Excretion and Homeostasis

Lessons

Senior 5 · Biology

Excretion and Homeostasis

4 lessons

Lesson 4 of 4

The kidney, the nephron and osmoregulation

25 min

180 litres a day

Every day your two kidneys filter about 180 dm³ of fluid out of your blood, more than the volume of a large oil drum, yet you pass only about 1.5 dm³ of urine. Almost everything is taken back. The kidneys do two jobs at once: they excrete urea and other wastes, and they osmoregulate, adjusting how much water and salt leave so that the water potential of the blood stays constant. Each kidney contains about a million tiny tubes called nephrons, and this lesson follows the fluid through one of them.

Definition

Osmoregulation: the control of the water potential of the body fluids, by adjusting the amounts of water and solutes in them.

Nephron: the functional unit of the kidney: a tube that filters blood and modifies the filtrate to form urine.

Ultrafiltration: filtration under high pressure, in the glomerulus, of small molecules and water out of the blood into the Bowman's capsule.

Selective reabsorption: the taking back into the blood of useful substances from the filtrate, mainly in the proximal tubule.

Antidiuretic hormone (ADH): a hormone, made in the hypothalamus and released from the posterior pituitary gland, that makes the collecting ducts more permeable to water.

Kidney and nephron

Each kidney receives blood from a renal artery and returns it through a renal vein; urine leaves through the ureter to the bladder. A cut kidney shows an outer cortex, an inner medulla arranged in cone-shaped pyramids, and a central cavity, the pelvis. The parts of each nephron lie in a fixed pattern across these regions (Figure 6).

1. Ultrafiltration in the glomerulus

The afferent arteriole that brings blood to the glomerulus is wider than the efferent arteriole that takes it away, so blood pressure in the glomerular capillaries is high. Water and small solutes are forced out through three layers:

the capillary endothelium, which has many pores;
the basement membrane, a mesh of protein fibres that acts as the actual filter, holding back blood cells and most plasma proteins;
the podocytes of the capsule wall, cells with foot-like processes and slits between them.

The filtrate therefore contains water, glucose, amino acids, urea, salts and vitamins, but almost no protein and no cells. The rate at which filtrate is made is the glomerular filtration rate (GFR), about 125 cm³ per minute in a healthy adult.

Substance	Plasma / g dm⁻³	Filtrate / g dm⁻³	Urine / g dm⁻³
Proteins	80	0.05	0
Glucose	1.0	1.0	0
Amino acids	0.5	0.5	0
Urea	0.3	0.3	20
Inorganic ions	7.2	7.2	15

(Typical textbook values; urine values vary a great deal with diet and water intake.)

A section through the cortex of a kidney, stained and seen with a light microscope. 1 marks a glomerulus, a knot of capillaries inside a Bowman's capsule; the double-headed arrow spans its width. 2 marks proximal convoluted tubules, whose thick walls of cube-shaped cells leave a narrow, fuzzy-edged space (the microvilli). 3 marks distal convoluted tubules, with lower walls and wider, clearer spaces. Photo: Uwe Gille, CC BY-SA 3.0, via Wikimedia Commons

2. Selective reabsorption in the proximal tubule

The proximal tubule takes back all the glucose and amino acids, most of the salts, and about two-thirds of the water. Its cells are built for it: microvilli on the side facing the filtrate give a large surface; there are many mitochondria to supply ATP; and the cells are close to blood capillaries.

Sodium–potassium pumps on the blood side of each cell actively pump sodium ions out into the tissue fluid, keeping the sodium concentration inside the cell low.
Sodium ions then diffuse from the filtrate into the cell through co-transporter proteins, each carrying a glucose or amino acid molecule with it (as in sucrose loading in Unit 7).
Glucose and amino acids diffuse out of the cell into the blood; water follows the solutes by osmosis.

The carriers can only work so fast. If blood glucose is very high, as in untreated diabetes, more glucose is filtered than they can reabsorb, and glucose appears in the urine.

3. The loop of Henle: making the medulla salty

The loop of Henle acts as a counter-current multiplier. The thick ascending limb actively pumps sodium and chloride ions out into the tissue fluid of the medulla, but its walls are impermeable to water. The descending limb is permeable to water, so water leaves it by osmosis into the salty medulla. Because the fluid flows down one limb and up the other, the effect builds up, and the deep medulla becomes very concentrated: it has a very low water potential. The longer the loop, the more concentrated the medulla, and the more concentrated the urine an animal can make.

4. Distal tubule and collecting duct: fine control by ADH

The distal tubule adjusts the salt and pH of the blood. The collecting duct runs down through the salty medulla. If its walls are permeable to water, water leaves it by osmosis, and a small volume of concentrated urine is made. Their permeability is controlled by ADH.

Osmoregulation by ADH: a negative feedback loop

Step	Blood too concentrated (e.g. after sweating)	Blood too dilute (e.g. after drinking a lot)
Receptor	Osmoreceptors in the hypothalamus lose water and shrink	Osmoreceptors swell
ADH	More ADH released from the posterior pituitary	Less ADH released
Collecting duct	ADH binds receptors; vesicles containing water channels (aquaporins) fuse with the membrane; walls become more permeable	Aquaporins are removed from the membrane; walls less permeable
Urine	Small volume, concentrated	Large volume, dilute
Result	Water potential of the blood rises back to normal	Water potential of the blood falls back to normal

Alcohol reduces the release of ADH, which is why drinking it produces large volumes of dilute urine and can lead to dehydration.

