Excretory System

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 wastes, 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.

Excretory organs of different organism

(1) 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.  

(2) 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.

(3) 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.  

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

(5) Nematoda : The round worms such as Ascaris have H-shaped excretory system. It is made up of a single Renette cell. in entire length of body. 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.

(6) Annelids (Earthworm) : In earthworm excretory system also known as nephridial system, which consist of 3 types of nephridia Pharyngeal, septal, and integumentary nephridia. All nephridia commonly called micronephridia. Earthworm excrete 40% urea, 20% ammonia, 40% amino acids. Earthworm mainly ureotelic. Chloragogen cells found in coelomic fluid also excretory in nature. Blood gland in earthworm found in 4, 5, 6 segment, serves for excretion, manufacture of blood corpuscles and Hb.

(7) Arthropods : 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.

Insects, centipedes, millipedes and arachnids like scorpion and spider posses Malpighian tubules as their principal excretory organs. 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. Spiders and scorpions possess Malpighian tubules and coxal glands both for excretion.  

(8) Molluscs : They have one or two pairs of kidneys or organs of bojanus and keber’s organ for excretion in unio.

(9) Echinoderms : Specialized excretory organs are absent in echinoderms (e.g., Starfish). The excretory products, chiefly ammonia, urea, and creatinine are eliminated by diffusion through dermal branchiae (primitive gills) and tube feet. Amoeboid coelomocytes also excretory.

Excretory products in different organisms

(1) Waste products of protein metabolism

(i) Amino acids : These are end products of protein digestion absorbed into the blood from small intestine. Certain invertebrates, like some molluscs (eg Unio, Limnae, etc.) and some echinoderms (eg Asterias) excrete excess amino acids as such. This is called aminotelic excretion or aminotelism.

(ii) Ammonia $\left(N{H}_4^{+}\text{or}N{H}_{\text{3}}\right)$ : In most animals, excess amino acids are deaminated, i.e. degraded into their keto and ammonia groups. The keto groups are used in catabolism for producing ATP, whereas ammonia is excreted as such or in other forms. Ammonia is highly toxic and highly soluble in water. Its excretion as such, therefore, requires a large amount of water. That is why, most of the aquatic arthropods, bony and freshwater fishes, amphibian tadpoles, turtles, etc excrete ammonia. This type of excretion is called ammonotelic excretion or ammonotelism.

(iii) Urea $CO{\left(N{H}_2\right)}_2$: This is less toxic and less soluble in water than ammonia. Hence, it can stay for some time in the body. Many land vertebrates (adult amphibians, mammals) and such aquatic animals which cannot afford to lose much water (e.g. elasmobranch fishes) marine bony fish, adult frog, earthworms, nematodes, turn their ammonia into urea for excretion. This type of excretion is called ureotic excretion or ureotelism.

(iv) Uric acid : Animals living in dry (arid) conditions, such as land gastropods, most insects, land reptiles (snakes and lizards), birds and kangroo rat (mammal) etc have to conserve water in their bodies. These, therefore, systhesize crystals of uric acid from their ammonia for the formation of uric acid xanthine oxidase enzyme is necessary. Uric acid crystals are nontoxic and almost insoluble in water. Hence, these can be retained in the body for a considerable time before being discharged from the body. Uric acid is the main nitrogenous excretory product discharged in solid form. This excretion is called uricotelic excretion or uricotelism.

(v) Trimethylamine oxide : Certain marine molluscs, crustaceans and teleost fishes first form trimethylamine from their ammonia by a process known as methylation. Then, the trimethylamine is oxidised to trimethylamine oxide for excretion.This oxide is soluble in water, but nontoxic.

(vi) Guanine : Spiders typically excrete their ammonia in the form of guanine. Some guanine is also formed in unio, penguin, birds and it is insoluble in water. Hence, no water is required for its excretion.

(2) Waste products of nucleic acid metabolism

As a result of nucleic acid digestion, nitrogenous organic bases – purines (adenine and guanine) and pyrimidines (cytosine, thymine and uracil) – are absorbed from intestine into the blood. Most of these are excreted out. About 5% of the total excretion of body accounts for these substances. In man, purines are changed to uric acid for excretion. In most other mammals, nitrogenous organic bases are excreted in the form of allantoin. Insects, amphibians, reptiles and birds also excrete these bases in the form of uric acid. Some freshwater molluscs and crustacean arthropods excrete these in the form of ammonia.

(3) Some sundry excretory substances (Others excretory products)

(i) Hippuric and ornithuric acids : Sometimes food of rabbit and other mammals may contain traces of benzoic acid, or this acid may be formed in small amounts during fat metabolism. It is highly toxic. As it is absorbed in blood, it is combined with glycine and changed into less toxic hippuric acid for excretion. In birds, benzoic acid is combined with ornithine and changed into ornithuric acid for excretion.

