Excretory Products and their Elimination: Class 11 Biology · Chapter 16

Wastes and the organs that remove them

A plate of rajma-chawal or a bowl of paneer gives you more protein than your body can store. The surplus amino acids are broken down for energy, and their nitrogen is stripped off as ammonia. Respiration adds carbon dioxide and water, and a salty snack brings in more $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$, $\mathrm{K}{\phantom{X}}^{+}$ and $\mathrm{C}\mathrm{l}{\phantom{X}}^{-}$ than you need; phosphate and sulphate ions pile up as well. Some of these must leave the body entirely, while others only need trimming back to the right level. This chapter is about how animals, and humans in particular, do that balancing and disposal (उत्सर्जन, excretion), with most of the attention on wastes that contain nitrogen.

Animals package surplus nitrogen in one of three main molecules, and each choice trades safety against water. Ammonia is the most poisonous of the three and dissolves very readily, so it is safe only if a large volume of water flushes it away at once. Urea is much less toxic and can stay in body fluids for a while. Uric acid is the least toxic and barely dissolves, so it can be expelled almost dry (Fig. 16.1).

Animals that live surrounded by water can afford the simplest option. Because ammonia dissolves so readily, it just diffuses out as ammonium ions ($\mathrm{N}\mathrm{H}{\phantom{X}}_4{\phantom{X}}^{+}$), across the general body surface or, in fish, across the gills, and the surrounding water carries it off; the kidneys hardly take part. Excreting nitrogen mainly as ammonia in this way is ammonotelism, and aquatic insects, aquatic amphibians and many bony fishes are ammonotelic.

You are ureotelic. Your liver turns ammonia into urea, the blood carries the urea to the kidneys, and the kidneys filter it into urine. Other mammals, many terrestrial amphibians and marine fishes do the same. Urea suits life on land: it is far less toxic than ammonia, so it can sit in body fluids for a while and be flushed out with much less water. Some ureotelic animals even keep a little urea back in the matrix of the kidney to hold the fluid there at the osmolarity they need, an idea you will meet again in Section 16.4.

Reptiles, birds, land snails and insects save even more water. They are uricotelic: their nitrogen leaves as uric acid, passed as a pellet or paste with hardly any water lost. The white part of a pigeon dropping on a balcony railing is mostly this uric acid paste.

Excretory structures across the animal kingdom

Look across the animal kingdom and the equipment for this work varies a great deal (Table 16.1). Invertebrates mostly rely on simple tubes; vertebrates have kidneys, elaborate organs built from many tubules. Whatever the design, most of these organs do two jobs at once. Besides getting rid of nitrogenous waste, they hold the volume and ion content of body fluids steady, a job called osmoregulation (परासरण नियमन).

16.1 Human Excretory System

Your own excretory system has four parts: a pair of kidneys (वृक्क) that make urine, a pair of ureters that carry it away, a urinary bladder that stores it, and a urethra through which it leaves the body (Fig. 16.2).

Put your hands on your back where your lowest ribs end, one on each side of the spine, and you are roughly over your kidneys. They lie against the dorsal inner wall of the abdominal cavity, spanning the levels from the last thoracic vertebra to the third lumbar vertebra. Each is a reddish-brown bean about the size of a closed fist: about 10–12 cm long, 5–7 cm wide and 2–3 cm thick, weighing 120–170 g. The notch on the bean's concave inner side is the hilum, and the ureter, blood vessels and nerves all pass through it.

Inside a kidney

Cut a kidney lengthwise and you see three zones from outside in (Fig. 16.3). A tough capsule wraps the surface. Beneath it is the outer cortex, and inside that the inner medulla, which is divided into a few cone-shaped medullary pyramids. Between neighbouring pyramids the cortex reaches inwards as strips of tissue called renal columns, or columns of Bertini. The tip of each pyramid projects into a cup-like calyx (plural: calyces). The calyces are projections of the renal pelvis, a broad funnel-shaped space just inside the hilum that narrows into the ureter.

The nephron

Each kidney holds about a million microscopic tubes called nephrons (वृक्काणु). The nephron is the kidney's functional unit: what the whole kidney does, each nephron does on a tiny scale. Every nephron has two parts, a knot of capillaries called the glomerulus and a long renal tubule (Fig. 16.4).

