Excretion- Example and Functions

Biology Doubts

What is Excretion ?

Excretion can be defined as the component structural and functional units of the bodies of all organism are cells which have been looked as "miniature chemical factories" because of continuous metabolism taking place in these. It yields certain waste products which are, not only useless, but harmful to the cells and the body. Cells, therefore, throw out these wates, by diffusion, into their surrounding medium. Finally, these wastes are eliminated by the body into its external environment. This is, thus an important vital activity of all organism. It is called excretion. Besides removing the metabolic wastes and impurities from the blood, the kidney also perform the important function of osmoregulation by regulating the amount of water in body fluids. The normally functioning kidneys produce a large volume of dilute urine when more water is taken, and a small volume of concentrated urine when water intake by the body is poor.

Excretory organs of different organism. 

(i) Protozoans : In protozoans like Amoeba and Paramecium carbon dioxide and ammonia are mostly excreted out by diffusion through general body surface. It is considered that the contractile vacuoles also play some role in the removal of excretory products.  

(ii) Sponges : In sponges, the nitrogenous metabolic waste (ammonia) leaves the body in the outgoing water current by diffusion.Most of the sponges are marine and have no problem of surplus water in their cells. A few sponges lie in hypotonic fresh water and have contractile vacuoles in most of their cells.  

(iii) Coelenterates : Hydra also lacks special excretory organs. The nitrogenous waste products like ammonia are removed through the general surface of the body by diffusion. Some nitrogenous waste products are also thrown along with indigestible matter through the mouth.  

(iv) Platyhelminthes : Planaria, liverfluke and tapeworm possess a large number of excretory cells called the flame cells (solenocytes) and long excretory ducts (also called canals of vessels). The flame cells open into the ductules which in turn open into the excretory duct.

Excretory canals are present on each lateral side or the collecting tubules of which one is dorsal and the other ventral. In the last proglottid, they join to form a pulsatile caudal vesicle, which is open to a exterior by excretory pore.

Excretory materials diffuse from the surrounding tissues into the flame cells. Vibrations of the cilia cause these materials to remove in the excretory ducts. The walls of the ducts reabsorb useful substances and remaining excretory materials (e.g., ammonia) are expelled out through the excretory pores.

(v) Aschelminthes : The round worms such as Ascaris have H-shaped excretory system. It is made up of a single Renette cell. It consists of two longitudinal excretory canals connected anteriorly by a network of transverse canals. A short terminal duct opens outside via excretory pore. Ascaris is excretes both ammonia and urea.  

(vi) Annelids : In annelids like Nereis, earthworm, leech, etc., the tubular coiled structures, the nephridia are excretory organs. A typical nephridium starts from a rounded ciliated funnel, the nephrostome which opens into coelom (body cavity). The nephrostome leads into a nephridial tubule with ciliated cells. A typical nephridium opens outside the body through a small aperture called nephridiopore. However, in earthworm three types of nephridia are found. The septal nephridia situated on the septa (behind 15th segment) and pharyngeal nephridia ( in three pairs of bundles in the 4th, 5th, and 6th segments) open into the alimentary canal and pour their excretory materials there. It is an adaptation for conservation of water. The integumentary nephridia (found scattered in the body wall in each segement except the first two segments) open directly on the body surface. Excretory materials help the earthworm in keeping the skin moist for cutaneous respiration.  

(vii) Arthropods : (a) The excretory system of the adult Prawn (crustacean) consists of a pair of antennary or green glands, a pair of lateral ducts and a single renal sac. Each green gland consists of an end sac, labyrinth (glandular plexus) and bladder. The end sac extracts nitrogenous waste products and excess water from the blood. The excretory fluid is transferred from the sacs to the labyrinth in which useful materials are absorbed and carried to the blood. The remaining excretory fluid called urine, flows from the labyrinth to the bladder. The excretory fluid also comes here from the renal sac. Urine is temporarily stored in the bladders. Later on urine is expelled out through ureters and renal pores.  

