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Excretion refers to the removal of the waste products of metabolism. Although lungs and skin also function as excretory organs, kidney is most important of all.


There are two bean shaped, dark red coloured kidneys placed just below the stomach, one on each side of the mid dorsal line. In case of man the left kidney is slightly on the lower side due to the fact that the left side of the abdominal cavity is occupied by the stomach. Each kidney is about 11 cm long, 6 cm broad and 3 cm thick. 

Outer surface is convex and inner surface is concave. This depression is called hilus from where the ureter originates and the renal artery and renal vein go in and out. Histologically each kidney is made-up of 2 zones.

(a) Outer dark red cortex  

(b) Inner pale red medulla Ureter entering through hilus expands forming Calyces. Medulla on the inner side has cone like structures called renal pyramids.

Each kidney contains about one million nephrons. Each nephron consists of a round malpighian body formed of Bowman’s capsule filled with capillary network (Afferent and efferent arterioles) called glomerulus. The malpighian tubule is divided into 3 parts: 

The proximal tubule—nearest Bowman’s capsule. A thin segment—forms the Henle’s loop. Distal tubule—join collecting tubule.

Composition of Urine 

Urine is acidic in nature. The pale-yellow colour of the urine is mainly due to the pigment urochrome. Quantity of urine formed in 24 hours in an adult normal individual varies from 600 ml to 1800 ml.

Tea, coffee and alcoholic beverages have got diuretic effects. Normally the freshly voided urine is clear and acidic in reaction with a pH value as low as 4.5 and as high as 8.6. The mean pH of the normal mixed 24 hours urine is approx 6.0. The urine voided immediately after a meal tends to be alkaline because of so called alkaline tide.
The odour of the normal urine is slightly aromatic and is due to the presence of large number of volatile organic substances particularly the bad-smelling substance-Urinod. When allowed to stand for sometime, the urine smells of ammonia due to the bacterial decomposition of urea to ammonia.

Functions of kidney

It excretes waste products, specially formed due to protein metabolism. These waste products are ammonia, urea and uric acid. Their composition is different in different animals. It helps to maintain water balance of the body and there by plasma volume. It helps to maintain the normal pH of the blood and other fluids of the body. It helps to maintain the optimum concentration of certain constituents of blood by the process of selective reabsorption in the kidney tubules.

Fat soluble vitamins: Vitamins A, D, E and K.
 Water soluble vitamins: Vitamin C and B-Complex.
 Vitamin which is associated with clotting of blood : Vitamin K.
 Vitamin which is destroyed by air and heat: Vitamin C.

It eliminates drugs and various toxic substances from the body. It helps to maintain the osmotic pressures in blood and tissues. It manufactures certain new substances like ammonia, hippuric acid and inorganic phosphates. Ammonia helps to maintain acid-base equilibrium of the body.

Hemodialysis or Artificial kidney

Uremia is a condition in which urea levels of blood rises abnormally. It occurs in those patients whose kidney tubules do not function properly so that the metabolic wastes start accumulating in the blood and they need to be eliminated frequently. This artificial removal of wastes is done by a process called hemodialysis or artificial kidney.

In this process, the blood of the patient is pumped from one of the main arteries and cooled to 0°C . It is then mixed with an anticoagulant heparin and passed through a cellophane tube. This cellophane tube is a semipermeable membrane which allows only the small molecules like urea, uric acid, creatinine and mineral ions to pass out but the macromolecules like proteins are not able to pass through it. The purified blood is mixed with antiheparin to restore its normal coagulability and is then pumped into the body through a vein.

