Excretory System

The process by which nitrogen-containing waste products of metabolism are eliminated from the body is called excretion. The main nitrogenous waste products are ammonia, urea, and uric acid.

Based on the excretory product, the following modes of excretion are known in animals:

1. Ammonotelism:

   • Ammonotelism is the elimination of nitrogenous waste in the form of ammonia. The organisms are called ammonotelic.
   • Ammonia is highly soluble in water, so it diffuses rapidly and is toxic. Therefore, it cannot be stored in the body.
   • A large amount of water is required to eliminate ammonia. So it is suitable for aquatic animals. 1 gm of NH₃requires about 300 to 500 ml of water.
   • Ex: fish, protista, earthworm, tadpole, bony fishes, salamander.
   • Chemical reaction: Amino acid + Oxygen → ketoacid + Ammonia

2. Ureatelism:

   • Ureatelism is the process of elimination of nitrogen chiefly in the form of urea. The animals are called ureotelic.
   • Ammonia combines with CO₂ in the liver of the organism with the expenditure of energy in the presence of an enzyme and finally converts to urea through the Urea cycle (Ornithine cycle).
   • Chemical reaction: Ammonia + CO₂ —(Arginase enzyme)→ Urea + water
   • Urea is 100,000 times less toxic than ammonia, so it can be stored in the body for a longer time without toxic effects.
   • In man, the concentration is 18 to 36 mg per 100 ml.
   • Ex: Humans, aquatic mammals like whales and seals, desert or terrestrial animals such as camels, kangaroo rats, turtles, frogs, toads, sharks, and ray fish.

3. Uricotelism:

   • Uricotelism is the process of elimination of nitrogen in the form of uric acid. The organism is called uricotelic.
   • It is a complex process and requires a larger amount of energy than ureatelism.
   • Uric acid is the least toxic, so it can be stored in cells for the longest time.
   • Uric acid can be stored or excreted in crystalline form, especially in birds (stored in the cloaca and comes out with feces).
   • Ex: Terrestrial reptiles (lizards and snakes), birds, land snails, etc.

Note:

• Protonephridia or Flame cells are the excretory structure in platyhelminthes (eg: Planaria), some annelids, and Cephalochordata (ex: Amphioxus).
• Nephridia are the excretory structure of earthworms.
• Malpighian tubules are the excretory structure of insects such as cockroaches.
• Antennal glands or green glands perform the excretory function in crustaceans like prawn.
• Ammonia is converted to guanine and no water is required for its excretion in Spiders.

Excretory System Overview

• Excretory Organs: Kidney, Ureter, Urinary bladder, Urethra.
• Physiology of Excretion:
  • Formation of Urine: Ultrafiltration, Selective reabsorption, Tubular secretion.
  • Micturition (Urination).

Human Urinary System

The human urinary system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

1. Kidney:

   These are paired structures and the main excretory organ in mammals. Kidneys are dark red, slightly flattened, bean-shaped organs, about 10-12 cm long, 5-7 cm wide, and 2-3 cm thick. They are placed against the back wall of the abdominal cavity just below the diaphragm, one on either side opposite the last thoracic (T₁₂) and first three lumbar vertebrae. Lower two pairs of ribs protect them. The left kidney is a little higher and larger than the right one due to space occupied by the liver on the right side. Each kidney has a concave inner surface (with a depression called the renal hilum) and a convex outer surface. Blood vessels, renal vessels, nerves, and ureters enter or leave the kidney through the hilum. Kidneys excrete nitrogenous waste in the form of urea.

2. Ureter:

   These are two ureters, one existing from each kidney. Each ureter is a thin muscular tube about 25 cm in length arising from the renal pelvis in the hilum region. It opens into the urinary bladder. Its inner wall is lined by transitional epithelium, middle muscular, and outer fibrous. Urine moves in the ureter by the peristaltic wave of its walls.

