Excretory Products and Their Elimination

Human Excretory System

Animals accumulate metabolic wastes such as urea, uric acid, ammonia, carbon dioxide, water, and ions (like $Na^+, K^+, Cl^-, phosphate, sulphate$). The removal of these metabolic wastes from the body is called excretion. Different animals have different excretory mechanisms depending on their habitat and the type of nitrogenous waste product they eliminate.

Nitrogenous Waste Products:

The main nitrogenous waste products in animals are ammonia, urea, and uric acid. The mode of excretion of these wastes determines the type of animal:

• Ammonotelism: Excretion of ammonia. Ammonia is highly toxic and requires a large amount of water for elimination. Common in aquatic animals. (e.g., bony fishes, aquatic amphibians, aquatic insects).
• Ureotelism: Excretion of urea. Urea is less toxic than ammonia and requires less water for elimination. (e.g., mammals, many terrestrial amphibians, marine fishes).
• Uricotelism: Excretion of uric acid. Uric acid is least toxic and requires minimal water for elimination, often excreted as a paste or pellet. Common in animals adapted to arid conditions. (e.g., reptiles, birds, land snails, insects).

Humans are ureotelic, primarily excreting urea as the main nitrogenous waste product.

Components of the Human Excretory System:

The human excretory system consists of:

1. A pair of Kidneys
2. A pair of Ureters
3. A Urinary Bladder
4. A Urethra

Kidneys:

• Pair of bean-shaped organs located in the abdomen, one on either side of the vertebral column, between the levels of the last thoracic and third lumbar vertebra. The right kidney is slightly lower than the left.
• Dimensions: Approximately 10-12 cm length, 5-7 cm width, 2-3 cm thickness. Weight is around 120-170 grams.
• Outer surface is convex, inner surface is concave. The concave side has a notch called the hilum, through which the renal artery and nerve enter, and the renal vein and ureter exit.
• Inside the hilum is a broad funnel-shaped space called the renal pelvis, with projections called calyces (singular: calyx).
• The kidney tissue has two distinct regions:
  • Outer Cortex: Granular in appearance. The cortex extends in between the renal pyramids as renal columns called Columns of Bertini.
  • Inner Medulla: Divided into conical masses called medullary pyramids (renal pyramids), projecting into the calyces.

*(Image shows a cutaway view of a human kidney showing the outer cortex, inner medulla with pyramids, renal pelvis, and ureter exiting from the hilum)*

Nephrons: The Functional Units of Kidney:

• Each kidney contains about one million complex tubular structures called nephrons. Nephrons are the structural and functional units of the kidney, responsible for urine formation.
• A nephron consists of two main parts:
  1. The Glomerulus: A tuft of capillaries formed by the afferent arteriole (a branch of the renal artery). Blood leaves the glomerulus through the efferent arteriole.
  2. The Renal Tubule: A long, coiled tube starting from the Bowman's capsule.
• The glomerulus is enclosed within a double-walled cup-like structure called Bowman's capsule. The glomerulus and Bowman's capsule together form the Malpighian body or renal corpuscle.
• The renal tubule extends from Bowman's capsule and consists of:
  • Proximal Convoluted Tubule (PCT): Highly coiled part located in the cortex.
  • Loop of Henle: A U-shaped portion extending into the medulla. It has a descending limb and an ascending limb.
  • Distal Convoluted Tubule (DCT): Highly coiled part located in the cortex, distal to the Loop of Henle.
  • The DCT opens into a larger tube called the Collecting Duct. Many collecting ducts merge and pass through the medullary pyramids to open into the renal pelvis through the renal papillae.

Types of Nephrons:

• Cortical nephrons: Majority of nephrons (about 85%). Their Loop of Henle is short and lies mainly in the cortex.
• Juxtamedullary nephrons: About 15% of nephrons. Their Loop of Henle is long and extends deep into the medulla. These nephrons play a crucial role in concentrating urine.