Follow blood and then filtrate through one nephron. Blood arrives in a fine branch of the renal artery, the afferent arteriole, which forms the knot of capillaries that is the glomerulus, and it leaves by a narrower efferent arteriole. Fluid squeezed out of the glomerulus is caught by Bowman's capsule, a double-walled cup that holds the glomerulus the way cupped hands hold a ball. Together the two make up the renal corpuscle, also called the Malpighian body. From the capsule the fluid enters the tightly coiled proximal convoluted tubule (PCT), runs down and back up a hairpin, Henle's loop, along its descending and ascending limbs, and then winds through a second coil, the distal convoluted tubule (DCT). Many DCTs share one straight collecting duct; collecting ducts merge with one another and finally empty into the renal pelvis at the tips of the medullary pyramids, inside the calyces.

The renal corpuscle, the PCT and the DCT all lie in the cortex; only Henle's loop dips into the medulla. How deep it goes divides nephrons into two kinds (the comparison below).

The efferent arteriole does not lead straight to a vein. It branches again into a fine network of peritubular capillaries around the tubule. One thin vessel of this network runs down beside Henle's loop and back up again in a U, the vasa recta. Blood in a nephron therefore passes through two capillary beds in a row, first the glomerulus and then the peritubular network, and both matter in the next three sections.

16.2 Urine Formation

Three processes make urine, each in its own part of the nephron: glomerular filtration in the renal corpuscle, reabsorption from the tubule back into the blood, and secretion from the blood into the tubule.

Glomerular filtration

The glomerulus works like a pressure filter, and the pressure comes from the blood itself: blood in the glomerular capillaries is under enough pressure to force fluid outwards. Three layers separate that blood from the space inside Bowman's capsule (Fig. 16.5). Nearest the blood is the endothelium of the capillary wall, outermost is the epithelium of the capsule, and sandwiched between them is a basement membrane. The capsule's epithelial cells, the podocytes, wrap the capillaries in an intricate interlocking pattern that leaves only tiny gaps, the filtration slits or slit pores.

How much blood passes through this filter? About 1100–1200 mL every minute, close to one-fifth of the volume that each ventricle of the heart pumps out in that minute. That is a large share for two organs that together weigh only about 300 g.

Only the plasma proteins, along with the blood cells, are too big to get through; water, glucose, amino acids, salts and urea all cross. Because the sorting happens at the scale of molecules, this is called ultrafiltration.

The volume of filtrate that both kidneys together form in one minute is the glomerular filtration rate (GFR). In a healthy adult it is about 125 mL per minute. That sounds modest until you scale it to a whole day:

Keeping GFR steady matters, and the nephron carries its own sensor for the job. Each DCT loops back and touches the afferent arteriole of its own glomerulus. At that point of contact, cells in both walls are specialised to form the juxtaglomerular apparatus (JGA). A drop in GFR prompts these JG cells to release renin, the first link in a chain that raises glomerular blood flow until GFR recovers. Section 16.5 follows the whole chain.

Reabsorption and secretion

Imagine filling a 180 L drum with filtrate every day but emptying only a 1.5 L bottle of urine. The other 178.5 L, close to 99 per cent, must go back into the blood; this return is reabsorption. Not everything returns the same way. Valuable solutes such as glucose, amino acids and $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ are carried back by active transport, which costs the tubule cells energy. Nitrogenous wastes only drift back passively, down their concentration gradients, and in the first segments of the tubule water too follows passively (Fig. 16.6).

Traffic also runs the other way. Tubule cells secrete $\mathrm{H}{\phantom{X}}^{+}$, $\mathrm{K}{\phantom{X}}^{+}$ and ammonia from the blood into the filtrate. Secretion is a genuine step of urine formation: by deciding how much acid and potassium to dump, the kidney keeps the ionic balance and the acid–base balance of body fluids steady.

16.3 Function of the Tubules

Each stretch of the tubule has its own set of jobs. The clearest way to remember them is to follow a drop of filtrate through the segments in turn (Fig. 16.6, Table 16.2).

Proximal convoluted tubule (PCT): bulk recovery. Almost every useful nutrient comes back here, along with 70–80 per cent of the water and electrolytes. To handle that load, the wall is built of simple cuboidal cells with a brush border of microvilli, which multiplies the surface area available for reabsorption. The PCT also tunes pH and ion balance: it pulls bicarbonate ($\mathrm{H}\mathrm{C}\mathrm{O}{\phantom{X}}_3{\phantom{X}}^{-}$) back out of the filtrate and adds $\mathrm{H}{\phantom{X}}^{+}$ and ammonia to it.

Henle's loop: two limbs, opposite rules. Going down, water can leave but electrolytes almost cannot, so the filtrate grows more concentrated. Coming up, the rule flips: water cannot cross the wall at all, while electrolytes leave it, actively or passively, so the filtrate grows more dilute. Reabsorption is least in the ascending limb, since no water can follow the salt it releases. Yet that salt is the point: it keeps the fluid between the tubules of the medulla, the medullary interstitium, at a high osmolarity.