3. Urinary Bladder:

   It lies in the pelvic cavity and is a pear-shaped, muscular, hollow, sac-like structure. It collects urine from both kidneys via ureters. It has a strong muscular wall and has the ability to stretch. The bladder has 3 openings: two from the ureters and one for the urethra, forming a triangle called the trigone. The wall of the urinary bladder is made up of three layers: inner smooth muscles, middle circular fibres, and outer layer of longitudinal fibres. It stores urine temporarily.

4. Urethra:

   It is a canal-like membranous tube which extends from the neck of the bladder and leads to the exterior. It carries urine at intervals from the urinary bladder to the outside. It opens by the urethral orifice in front of the vaginal aperture in females, whereas in males, the urethra is much longer and opens out at the tip of the penis by the urinogenital aperture. Thus, the urethra of the male carries both urine and semen. There is a urethral muscular sphincter present close to the junction of the urethra and bladder which regulates urination.

Internal Structure of Kidney

In a vertical section through the kidney, the following regions can be seen:

1. Capsule: It is the structure covering the kidney. It has three layers: inner thin fibrous connective tissue, middle Renal fat (Adipose capsule), and outer renal fascia.
2. Renal Cortex: It is the outermost region towards the convex surface of the kidney. It contains Bowman's Capsule as well as proximal and distal convoluted tubules.
3. Renal Medulla: It is a region just inner to the renal cortex and is divided into a few conical masses called medullary pyramids. It consists of renal tubules, collecting ducts, and blood vessels. The medullary pyramids project into calyces of the renal pelvis.
4. Renal Pelvis: It is a large funnel-shaped structure formed by the union of major calyces towards the hilum of the kidney. Urine is collected in the renal pelvis and is passed down to the ureter. The urine is drained into the renal pelvis through minute openings called papillae of the pyramid (blunt posterior end). The substances of the cortex extend between the pyramids forming the renal columns of Bertini.

Structure of Nephron

The nephron is the functional unit of the kidney. Each kidney may have more than 1.2 million nephrons. Each nephron is about 3 cm long, originating in the cortex region and extending into the medulla region. The nephron works as an independent unit and produces urine which is collected by collecting tubules and finally pours into the renal pelvis.

The nephron consists of a long coiled tube which is closed at one end and opens into collecting tubules. The closed end forms the Malpighian body, which is situated in the cortex. The nephron consists of 5 anatomical regions:

1. Renal Corpuscles or Malpighian Body:

   Consists of 2 parts:

   • a) Bowman's Capsule: A large double-walled, cup-shaped hollow structure. It lies in the renal cortex. It consists of an outer parietal layer made of simple squamous epithelium and an inner visceral layer that consists of podocytes (specialized flattened cells). The space between the parietal and visceral layers is called the capsular space, which is filled with glomerular filtrate.
   • b) Glomerulus: It is a network of capillaries present in the hollow of the Bowman's capsule. An afferent arteriolecarries blood to the glomerulus while an efferent arteriole collects blood from it. The diameter of the efferent arteriole is much smaller than the afferent arteriole.

2. Proximal Convoluted Tubule (PCT):

   Starts from the neck of the Bowman's capsule and is greatly twisted, also lying in the renal cortex. The wall of the tubule is made of simple cuboidal cells which become ciliated at the neck region. These epithelial cells of PCT have a brush border due to the presence of prominent microvilli on the free surface and rest on a basement membrane. These microvilli and mitochondria increase the surface area and absorption by active process. It transports glomerular filtrate from Bowman's capsule into the Loop of Henle.

3. Loop of Henle:

   It is a U-shaped loop consisting of a continuation from the PCT. It descends down from the cortex into the medulla region and again returns back after forming a loop. It consists of:

   • a) Descending limb of Loop of Henle: A thin-walled long tube which is permeable to water. It is lined with flattened cuboidal epithelial cells with very few microvilli.
   • b) Ascending limb of Loop of Henle: It is thicker and made up of cuboidal epithelium which is impermeable to water but allows electrolyte passage.

