Other excretory organ - class-XI

The skin acts as an excretory organ by releasing sweat, which contains water, excessive mineral salts, and small amounts of urea, helping to regulate body temperature and remove metabolic waste.

The statement "skin is the main excretory organ" is incorrect because in all chordates kidneys act as main excretory organ.

Sweat glands are coiled and tubular in structure. Coiled part of this gland is called as secretory unit. Around secretory unit, myoepithelial cells are present. When they contract sweat comes out from sweat gland. This secretion is called as sweat. 

The skin has sweat glands that excrete perspiration (salt, water, and nitrogenous wastes) through its pores. 

Skin serves in excretion of waste material through the process of sweating. Large intestine serves to reabsorb water and essential minerals from the undigested food and to store it till it is excreted at the anus. Lungs serve in excretion via expiration. The liver serves in the formation of urea which is then excreted by kidneys. Kidneys are the organs of urine formation. This makes option B correct answer as large intestine only stores the undigested material; it does not excrete it directly. 

Evaporation or the loss of water from the body due to heat is called as perspiration. The heat present in the surrounding causes water to be lost from the body in the form of sweat through the sweat glands on the skin. The water is deposited on the surface of the organism's body leading to a cooling effect. Thus the correct answer is option A.

Sweat glands secrete a fluid called sweat that contains water and dissolved salts of Calcium, Sodium, Magnesium etc Thus, it helps in excreting water and salts to some extent by a process called sweating or perspiration. 

Insoluble waste products, including certain mineral salts like calcium phosphate that are not absorbed by the body, are eliminated through the large intestine as part of feces.

Since terrestrial animals do not have automatic access to either fresh or salt water, they must regulate water content in other ways, balancing off gains and losses.

Ammonia which is the major form of nitrogenous waste generated in the body is converted urea in the liver and excreted through the urine by the kidneys. Thus, the correct option is C.

The accessory excretory organs are:-

(A) Spleen - Primarily acts as blood filter. 

Metabolic wastes are substances left over from a metabolic process which cannot be used by the organisms and must, therefore, be excreted. This includes nitrogen compounds. The nitrogen compounds through which excess nitrogen is eliminated from the organism are called nitrogenous wastes. They are ammonia, urea, uric acid, and creatinine. 

The large intestine is the part of the digestive system. Whereas the excretory system of humans consists of two kidneys, two ureters, one urinary bladder, and one urethra. Urine is found in almost all excretory system parts(little or more).

The epidermis is composed of 4 or 5 layers depending on the region of skin being considered. The Malpighian layer (stratum malpighii) is defined as both the stratum basale and stratum spinosum. In spinous layer (stratum spinosum), keratinocytes become connected through desmosomes and start to produce lamellar bodies, from within the golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes. Basal/germinal layer (stratum basale/germinativum) is composed of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips. 

The sweat on the surface of the skin is evaporated by the body heat. As the water is evaporated by heat released from the body, the body cools off. Each gram of water on the body surface absorbs about 0.58 calories of energy before evaporating, which is a significant source of heat loss.

In humans, kidneys are the chief excretory organ, other than kidneys, lungs, liver, and skin also help in the elimination of excretory wastes. Lungs remove a large amount of carbon dioxide and the significant amount of water every day. The liver secretes bile containing substances like bilirubin, biliverdin, degraded steroid hormones, vitamins. Most of these substances pass along with digestive wastes. Skin contains sweat glands through which NaCl, the small amount of urea, lactic acid etc are secreted. The intestine is not an excretory organ but it is a digestive organ.

Sebum is an oily secretion produced by sebaceous glands, tiny ducts adjacent to hair follicles. Sebum is secreted into the follicle, from which it spreads over the hair and skin. The main role of sebum is to waterproof the skin and hair. Sweat is a salty, watery solution produced by sweat glands, numerous microscopic channels opening onto the skin surface. As sebum and sweat mix up on the skin surface, they form a protective layer often referred to as the acid mantle . Acid mantle has a particular level of acidity characterized by pH from about 4 to 5.5. A pH of 7 is considered neutral, above 7 is alkaline, and below is acidic.