Excretion

1. What This Chapter Covers

There is no factory that can make a product without generating waste, and the body is a living cellular factory. While performing metabolic activities, wastes are generated at regular intervals from the bodies of all living organisms.

The book opens with the questions it intends to answer: where the wastes are produced, how they are produced, what substances are in them, and whether the composition varies in the same organism in different situations.

Organisms need energy to survive and to carry out metabolic activities, which are either anabolic or catabolic. Different products are generated by different activities, and the book's Table 1 asks you to fill in what photosynthesis, respiration and digestion each produce.

Some of those products would harm the organism if kept, so they are removed from the body or packed and stored in another form. Excretion is the term for all the biological processes involved in separating and removing wastes or non-useful products from the body. In Latin, ex means out and crenere means shift.

The index allots the chapter 10 periods in August and runs it from page 78 to page 99.

2. What Counts as a Waste (Textbook 4.1)

Many reactions take place during metabolism. Useful substances and energy are produced, but at the same time toxic wastes may be produced, water content may increase, and the ionic balance, homeostasis, may be disturbed.

The waste products include carbon dioxide, water, nitrogenous compounds like ammonia, urea and uric acid, bile pigments and excess salts. Of all of them, ammonia is the most poisonous.

Reading a real report

The book prints two actual laboratory reports and asks you to compare them — which substances appear in blood, which in urine, which in both, and which are above the normal level.

For the 24-hour test, urine is collected for a whole day and a 100-150 ml sample is tested. Reading the two tables together is the exercise: creatinine and uric acid are above range in both, urea is normal in blood but raised in urine, and calcium is normal in blood but raised in urine.

3. The Excretory System (Textbook 4.2)

In human beings excretion occurs mainly through the urinary system: a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.

Lab Activity — a sheep or goat kidney

Wash the kidney thoroughly before bringing it to the lab so the blood drains completely. Put it in the tray and note the external features, then take a longitudinal section with a sharp blade under your teacher's guidance and observe the internal structure.

The observations to record are the shape and colour of the kidney, whether any structure is attached to the upper portion, the colour of the outer part, where the dark brown portion lies in the L.S., and how many tubes come out from the fissure, the hilum. Wash your hands with antibacterial lotion afterwards.

The kidneys (4.2.1)

There is a pair of bean-shaped, reddish brown kidneys in the abdominal cavity, attached to the dorsal body wall, one on either side of the backbone.

The right kidney sits slightly lower than the left, and the book supplies the reason: the right side of the abdominal cavity is occupied by the liver.

Each kidney is about 10 cm long, 5-6 cm broad and 4 cm thick, convex on the outer side and concave on the inner. The inner side has a fissure, the hilum, for the entry of the renal artery, the exit of the renal vein and the ureter.

The renal artery brings oxygenated blood loaded with waste products from all over the body to the kidney; the renal vein sends out deoxygenated blood from it.

Internal structure (4.2.2)

A longitudinal section shows two distinct regions: a dark brown outer zone, the cortex, and a pale inner zone, the medulla.

Each kidney has more than a million microscopic renal tubules — the book gives 1.3 to 1.8 million. These functional units are the nephrons, also called uriniferous tubules.

After the age of 40 the number of functioning nephrons usually decreases by about 10 per cent every 10 years.

4. Structure of a Nephron (Textbook 4.2.3)

The Malpighian body

At one end of a nephron is a blind, cup-shaped broad structure, the Bowman's capsule, containing a network of fine blood capillaries called the glomerulus. Together they make the Malpighian body.

The afferent arteriole — an arteriole being the finer branch of an artery — enters the Bowman's capsule, divides into the capillary network, and leaves as the efferent arteriole, which has a smaller diameter.

That difference in diameter is the whole mechanism. Because the outgoing vessel is narrower, pressure builds in the glomerulus, and the glomerulus therefore works as a filtration unit.

The Bowman's capsule, which accommodates one glomerulus, is lined by a single layer of squamous epithelial cells called podocytes, with fine pores between them to let filtered material through.

