Excretion is the process by which metabolic waste products are eliminated from the body. The most important metabolic waste is ammonia, produced by the breakdown of amino acids (deamination). The nature of the nitrogenous waste product depends on the availability of water and the habitat of the organism. Ammonia is the most toxic and requires the most water for elimination, while uric acid is the least toxic and requires the least water. On this basis, animals are classified into ammonotelic, ureotelic and uricotelic organisms.
The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. The kidney, which is the main excretory organ, filters the blood to form urine through a remarkable microscopic unit called the nephron. This chapter describes the structure of the human excretory system, the mechanism of urine formation (glomerular filtration, reabsorption and secretion), the countercurrent mechanism for concentrating urine, and the regulation of kidney function by hormones. It also describes the disorders of the excretory system, including renal failure and the process of dialysis.
Animals that excrete ammonia are called ammonotelic. Ammonia is highly toxic and soluble in water, and it is readily soluble and requires a large amount of water for its elimination. Ammonotelic animals include most aquatic invertebrates, bony fishes (e.g., rohu) and aquatic amphibians and reptiles. Ammonia is excreted by diffusion across the body surface or through the gills.
Animals that excrete urea are called ureotelic. Urea is less toxic than ammonia and requires less water for elimination. Urea is produced in the liver by the ornithine cycle and is transported to the kidneys. Ureotelic animals include terrestrial amphibians, marine fishes and mammals. Humans excrete urea and are therefore ureotelic.
Animals that excrete uric acid are called uricotelic. Uric acid is the least toxic and is almost insoluble in water; it is excreted as a semi-solid paste with the loss of very little water. This is an adaptation for conserving water, especially in eggs. Uricotelic animals include reptiles, birds, land snails and insects.
The human excretory system consists of: - A pair of kidneys: Bean-shaped, reddish-brown organs situated on either side of the vertebral column in the abdominal cavity, between the levels of the last thoracic and third lumbar vertebrae. The right kidney is slightly lower than the left. The concave inner surface of each kidney has a notch called the hilum, through which the renal artery enters, the renal vein and ureter exit, and nerves pass. - A pair of ureters: Thin, muscular tubes that carry urine from the kidneys to the urinary bladder. - A urinary bladder: A sac that stores urine. - A urethra: The tube that carries urine from the bladder to the exterior.
Each kidney is covered by a fibrous capsule and has an outer cortex and an inner medulla. The medulla is divided into conical masses called the renal pyramids, which project into the renal pelvis. The pelvis has subdivisions called major and minor calyces. Each kidney contains about a million functional units called nephrons, which are the structural and functional units of the kidney.
Each nephron consists of two parts: the renal corpuscle (Malpighian body) and the renal tubule. - Renal corpuscle: Composed of the glomerulus and Bowman's capsule. The glomerulus is a tuft of capillaries formed by the afferent arteriole. The Bowman's capsule is a double-walled cup surrounding the glomerulus. The afferent arteriole brings blood to the glomerulus and is wider than the efferent arteriole, which takes blood away. - Renal tubule: Continues from the Bowman's capsule as the proximal convoluted tubule (PCT), the loop of Henle (a hairpin-shaped tube with a descending and an ascending limb), and the distal convoluted tubule (DCT). The DCTs of many nephrons join to form the collecting duct, which passes through the medulla and opens into the renal pelvis.
There are two types of nephrons: cortical nephrons (with a short loop of Henle, lying in the cortex) and juxtamedullary nephrons (with a long loop of Henle, extending deep into the medulla). The juxtamedullary nephrons are important for the concentration of urine.
Urine formation involves three main processes: glomerular filtration, tubular reabsorption and tubular secretion.
Glomerular filtration is the first step of urine formation, occurring in the renal corpuscle. The glomerular capillaries have a high pressure due to the wider afferent arteriole, and the glomerular capillary wall plus the Bowman's capsule wall form a filtration membrane that is selectively permeable. Blood is filtered through this membrane, and about 110 to 125 millilitres of filtrate is produced per minute; this is called the glomerular filtration rate (GFR). The filtrate contains glucose, amino acids, salts, urea and other small molecules, but not blood cells and large proteins.
