The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR).

Two points worth adding. First, the filtrate is produced by ultra filtration at the glomerulus, driven by the glomerular capillary blood pressure, and contains almost all the constituents of the plasma except the proteins. Second, compare 180 litres filtered with the 1.5 litres of urine actually released, and you see at once that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules.

The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. One such efficient mechanism is carried out by juxta glomerular apparatus (JGA).

What the JGA is

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.

How it works

A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular blood flow and thereby the GFR back to normal.

The fuller chain, from Section 16.5, is this:

1. A fall in glomerular blood flow / glomerular blood pressure / GFR can activate the JG cells to release renin.
2. Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II.
3. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR.
4. Angiotensin II also activates the adrenal cortex to release Aldosterone, and 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.

Only (b) is false. It is the one statement deliberately inverted, and it tests whether you have read the meaning of “antidiuretic”.

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

What makes it “counter current”

• 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 gradient it maintains

The proximity between the Henle's loop and vasa recta, as well as the counter current in them, help 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.

How the two solutes are recycled

• 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.
• 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.

Why it matters

Presence of such 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 concentrated than the initial filtrate formed.

Other than the kidneys, lungs, liver and skin also help in the elimination of excretory wastes.

Lungs

Our lungs remove large amounts of CO₂ (approximately 200 mL/minute) and also significant quantities of water every day.

Liver

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.

Remember also, from Part 1, that the liver has a second and even more fundamental excretory role in a ureotelic animal: ammonia produced by metabolism is converted into urea in the liver and released into the blood for the kidneys to filter. Without the liver there would be no urea to excrete.

Skin

The sweat and sebaceous glands in the skin 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, etc. 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 mentioned above.
• Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin.

And one more route: small amounts of nitrogenous wastes could be eliminated through saliva too.

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).

The sequence

1. This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
2. In response, the stretch receptors on the walls of the bladder send signals to the CNS.
3. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, causing the release of urine.

Facts about the urine released

• 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 gm of urea is excreted out per day.
• Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney — for example glycosuria and ketonuria indicate diabetes mellitus.

Osmoregulation means the regulation of the osmotic concentration of the body fluids — that is, maintaining a proper balance of water and electrolytes in the body, so that the concentration of body fluids is kept within the narrow range the cells can tolerate.

Where the chapter uses the term

• Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation.
• Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance.
• Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation.

How the human kidney does it

• By adjusting how much water is reabsorbed — conditional reabsorption of Na⁺ and water in the DCT and large amounts of water in the collecting duct, under the control of ADH.
• By adjusting how much Na⁺ is retained, under the control of aldosterone.
• By tubular secretion of H⁺, K⁺ and ammonia, which helps in the maintenance of ionic and acid base balance of body fluids.
• By producing urine of variable concentration, using the counter current mechanism — an excellent mechanism of conservation of water.

Because ammonia excretion costs far too much water for an animal living on land.

The reasoning, step by step

• 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.
• An aquatic animal has water to spare. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces as ammonium ions, and is diluted instantly by the surrounding water. Kidneys do not play any significant role in its removal.
• A terrestrial animal has no such supply. Water must be conserved, so a waste that can be excreted in a small volume is essential. As the chapter puts it: terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water.

The two solutions adopted

• Ureotelism. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood, which is filtered and excreted out by the kidneys. Urea is far less toxic than ammonia, so it can be carried in the blood and excreted in a moderate volume of water.
• Uricotelism. Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water. This is the most water-thrifty solution of all, which is why it is found in animals that fly, live in dry places, or must seal their wastes inside an egg.

A useful extra point: even marine fishes are ureotelic although they live in water, because sea water is more concentrated than their body fluids, so they are constantly losing water by osmosis. The real principle is therefore not “land versus water” but availability of water — exactly as the chapter's summary states.

The JGA is the kidney's own device for regulating its filtration rate, and it does so without any instruction from outside.

What it is

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.

Its significance

• Autoregulation of GFR. The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. One such efficient mechanism is carried out by juxta glomerular apparatus. A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular blood flow and thereby the GFR back to normal.
• It starts the renin-angiotensin mechanism. Renin 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.
• It brings in a hormone from another gland. Angiotensin II also activates the adrenal cortex to release Aldosterone, and 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.
• It helps regulate blood pressure for the body as a whole, not only for the kidney — since angiotensin II constricts vessels everywhere and aldosterone adds volume to the circulation.
• It plays a complex regulatory role, as the chapter itself puts it, being one of the three systems — with the hypothalamus and, to a certain extent, the heart — that monitor and regulate kidney function.

Note on (a): the word chordate is the whole point of the question. Planaria and rotifers also have flame cells but are not chordates; Amphioxus is the only chordate in the list.

A note on (a). Getting the order right matters. The descending limb lets water out, so the filtrate concentrates as it moves down; the ascending limb lets electrolytes out but not water, so the filtrate gets diluted as it rises. Reversing the two makes the counter current mechanism impossible to explain.