(b) Most insects, centipedes and millipedes, possess Malpighian tubules as their principal excretory organs. They are fine, spiral or convoluted, thread-like tubules which are attached to the alimentary canal. The distal closed end of each Malpighian tubule float freely in the haemolymph (blood). These tubules extract metabolic wastes like potassium and sodium urate, water and carbon dioxide from the blood. In the Malpighian tubules bicarbonates of potassium and sodium, water and uric acid are formed. A large amount of water and bicarbonates of potassium and sodium are reabsorbed by the cells of Malpighian tubules and then transferred to the blood (haemolymph). Uric acid is carried to the alimentary canal of the insect and is finally passed out through anus.  

(c) Spiders and scorpions possess Malpighian tubules or coxal glands or both for excretion.  

(viii) Molluscs : They have one or two pairs of kidneys which discharge excretory matter into the mantle cavity which is finally passed out of the body along with the out flowing water.

(ix) Echinoderms : Specialized excretory organs are absent in echinoderms (e.g., Starfish). The excretory products, chiefly ammonia, are eliminated by diffusion through dermal branchae (primitive

Excretory system of man

Mammalian (human) urinary system consists of a pair of kidneys, a pair of ureter, a urinary bladder and a urethra.

Excretory system of man structure and Function of Kidney

Kidneys : The kidneys are dark-red, bean-shaped organs about 11 cm long, 5 cm wide and 3 cm thick, each weight about 150 gm in an adult male and about 135 gm in adult female. They are placed against the back wall of the abdominal cavity just below the diaphragm, one on either side opposite the last thoracic and first three lumber vertebrae. The lower two pairs of ribs protect them.The kidneys are covered by peritoneum on the front (ventral) side only. thus, they are retroperitoneal. The right kidney is attached more anterior than the left in rabbit. This asymmetry is just the reverse of that found in man. 

In man left kidney occurs at a slightly higher level than the right one, because right side has prominent right liver lobe. In rabbit the condition is little differ due to quadropedilism i.e. left kidney is in normal position while the right kidney shift ached to provide place for stomach below it.

In mammals, the kidney is bean-shaped i.e. concavo convex. The center of concave inner surface is called as hilum or hilus which gives out a ureter. From this hilus surface the renal artery enters into the kidney, the renal vein comes out and the renal nerves enter into the kidney. 

Structure of kidney 

The kidneys are metanephric in mammals. The kidney is divisible into two parts outer-cortex and inner-medulla. Renal pyramids or medullary pyramids : The medulla is subdivided into 10 to 12 conical masses – the renal pyramid, each having broad base towards the cortex and a narrow end called renal papilla towards the pelvis.

Excretion

Renal columns of bertini : Between the pyramids, the cortex extends into the medulla or renal columns of bertini.

Calyx : Each renal papilla projects into the cavity of a minor calyx, minor calyx join to form major calyx. The major calyx open into a wide funnel like structure, the pelvis. The latter leads into the ureter. In rabbit, the pelvis is unbranched hence, it is without calyx.In frog ventral surface of each kidney has many ciliated funnels called nephrostomes. They drain wastes from body cavity (coelom) and connect to renal veins in frog or to uriniferous tubules in tadpoles.  

Histology of kidney : Histologically a kidney is made of innumerable thin, long, much convoluted tubular units called uriniferous tubule or nephron.Nephron is the structural and functional unit of kidney. One human kidney may contain about one million (10 lac nephron) nephron (In rabbit each kidney bear about 2 lac nephron). In frog each kidney bears about 2 thousand nephron.

(b) Structure and types of nephron : A nephron or uriniferous tubules is made of two parts 

(1) Malpighian body : The proximal end of each nephron forms a blind or closed, enlarged and double walled cup, the Bowman's capsules in the cortex. (name Bowman's capsule is based on english physiologist and histologist William Bowman).

Each capsule contains a network of blood capillaries the glomerulus which receives blood through afferent arteriole and the blood comes out through the efferent arteriole .The diameter of the efferent arteriole is comparatively lesser. (Bowman's capsule and glomerulus receives about 20 – 25% of the cardiac out put (blood) at rest.

The composite structure of Bowman's capsule and glomerulus is known as Malpighian body or Malpighian corpuscles after the Italian microscopist Marcello Malpighi.