Diseases Caused by Bacteria
DiseasesCausal OrganismMode of transmission and incubation periodMain symptoms of the disease
Septic sore throatStreptococcus (sp.)Bacteria infect throat and nasal membranes by droplet and direct contact; 3-5 daysSore throat often accompanied by fever and cough.
DiphtheriaCorynebacterium diptheriaBacteria infect respiratory tract by carrier, direct contact, droplet and food; 1-7 daysSore throat, fever, vomiting, formation of a grey membranous deposit in the throat, difficult breathing.
PneumoniaDiplococcus pneumoniaeBacteria transmitted to respiratory tract, including the lungs by droplet infection; variableChills, pain in the chest, rusty sputum, rapid breathing, abdominal pain, jaundice.
TuberculosisMycobacterium tuberculosisBacteria transmitted to lungs, bones and other organs by direct contact, droplet infection, food and milk, variable.Symptoms vary with the organ affected, cough, fever in the evening, fatigue, loss of weight, X-ray pictures show infection in the lungs.
Plague or Bubonic plagueYersinia pestisRat flea spreads disease from rat to man; 2-10 daysSudden onset, high fever, vomitting, hot dry skin, thirst, black spots on skin, lymph nodes in groin swollen.
Tetanus or LockjawClostridium tetaniBacteria in soil, enter through wound; 2-40 daysSpasms of muscles and convulsions, lockjaw.
TyphoidSalmonella typhiFlies, food, faeces, water and carriers; 10-14 daysFever, nausea, vomiting, severe abdominal pain, chills and diarrhoea.
CholeraVibrio choleraeFlies, food, stools, water and carriers; 1-2 daysAcute severe diarrhoea with rice water stools, vomiting, rapid dehydration, muscular cramps and stoppage of urine (anuria).
Whooping coughHemophilus pertussisDroplets projected during coughing and sneezing, 7-14 daysStarts with cold and a dry hacking cough, later cough becomes violent. An attack consists of 10 to 12 explosive coughs followed by intake of breath and a ‘whoop’.
Gonorrhoea (Clap)Neisseria gonorrhoeaeSexual intercourse; 2-8 daysRedness, swelling, pus discharge through urethra, frequent and burning urination.
SyphilisTreponema pallidumDirect contact, chiefly sexual intercourse; 10-90 daysA hard, painless sore or chance (ulcer) on the genitalia, variable types of skin eruptions, and serious tissue destruction in any part of the body.
LeprosyMycobacterium lepraeLong and close contact within infected personsUlcers, nodules, scaly scabs, deformities of fingers and toes, and wasting of body parts.
BotulismClostridium botulinumOrganism produces poison in food; 18-66 hoursSevere gastrointestinal upset, vomiting and diarrhoea, fatigue, disturbance of vision, paralysis.


Female reproductive organs

Two ovaries are attached to the dorsal wall of the abdominal cavity behind the kidneys. Their germinal epithelium produces a succession of eggs. The ovaries are oval bodies showing blisters like Graffian follicles. Close to each ovary is a fimbriated funnel leading into an oviduct.

The fimbriated funnel receives the eggs when released from ovary into the body cavity. The oviduct behind the funnel is differentiated into two parts—first is a “fallopian tube” of narrow diameter and the second is the `Uterus.’ It is much thicker than the fallopian tube. The two uteri join along the median line to form a common tube called vagina. A vaginal septum separates the opening of the two uteri.

Ventral to the vagina lies a urinary bladder whose neck fuses with the vagina to form a short ‘urogenital canal’ or vestibule that opens to the outside by a slit like ‘Vulva’.
A pair of “bartholin’s glands” open into the dorsal wall of the vestibule serving to lubricate the opening of the vagina, they correspond to the cowper’s glands of the male.
At the ventral side of the vestibule lies a ‘clitoris’ which is homologous with the penis of the male and consists of erectile tissue with a highly sensitive tip.

Diseases Caused by Protozoa
DiseasesCausal OrganismHabitat and TransmissionMain symptoms
MalariaPlasmodiumPasses through a developmental phase in liver, resides inside R.B.Cs, and carried by blood to all organs. Female anophelese mosquito.Three stages: Cold stage—headache, shivering and rising temperature;
Amoebic dysentery or ‘amoebiasis’Entamoeba histolyticaLarge intestine. Ingestion of contaminated food.Fever stage—fever rises to its maximum, severe headache, pain in back and joints, vomiting; Sweating stage—profuse sweating, fall in temperature, pain relieved
Sleeping sickness (Trypanosomiasis)Trypanosoma bruceiReaches lymph nodes via lymphatics, blood and infests brain. By bite of tsetse fly.Acute dysentery with blood and mucous in stools, and severe abdominal pain. Secondary complications include formation of multiple abscesses in liver, lung, brain, spleen and ulceration of skin, vagina and penis.
Oriental sore of ‘Delhi boil’Leishmania tropicaReticuloendothelial cells. Bite of sandfly.Fever, severe headache, enlargement of glands at back of neck, rash on the back and chest, joint pains, swelling of eyelids, ankles and hands, trembling, loss of appetite, no desire to work except sit or sleep, mental disturbances, coma and death.
Kala-azar or black sicknessLeishmania donovaniReticuloendothelial cells. Bite of sandfly.Ulcer formation wherever sandfly has bitten, ulcers usually found on face, arms and legs Enlargement of spleen, liver, fever, jaundice, skin becomes dark through pigmentation.
Diarrhoea ‘Giardiasis’Giardia intestinalisBile tract, duodenum. By contaminated food.Malabsorption of fat, persistent looseness of bowels with passage of yellowish, greasy stools with excess fat, fever, anaemia and allergic manifest-ations.