Distal convoluted tubule (DCT): fine-tuning. How much $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ and water this segment takes back is conditional: more when the body is short of them, less when it has plenty. Like the PCT, it returns $\mathrm{H}\mathrm{C}\mathrm{O}{\phantom{X}}_3{\phantom{X}}^{-}$ to the blood, and it selectively adds $\mathrm{H}{\phantom{X}}^{+}$, $\mathrm{K}{\phantom{X}}^{+}$ and $\mathrm{N}\mathrm{H}{\phantom{X}}_3$ to the filtrate, keeping the blood's pH and its sodium–potassium balance on target.

Collecting duct: the final say. This last stage decides how concentrated the urine will be. The duct runs all the way from the cortex down into the inner medulla, through ever saltier surroundings, and when the body needs to save water, large volumes leave it here. It also lets a little urea escape into the medullary interstitium, which helps keep that region's osmolarity high, and like the DCT it selectively secretes $\mathrm{H}{\phantom{X}}^{+}$ and $\mathrm{K}{\phantom{X}}^{+}$.

16.4 Mechanism of Concentration of the Filtrate

A camel crossing the Thar, or you after an afternoon of cricket without a water bottle, must keep excreting waste while losing as little water as possible. Mammals manage this by making urine that is more concentrated than their blood. Henle's loop and the vasa recta make it possible.

The key word is countercurrent: two streams flowing side by side in opposite directions. Henle's loop has one, with filtrate running down one limb and up the other, and the vasa recta has another, with blood doing the same. Lying close together, these two hairpins hold the medullary interstitium at an osmolarity that climbs with depth, from about 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. NaCl and urea supply most of it.

How the gradient builds

A simple model shows why a hairpin can build such a steep gradient (Fig. 16.7). At each level, the ascending limb pushes salt out into the interstitium but does not let water follow, so the interstitium becomes saltier than the ascending fluid. The descending limb beside it then loses water until it matches the saltier interstitium. On its own this makes only a modest side-to-side difference. But the fluid keeps moving, carrying the saltier fluid deeper, where the next round of pumping adds to it. Round after round, small sideways differences stack up into a large top-to-bottom gradient, which is why the loop is often called a countercurrent multiplier.

Holding the gradient: NaCl and urea

Two flows of solute keep the gradient in place (Fig. 16.8). NaCl leaves the ascending limb of Henle's loop into the interstitium. Blood flowing down the descending limb of the vasa recta picks some of this salt up, and blood flowing back up the ascending limb of the vasa recta returns it to the interstitium, so the blood leaves the medulla without washing the salt away. Urea cycles too. Small amounts of urea leak from the deep collecting duct into the interstitium, pass into the thin part of the ascending limb, travel on with the filtrate to the collecting duct, and leak back out into the interstitium.

This transport of NaCl and urea, made possible by the hairpin arrangement of Henle's loop and the vasa recta, is the countercurrent mechanism. It pays off in the collecting duct. As the duct runs down through ever saltier interstitium, water is drawn out of it by osmosis, and the urine inside becomes steadily more concentrated. Human kidneys can make urine nearly four times as concentrated as the original filtrate, about 1200 mOsmol/L against 300.

16.5 Regulation of Kidney Function

The kidneys do not run on a fixed setting. Hormonal feedback loops run by the hypothalamus, the JGA and, to a lesser degree, the heart keep adjusting their work (Fig. 16.9).

ADH: saving water

Picture a long bus journey in May with no water stop. You sweat steadily and your body fluid shrinks. Osmoreceptors, which keep watch on blood volume, body-fluid volume and ion concentration, detect the change and stimulate the hypothalamus. The hormone that results is antidiuretic hormone (ADH), or vasopressin, which enters the blood from the neurohypophysis. Its target is the late part of the tubule, the DCT and collecting duct, where it lets more water return to the blood. Less water reaches the urine, so ADH works against diuresis, the passing of large volumes of dilute urine. Drink a bottle of water at the next stop and the loop runs the other way: the extra fluid switches the osmoreceptors off, ADH release falls and the kidneys let more water go.

ADH also narrows blood vessels. That raises blood pressure, which sends more blood into the glomerulus and pushes GFR up.

