Excretion, Reproduction | Science & Technology for UPSC CSE PDF Download

Excretion

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.

kidney

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.

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.

Reproduction

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.

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.

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.

Ovlulation

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.