EXCRETORY SYSTEM

EXCRETION The process of removal of metabolic wastes from the body.

Osmoregulation The process by which living organisms maintain an osmotic pressure in the body by regulating the amount of water and salts.

Kinds of wastes excreted Nitrogenous wastes: These are the major waste products formed during the breakdown of extra amino acids and nucleic acids. Nitrogenous wastes: These are the major waste products formed during the breakdown of extra amino acids and nucleic acids. Non- nitrogenous wastes: oxalic acid and lactic acid. Non- nitrogenous wastes: oxalic acid and lactic acid. Excess chemicals: like drugs, vitamins, hormones, cholesterol etc. Excess chemicals: like drugs, vitamins, hormones, cholesterol etc. Bile pigments: like bilirubin and biliverdin. Bile pigments: like bilirubin and biliverdin. Carbon dioxide Carbon dioxide Excess water. Excess water.

Classification of living Organisms ( on the basis of type of nitrogenous waste ) Ammonotelic organisms: Organisms which excrete waste as ammonia. Organisms which excrete waste as ammonia. Ammonia being highly toxic needs large amount of water an immediate expulsion. Ammonia being highly toxic needs large amount of water an immediate expulsion. All aquatic animals are ammonotelic All aquatic animals are ammonotelic

Classification of living Organisms ( on the basis of type of nitrogenous waste ) Ureotelic organisms: Organisms which eliminate waste as urea. Organisms which eliminate waste as urea. Urea being less toxic than ammonia can be retained for a longer time and also needs less dilution for elimination. Urea being less toxic than ammonia can be retained for a longer time and also needs less dilution for elimination. Ammonia is converted to urea in the liver in human beings. Ammonia is converted to urea in the liver in human beings. All animals dwelling in moderate conditions are ureotelic. All animals dwelling in moderate conditions are ureotelic.

Classification of living Organisms ( on the basis of type of nitrogenous waste ) Uricotelic organisms: Organisms which eliminate waste as uric acid. Organisms which eliminate waste as uric acid. Uric acid being least toxic can be retained for a longer time and also needs very less dilution for elimination. Uric acid being least toxic can be retained for a longer time and also needs very less dilution for elimination. Uric acid is eliminated as semi-solid waste. Uric acid is eliminated as semi-solid waste. Birds, some desert animals are uricotelic. Birds, some desert animals are uricotelic.

Excretion in Unicellular organisms No excretory organs present No excretory organs present Excretion by simple diffusion across the plasma membrane. Excretion by simple diffusion across the plasma membrane. Cells have contractile vacuoles to collect and eliminate waste. Cells have contractile vacuoles to collect and eliminate waste.

Excretion in Multicellular organisms Sponges and coelentrates: no specific excretory organs. Sponges and coelentrates: no specific excretory organs. Platyhelminthes: Flame cells Platyhelminthes: Flame cells Annelida: Nephrida Annelida: Nephrida Insects: malpighian tubules. Insects: malpighian tubules. Mollusca and all vertebrates: Kidneys Mollusca and all vertebrates: Kidneys

Excretory system in Human beings

The excretory system is made up of the following parts: Kidneys A pair of bean shaped (reniform) organs situated in the abdominal cavity. A pair of bean shaped (reniform) organs situated in the abdominal cavity. Left kidney lies at a higher level than the right. Left kidney lies at a higher level than the right. The Renal artery carrying oxygenated blood enters the kidney through the hilus. The Renal artery carrying oxygenated blood enters the kidney through the hilus. The renal artery drains the de-oxygenated blood from the kidney. The renal artery drains the de-oxygenated blood from the kidney. Each kidney contains about 1 million nephrons for filtering blood Each kidney contains about 1 million nephrons for filtering blood

Structure of Kidney Each kidney is divided into two parts: Each kidney is divided into two parts: (a) outer cortex (a) outer cortex (b) inner medulla (b) inner medulla The renal cortex consists of the malpighian tubules The renal cortex consists of the malpighian tubules The renal medulla consists of the collecting ducts of nephrons. The renal medulla consists of the collecting ducts of nephrons.

Ureters A pair of distensible tubes arising from hilus. A pair of distensible tubes arising from hilus. Ureters carry urine by peristaltic movements, from the kidneys and drain it into the urinary bladder. Ureters carry urine by peristaltic movements, from the kidneys and drain it into the urinary bladder.

Urinary Bladder A pear shaped muscular bag lies in the lower abdominal cavity and is the structure for temporary storage of urine. A pear shaped muscular bag lies in the lower abdominal cavity and is the structure for temporary storage of urine.Urethra Tube that takes urine from the bladder to the outside. Tube that takes urine from the bladder to the outside. Its opening is guarded by a ring of muscles called the urethral sphincter. Its opening is guarded by a ring of muscles called the urethral sphincter. Urethra is separate in females but common with the reproductive tract in males. Urethra is separate in females but common with the reproductive tract in males.