4. Distal Convoluted Tubule (DCT):

   It is a second coil of twisted tubules that continues from the ascending limb of the loop of Henle. It is located in the renal cortex, is shorter in length, and is less convoluted than the PCT. It is lined by cuboidal epithelium and lacks microvilli.

5. Collecting Tubule:

   These are large tubes, each receiving the collecting tubules of several nephrons. They pass into the renal medulla and join with each other forming still larger Ducts of Bellini. These run through medullary pyramids and open into calyces which lead into the pelvis. The collecting ducts are lined by cuboidal cells which are ciliated and columnar cells.

* Vasa Rectae: These are parallel, wide, thin-walled, and straight capillaries that arise from the efferent arteriole and run parallel to the loop of Henle and collecting duct in the medulla. It maintains high osmotic pressure and also helps in the concentration of urine in the loop of Henle.

Types of Nephron

There are 2 different types of nephron according to their position in the kidney:

Physiology of Excretion

Urine formation of nephron involves 3 basic steps:

1) Glomerular filtration or Ultrafiltration:

The first step in urine formation is a filtration of blood which is carried out by the glomerulus, hence called glomerular filtration. Blood enters the glomerulus through the wider afferent arterioles and leaves through narrow efferent arterioles. So, the filtration occurs under high hydrostatic pressure, and smaller molecules of glucose, water, salt, and ammonia (except protein) diffuse from the glomerulus to the Bowman's Capsule. This is called ultrafiltration.

The glomerular capillary blood pressure causes filtration of the blood through 3 layers: the endothelium of the glomerular blood pressure, the epithelium of Bowman's capsule, and a basement membrane between these 2 layers. The epithelial cell of Bowman's capsule is called podocytes; podocytes in between them have slit pores.

The amount of filtrate formed by the kidney per minute is called the Glomerular Filtration Rate (GFR), which is approximately 125 ml per minute, i.e., 180 liters per day.

Glomerular Filtration Pressure (GFP) is the total pressure that causes filtration. It is determined by:

• Glomerular Blood Hydrostatic Pressure (GBHP) ≈ 60 mmHg
• Blood Colloidal Osmotic Pressure (BCOP) ≈ 32 mmHg (Plasma Protein)
• Capsular Hydrostatic Pressure (CHP) ≈ 18 mmHg

Hence, GFP = GBHP - (BCOP + CHP) = 60 - (32 + 18) = 10 mmHg.

Thus, a pressure of 10 mmHg causes blood plasma to filter from the glomerulus into the Bowman's capsule. About 1100 to 1200 ml of blood circulates per minute through the kidney, out of which 652-700 ml is plasma filtrate called Renal Plasma Flow (RPF).

2) Selective Reabsorption:

It means the substances which are necessary to our body are absorbed. About 180 liters of glomerular filtrate is produced per day; only 1.5 liters is put out as urine. As a result of tubular reabsorption, only useful substances of filtrate pass out of the nephron tubule. So 99% of material filtered are reabsorbed, preventing the loss of water, nutrients, and ions from the body. It involves active and passive mechanisms occurring in different regions.

• At PCT: The cells of PCT are provided with microvilli (brush border). About 70% of filtrate is absorbed by active transport. So the filtrate of PCT is isotonic and substances reabsorbed are water, Na⁺, K⁺, chloride, Ca²⁺, and Vitamin C.
• Loop of Henle: It is composed of a thin descending limb and a thick ascending limb. The descending limb is permeable to water, so by osmosis, the water is reabsorbed and the filtrate becomes hypertonic. Now the ascending limb reabsorbs electrolytes, which are reabsorbed at the DCT.
• DCT: The filtrate passes through the DCT, which has a collecting duct where reabsorption of NaCl and chloride from the filtrate takes place under the influence of hormones like Aldosterone and Antidiuretic Hormone (ADH).