Blood Supply to Nephron:

• The renal artery branches into afferent arterioles, which supply blood to the glomerulus.
• The efferent arteriole carries blood away from the glomerulus.
• The efferent arteriole forms a fine capillary network around the renal tubule called the peritubular capillaries.
• In juxtamedullary nephrons, the peritubular capillaries form a U-shaped vessel called the vasa recta, which runs parallel to the Loop of Henle.

*(Image shows a diagram of a nephron illustrating all its parts and the associated blood vessels)*

Ureters, Bladder, and Urethra:

• Ureters: A pair of tubes that carry urine from the renal pelvis of each kidney down to the urinary bladder.
• Urinary Bladder: A muscular, distensible sac that stores urine temporarily.
• Urethra: A tube that carries urine from the urinary bladder to the outside of the body. It is shorter in females than in males. The opening is guarded by sphincters.

Urine Formation

Urine formation is a complex process that takes place in the nephrons. It involves three main steps:

1. Glomerular Filtration (Ultrafiltration)
2. Reabsorption
3. Secretion

These steps occur simultaneously as blood flows through the kidney.

1. Glomerular Filtration (Ultrafiltration):

• This is the first step, occurring in the Malpighian body (renal corpuscle).
• Blood flows into the glomerulus through the afferent arteriole and leaves through the efferent arteriole. The diameter of the efferent arteriole is slightly narrower than the afferent arteriole, which creates pressure in the glomerulus.
• The filtration occurs across a very thin membrane called the filtration membrane or glomerular membrane. This membrane is formed by:
  • The endothelium of glomerular capillaries.
  • The epithelium of Bowman's capsule (podocytes).
  • A thin basement membrane between the two layers.
• This membrane allows the filtration of water and small solutes from the blood plasma into the Bowman's capsule, but it prevents the passage of larger molecules like proteins and blood cells.
• The pressure that drives filtration is the Glomerular Hydrostatic Pressure (GHP) (about 55 mm Hg).
• GHP is opposed by the Blood Colloid Osmotic Pressure (due to plasma proteins, about 30 mm Hg) and Capsular Hydrostatic Pressure (pressure exerted by the filtrate in Bowman's capsule, about 15 mm Hg).
• The Net Filtration Pressure (NFP) is the effective pressure causing filtration:

  $ NFP = GHP - (\text{Blood Colloid Osmotic Pressure} + \text{Capsular Hydrostatic Pressure}) $

  $ NFP = 55 \text{ mm Hg} - (30 \text{ mm Hg} + 15 \text{ mm Hg}) = 55 - 45 = 10 \text{ mm Hg} $

• The fluid that filters into Bowman's capsule is called glomerular filtrate. It is essentially plasma minus proteins and blood cells.

Glomerular Filtration Rate (GFR):

• GFR is the volume of glomerular filtrate formed per minute by both kidneys.
• Average GFR in a healthy adult is about 125 mL/minute, or 180 litres per day!

*(Image shows the glomerulus within Bowman's capsule, with afferent and efferent arterioles, illustrating the filtration membrane and showing movement of water/solutes from blood to Bowman's capsule)*

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2. Reabsorption:

• The glomerular filtrate contains many useful substances (like glucose, amino acids, electrolytes, water) in addition to waste products. If all of this filtrate were excreted, the body would lose essential substances.
• Reabsorption is the process by which these useful substances are transported back from the renal tubule into the blood in the peritubular capillaries.
• Reabsorption is highly selective.
• It occurs along the entire length of the renal tubule (PCT, Loop of Henle, DCT, Collecting Duct).
• Reabsorption can be active (requires ATP, e.g., glucose, amino acids, $Na^+$ ions) or passive (down concentration/electrochemical gradients, e.g., water by osmosis, some ions).
• About 99% of the glomerular filtrate is reabsorbed, leaving only about 1.5 litres of urine excreted per day.