The renal tubule

The tubule has three parts: the Proximal Convoluted Tubule (PCT), the U-shaped loop of Henle, and the Distal Convoluted Tubule (DCT).

Distal convoluted tubules open into a collecting tube. Collecting tubules form pyramids and calyces, which open into the pelvis, and the pelvis leads into the ureter.

All parts of the renal tubule are surrounded by a network of peritubular capillaries formed from the efferent arteriole. These join to form a renal venule, which joins others to form the renal vein.

Every question on urine formation is really asking where in this chain something happens. Reabsorption is the one stage a dialysis machine cannot reproduce, and that single fact answers the standard "how does an artificial kidney differ from a real one" question.

5. Mechanism of Urine Formation (Textbook 4.3)

Stage 1 — glomerular filtration. Blood from the renal artery flows into the afferent arteriole of the glomerulus and is filtered under high pressure. Waste materials, along with some water and some useful substances, are filtered out into the Bowman's capsule. The filtrate is called primary urine.

Stage 2 — tubular reabsorption. Primary urine is almost the same as blood in chemical composition except that it contains no blood cells. The peritubular capillaries around Henle's loop reabsorb essential substances and excess water from it.

Stage 3 — tubular secretion. After reabsorption, the urine travels along the loop of Henle, and waste materials that were left unfiltered in the blood during glomerular filtration are secreted into the loop from the peritubular capillaries around it.

Stage 4 — formation of concentrated urine. Urine reaching the collecting tubule is further concentrated in the presence of the hormone vasopressin. A deficiency of vasopressin causes excessive dilute urination, a condition called diabetes insipidus.

6. The Rest of the System (Textbook 4.3.1 to 4.5)

Ureters

A pair of whitish, narrow, distensible, muscular tubes about 30 cm long. Each arises from the hilum of a kidney, runs downward and obliquely, and opens into the urinary bladder. Urine moves along the ureter by peristalsis.

Urinary bladder

A median, pear-shaped, distensible sac in the pelvic region, on the ventral side of the rectum. It stores 300-800 ml of urine temporarily, brought by the two ureters.

Urethra

A tube taking urine from the bladder to the outside. The opening of the bladder into the urethra is guarded by a ring of muscles, a sphincter. The urethra is about 4 cm long in females, opening into the vestibule, and about 20 cm long in males. Its opening is separate in females but shared with the reproductive tract in males, making a urino-genital duct.

Micturition (4.4)

There are two sets of circular sphincter muscles in the bladder. While the bladder is filling, both are constricted and the exit is closed. As pressure rises the bladder walls are stretched, which triggers an automatic reflex relaxing the upper sphincter.

The lower sphincter is under the control of the brain, so urination is voluntary. Very young children do not possess this control; it is gradually learned.

The bladder can store a maximum of 700-800 ml, and the urge to urinate arises at 300-400 ml. The stretched bladder stimulates nerve endings, which signal the brain, and the urge eventually becomes painful, leading to voluntary micturition. The total urine excreted per day is about 1.6 to 1.8 litres, rising with larger fluid intake and falling with less.

Composition of urine (4.5)

Urine is a light yellowish, transparent fluid. Its colour comes from urochrome, formed by the breakdown of haemoglobin.

Composition varies considerably. A protein-rich diet gives more urea, because proteins are de-aminated in the liver with subsequent formation of urea. Even sugar can appear in a normal person after a heavy intake of carbohydrates, and a large intake of liquids raises the volume of water in the blood so urine is passed more often.

Normal urine is 96 per cent water, 2.5 per cent organic substances — urea, uric acid, creatine, creatinine, water-soluble vitamins, hormones, oxalates — and 1.5 per cent inorganic solutes such as sodium, chloride, phosphate, sulphate, magnesium, calcium and iodine.

It is acidic at first, at pH 6.0, and gradually becomes alkaline as urea breaks down to form ammonia.

7. When Kidneys Fail (Textbook 4.6, 4.7)

Complete and irreversible kidney failure is End Stage Renal Disease (ESRD). If the kidneys stop working the body fills with extra water and waste products, a condition called uremia. The hands and feet may swell, and the person feels tired and weak because the blood is not being purified.