The Nephron

Nephron Def: The fundamental structural and functional unit of the excretory system (kidneys)

Structure of a nephron A nephron is made up of two main parts: Malpighian body Renal tubule

Malpighian Body: Also called renal corpuscle. It is made up of a cup shaped structure called the Bowman’s capsule which contains a bunch of fine blood vessels or capillaries arising from the Afferent arteriole (a branch of the renal artery). This bunch is called Glomerulus. which contains a bunch of fine blood vessels or capillaries arising from the Afferent arteriole (a branch of the renal artery). This bunch is called Glomerulus. The bunch comes out of the Bowman’s capsule as a narrower efferent arteriole. The Bowman’s Capsule and the Glomerulus make up the Malpighian Body, which is the main filtering unit of the kidneys.

Renal Tubule: It has three parts – (a) Proximal Convoluted tubule, (b) Loop of Henle and (c) Distal convoluted Tubule It has three parts – (a) Proximal Convoluted tubule, (b) Loop of Henle and (c) Distal convoluted Tubule DCT opens into the collecting ducts that give rise to pyramids called renal pyramids. DCT opens into the collecting ducts that give rise to pyramids called renal pyramids. Pyramids form calyces which open into the pelvis. Pyramids form calyces which open into the pelvis. Pelvis leads into the ureter. Pelvis leads into the ureter. All the parts of the renal tubule are covered by a network of capillaries called Peri-tubular capillaries, formed from the efferent arteriole All the parts of the renal tubule are covered by a network of capillaries called Peri-tubular capillaries, formed from the efferent arteriole

Mechanism of Urine formation It has 4 major components: 1. Glomerular filtration: Due to the difference in the diameter of the afferent and efferent arterioles the amount of blood entering the glomerulus is much more than the volume of blood leaving it. This leads to the development of a pressure. This pressure leads to the filtration of blood. The glomerular filtrate so obtained contains glucose, ions, vitamins, amino acids, uric acid, Sodium, Potassium, Chloride, bicarbonates and water. The volume of Glomerular filtrate is 125 ml/ min

Mechanism of Urine formation 2. Reabsorption: Glomerular filtrate passes through the PCT. Glomerular filtrate passes through the PCT. As it travels down the renal tubule useful substances like glucose, amino acids, vitamins c, calcium, Potassium, Sodium, Chloride, bicarbonates and water (75%) are absorbed into the Peri-tubular capillaries. The reabsorption is either active or passive

Mechanism of Urine formation 3. Tubular Secretion: Occurs mostly in the DCT and some in PCT. It is an active process, in which blood capillaries secrete waste products into the renal tubule. These waste products include urea, uric acid, creatinine, extra salts.

Mechanism of Urine formation 4. Concentration of Urine: Most of the water (70%) in the glomerular filtrate is re-absorbed into the peritubular network from the PCT and some (10 %) by osmosis from Loop of Henle. Most of the water (70%) in the glomerular filtrate is re-absorbed into the peritubular network from the PCT and some (10 %) by osmosis from Loop of Henle. The Anti- Diuretic Hormone from the Pituitary also regulated the concentration of urine. The Anti- Diuretic Hormone from the Pituitary also regulated the concentration of urine. When more water is taken in ADH is released to absorb more water from the nephrons. Low ADH production leads to discharge of frequent and highly diluted urination (Diabetes insipidus)

Composition of Urine Transparent, amber coloured (due to urochrome) Transparent, amber coloured (due to urochrome) 96% Water 96% Water 2.5% organic substances (urea, uric acid, creatine, water soluble vitamins, hormones, oxalates) 2.5% organic substances (urea, uric acid, creatine, water soluble vitamins, hormones, oxalates) 1.5 % inorganic solutes (sodium, chloride, phosphate, sulphate, magnesium, calcium, iodine) 1.5 % inorganic solutes (sodium, chloride, phosphate, sulphate, magnesium, calcium, iodine)

Micturition The process of excreting urine. Micturition can be voluntary as well as involuntary.

Functions of kidneys Removal of waste products Removal of waste products Maintaining water balance. Maintaining water balance. Elimination of excess water soluble substances. Elimination of excess water soluble substances. Regulation of salts content. Regulation of salts content. Maintenance of pH. Maintenance of pH. Regulation of blood pressure. Regulation of blood pressure.

Accessory Excretory Organs

Skin as an Excretory organ Contains sweat and sebaceous glands. Contains sweat and sebaceous glands. Secrete sweat and wax or sebum. Secrete sweat and wax or sebum.

Liver as an Excretory organ Extra amino acids are deaminated and toxic ammonia converted to urea. Extra amino acids are deaminated and toxic ammonia converted to urea. Degrades hemoglobin into bilirubin and biliverdin Degrades hemoglobin into bilirubin and biliverdin Passes cholesterol, excess vitamins, drugs for elimination. Passes cholesterol, excess vitamins, drugs for elimination.

Dialysis The process of filtering blood artificially using a dialysis machine is called dialysis. The process of filtering blood artificially using a dialysis machine is called dialysis. Dialysis involves separation of smaller solutes or ions from larger particles, using an ultrafilter. Dialysis involves separation of smaller solutes or ions from larger particles, using an ultrafilter. Blood from an artery or a vein fitted to a pump and mixed with heparin (anti-coagulant) is cooled to 0 deg. C and passes through cellophane tubes for filtration. Blood from an artery or a vein fitted to a pump and mixed with heparin (anti-coagulant) is cooled to 0 deg. C and passes through cellophane tubes for filtration. The filtered blood is returned into another vein. The filtered blood is returned into another vein. The whole process takes 3-4 hours. The whole process takes 3-4 hours.