Excretory Products and Their Elimination
How organisms remove metabolic wastes — from simple diffusion in aquatic forms to the sophisticated counter-current mechanism of the mammalian kidney
In this chapter
Animals accumulate a range of metabolic by-products — ammonia, urea, uric acid, carbon dioxide, water, and ions such as Na+, K+, Cl−, phosphate, and sulphate — either through their own metabolic activities or by excess ingestion. These substances must be removed either totally or partially. The mechanisms by which they are eliminated form the focus of this chapter, with special emphasis on the common nitrogenous wastes.
Ammonia, urea, and uric acid are the three major forms of nitrogenous wastes excreted by animals. Ammonia is the most toxic form and demands large quantities of water for its elimination. In contrast, uric acid, being the least toxic, can be removed with minimal water loss.
Ammonotelism
The process of excreting ammonia. Many bony fishes, aquatic amphibians, and aquatic insects are ammonotelic. Ammonia, being highly soluble, diffuses across body surfaces or through gill surfaces as ammonium ions. Kidneys play no significant role in its removal.
Ureotelism
Mammals, many terrestrial amphibians, and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver, released into blood, and filtered and excreted by the kidneys.
Uricotelism
Reptiles, birds, land snails, and insects excrete nitrogenous wastes as uric acid in the form of a pellet or paste with minimal water loss. These are called uricotelic animals.
Terrestrial Adaptation
Terrestrial adaptation necessitated the production of less toxic nitrogenous wastes like urea and uric acid for conservation of water. Some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity.
A survey of the animal kingdom reveals a wide variety of excretory structures. In most invertebrates, these are simple tubular forms, while vertebrates possess complex tubular organs called kidneys.
- Protonephridia (flame cells) — Found in Platyhelminthes (e.g., Planaria), rotifers, some annelids, and the cephalochordate Amphioxus. Primarily concerned with osmoregulation.
- Nephridia — Tubular excretory structures of earthworms and other annelids. Help remove nitrogenous wastes and maintain fluid and ionic balance.
- Malpighian tubules — Excretory structures of most insects, including cockroaches. Aid in removal of nitrogenous wastes and osmoregulation.
- Antennal glands (green glands) — Perform the excretory function in crustaceans like prawns.
16.1 Human Excretory System
In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder, and a urethra. Kidneys are reddish-brown, bean-shaped structures situated between the levels of the last thoracic and third lumbar vertebrae, close to the dorsal inner wall of the abdominal cavity.
Kidney Dimensions
Each kidney of an adult human measures 10–12 cm in length, 5–7 cm in width, 2–3 cm in thickness, with an average weight of 120–170 g.
Nephrons
Each kidney contains nearly one million complex tubular structures called nephrons — the functional units of the kidney.
Gross Anatomy of the Kidney
Towards the centre of the inner concave surface is a notch called the hilum, through which the ureter, blood vessels, and nerves enter. Inner to the hilum is a broad, funnel-shaped space called the renal pelvis, with projections called calyces.
The outer layer of the kidney is a tough capsule. Inside, there are two zones: an outer cortex and an inner medulla. The medulla is divided into a few conical masses called medullary pyramids that project into the calyces. The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
Structure of the Nephron
Each nephron has two parts — the glomerulus and the renal tubule.
The glomerulus is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery. Blood from the glomerulus is carried away by an efferent arteriole.
The renal tubule begins with a double-walled, cup-like structure called Bowman's capsule, which encloses the glomerulus. The glomerulus together with Bowman's capsule is called the Malpighian body or renal corpuscle.
The tubule continues to form a highly coiled network — the proximal convoluted tubule (PCT). A hairpin-shaped Henle's loop is the next part, with a descending and an ascending limb. The ascending limb continues as another highly coiled tubular region called the distal convoluted tubule (DCT). The DCTs of many nephrons open into a straight tube called the collecting duct, many of which converge and open into the renal pelvis through medullary pyramids in the calyces.
Cortical vs. Juxtamedullary Nephrons
The Malpighian corpuscle, PCT, and DCT of the nephron are situated in the cortical region, whereas the loop of Henle dips into the medulla. In most nephrons, the loop of Henle is short and extends only slightly into the medulla — these are called cortical nephrons. In some nephrons, the loop of Henle is very long and runs deep into the medulla — these are called juxtamedullary nephrons.
The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries. A minute vessel of this network runs parallel to Henle's loop, forming a 'U'-shaped vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.
16.2 Urine Formation
Urine formation involves three main processes: glomerular filtration, reabsorption, and secretion, which take place in different parts of the nephron.
