Q5:

Describe the role of liver, lungs and skin in excretion.

1. The Role of the Liver in Excretion

The liver acts as a primary metabolic center for detoxification and the conversion of nitrogenous wastes. Its excretory functions are twofold:

• Urea Synthesis (Ornithine Cycle): The liver converts toxic ammonia ($NH_3$), derived from the deamination of amino acids, into urea ($CO(NH_2)_2$). This is a less toxic form of nitrogenous waste, which is subsequently transported to the kidneys for filtration [Per the Ornithine Cycle biochemistry].
• Bile Pigment Secretion: The liver facilitates the breakdown of senescent red blood cells. The degradation of hemoglobin yields bile pigments, specifically bilirubin and biliverdin. These pigments are excreted via bile into the gastrointestinal tract, where they are eliminated through feces [Reference: Hepatic portal system and digestive secretion pathways].
• Cholesterol and Drug Metabolism: The liver serves as a clearance site for cholesterol, steroid hormones, vitamins, and pharmacological agents, secreting these substances into bile for intestinal excretion.

2. The Role of the Lungs in Excretion

The lungs function as the principal organs for the elimination of volatile gaseous waste products produced by cellular respiration.

• Carbon Dioxide ($CO_2$) Elimination: During cellular respiration, $CO_2$ is generated as a metabolic byproduct. This gas diffuses from the tissues into the blood, where it is transported primarily as bicarbonate ions ($HCO_3^-$) and carbaminohemoglobin to the pulmonary capillaries [Per the Bohr Effect and the Chloride Shift mechanism].
• Water Vapor: The lungs also expire a significant volume of water in the form of water vapor with every exhalation, contributing to the regulation of body water balance and thermoregulation.

3. The Role of the Skin in Excretion

The integumentary system utilizes specialized glands to manage the homeostasis of electrolytes and metabolic wastes.

• Sweat Glands (Sudoriferous Glands): These glands secrete a fluid—sweat—which contains water, sodium chloride ($NaCl$), small amounts of urea, and lactic acid. While the primary function of sweat is thermoregulation (via evaporative cooling), it serves as a secondary excretory route for salt and nitrogenous wastes [Reference: Integumentary physiology].
• Sebaceous Glands: These glands secrete sebum, an oily substance consisting of waxes, sterols, and fatty acids. Sebum serves to provide a protective, waterproof coating to the skin, while simultaneously acting as a mechanism for the excretion of certain lipid-soluble metabolic byproducts.

Summary Comparison Table

Visual Representation of Excretory Pathways

Final Solution: The liver, lungs, and skin collectively maintain physiological homeostasis by eliminating diverse metabolic waste products: the liver processes nitrogenous wastes and bile pigments via the biliary system; the lungs eliminate gaseous carbon dioxide and water vapor through ventilation; and the skin excretes water, salts, and organic compounds through sweat and sebum secretions.