(2) Tubule : The tubule is differentiated in to 3 parts P.C.T., Henle's loop and D.C.T. The Bowman's capsule opens into a proximal convoluted tubule (P.C.T.) the anterior part of the P.C.T. is more coiled where as its posterior part is almost straight. The P.C.T. opens into a Henle's loop. The Henle's loop is a U- shaped structure which has a distinct descending limb and an ascending limb. The ascending limb opens in to the distal convoluted tube. The D.C.T. is a coiled structure. Many D.C.T. unit to form a collecting duct. The collecting ducts of one pyramid unit to form a duct of Bellini. The duct of Bellini lead into the pelvis part.

Arrangement of nephron : The malpighian body and a part of P.C.T. and D.C.T. are situated in the cortex. Most of the part of P.C.T. and D.C.T., Henle's loop and collecting ducts are found in the medulla.

Vasa recta : The efferent arteriole of juxta-glomerular nephron forms a peritubular capillary system around the Henle's loop which is called vasa recta. Each of the vasa recta makes U turn at the inner most part of the medulla and return to the venous circulation near the junction of medulla and cortex. The efferent arteriole and peritubular capillaries technically constitute a renal portal system. In all amniotes as reptiles, birds and mammals have a renal portal system.  

Types of nephron : Nephrons are of two types cortical and juxtamedullary, with regard to their location in the kidney. The cortical nephrons form about 80% to 90% of total nephron. They lie in the renal cortex and have very short loops of Henle that extend only little into the medulla.

The juxta medullary nephron have their Bowman's capsule close to (Juxta) the junction of the cortex and the medulla and have very long loops of Henle, extending deep into the medulla. This type of nephron is present in only birds and mammals. The cortical nephrons control the plasma volume when water supply is normal. The juxtamedullary nephrons regulate the plasma volume when water is in short supply (In advarse condition).

Differences between cortical and Juxtamedullary nephrons

Cortical Nephrons                                Juxtamedullary Nephron

1.Form 80% of total nephrons. 1.Form only 20% of total nephrons.

2.Are small in size.                               2.Are large in size.

3.Lie mainly in the renal cortex. 3.Have Bowman's capsules in the cortex near its junction with the medulla.

4.Henle's loops are very short and extend only a little into the medulla 4.Henle's loop are very long and extend deep into the medulla.

5.Control plasma volume when water supply is normal. 5.Control plasma volume when water supply is short.

Histology of nephron

Glomerulus : Glomerulus is a network of up to 50 parallel branching and anastomosing capillaries covered by endothelium, basement membrane and epithelium made of podocytes which has slit pores that restrict passage of colloids. However, small molecules and water can easily pass through them in to the P.C.T.

Bowman's capsule : The podocytes forming the inner wall of the Bowman's capsule have gaps (about 25 nm wide) the slit pores.The outer wall of the Bowman's capsule consists of unspecialized squamous epithelium (flattened).

Proximal convoluted tube : P.C.T. is made up of simple columnar epithelium. It has microvilli so it is also known as brush border epithelium.

Loop of Henle : The epithelium of descending limb of loop of Henle is very thin and composed of squamous epithelium and ascending limb is lined by cuboidal epithelium. The ascending limb is impermeable to water and permeable to NaCl.

Distal convoluted tube : It is made up of cuboidal epithelium which is glandular in nature.

Collecting ducts : The collecting ducts are lined by cuboidal and columnar epithelium in different regions. At intervals, the cuboidal cells are ciliated.

Juxta-glomerular apparatus : This specialized cellular apparatus is located where the distal convoluted tube passes close to the Bowman's capsule and afferent arteriole. Cells of the D.C.T. epithelium in contact with afferent arteriole are denser than other epithelial cells known as maculla densa. Maculla densa has special Lacis cell or Polkisson's cell. These cells secrete renin hormone that modulate blood pressure and thus renal blood flow and G.F.R. are regulated.

(d) Origin and types of kidneys in different vertebrate : Kidney tubules (nephrons) arise in the embryo in a linear series from a special part of mesoderm called mesomeare or nephrotome.