The filtrate passes through the renal tubules, where useful substances are reabsorbed into the blood. Reabsorption occurs by active transport (e.g., glucose, amino acids, most sodium) and by passive transport (e.g., water, urea). Nearly all of the glucose and amino acids are reabsorbed in the proximal convoluted tubule. Water is reabsorbed from the PCT, the descending limb of the loop of Henle, and the collecting ducts.
Tubular secretion is the process by which certain substances such as hydrogen ions, potassium ions, ammonia and some drugs are secreted from the blood into the tubular fluid, helping in the maintenance of pH and ionic balance.
The concentration of urine in the kidney is achieved by the countercurrent mechanism, which operates in the loop of Henle and the collecting duct. The countercurrent mechanism involves the flow of fluid in opposite directions in the descending and ascending limbs of the loop of Henle, which establishes a concentration gradient in the medullary interstitial fluid.
The kidney function is regulated by hormonal mechanisms: - Renin-angiotensin system: When there is a fall in the glomerular blood flow and the glomerular filtration rate, the juxtaglomerular (JG) cells of the kidney release renin. Renin converts angiotensinogen in the blood into angiotensin I, which is converted into angiotensin II by the enzyme ACE (angiotensin converting enzyme) in the lungs. Angiotensin II is a powerful vasoconstrictor that increases the glomerular blood pressure and GFR, and it also stimulates the adrenal cortex to release aldosterone. Aldosterone stimulates the reabsorption of sodium and water, increasing blood volume and pressure. - Atrial natriuretic factor (ANF): The ANF is secreted by the atria of the heart when blood volume increases. ANF opposes the renin-angiotensin system, causing vasodilation and reducing blood pressure. The ANF can cause the dilation of blood vessels, countering the effects of angiotensin II. - Antidiuretic hormone (ADH): ADH, secreted by the posterior pituitary, acts on the collecting ducts and makes them more permeable to water, increasing the reabsorption of water. This concentrates the urine and reduces urine volume. A deficiency of ADH causes diabetes insipidus, characterised by the production of a large volume of dilute urine.
| Type | Product | Toxicity | Water needed | Animals |
|---|---|---|---|---|
| Ammonotelism | Ammonia | Most toxic | Maximum | Bony fishes, aquatic animals |
| Ureotelism | Urea | Less toxic | Less | Mammals, amphibians |
| Uricotelism | Uric acid | Least toxic | Minimum | Reptiles, birds, insects |
| Part | Function |
|---|---|
| Glomerulus | Filtration of blood |
| PCT | Reabsorption of glucose, amino acids, water |
| Loop of Henle | Countercurrent concentration |
| DCT | Reabsorption and secretion, regulation |
| Collecting duct | Water reabsorption under ADH, concentration |
| Hormone | Source | Action |
|---|---|---|
| Renin | JG cells of kidney | Raises GFR and blood pressure |
| Aldosterone | Adrenal cortex | Reabsorption of Na+ and water |
| ADH | Posterior pituitary | Water reabsorption in collecting ducts |
| ANF | Atria of heart | Lowers blood pressure, opposes RAAS |
Excretion is the essential counterpart of metabolism, ensuring that toxic nitrogenous wastes do not accumulate in the body. The classification of animals into ammonotelic, ureotelic and uricotelic reflects their evolutionary adaptations to water availability. The human excretory system, centred on the kidney, is a marvel of precision engineering, in which the nephron filters blood, reabsorbs useful substances and secretes waste products to produce urine. The countercurrent mechanism and the hormonal regulation by renin, aldosterone, ADH and ANF allow the kidney to adjust urine concentration precisely to the body's needs. Understanding the disorders of the excretory system and the role of dialysis highlights the clinical importance of these mechanisms, completing the picture of how the body maintains its internal environment.