure or GFR, and release a peptide hormone called renin. Renin converts angiotensinogen in the blood into angiotensin I, which is further converted into angiotensin II.
Angiotensin II, a powerful vasoconstrictor, constricts the blood vessels. thereby increasing blood pressure, which stimulates the cortex of the adrenal gland, to secrete aldosterone. Aldosterone, increases the reabsorption of sodium and water, from the distal parts of the renal tubule, resulting in an increase in blood volume, which increases blood pressure and GFR. This complex mechanism is commonly known as the renin-angiotensin mechanism.
Apart from the hypothalamus and JGA, the heart also regulates the functioning of the kidneys to a certain extent. The muscles of the heart release atrial natriuretic factor or ANF when the blood pressure in the atria increases. ENF, a peptide hormone, is a vasodilator. and also a diuretic that dilates the blood vessels and helps to decrease the blood sodium and water levels.
It exhibits an inhibitory effect on the renin angiotensin mechanism. Hence, hormonal feedback mechanisms efficiently monitor and control the functioning of the kidneys, leading to the formation of urine which passes into the urinary bladder where it is stored until a signal is received by the central nervous system or CNS. As the urinary bladder gets filled with urine, the bladder is stretched and causes the stretch receptors on its walls to send a signal to the CNS.
The CNS in turn sends motor messages that make the smooth bladder muscles contract and the urethral sphincter relax. resulting in the release of urine. This process of disposing urine is called micturition and the neural mechanism responsible for the process is known as the micturition reflex.
On average, an adult human releases about 1 to 1.5 liters of urine every day. The urine released is a watery fluid that is light yellowish in color. Moreover, it has a characteristic odor and is slightly acidic, with a pH value of 6. The characteristics of urine can change according to different body conditions. In fact, a urine test helps to diagnose many metabolic disorders in the body and any malfunction in the kidneys.
For instance, The presence of glucose in urine, which is called, glycosuria, is indicative of diabetes mellitus. Moreover, the urine of diabetic patients, sometimes shows ketone bodies, which is called, ketone urea. The malfunctioning of the kidneys can also cause, uremia, a condition where large amounts of urea, accumulate in the blood. Uremia. can even lead to kidney failure.
The life-saving process for uremic patients is hemodialysis, wherein excess urea in the blood is removed. In this process, blood from a convenient artery is drained into a dialyzing unit after adding an anticoagulant like heparin. In the dialyzer of the unit, a coiled cellophane tube surrounded by a dialyzing fluid, with a composition similar to plasma except for the nitrogenous waste, is placed. The membrane of the cellophane tube is porous, and allows molecules to pass through it, based on the concentration gradient.
These molecules are the nitrogenous wastes to be removed from the blood. The absence of nitrogenous wastes in the dialyzing fluid helps urea to easily move out, thereby clearing the blood. Antiheparin is added to this cleared blood, and it is pumped back into the body, through a vein. However, in cases of acute renal or kidney failure, the only option is kidney transplantation.
In this process, a functioning kidney from a donor is transplanted in the patient. Usually a close relative of the patient is the preferred donor, to minimize the chances of the patient's immune system rejecting the kidney. Other disorders due to the malfunctioning of the kidneys, include the formation of stones or insoluble masses of crystallized salts within the kidney. A condition known as renal calculi.
These stones are usually salts of calcium, mainly calcium oxalate, or calcium phosphate. Such stones are also formed in the ureter, and the urinary bladder. Another disorder affecting the kidneys is, glomerulonephritis. Wherein, the glomeruli of the kidneys get inflamed.
Therefore, the functioning of the kidneys is efficiently monitored and controlled by hormonal feedback mechanisms and even a slight dysfunction or disorder may lead to severe diseases of the kidneys.