Number, complexity and arrangement of Nephrons are differ in different groups of vertebrates. A nephron is differentiated into three parts – peritoneal funnel, tubule and malpighian body. Peritonial funnel (nephrostome) are normally present in embryos and larvae and considered as vestigeal organ of hypothetical primitive kidneys.

Archeonephros kidney : Archeonephros is the name given to the hypothetical primitive kidney of ancestral vertebrate. It is also called as holonephros or complete kidney. (It extended entire length of coelom) It tubules are segmentally arragned and nephrostome is present. Glomerulus is external (without capsule). It duct is called as archeonephric duct. Ex. Larva of myxine.

Modern vertebrates exhibits three different kinds of adult kidney Pronephros, Mesonephros and Metanephros.

(1) Pronephros : It originates from the anterior part of the nephrotome. It is also termed head kidney due to its anterior position. There are only 3 pronephrine tubule (nephron) in frog embryo, 7 in human embryo, and about 12 in chick embryo which are segmentary arranged. Nephrostome present, glomerulus is external and unite to form glomus in some cases. Duct is pronephric duct or mullerian duct. Pronephros is functional in all embryos and larval stages. It is mostly transitory and soon replaced by the next stage or mesonephros.

Example – Adult myxine and petromyzones (cyclostomes) and some fishes but non urinary and lymphoid in function.

(2) Mesonephros : It originates from the middle part of the nephrotome. Duct is mesonephric or Wolffian duct. Nephrostome is absent except some embryos of anamniotes. Example – In amniotes (reptiles, birds and mammals) mesonephros is functional only in the embryos, replaced by metanephros in the adult. In anamniotes (fishes and amphibian) mesonephros is functional in both embryo as well as adults.

(3) Metanephros : It originates from the posterior part of the nephrotome. When metanephric tubules develop, all the mesonephric tubules disappear except those associated with the testes in male and forming vasa efferentia. Nephrostome absent. A thin, U-shaped loop of Henle forms between P.C.T. and D.C.T. which is incomplete in Reptiles and Birds and well developed in mammals. Duct is metanephric or ureter. Reproductive duct is separate. The kidney is highly compact which possesses innumerable nephrons.  Example – All amniotes – Reptile, Birds and mammal.

Excretory system of man structure and Function of Ureters 

Ureters : From the hilum of each kidney emerges a whitish tube the ureter. The ureters are about 28 cm long. Their wall consists of transitional epithelium surrounded by a layer of muscle fibres. Openings of the two ureters in the bladder are separate, but closely placed. These are oblique, so that the urine cannot regurgitate into the ureters when the bladder contracts. Peristalsis of ureters also cheeks regurgitation of urine. 

(iii) Urinary bladder and Urethra : The urinary bladder is pear-shaped which is made up of smooth and involuntary muscles. The muscles is also known as detrusor muscles (muscles that has the action of expelling a substance). The lower part or neck of the bladder leads into the urethra. There is a smooth triangular area, called trigonium vesicae. The lumen of the urinary bladder is lined by transition epithelium which has great power of streaching. The neck of bladder is guarded by two sphincters, inner is involuntary controlled by spinal reflex and outer is voluntary controlled by cerebral cortex. A person feels the sensation of micturation when the quantity of urine in the bladder is about 300 c.c.

Urethra : The urinary bladder leads into the urethra. In a female, it is quite short, only about 3 to 5 cm long, and carries only urine. It opens by urethral orifice, or urinary aperture in the vulva infront of the veginal or genital aperture. In a male urethra is much longer, about 20 cm and carries urine as well as spermatic fluid. It passes through the prostate gland and the penis. It opens out at the tip of the penis by urinogenital aperture. 

Differences between male and female urethra

Male urethra                                                       Female urethra

1.It is about 20 cm long. 1.It is just 3 – 5 cm long.

2.It has 3 regions : prostatic urethra (3–4 cm), membranous (1 cm) and penial (15 cm) 2.It is not differentiated into regions.

3.It opens out at the tip of the penis by urinogenital aperture. 3.It opens into the vulva by urinary aperture.

4.It carries urine as well as semen to the exterior. 4.It carries only urine to the exterior.

5.It has 2 sphincters. 5.It has a single sphincter.

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