3. Secretion:

• Secretion is the process by which certain substances (like $K^+$ ions, $H^+$ ions, creatinine, metabolic wastes, drugs) are transported from the blood in the peritubular capillaries into the lumen of the renal tubule.
• This process is usually active.
• Secretion is important for:
  • Eliminating certain waste products and foreign substances.
  • Maintaining the ionic balance and pH of the body fluids.
• Secretion occurs mainly in the PCT, DCT, and Collecting Duct.

Thus, urine formation is a result of the combined processes of filtration (non-selective), reabsorption (selective return of useful substances to blood), and secretion (selective removal of wastes and excess ions from blood into the filtrate).

*(Image shows a nephron tubule and associated capillary, with arrows indicating filtration from glomerulus to Bowman's capsule, reabsorption from tubule to capillary, and secretion from capillary to tubule at different parts of the tubule)*

Function Of The Tubules

Each segment of the renal tubule plays a specific role in modifying the glomerular filtrate, contributing to reabsorption and secretion, and ultimately determining the final composition of urine.

Proximal Convoluted Tubule (PCT):

• Located in the cortex. The epithelial cells are cuboidal and have a brush border of microvilli, significantly increasing the surface area for reabsorption.
• Major site of reabsorption: About 70-80% of electrolytes, all glucose and amino acids, and a large amount of water are reabsorbed here.
  • Glucose and amino acids are reabsorbed by active transport.
  • Electrolytes ($Na^+, K^+, Cl^-$) are reabsorbed actively and passively.
  • Water is reabsorbed passively by osmosis, coupled with solute reabsorption.
• Secretion: Secretes $H^+$ ions, $NH_3$, and some drugs into the filtrate.
• Function: Maintains the pH and ionic balance of body fluids. Reabsorbs most essential nutrients.

Loop Of Henle:

• U-shaped part of the tubule extending into the medulla.
• Crucial role in creating a concentration gradient in the medulla, which helps in concentrating urine.
• Descending Limb: Permeable to water but almost impermeable to electrolytes.
  • As filtrate moves down into the hyperosmolar medulla, water moves out by osmosis into the interstitial fluid.
  • Filtrate becomes progressively more concentrated as it moves down.
• Ascending Limb: Impermeable to water but permeable to electrolytes.
  • Electrolytes ($Na^+, K^+, Cl^-$) are reabsorbed here. Passive diffusion in the thin segment, active transport in the thick segment.
  • As filtrate moves up, electrolytes are removed, and water is retained, making the filtrate less concentrated as it moves towards the cortex.
• Function: Helps in creating the medullary osmotic gradient for urine concentration (Countercurrent Mechanism).

Distal Convoluted Tubule (DCT):

• Located in the cortex.
• Conditional reabsorption: Reabsorption of $Na^+$ ions and water occurs here under hormonal control (e.g., by aldosterone and ADH).
• Secretion: Secretes $H^+$ ions, $K^+$ ions, and $NH_3$ into the filtrate to maintain ionic balance and pH.
• Function: Fine-tunes the ionic and water balance. Helps regulate blood pressure and volume.

Collecting Duct:

• Extends from the cortex to the inner medulla. Many nephrons' DCTs open into a single collecting duct.
• Reabsorption: Reabsorbs a large amount of water under the control of ADH, especially in the medulla. This helps in concentrating the urine.
• Permeable to small amounts of urea in the inner medullary part, which contributes to the medullary osmotic gradient.
• Secretion: Secretes $H^+$ and $K^+$ ions to maintain pH and ionic balance.
• Function: Collects urine from multiple nephrons. Plays a major role in concentrating urine by reabsorbing water.

*(Image shows a diagram of a nephron tubule, indicating reabsorption and secretion of various substances (water, ions, glucose, amino acids, urea, H+, K+) in different parts)*

The selective reabsorption and secretion processes along the renal tubules ensure that the body retains essential substances while eliminating waste products and excess ions, thereby regulating body fluid composition and volume.