3) Tubular Secretion:

Tubular secretion is an important step in urine formation as it helps in the maintenance of ionic and acid-base balance of body fluid. Certain chemicals in the efferent vessel also contain unwanted substances that are removed by peritubular capillaries in the DCT. The metabolic waste products like ammonia, hippuric acid, creatinine, H⁺, etc., present in the blood diffuse into the DCT. Now this filtrate is called urine.

Micturition

Micturition is the process by which the urine from the urinary bladder is exerted. As the urine accumulates, the muscular wall of the bladder expands. The walls stimulate the sensory nerves in the bladder, setting up a reflex action. This reflex stimulates the urge to pass out urine. So, to discharge urine, the urethral sphincter relaxes and the smooth muscles of the bladder contract. This forces the urine out from the bladder.

Urine: In a healthy person, the volume and composition of urine widely vary from day to day. It depends on: i) fluid intake and type of food consumed, ii) physical activity, sweating, and temperature. Profuse sweating, heavy work, and high temperature decrease the volume of urine.

So urine is a light yellow coloured watery fluid, which is formed after selective reabsorption and secretion from the filtrate. It constitutes about 95% water and 5% of other organic and inorganic substances. Organic substances in urine are nitrogen, urea, creatinine, ammonia, uric acid, vitamins, hormones, and enzymes, while inorganic substances include chloride, phosphate, sulphate, potassium, calcium, and magnesium, etc.

Disorders of Excretory System

1. Uremia: It is an abnormal rise of blood urea level that occurs due to kidney failure.
2. Proteinuria: Presence of excess protein in urine.
3. Ketonuria: Presence of ketone bodies in urine.
4. Haematuria: It is the presence of blood cells in urine (Ex: stone, cystitis).
5. Pyuria: Presence of pus in urine.
6. Glycosuria: A condition when glucose appears in urine as in diabetes mellitus. The threshold value of glucose is 180 mg / 100 ml, above which it is not absorbed from glomerular filtrate.
7. Renal failure: It is also called kidney failure in which glomerular filtration is restricted and both kidneys stop working. Kidney transplant is the ultimate method in correction.
8. Renal calculi: Formation of stone or insoluble mass of crystallized salts within the kidney.
9. Glomerulonephritis (Bright's disease): Inflammation of glomeruli of the kidney due to entry of protein or red blood corpuscles into the filtrate, often due to injury.
10. Gout: High level of uric acid accumulation in the body which tends to solidify into crystals and are redeposited in the joints and kidney. Due to which joints become painful.
11. Diuresis: The problem of increased urination.
12. Diabetes insipidus: Deficiency of ADH leads to diabetes insipidus, a condition marked by the output of huge amounts of urine and intense thirst (polydipsia, dehydration, hyperkalemia). Whereas diabetes mellitus (mel - honey) is where insulin deficiency causes large amounts of blood sugar to be lost in the urine.

Artificial Kidney (Hemodialysis)

Hemodialysis is an artificial process of removing toxic substances from the blood in patients with kidney failure. The hemodialysis machine is therefore known as the artificial kidney.

Hemodialysis Procedure:

A hemodialysis unit is called an artificial kidney because it performs the same function. It helps in removing the nitrogenous waste from the blood without losing plasma protein. This method is a boon for people suffering from uremia.

In this process, the dialysis unit contains a coiled cellophane tube surrounded by dialyzing fluid having the same composition as plasma except for the nitrogenous wastes. Blood drained from a convenient artery is pumped into the dialyzing unit after adding an anti-coagulant like heparin.

The porous cellophane membrane of the tube allows the passage of molecules based on the concentration gradient. As nitrogenous wastes are absent in the dialyzing fluid, these substances freely move out, thereby clearing the blood. The cleaned blood is pumped back to the body through a vein after adding anti-heparin to it.

Functions of Kidney (Mnemonic: A WET BED)

1. A - Maintaining Acid-base balance
2. W - Maintaining Water balance (Osmoregulation)
3. E - Electrolyte balance
4. T - Toxin removal (nitrogenous waste)
5. B - Blood pressure control
6. E - Making Erythropoietin
7. D - Vitamin D metabolism