Dialysis (4.6)

A dialysis machine filters the blood when both kidneys are damaged, and the process is haemodialysis.

Blood is taken from a main artery, mixed with an anticoagulant such as heparin, and pumped into an apparatus called the dialyzer. Inside, blood flows through channels or tubes made of cellophane, embedded in the dialysing fluid. A thin membrane separates blood from fluid.

The dialysing fluid has the same composition as plasma except that it contains no nitrogenous wastes. Because those wastes are absent outside, they move freely out of the blood, cleaning it.

This resembles the kidney's work but differs in one crucial way: there is no reabsorption. The cleaned blood is pumped back through a vein after adding anti-heparin. Each session lasts 3 to 6 hours.

Kidney transplantation (4.7)

The best long-term solution for acute renal failure is transplantation. A functioning kidney is taken from a donor, preferably a close relative, and must be a good match to minimise the chance of rejection by the recipient's immune system. Modern clinical procedures have increased the success rate.

Organs are also collected from brain dead patients and transplanted, which is organ donation. The book's annexure notes that transplantation of organs from brain dead patients to another person is called cadaver transplantation, that in Hyderabad the facility exists in two government hospitals, NIMS and Osmania, and in more than ten corporate hospitals, and that cornea, kidney, liver, heart, lungs, skin, bone, intestines and pancreas can all be transplanted this way.

8. Accessory Excretory Organs (Textbook 4.8)

The kidney is the chief excretory organ, but the lungs, skin, liver and large intestine all carry out excretion as a secondary function.

Lungs remove carbon dioxide and water vapour in the respiratory process.

Skin carries a large number of sweat glands richly supplied with blood capillaries, from which they extract sweat and some metabolic wastes from the blood. It sends out plenty of water and a small amount of salts as sweat. Sebaceous glands in the skin secrete sebum, containing waxes, sterols, hydrocarbons and fatty acids.

Liver. Red blood cells live 120 days and are destroyed in the liver, which produces bile pigments — bilirubin, biliverdin and urochrome — as metabolic wastes of the haemoglobin of dead RBCs. Urochrome is eliminated through urine; biliverdin and bilirubin are excreted through bile, along with cholesterol, steroid hormones, extra drugs, vitamins and alkaline salts. The liver also plays a key role in urea formation.

Large intestine. Excess salts of calcium, magnesium and iron are separated by the epithelial cells of the colon and eliminated with the faeces.

Small amounts of nitrogenous waste also leave through saliva and tears.

9. Excretion in Other Organisms (Textbook 4.9)

Unicellular organisms have no specific excretory organs and remove wastes by simple diffusion from the body surface into the surrounding water.

Fresh water organisms like Amoeba and Paramoecium have an osmoregulatory organelle, the contractile vacuole. It collects excess water and waste from the cell, moves slowly through the cytoplasm to the surface, and bursts to release its contents outside. Even in these organisms, the main excretion is still by diffusion through the body surface.

In multicellular organisms the structural and functional complexity of excretory organs increases from sponges to humans.

Excretory organs are first seen in Platyhelminthes, as flame cells.

10. Excretion in Plants (Textbook 4.10)

Plants have no specific organs to excrete nitrogenous wastes. They break waste substances down much more slowly than animals, so production and accumulation are also much slower.

Green plants in darkness, and plants that have no chlorophyll, produce carbon dioxide and water as respiratory wastes. Oxygen generated during photosynthesis leaves through the stomata of leaves and lenticels of stems.

Plants get rid of excess water by transpiration and guttation. Waste products may be stored in leaves, bark and fruits, and are disposed of when those ripened parts fall from the tree. Waste stored in fruits takes the form of solid bodies called raphides.

Several compounds are synthesized by plants for their own use, especially for defence. Many store waste materials in roots, leaves and seeds as protection against herbivores; most of these chemicals taste unpleasant, so herbivores avoid such plants, and some are toxic enough to kill an animal that eats them.

Some plants secrete chemicals when injured, which seal the wound and help recovery. Others release attractants that help with pollination, seed dispersal or nutrition — plants with root nodules secrete chemicals to attract Rhizobium bacteria into the soil around the roots and give them shelter, forming a symbiotic relationship.

