All living organisms generate metabolic wastes that, if allowed to accumulate, would become toxic and disrupt the delicate internal balance, or homeostasis, essential for life. The process of eliminating these metabolic wastes from the body is called excretion. The most significant of these wastes are nitrogenous compounds, which are primarily produced from the catabolism of proteins and nucleic acids. Depending on the organism and its habitat, this nitrogenous waste is excreted in one of three main forms: ammonia, urea, or uric acid. Animals that excrete ammonia (ammonotelic) are typically aquatic, as ammonia is highly toxic and requires large amounts of water for its removal. Terrestrial animals, to conserve water, convert ammonia into less toxic substances. Mammals and many amphibians excrete urea (ureotelic), while reptiles, birds, and insects excrete uric acid (uricotelic), which is the least toxic and requires minimal water.

The excretory structures vary widely across the animal kingdom, reflecting an evolutionary progression from simple to complex. Invertebrates like flatworms possess simple tubular structures called protonephridia (flame cells), while earthworms have more complex tubules called nephridia. Insects utilize Malpighian tubules. In vertebrates, the primary excretory organs are a pair of sophisticated organs called the kidneys.

The human excretory system is a highly efficient apparatus for filtering blood and forming urine. It consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. The kidneys, reddish-brown, bean-shaped organs, are the main sites of urine formation. Internally, each kidney is divided into an outer cortex and an inner medulla. The functional unit of the kidney, responsible for filtering blood and forming urine, is the nephron. Each kidney contains about a million of these microscopic tubules.

A nephron has two main parts: the glomerulus and the renal tubule. The glomerulus is a tuft of capillaries that receives blood from an afferent arteriole. It is enclosed by a cup-shaped structure called Bowman’s capsule. Together, the glomerulus and Bowman’s capsule form the Malpighian body or renal corpuscle. The renal tubule extends from Bowman’s capsule and is a long, coiled structure divided into three main regions: the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT).

The process of urine formation involves three crucial steps: glomerular filtration, tubular reabsorption, and tubular secretion.

1. Glomerular Filtration: This is the first step, where blood is filtered under high pressure as it passes through the glomerulus. Water, glucose, salts, amino acids, and urea are forced out of the blood into Bowman’s capsule, forming the glomerular filtrate. This process is non-selective, except for filtering out large proteins and blood cells.

2. Tubular Reabsorption: As the filtrate passes through the renal tubule, the body reclaims essential substances that it needs. This is a highly selective process. The majority of water, glucose, amino acids, and vital ions are reabsorbed back into the blood, primarily in the PCT. The loop of Henle is crucial for reabsorbing water and salts, which helps in concentrating the urine. Further reabsorption occurs in the DCT under hormonal control.

3. Tubular Secretion: In this final step, certain waste products and excess ions (like hydrogen ions, potassium ions, and ammonia) are actively secreted from the blood into the filtrate within the tubule. This process helps in maintaining the ionic and acid-base balance of the body fluids.

A key function of the mammalian kidney is its ability to produce concentrated urine, which is vital for conserving water. This is achieved through the countercurrent mechanism, involving the loop of Henle and the vasa recta (a network of capillaries running parallel to the loop). This mechanism creates and maintains a high concentration of solutes in the medullary interstitium, allowing for the passive reabsorption of water from the collecting duct under the influence of hormones.

The functioning of the kidneys is meticulously regulated by hormonal feedback mechanisms. The renin-angiotensin-aldosterone system (RAAS) and antidiuretic hormone (ADH) play pivotal roles in controlling blood volume, blood pressure, and osmolarity. When the body is dehydrated, ADH is released, increasing water reabsorption. The RAAS system is activated in response to low blood pressure, leading to vasoconstriction and increased reabsorption of sodium and water.

Finally, the urine formed in the nephrons is collected in the urinary bladder and is expelled from the body through the urethra in a process called micturition, or urination. While the kidneys are the primary excretory organs, other organs like the lungs (eliminating CO₂), skin (eliminating sweat), and liver also play supplementary roles in excretion.