The renin–angiotensin mechanism

The JGA runs a relay, triggered by a drop in glomerular blood flow, glomerular blood pressure or GFR. First, JG cells release the enzyme renin into the blood. Second, renin acts on the plasma protein angiotensinogen to give angiotensin I. Third, angiotensin I is converted into angiotensin II, the active product. Angiotensin II then fixes the problem twice over. As a powerful vasoconstrictor, it narrows blood vessels and pushes glomerular pressure and GFR back up. It also signals the adrenal cortex to release aldosterone, so the distal parts of the tubule keep more $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ and, with it, water; the extra fluid raises blood pressure and GFR further. The whole relay is the renin–angiotensin mechanism.

ANF: the brake

The heart supplies the counterweight. If extra blood floods into the atria and stretches their walls, the walls release atrial natriuretic factor (ANF). ANF widens blood vessels (vasodilation), so blood pressure drops. Because it pulls pressure down while renin–angiotensin pushes it up, ANF keeps that mechanism in check.

16.6 Micturition

Think of a long train journey. Urine reaches the bladder all the time, and as the bladder fills, its wall stretches. Stretch receptors in the wall report the stretch to the central nervous system (CNS), which is why you feel the urge. Nothing happens, though, until you decide the moment has come, because the release is voluntary. The CNS then sends motor signals that do two things together: the smooth muscle of the bladder wall contracts and the urethral sphincter relaxes, so urine leaves through the urethra (Fig. 16.10). Passing urine is micturition, and the nerve circuit that runs it is the micturition reflex.

Table 16.3 gives the typical figures for a healthy adult: 1–1.5 L a day of a light yellow, watery, slightly acidic fluid with its own smell, carrying away 25–30 g of urea.

Doctors read urine almost like a report on the blood. Its make-up shifts with many conditions, so a routine urine test can flag kidney trouble and metabolic disorders alike. Sugar and ketone bodies in the urine (glycosuria and ketonuria) point to diabetes mellitus, in which blood glucose climbs so high that the tubules cannot reclaim all of it.

16.7 Role of Other Organs in Excretion

The kidneys do most of the work, but the lungs, the liver and the skin also remove wastes (Fig. 16.11).

Lungs. Your lungs breathe out large amounts of carbon dioxide, about 200 mL every minute, and they also lose a fair amount of water as vapour every day. On a cold morning in Shimla you can watch that vapour condense as you breathe out.

Liver. The largest gland in the body uses bile as a disposal route. Into it go the bile pigments bilirubin and biliverdin, cholesterol, vitamins, drugs and the breakdown products of steroid hormones. Bile drains into the intestine, so most of these leave the body with the faeces.

Skin. On a sticky monsoon afternoon you can taste salt on your upper lip. Sweat from the sweat glands is mostly water, with NaCl, some lactic acid and a trace of urea dissolved in it. Its real purpose is to cool the body surface as it evaporates, so the wastes it carries are a bonus. The skin's other secretion, sebum from the sebaceous glands, is oily: it takes some sterols, hydrocarbons and waxes out of the body and leaves a protective film on the skin.

16.8 Disorders of the Excretory System

If the kidneys stop working properly, urea accumulates in the blood. This condition, uremia, is very harmful and can lead to kidney failure. Urea can be cleared from such a patient's blood by haemodialysis (Fig. 16.12).

Haemodialysis works by diffusion across a membrane. Inside the dialyser, also called an artificial kidney, blood runs through a coiled tube of porous cellophane surrounded by dialysing fluid. The fluid is made to match plasma in every respect but one: it contains no nitrogenous wastes. Small molecules cross the cellophane down their concentration gradients, so urea and the other nitrogenous wastes, plentiful in the blood and absent outside, move out freely. Getting the blood there and back takes two extra steps. Blood drawn from a convenient artery first receives heparin, an anticoagulant, so it does not clot in the tubing. After cleaning it receives anti-heparin to restore normal clotting and is pumped back into a vein. For people with uremia around the world, this has been life-saving on a very large scale.

Why must the dialysing fluid match plasma so closely? If it lacked glucose or salts, those would leak out of the blood as well. By matching everything except the wastes, the fluid makes sure that only the wastes have a gradient to move down.

When the kidneys fail for good, dialysis can only buy time. The lasting remedy is kidney transplantation, placing a healthy, working kidney from a donor into the patient. The recipient's immune system may attack the new organ as foreign, so doctors prefer a donor who is a close relative; this lowers the chance of rejection. Modern surgery and medicines have made this complicated operation far more successful than it once was.

Before the summary, rebuild the chapter's cause-and-effect chains yourself: the route of filtrate, the ADH and renin–angiotensin loops, the ANF brake, the micturition reflex and the dialysis circuit (Fig. 16.13).