Mechanism Of Concentration Of The Filtrate

Mammals have the ability to produce urine that is more concentrated than their blood plasma. This is crucial for conserving water, especially in terrestrial environments. The concentration of filtrate (and thus urine) is achieved through a mechanism called the Countercurrent Mechanism, which operates in the medulla of the kidney.

The Countercurrent Mechanism:

This mechanism involves the interaction between the flow of filtrate in the Loop of Henle and the flow of blood in the vasa recta (capillaries parallel to the loop). The flow of fluid in the descending and ascending limbs of the loop of Henle is in opposite directions (countercurrent), and the flow of blood in the descending and ascending limbs of the vasa recta is also in opposite directions, countercurrent to the Loop of Henle.

Creation and Maintenance of Medullary Osmotic Gradient:

The interstitial fluid of the renal medulla becomes progressively hyperosmolar (more concentrated) from the cortex towards the inner medulla. The osmolarity increases from about 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. This gradient is maintained by:

1. Passive diffusion of NaCl: From the thin ascending limb of the Loop of Henle into the medullary interstitium.
2. Active transport of NaCl: From the thick ascending limb of the Loop of Henle into the medullary interstitium.
3. Diffusion of Urea: From the collecting duct into the medullary interstitium.

How the Gradient Concentrates Urine:

The established osmotic gradient in the medulla drives the movement of water out of the renal tubule:

1. Descending Limb of Loop of Henle: As filtrate passes down, water moves out by osmosis into the hyperosmolar medulla because the membrane is permeable to water. This concentrates the filtrate.
2. Ascending Limb of Loop of Henle: As filtrate moves up, electrolytes are removed (passively then actively). The membrane is impermeable to water, so water remains in the tubule. This dilutes the filtrate compared to the descending limb.
3. DCT and Collecting Duct: The filtrate entering the DCT is relatively dilute (around 300 mOsmol/L). However, as it passes through the DCT and especially the Collecting Duct (which passes through the hyperosmolar medulla), a large amount of water is reabsorbed by osmosis, particularly under the influence of ADH. The collecting duct is permeable to water but not to salts in the outer medulla. In the inner medulla, it is permeable to urea.

Role of Vasa Recta:

• The vasa recta also operate on a countercurrent principle. Blood flows down into the medulla (descending limb) and then back up towards the cortex (ascending limb).
• As blood descends into the hyperosmolar medulla, NaCl diffuses into the blood, and water diffuses out.
• As blood ascends towards the cortex, NaCl diffuses out of the blood, and water diffuses in.
• This countercurrent flow in the vasa recta prevents the wash-out of the medullary osmotic gradient, preserving the concentration difference essential for concentrating urine.

*(Image shows a juxtamedullary nephron with the Loop of Henle and vasa recta running parallel in the medulla, illustrating the increasing osmotic gradient in the interstitium from cortex to medulla and the movement of water, NaCl, and urea in the tubules and vasa recta)*

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The coordinated function of the Loop of Henle, vasa recta, and collecting duct in the medullary osmotic gradient allows the kidney to produce highly concentrated urine, conserving water in the body. Humans can produce urine nearly four times as concentrated as their blood plasma (e.g., up to 1200 mOsmol/L compared to 300 mOsmol/L).

Regulation Of Kidney Function

The functioning of the kidneys is highly regulated to maintain homeostasis, particularly concerning blood volume, blood pressure, ionic balance, and osmolarity.