Osmoregulation in other animals

Freshwater bony fish, such as tilapia, gain water by osmosis all the time. They drink almost nothing, make large volumes of very dilute urine, and actively take up salts through their gills.
Marine bony fish lose water to the sea. They drink sea water, make very little urine, and actively excrete salt through their gills.
Desert mammals, such as the kangaroo rat, have very long loops of Henle and make extremely concentrated urine; they may never need to drink, living on water from food and respiration.

Worked example 1: how much is reabsorbed?

A person's glomerular filtration rate is $125\,\mathrm{cm^3\,min^{-1}}$ , and they pass 1.5 dm³ of urine a day. Calculate (a) the volume of filtrate made per day, (b) the percentage of the filtrate that is reabsorbed.

(a) $125 \times 60 \times 24 = 180\,000\,\mathrm{cm^3} = 180\,\mathrm{dm^3}$ per day.
(b) Reabsorbed: $180 - 1.5 = 178.5\,\mathrm{dm^3}$ . $\frac{178.5}{180} \times 100 = 99.2\%$

Meaning: less than 1% of the filtrate becomes urine. A small change in reabsorption therefore makes a large change in urine volume, which is why ADH is so effective.

Worked example 2: concentrating urea

Use the table. (a) By how many times is urea more concentrated in urine than in the filtrate? (b) Explain how this happens, given that urea is not actively secreted into the tubule.

(a) $\frac{20}{0.3} \approx 67 \text{ times}$
(b) About 99% of the water in the filtrate is reabsorbed, but much of the urea stays in the tubule. Removing water from around the urea concentrates it, as boiling down a sauce concentrates its salt. (Some urea is reabsorbed passively, so the factor is smaller than the 120 times that removing 99.2% of the water would give.)

Warning

Common mistakes

"ADH makes more urine." ADH stands for antidiuretic hormone: it makes less, more concentrated urine.
"Glucose is filtered out and stays in the urine." Glucose is filtered, but all of it is normally reabsorbed in the proximal tubule.
"The glomerulus filters by active transport." Ultrafiltration is driven by blood pressure; reabsorption is what uses ATP.
Mixing up the limbs. Descending: permeable to water. Ascending: pumps out salt, impermeable to water.

Miss Estruch Biology: "Structure of the nephron. Ultrafiltration and selective reabsorption in the kidney". Watch for the three layers of the filter and for the co-transport of glucose with sodium ions in the proximal tubule; compare them with Figure 6.

Tip

Memory hook for the nephron, in order: "Good Bananas Please Little Donkeys Carefully": Glomerulus, Bowman's capsule, Proximal tubule, Loop of Henle, Distal tubule, Collecting duct. Then attach one job to each: filter, collect, reabsorb, make the medulla salty, adjust, concentrate.

✦

Practice: the kidney and osmoregulation

Name the three layers that the filtrate passes through in the glomerulus, and say which acts as the filter.
Explain why the blood pressure in the glomerular capillaries is high.
Explain why there is almost no protein in the glomerular filtrate.
Describe two features of proximal tubule cells and say how each helps reabsorption.
A person's GFR is 110 cm³ min⁻¹ and their urine output is 1.2 dm³ a day. Calculate the percentage of filtrate reabsorbed.
Net filtration pressure = blood pressure in the glomerulus − (pressure of fluid in the capsule + solute potential effect of plasma proteins). Calculate it when these are 7.3 kPa, 2.0 kPa and 4.0 kPa.
Explain how a long loop of Henle allows a desert mammal to make very concentrated urine.
After a long football match on a hot day, a player's urine is dark and small in volume. Explain this using ADH and negative feedback.

Show answersHide answers

Capillary endothelium, basement membrane, podocytes; the basement membrane is the filter.
The afferent arteriole is wider than the efferent arteriole, so blood enters faster than it can leave, and the pressure builds up.
Plasma proteins are too large to pass through the basement membrane.
Any two: microvilli give a large surface for absorption; many mitochondria supply ATP for active transport; co-transporter proteins carry glucose with sodium; close contact with capillaries keeps the diffusion path short.
Filtrate $= 110 \times 60 \times 24 = 158\,400\,\mathrm{cm^3} = 158.4\,\mathrm{dm^3}$ ; reabsorbed $= \frac{158.4 - 1.2}{158.4} \times 100 = 99.2\%$ .
$7.3 - (2.0 + 4.0) = 1.3\,\mathrm{kPa}$ .
The longer the ascending limb, the more sodium and chloride it pumps into the medulla, so the medulla becomes more concentrated; water then leaves the collecting duct by osmosis down a steeper gradient, so the urine can become more concentrated.
Sweating lost water, so the blood's water potential fell; osmoreceptors in the hypothalamus detected it and more ADH was released from the posterior pituitary; ADH made the collecting ducts more permeable to water, so more water was reabsorbed, giving a small volume of concentrated urine. This restores the blood's water potential: negative feedback.

Key points

Glomerulus: ultrafiltration under high pressure through endothelium, basement membrane (the filter) and podocytes.
Proximal tubule: selective reabsorption of all glucose and amino acids, most salts and water, using microvilli, mitochondria and Na⁺ co-transport.
Loop of Henle: counter-current multiplier that makes the medulla salty.
ADH from the posterior pituitary inserts aquaporins into the collecting duct: less, more concentrated urine.

1180 litres a day

2Kidney and nephron

31. Ultrafiltration in the glomerulus

42. Selective reabsorption in the proximal tubule

53. The loop of Henle: making the medulla salty

64. Distal tubule and collecting duct: fine control by ADH

7Osmoregulation by ADH: a negative feedback loop

8Osmoregulation in other animals