Primary and secondary metabolites

The biochemical substances produced in plants are of two types. Primary metabolites are the materials needed for normal growth and development: carbohydrates, fats and proteins.

Secondary metabolites are those not required for normal growth and development: alkaloids, tannins, resins, gums and latex. Plants make them for their own use, but we have found uses of our own for them, and they are generally coloured and fragrant.

Alkaloids (4.10.1)

These are nitrogenous byproducts and poisonous, stored in different parts of the plant.

Tannins, resins, gums and latex (4.10.2 to 4.10.5)

Tannins are carbon compounds, stored in different parts of the plant and deep brown in colour. They are used in tanning leather and in medicines, from plants such as Acacia and Cassia.

Resins occur mostly in gymnosperms, in specialized channels called resin passages, and are used in varnishes; the example is Pinus.

Gums ooze from plants like neem and Acacia as a sticky substance when branches and stems are cut. Gum swells by absorbing water and helps heal the damaged part. Gums are economically valuable as adhesives and binding agents and in preparing medicines and food.

Latex is a sticky, milky white substance secreted by some plants and stored in laticifers, which are cells and vessels. Rubber is prepared from the latex of Hevea brasiliensis.

Do roots secrete?

The botanist Brugman proved by experiment that roots not only absorb water and minerals but also secrete substances back into the soil.

The book's example is the apple: where the crop is grown continuously for four or five years in the same soil, it fails to produce fruits, and will not yield properly even with a lot of fertiliser. The question it leaves you with is whether the fall in yield is related to those root secretions.

Chewing gum has historical evidence going back 5000 years, and modern chewing gum was originally made of natural latex from plants such as chicle and sapota. Pollen grains that enter our body cause allergy because of the nitrogenous substances in them, producing skin allergy and asthma; the book's example is Parthenium.

11. Excretion versus Secretion (Textbook 4.11)

The two are alike in that both involve the transport and elimination of unwanted components. Excretion is the removal of materials from a living being, while secretion is the movement of material from one point to another. On the book's own reckoning, secretion is active and excretion is passive in nature.

Humans excrete tears, urine, carbon dioxide and sweat; they secrete enzymes, hormones and saliva. Plants excrete through the roots into the surroundings and by shedding leaves, fruits and bark; their secretions take the form of latex, resins and gums.

Key words from the chapter

Creatinine, peritubular network, podocyte, afferent arteriole, efferent arteriole, glomerulus, proximal convoluted tubule, distal convoluted tubule, loop of Henle, calyces, micturition, urochrome, dialyser, haemodialysis, anticoagulant, alkaloids, biodiesel.

12. Summary

Metabolism produces many products, and the process of separating and removing the toxic wastes among them from the body is excretion.

The human excretory system comprises a pair of kidneys, a pair of ureters, the urinary bladder and the urethra. Each kidney holds approximately 1.3 to 1.8 million uriniferous tubules, or nephrons, which are the structural and functional units of the kidney.

A nephron comprises the Bowman's capsule, the glomerulus, the proximal convoluted tubule, Henle's loop, the distal convoluted tubule and the collecting tubule.

Urine formation involves four stages: glomerular filtration, tubular reabsorption, tubular secretion, and the formation of concentrated urine.

Kidneys remove nitrogenous waste, and maintain water balance — osmoregulation — as well as salt concentration, pH and blood pressure.

A dialysis machine is an artificial kidney that filters the blood to remove metabolic wastes, and kidney transplantation is the permanent solution for renal failure patients.

Different animals have different excretory organs: contractile vacuole in Amoeba, flame cells in Platyhelminthes, nephridia in annelids, Malpighian tubules in arthropods, and kidneys in reptiles, birds and mammals.

There are no special organs for excretion in plants. Plants store waste materials in leaves, bark, roots and seeds, which fall off after ripening.

Plant metabolites are of two types: primary metabolites such as proteins, carbohydrates and fats, and secondary metabolites such as alkaloids, gums, tannins, latex and resins, all of which have economic importance.