Key regulatory mechanisms involve hormonal feedback loops:

1. Regulation by Antidiuretic Hormone (ADH) / Vasopressin:

• Produced by the hypothalamus and released by the posterior pituitary gland.
• Release is stimulated by:
  • Increase in body fluid osmolarity (e.g., due to dehydration).
  • Decrease in blood volume.
  • Decrease in blood pressure.
• Action: ADH increases the permeability of the collecting duct and DCT to water. This increases the reabsorption of water from the filtrate back into the blood, reducing water loss in urine (producing concentrated urine) and increasing blood volume.
• ADH also causes vasoconstriction (narrowing of blood vessels), which increases blood pressure (hence called Vasopressin).
• Absence of ADH (e.g., due to damage to pituitary) leads to Diabetes Insipidus, characterised by excessive urination (dilute urine) and thirst.

*(Image shows hypothalamus/pituitary releasing ADH in response to increased blood osmolarity, and ADH increasing water reabsorption in the collecting duct)*

2. Regulation by Renin-Angiotensin-Aldosterone System (RAAS):

• This system is activated by a decrease in Glomerular Filtration Rate (GFR), blood pressure, or blood volume.
• Juxtaglomerular Apparatus (JGA): A specialised region formed by cellular modifications in the DCT and the afferent arteriole at their point of contact. JGA secretes Renin.
• Steps:
  1. A drop in GFR/blood pressure/blood volume is sensed by the JGA, which releases Renin.
  2. Renin converts angiotensinogen (a plasma protein produced by the liver) into Angiotensin I.
  3. Angiotensin I is converted to Angiotensin II in the lungs (by Angiotensin Converting Enzyme - ACE).
  4. Angiotensin II is a powerful vasoconstrictor, which increases blood pressure.
  5. Angiotensin II also stimulates the adrenal cortex to release Aldosterone.
  6. Aldosterone acts mainly on the DCT and collecting duct, stimulating the reabsorption of $Na^+$ and water, and increasing $K^+$ and $H^+$ secretion. This increases blood volume and blood pressure.
• RAAS is a major system for regulating blood pressure and volume.

*(Image shows a flowchart illustrating the RAAS pathway: Low BP/GFR $\rightarrow$ Renin release from JGA $\rightarrow$ Angiotensinogen $\rightarrow$ Angiotensin I $\rightarrow$ Angiotensin II $\rightarrow$ Vasoconstriction + Aldosterone release $\rightarrow$ Increased Na+/water reabsorption $\rightarrow$ Increased BP/volume)*

3. Regulation by Atrial Natriuretic Factor (ANF):

• ANF is a peptide hormone secreted by the atrial wall of the heart when blood pressure increases (e.g., due to increased blood volume).
• Action: ANF acts as a vasodilator (widens blood vessels), decreasing blood pressure. It also inhibits the release of Renin and Aldosterone, reducing $Na^+$ and water reabsorption.
• ANF thus acts antagonistically to RAAS, promoting $Na^+$ and water excretion and decreasing blood pressure.

These hormonal mechanisms interact to finely regulate kidney function and maintain fluid and electrolyte balance and blood pressure.

Micturition

Micturition is the process of expulsion of urine from the urinary bladder to the outside of the body. It is a neural mechanism called the micturition reflex.

The Micturition Reflex:

• The urinary bladder stores urine until it is full. As the bladder fills with urine, the stretch receptors in its wall are activated.
• Stretch receptors send signals to the central nervous system (spinal cord and pons).
• The CNS initiates a reflex:
  • Stimulation of parasympathetic nerves causes contraction of the smooth muscles in the bladder wall.
  • Simultaneously, it causes relaxation of the urethral sphincter (the muscle guarding the opening of the urethra).
• This reflex leads to the expulsion of urine.

Voluntary Control:

• In adults, the micturition reflex can be voluntarily controlled to some extent.
• Neural signals from the brain can override the reflex, allowing a person to delay urination until an appropriate time and place.
• This voluntary control develops as the nervous system matures (usually by around 2-3 years of age).

*(Image shows a simplified diagram showing bladder filling, stretch receptors, neural pathway to spinal cord, efferent pathway to bladder muscle (contraction) and sphincter (relaxation))*

The urge to urinate arises when the bladder contains about 150-250 mL of urine, but the bladder can hold up to 500 mL or more.