Glomerular Filtration
The first step in urine formation is the filtration of blood, carried out by the glomerulus. On an average, 1100–1200 ml of blood is filtered by the kidneys per minute, constituting roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute.
The glomerular capillary blood pressure causes filtration of blood through three layers:
- The endothelium of glomerular blood vessels
- The epithelium of Bowman's capsule
- A basement membrane between these two layers
The epithelial cells of Bowman's capsule, called podocytes, are arranged in an intricate manner so as to leave minute spaces called filtration slits or slit pores. Blood is filtered so finely through these membranes that almost all constituents of the plasma except the proteins pass onto the lumen of the Bowman's capsule. This process is therefore considered as ultra filtration.
The amount of filtrate formed by the kidneys per minute is called the glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute, i.e., 180 litres per day!
Regulation of GFR by JGA
The kidneys have built-in mechanisms for the regulation of GFR. One such mechanism is carried out by the juxtaglomerular apparatus (JGA). JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular blood flow and thereby bring the GFR back to normal.
Reabsorption
A comparison of the volume of filtrate formed per day (180 litres) with that of urine released (1.5 litres) suggests that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. The tubular epithelial cells in different segments of the nephron perform this either by active or passive mechanisms.
For example, substances like glucose, amino acids, Na+, etc., in the filtrate are reabsorbed actively, whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron.
Tubular Secretion
During urine formation, the tubular cells secrete substances like H+, K+, and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid–base balance of body fluids.
16.3 Function of the Tubules
Proximal Convoluted Tubule (PCT)
PCT is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption. Nearly all of the essential nutrients, and 70–80 per cent of electrolytes and water are reabsorbed by this segment. PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions and ammonia into the filtrate and by absorption of HCO3− from it.
Henle's Loop
Reabsorption is minimum in its ascending limb. However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. The descending limb of the loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate passes upward, it becomes diluted.
Distal Convoluted Tubule (DCT)
DCT has similar functions as PCT — it reabsorbs Na+ and water. It also secretes K+ and H+ into the filtrate while reabsorbing HCO3−. Hence, it plays a role in the maintenance of pH and ionic balance of body fluids.
Collecting Duct
This long duct extends from the cortex of the kidney to the inner parts of the medulla. Large amounts of water could be reabsorbed from this region to produce a concentrated urine. This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity. It also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of H+ and K+ ions.
Summary — Functions of Nephron Segments
| Segment | Key Functions |
|---|---|
| PCT | Reabsorbs ~70–80% electrolytes, water, and all essential nutrients; secretes H+, NH3; absorbs HCO3− |
| Henle's Loop | Descending limb: permeable to water, impermeable to solutes. Ascending limb: impermeable to water, active/passive transport of electrolytes |
| DCT | Reabsorbs Na+ and water; secretes K+, H+; reabsorbs HCO3− |
| Collecting Duct | Reabsorbs large amounts of water for concentrated urine; allows urea into medullary interstitium; secretes H+, K+ |
16.4 Mechanism of Concentration of the Filtrate
Mammals have the ability to produce a concentrated urine. Henle's loop and vasa recta play a significant role in this.
Counter-Current Mechanism
The flow of filtrate in the two limbs of Henle's loop is in opposite directions and thus forms a counter-current. The flow of blood through the two limbs of vasa recta is also in a counter-current pattern.
The proximity between Henle's loop and vasa recta, as well as the counter-current flow in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium — from 300 mOsmol L−1 in the cortex to about 1200 mOsmol L−1 in the inner medulla. This gradient is mainly caused by NaCl and urea.
- NaCl is transported by the ascending limb of Henle's loop, which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.
- Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop and are transported back to the interstitium by the collecting tubule.
The transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called the counter-current mechanism. This mechanism helps to maintain a concentration gradient in the medullary interstitium. The presence of such a gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times more concentrated than the initial filtrate formed.
16.5 Regulation of Kidney Function
The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and to a certain extent, the heart.
ADH and Osmoreceptors
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume, and ionic concentration. An excessive loss of fluid from the body can activate these receptors, which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis.
An increase in body fluid volume can switch off the osmoreceptors and suppress ADH release to complete the feedback. ADH can also affect kidney function by its constrictory effects on blood vessels, causing an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.
Renin–Angiotensin Mechanism
The JGA plays a complex regulatory role. A fall in glomerular blood flow, glomerular blood pressure, or GFR can activate the JG cells to release renin, which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR.
Angiotensin II also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the Renin–Angiotensin mechanism.
ANF — A Check on the System
An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decrease blood pressure. The ANF mechanism acts as a check on the renin–angiotensin mechanism.