The testis are avoid bodies which arise near the kidneys and in most mammals they descent before birth from the body cavity into a special portion of the coelom called the vaginal coelom, which is placed into pair of “Scrotal Sacs” covered externally by hairy skin. Each testis remains connected with its original position by a spermatic cord.
Male Reproductive Organs

In passing downwards the testis carries with it a part of the embryonic kidney which is called ‘epididymis.’ It lies along side of the testis. The testis has ‘seminiforous tubules’ whose walls are lined by germinal epithelium which produces sperms. In between the germinal epithelial cells are largesized “Sertoli” or “Nurse” cells which nourish the sperms. On the neck of the urinary bladder lies seminal vesicle in which sperms are stored. The urethra serves as a passage both for urine and the spermatic fluid.
A large ‘prostrate gland’ lies at the base of the seminal vesicle. It opens into the urethra by numerous ducts. Below the prostrate is a pair of ‘cowper’s glands”. The penis is a cylindrical organ with spongy body. It is invested by a loose skin, the skin hangs over the tip of the penis as the ‘prepuce’. The tip of the penis which is enclosed by the prepuce is called ‘glans penis’ which is highly sensitive.

Ovulation At birth each ovary contains about 2,50,000-5,00,000 ova but the majority degenerate during the life time. Only about 400 ova mature and are released in a normal human life span. Ovulation occurs in 14 days before the next menstrual cycle. The duration of pregnancy is on average 280 days or 40 weeks from the last menstrual cycle. By the 12th week, all essential features, recognisable in later life, are present in the foetus, though not fully developed. During the last 4 weeks the foetus is fully matured.

Menstrual Cycle The menstrual cycle starts at the age of 12-13 years (puberty) and lasts up to 40-45 years (menopause) when the reproductive capacity of the female is arrested. It is also absent during pregnancy.

Menstrual phase

It starts on 28th day of the cycle and lasts upto 4 to 6 days. During this period of soft epithelium tissues (myometrium) and the uterine epithelium glands and connective tissues (endometrium) breaks down due to the lack of progesterone as a result bleeding occurs. The unfertilized ovum is also discharged. Corpus luteum formed in the ovary also degenerates as placental gonadotropins, essential for its maintenance and growth are absent. Placenta is formed only if pregnancy occurs.

In the absence of pregnancy placenta will not be formed.

Diseases Caused by Fungi   
DiseaseCausal organismTransmissionSymptoms
Ringworm (tinea)Microsporum, TrichophytonDirect contact from unbathed cats and dogs or objects handled by infected individualsSores begin as small, slightly raised reddish areas, enlarge, become redder and contain one or more blistered areas on skin and scalp. Cau-se partial and tempo-rary baldness in children.
Athlete’s footTrichophytonBad foot hygiene where skin remains warm and moist for long periods, fungi find optimum condition to invade dead outer layer of skin.Painful itching or burning sensation in the infected areas. Crack appears in the skin at the base of 5th toe or between 4th and 5th toes, mass of loose dead skin clings between toes, otherwise reddening, scaling and thickening of skin between toes.
Madura footMadurella mycetomiFungi gain entry through some minor injury to the skin.Produce a choleric, granulating infection of the lower extremities, affect part becomes enlarged and develops many deep sores, extensive bone destruction leading to

Post-menstrual or Follicular phase Under the influence of FSH from anterior pituitary, new follicle starts developing from the ovary. This is due to the absence of the inhibitory action of progesterone, and estrogen secretion stants. In the uterns repair and renewal of uterine endometrium occurs. A slow proliferatory phase starts. This phase lasts for a week.