Role Of Other Organs In Excretion

While the kidneys are the primary excretory organs in humans, other organs also play a role in eliminating certain waste products from the body.

1. Lungs:

• Lungs are primarily involved in the exchange of respiratory gases.
• They eliminate large amounts of carbon dioxide ($CO_2$) (about 200 mL per minute) and a significant amount of water vapour during breathing.

2. Skin:

• The skin has sweat glands and sebaceous glands, which eliminate certain substances.
• Sweat glands: Eliminate sweat, which contains water, small amounts of urea, lactic acid, salts (like NaCl). Sweat helps in regulating body temperature and removing some waste products.
• Sebaceous glands: Eliminate substances like sebum through the skin surface. Sebum contains steroids, hydrocarbons, and waxes. This elimination mainly provides a protective oily covering to the skin and hair.

3. Liver:

• The liver is a major metabolic organ that processes and detoxifies various substances.
• It produces urea from the breakdown of amino acids (ornithine cycle). Urea is then transported to the kidneys for excretion.
• It breaks down haemoglobin pigments, forming bile pigments (bilirubin and biliverdin). These pigments are released into the bile and are eliminated from the body along with faeces.
• Liver also excretes excess vitamins, drugs, and toxins into the bile.

Although kidneys are the main organs for filtering blood and forming urine to eliminate nitrogenous wastes, excess water, and salts, the lungs, skin, and liver contribute to the overall process of waste elimination from the body.

Disorders Of The Excretory System

Malfunctioning of the kidneys or other parts of the excretory system can lead to various disorders affecting the body's ability to eliminate waste, regulate fluid balance, and maintain blood pressure.

Common Disorders:

• Uremia: Accumulation of urea in the blood due to kidney failure. This can be highly toxic and lead to kidney failure. Patients with uremia require haemodialysis or kidney transplant.
• Renal Calculi (Kidney Stones): Formation of stones or insoluble masses of crystallised salts (e.g., calcium oxalates, phosphates) within the kidney or urinary tract. These stones can block the passage of urine, causing severe pain.
• Glomerulonephritis: Inflammation of the glomeruli of the kidneys. This affects the filtration process, leading to protein and blood in the urine and impaired kidney function. Can be caused by infections or autoimmune diseases.
• Kidney Failure (Renal Failure): A condition where the kidneys lose their ability to adequately filter waste products from the blood. Can be acute (sudden) or chronic (gradual loss of function over time). Causes include diabetes, high blood pressure, chronic infections, etc.

Treatment for Kidney Failure:

• Haemodialysis ('Artificial Kidney'): A process used for patients with kidney failure to filter their blood outside the body.
  • Blood from an artery is passed into a dialysing unit (artificial kidney).
  • The unit contains a coiled cellophane membrane surrounded by dialysing fluid.
  • The dialysing fluid has the same composition as plasma, except for nitrogenous wastes.
  • Waste products from the patient's blood diffuse across the semipermeable cellophane membrane into the dialysing fluid (due to the concentration gradient).
  • The filtered blood, free of urea and other wastes, is then pumped back into the patient's vein.
  • This process is typically performed regularly (e.g., a few times a week) to remove accumulated wastes.

  *(Image shows a diagram of a haemodialysis setup with blood being drawn from a patient's artery, flowing through a dialyzer with dialysing fluid, and returned to the patient's vein)*

• Kidney Transplant: The ultimate method in the correction of acute renal failures (kidney failure). A healthy kidney from a suitable donor (living relative preferred to minimise rejection) is transplanted into the patient's body. The recipient's immune system is usually suppressed using medications to prevent rejection of the transplanted kidney.

Maintaining a healthy lifestyle, controlling blood pressure and diabetes, and avoiding excessive use of certain medications can help prevent or manage kidney disorders.