ADH (Vasopressin) — Released from neurohypophysis; facilitates water reabsorption; prevents diuresis; constricts blood vessels to raise BP.
Renin → Angiotensin II — Activated by low GFR; angiotensin II is a powerful vasoconstrictor that raises glomerular BP and GFR.
Aldosterone — Released by adrenal cortex under angiotensin II stimulation; increases Na+ and water reabsorption from distal tubule.
ANF — Released by heart atria; causes vasodilation; decreases BP; acts as a check on the renin–angiotensin mechanism.
16.6 Micturition
Urine formed by the nephrons is ultimately carried to the urinary bladder, where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
In response, the stretch receptors on the walls of the bladder send signals to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, causing the release of urine. The process of release of urine is called micturition, and the neural mechanisms causing it are called the micturition reflex.
An adult human excretes, on an average, 1 to 1.5 litres of urine per day. The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH 6.0) and has a characteristic odour. On an average, 25–30 g of urea is excreted per day.
Various conditions can affect the characteristics of urine. Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney. For example, presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine are indicative of diabetes mellitus.
16.7 Role of other Organs in Excretion
Other than the kidneys, lungs, liver, and skin also help in the elimination of excretory wastes.
Lungs
Remove large amounts of CO2 (approximately 200 mL/minute) and also significant quantities of water every day.
Liver
The largest gland in the body. Secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. Most of these pass out along with digestive wastes.
Sweat Glands
Produce a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. Though the primary function is cooling, it also helps in the removal of some wastes.
Sebaceous Glands
Eliminate certain substances like sterols, hydrocarbons, and waxes through sebum. This secretion provides a protective oily covering for the skin.
Small amounts of nitrogenous wastes could be eliminated through saliva too!
16.8 Disorders of the Excretory System
Malfunctioning of kidneys can lead to the accumulation of urea in blood, a condition called uremia, which is highly harmful and may lead to kidney failure. In such patients, urea can be removed by a process called hemodialysis.
Hemodialysis
During the process of hemodialysis, blood drained from a convenient artery is pumped into a dialyzing unit called an artificial kidney. Blood is pumped into the dialyzing unit after adding an anticoagulant such as heparin. The dialyzing unit contains a porous cellophane membrance of a tube. The composition of the dialyzing fluid is the same as that of plasma except the nitrogenous wastes. The porous cellophane membrane allows the passage of molecules based on concentration gradient. As nitrogenous wastes are absent in the dialyzing fluid, these substances freely move out, thereby clearing the blood. The cleared blood is pumped back to the body through a vein after adding anti-heparin to it. This method is a boon for thousands of uremic patients all over the world.
Kidney Transplantation
Kidney transplantation is the ultimate method in the correction of acute renal failures (kidney failure). A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimise the chances of rejection by the immune system of the host. Modern clinical procedures have increased the success rate of such a complicated technique.
Uremia — Accumulation of urea in blood due to kidney malfunction; highly harmful; may lead to kidney failure.
Glycosuria — Presence of glucose in urine; indicative of diabetes mellitus.
Ketonuria — Presence of ketone bodies in urine; also indicative of diabetes mellitus.
✦ Summary
• Many nitrogen-containing substances, ions, CO2, water, etc., accumulate in the body and must be eliminated. The nature of nitrogenous wastes and their excretion vary among animals, mainly depending on the habitat (availability of water). Ammonia, urea, and uric acid are the major nitrogenous wastes excreted.
• Protonephridia, nephridia, Malpighian tubules, green glands, and kidneys are the common excretory organs in animals. They not only eliminate nitrogenous wastes but also help maintain ionic and acid–base balance of body fluids.
• In humans, the excretory system consists of one pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. Each kidney has over a million nephrons — the functional units of the kidney — comprising a glomerulus and renal tubule (PCT, Henle's loop, DCT).
• Urine formation involves three main processes: filtration, reabsorption, and secretion. About 1200 ml of blood is filtered by the glomerulus per minute to form 125 ml of filtrate (GFR). JGA plays a significant role in the regulation of GFR.
• Nearly 99 per cent reabsorption of the filtrate takes place through different parts of the nephrons. PCT is the major site of reabsorption and selective secretion. Henle's loop primarily helps to maintain the osmolar gradient (300–1200 mOsmol L−1) within the kidney interstitium.
• A counter-current mechanism operates between the two limbs of the loop of Henle and those of vasa recta. The filtrate gets concentrated as it moves down the descending limb but is diluted by the ascending limb. DCT and collecting duct concentrate the filtrate about four times.
• Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its release through the urethra — micturition. Skin, lungs, and liver also assist in excretion.