(ii) Creatine and creatinine : Muscle cells contain molecules of creatine phosphate, which are high energy molecules and serve for storage of bioenergy like ATP. It is synthesised by 3 amino acids (G.A.M.) (Glycine, Argenine and Methionine). Excess amount of this phosphate is, however, excreted out as such, or after being changed into creatinine.

Excretory system of man

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

(1) 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 11^th and 12^th 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 ahead to provide place for stomach below it.

In mammals, the kidney is 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.  

(i) Structure of kidney : The kidneys are metanephric in mammals. The kidney is divisible into two parts outer-cortex and inner-medulla. Three layers of tissue surround each kidney.

(a) The innermost, renal capsule made up of fibrous connective tissue.

(b) The intermediate layer, adipose capsule of fatty tissue.

(c) The outermost, renal fascia of dense connective tissue. 

Renal pyramids or medullary pyramids : The medulla is subdivided into 8 to 18 conical masses – the renal pyramid, each having broad base towards the cortex and a narrow end called renal papilla towards the pelvis.

Path of urinary drainage : Collecting duct $\to$ Papillary duct in renal pyramid $\to$ Minor calyx $\to$ Major calyx $\to$ Renal pelvis $\to$ Ureter $\to$ Urinary bladder

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.  

(ii) 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.

(a) Structure of nephron : A nephron or uriniferous tubules consist of two parts :

Malpighian body / Renal Corpuscles : The proximal end of each nephron forms a blind or closed, enlarged and double walled cup, the Bowman&aposs capsules in the cortex. (name Bowman&aposs 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&aposs capsule and glomerulus receives about 20 – 25% of the cardiac out put (blood) at rest.

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

Tubule : The tubule is differentiated in to 3 parts P.C.T., Henle&aposs loop and D.C.T.

The Bowman&aposs 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&aposs loop. The Henle&aposs loop is a U- shaped structure or makes hair pin turn, which has a distinct descending limb and an ascending limb. The ascending limb opens in to the distal convoluted tubule. 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.

(b) Arrangement of nephron : The malpighian body and most of the P.C.T. and D.C.T. are situated in the cortex. Henle&aposs 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&aposs 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 this renal portal system of efferent arteriole and peritubular capillaries.

(c) 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 85% of total nephron. They lie in the renal cortex and have very short loops of Henle that extend only little into the medulla.

Remaining 15 – 20% are juxta medullary nephron have their Bowman&aposs 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).

(d) 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&aposs capsule : The podocytes forming the inner wall of the Bowman&aposs capsule have gaps (about 25 nm wide) the slit pores.

The outer wall of the Bowman&aposs capsule consists of unspecialized squamous epithelium (flattened).

Proximal convoluted tube : P.C.T. is made up of simple cuboidal epithelium . It has microvilli so it is also known as brush border epithelium. P.C.T. is most important site for selective reabsorption.

Loop of Henle : The epithelium of descending limb of loop of Henle is very thin and composed of squamous epithelium and ascending limb is made up of two parts. First is thin ascending limb lined by squamous epithelium and second thick ascending limb lined by cuboidal epithelium. The ascending limb is impermeable to water and permeable to NaCl.

Distal convoluted tube : D.C.T. is made up of cuboidal epithelium which is glandular in nature.

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

Juxta-glomerular apparatus : This specialized cellular apparatus is located where the distal convoluted tubule passes close to the Bowman&aposs 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&aposs cell. These cells secrete renin hormone that modulate blood pressure and thus renal blood flow and G.F.R. are regulated.

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.

(1) 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 and some apodan amphibians. 

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

(i) 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. A pair of pronephros appear in all vertebrate embryos but they becomes functions kidneys in adult myxine and embryos of all anamniotes (fish, amphibian). This kidney found as transitory kidney in all vertebrates embryos.

(ii) 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. Also found in adult petromyzon.

$\bullet$ In fish and amphibians, tubules extend through length of coelom behind pronephros and formed from entire nephrotome left behind the pronephrus called opisthonephric kidney. The opisthonephric kidney specially in males differentiated into anterior genital and post renal part.      

$\bullet$ In frog mesonephric duct is also known as Bidder&aposs canal which carry sperm and urine both.

(iii) 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.਎xample – All amniotes – Reptile, Birds and mammal.

(2) 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. Like kidneys, the ureters are retroperitoneal.

(3) Urinary bladder and Urethra : The urinary bladder is pear-shaped hollow muscular organ situated in pelvic cavity, 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. The average capacity of urinary bladder is 700 – 800 ml. In general, urinary bladder capacity is smaller in females because the uterus occupies the space just superior to bladder. Mucosa of bladder with folds called rugae. (rugae also present is stomach and vagina).

(4) 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. In males the epithelium of spongy urethra is stratified or pseudostratified columnar epithelia, except near external urethral orifice, which is non keratinized stratified squamous epithelia. The prostatic urethra lined by transitional epithelia, while membranous urethra lined by pseudostratified columnar.