Excretory Products and their Elimination

1. Chapter at a Glance

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl⁻, phosphate and sulphate, either by metabolic activities or by excess ingestion. These substances must be removed totally or partially. This chapter deals with the elimination of the common nitrogenous wastes — ammonia, urea and uric acid. It covers the modes of excretion, excretory structures in animals, the human excretory system, the structure of the kidney and nephron, the formation of urine (filtration, reabsorption and secretion), the counter-current mechanism, regulation of kidney function, micturition, the role of other organs, and disorders. NEET asks about two questions every year, usually on GFR, the functions of nephron parts, the counter-current mechanism and hormonal regulation (ADH, renin–angiotensin, ANF).

2. Modes of Nitrogenous Excretion

Ammonia is the most toxic form and requires a large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water.

3. Excretory Structures in Animals

A survey of the animal kingdom presents a variety of excretory structures. In most invertebrates, these structures are simple tubular forms, whereas vertebrates have complex tubular organs called kidneys.

4. Human Excretory System

• It 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 vertebra, close to the dorsal inner wall of the abdominal cavity.
• Each kidney of an adult human measures 10–12 cm in length, 5–7 cm in width and 2–3 cm in thickness, with an average weight of 120–170 g.
• Towards the centre of the inner concave surface of the kidney 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 the kidney there are two zones: an outer cortex and an inner medulla. The medulla is divided into a few conical masses, the medullary pyramids, projecting into the calyces. The cortex extends in between the medullary pyramids as renal columns called columns of Bertini.

The nephron

Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units. 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 along 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, which has 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 collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.
• The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney, whereas the loop of Henle dips into the medulla.
• In the majority of nephrons, the loop of Henle is too short and extends only very little into the medulla; these are cortical nephrons. In some nephrons, the loop is very long and runs deep into the medulla; these are 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 the Henle's loop, forming a U-shaped vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.

5. Urine Formation

Urine formation involves three main processes: glomerular filtration, reabsorption and secretion, which take place in different parts of the nephron.

Glomerular filtration

• On average, 1100–1200 mL of blood is filtered by the kidneys per minute, which constitutes 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, and a basement membrane between these two.
• The epithelial cells of Bowman's capsule, called podocytes, are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores.
• Blood is filtered so finely through these membranes that almost all the constituents of the plasma except the proteins pass onto the lumen of Bowman's capsule. Therefore it is considered a process of ultrafiltration.
• 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.

Reabsorption

• A comparison of the volume of filtrate formed per day (180 litres) with that of the urine released (1.5 litres) suggests that nearly 99% of the filtrate has to be reabsorbed by the renal tubules.
• 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 and Na⁺ in the filtrate are reabsorbed actively, whereas nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron.

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 the ionic and acid–base balance of body fluids.

6. Function of the Tubules

7. Mechanism of Concentration of the Filtrate

Mammals have the ability to produce concentrated urine. The Henle's loop and vasa recta play a significant role in this.

• 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 the Henle's loop and vasa recta, as well as the counter current in them, helps in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e. 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, which is transported back to the interstitium by the collecting tubule.
• The above-described 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.
• Presence of such an interstitial 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 as concentrated as the initial filtrate formed.

8. Regulation of Kidney Function

The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, the JGA and, to a certain extent, the heart.

• ADH (vasopressin): 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) 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, which can increase the glomerular blood flow and thereby GFR.
• Renin–angiotensin mechanism: 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, which also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the renin–angiotensin mechanism.
• ANF (atrial natriuretic factor): an increase in blood flow to the atria of the heart can cause the release of ANF. ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. The ANF mechanism therefore acts as a check on the renin–angiotensin mechanism.

9. Micturition

• Urine formed by the nephrons is ultimately carried to the urinary bladder, where it is stored until 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 the CNS. The CNS passes on motor messages to initiate the contraction of the 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 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 average, 25–30 g of urea is excreted out per day.
• Analysis of urine helps in the 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 is indicative of diabetes mellitus.

10. Role of Other Organs in Excretion

• Lungs remove large amounts of CO₂ (about 200 mL per minute) and also significant quantities of water every day.
• Liver, the largest gland in our body, secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out along with digestive wastes.
• Skin: the sweat and sebaceous glands can eliminate certain substances through their secretions. Sweat produced by the sweat glands is a watery fluid containing NaCl, small amounts of urea, lactic acid and so on. Though the primary function of sweat is to facilitate a cooling effect on the body surface, it also helps in the removal of some of the wastes. Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum, which provides a protective oily covering for the skin.
• Small amounts of nitrogenous wastes can be eliminated through saliva too.

11. Disorders of the Excretory System

12. Master Summary Table

13. Mind Map

14. Tips and Tricks

15. Important One-liners

• Ammonia is the most toxic nitrogenous waste; uric acid the least.
• Urea is synthesised in the liver.
• Flame cells are excretory structures of flatworms.
• Malpighian tubules are excretory organs of insects.
• Each human kidney weighs 120–170 g and has about one million nephrons.
• Columns of Bertini are cortical extensions between medullary pyramids.
• Podocytes form filtration slits in Bowman's capsule.
• Maximum reabsorption occurs in the PCT.
• Human kidneys can produce urine about four times as concentrated as the initial filtrate.
• Heparin is added to blood before haemodialysis.
• Presence of glucose and ketone bodies in urine indicates diabetes mellitus.

16. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

17. Quick Revision Summary