Proliferative or Follicular phase

In this the graffian follicle matures in the ovary under the influence of FSH which finally is inhibited by high estrogen levels. In this vascular supply to uterus increases and endometrium enlarges. It lasts upto 14th day of the cycle.


On the 14th day ovum is released by the rupture of the graffian follicle of the ovary in the abdominal cavity. It is received by the fimbriae of the fallopian tube. It remains viable for two days.

Pre-menstrual or Luteal phase

It lasts from 15th to 28th day of the cycle. The ruptured place from where ovum is released forms the corpus luteum which further starts secreting progesterone. The corpus lutelum is fully formed on the 19th day and lasts to 27th day and on 28th day it degenerates if the ovum is not fertilized. Both the formation of corpus luteum and secretion of progesterone is controlled by LH from anterior pituitary. If the ovum is fertilized by sperm in the fallopian tube, then it settles down on the endometrial tissues of the uterus (implantation) and its further development starts (pregnancy). If the voum is not fertilized then again it passes out along with endometrial tissues as menstrual flow.

Esterous cycle

In non-primate animals, a different cycle operates which is known as esterous cycle. It is similar to menstrual cycle in all types of events and changes except for two differences :
(i) There is no menstrual flow of blood at the end of esterous cycle though the breakdown of tissues occur in the female reproductive tract;
(ii) At the time of ovulation in the esterous cycle there is high level of estrogen hormone and this arises a strong sexual urge in the female. This is known as “heat-period” and during this period the female should get a male to mate. However, this heat-period is of short duration and is different for different animals e.g., 18 hours in cows.

Summary of Urine formation
Parts of tubuleFunctionSubstances removed


Proximal tubule and loop of Henle



Distal tubule and collecting tubules.


Reabsorption by active transport. Reabsorption by diffusion. Obligatory water reabsorption by osmosis.

Reabsorption by active transport. Facultative water reabsorption by osmosis. Reabsorption by diffusion. Secretion by active transport.

Water: All solutes except colloids such as blood proteins.

Na+ and some other ions, glucose and amino acids. Cl, HCO3,Water

Na+ and some other ions.

Water NH3

K+, H+, and some drugs.

The document Excretion, Reproduction | Science & Technology for UPSC CSE is a part of the UPSC Course Science & Technology for UPSC CSE.
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FAQs on Excretion, Reproduction - Science & Technology for UPSC CSE

1. What is excretion?
Ans. Excretion is the process by which waste products, toxins, and excess substances are removed from the body. It is an essential function to maintain homeostasis and prevent the buildup of harmful substances in the body.
2. How does the excretory system work?
Ans. The excretory system consists of several organs, including the kidneys, liver, lungs, and skin. Each organ plays a specific role in eliminating waste products. The kidneys filter blood and produce urine, which contains waste materials and excess water. The liver processes and detoxifies harmful substances, while the lungs remove carbon dioxide and other gases. The skin eliminates sweat, which contains metabolic waste products.
3. What are the main functions of the kidneys in excretion?
Ans. The kidneys are the main organs responsible for excretion. They filter waste products, toxins, and excess substances from the blood to produce urine. The kidneys regulate the balance of electrolytes, such as sodium, potassium, and calcium, in the body. They also help maintain the pH level and control blood pressure by producing hormones.
4. How does reproduction occur in mammals?
Ans. In mammals, reproduction involves the fertilization of an egg by a sperm. The male reproductive system produces sperm, which are transferred to the female reproductive system during sexual intercourse. The sperm then travel to the fallopian tubes, where they may fertilize an egg released by the female's ovary. If fertilization occurs, the fertilized egg implants in the uterus and develops into an embryo.
5. What are the different methods of reproduction in animals?
Ans. Animals employ various methods of reproduction, including sexual and asexual reproduction. Sexual reproduction involves the fusion of gametes from two parents, resulting in offspring with genetic variation. Asexual reproduction, on the other hand, does not involve the fusion of gametes and produces genetically identical offspring. Examples of asexual reproduction include budding, fission, fragmentation, and parthenogenesis.
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