The endocrine system, hormones, the kidney and the nephron: control by chemistry

The endocrine system runs homeostasis slowly and widely: a gland releases a hormone into the blood, every cell with the matching receptor responds, and negative feedback shuts the gland off when the variable is back. The kidney is where two of those hormones do their best-known work. Each of its million or so nephrons filters blood at the glomerulus, takes back almost everything useful along the tubule, and leaves urine, with ADH deciding how much water comes back and aldosterone how much sodium. Below, how a hormone acts, the hormones the course asks for, one nephron traced step by step with the volumes, and the two hormones students swap.

The endocrine system: how hormones work, by receptor, response and feedback

A hormone is a chemical messenger made by a gland and carried in the blood to the whole body. It acts only on target cells, the ones that carry a receptor for it; every other cell lets it pass. Compare the two control systems in three lines.

	Nervous control	Endocrine control
Speed	Milliseconds	Seconds to hours
Duration	Brief	Long-lasting
Reach	One target along a wire	Every cell with the receptor

Two chemical kinds, steroid vs peptide hormones, act in two ways. Water-soluble hormones (peptides and most amino-acid-derived hormones, insulin and adrenaline among them) cannot cross the membrane; they bind a receptor on the cell's surface, and the receptor sets off a second messenger inside the cell, which changes what enzymes are doing within seconds. Fat-soluble hormones (the steroids, such as cortisol, aldosterone and the sex hormones, and thyroxine) cross the membrane and bind a receptor inside the cell; the hormone–receptor pair then changes which genes are transcribed, which takes longer and lasts longer. A receptor, like an enzyme, is a protein with a binding site shaped for one molecule, which is where the specificity comes from.

Every hormone sits in the five-part loop of homeostasis with a gland as the effector: the variable moves, a sensor reads it, the gland releases, the target responds, the variable returns and the release stops.

The hormones SBI4U asks for, in one table

Gland	Hormone	Target	Effect	The loop
Hypothalamus	Releasing hormones	Anterior pituitary	Tells the pituitary what to release	Master control
Posterior pituitary	ADH (antidiuretic hormone)	Collecting duct of the nephron	More water reabsorbed	Water balance
Anterior pituitary	Growth hormone, and the hormones that drive the thyroid, adrenal cortex and gonads	Bone, muscle, the other glands	Growth; control of the other glands	Several
Thyroid	Thyroxine	Most cells	Raises metabolic rate	Metabolism, temperature
Adrenal cortex	Aldosterone; cortisol	Distal tubule and collecting duct; most cells	More sodium reabsorbed; the long stress response, glucose raised	Salt and blood pressure; stress
Adrenal medulla	Adrenaline	Heart, lungs, liver, muscle	The fast stress response	Fight or flight
Pancreas	Insulin; glucagon	Liver, muscle, fat	Glucose lowered; glucose raised	Blood glucose
Gonads	Testosterone; oestrogen and progesterone	Reproductive tissues	Development and the cycle	Reproduction

The pituitary is two glands in one: the posterior part releases hormones the hypothalamus made (ADH is one), and the anterior part makes its own under the hypothalamus's instruction. One card per gland, front the gland and back the hormone, target and effect, is the quickest way to make the table stick.

Blood glucose: glucagon and insulin, the antagonistic pair traced

After a meal, blood glucose rises. The β cells of the pancreas sense it and release insulin. Insulin makes muscle and fat cells take glucose up, and the liver store it as glycogen. Glucose falls toward its set point, and insulin release falls with it.

Between meals, blood glucose falls. The α cells release glucagon. Glucagon makes the liver break glycogen down and release glucose. Glucose rises toward the set point, and glucagon release falls.

Two hormones, opposite effects, one set point: an antagonistic pair. Neither has to be switched off by a third signal; each simply stops being released as the condition that called for it goes away. The glucose that insulin stores is the fuel that cellular respiration burns. One sentence on disease, and no more: in type 1 diabetes insulin is absent; in type 2 the cells respond to it less.

Kidney function: the four jobs, and the nephron's parts in order

The kidney does four things to the blood: filtration (blood to tubule, by pressure), reabsorption (tubule back to blood, of what the body wants to keep), secretion (blood to tubule, by transport, of what the body wants to lose), and excretion (what leaves as urine). Get the directions right here, because the swap below is a direction swap: filtration and secretion both add to the tubule's fluid; reabsorption takes from it.

The nephron is the kidney's working unit, about a million of them per kidney, the exact number varying from person to person. The parts, in the order the fluid meets them, every time:

renal artery → afferent arteriole → glomerulus inside Bowman's capsule → proximal tubule → loop of Henle (descending limb, then ascending limb) → distal tubule → collecting duct → renal pelvis → ureter → bladder

How does the nephron work: traced from about 180 litres of filtrate to about 1.5 litres of urine (nephron function)

Follow one day's fluid through one nephron, with the volumes for the whole kidney pair.

1. Filtration at the glomerulus. Blood arrives under pressure in a knot of capillaries, and the pressure pushes water and small solutes (glucose, salts, amino acids, urea) out through the capillary walls into Bowman's capsule. Proteins and blood cells are too large and stay in the blood. The filtrate is blood plasma without its proteins. For an adult man, the two kidneys make about 180 litres of filtrate a day (about 150 for a woman); the number is the glomerular filtration rate summed over a day.
2. The proximal tubule. Most of the reabsorption happens here. All of the glucose and amino acids, most of the Na⁺, and most of the water are moved back into the blood: the solutes by active transport, the water following them by osmosis. The filtrate shrinks a great deal but stays about as concentrated as blood.
3. The descending limb of the loop of Henle. Permeable to water, not to salt. The loop dips into the medulla, which is salty, so water leaves the tubule by osmosis and the filtrate concentrates as it descends.
4. The ascending limb. Permeable to salt, not to water. Na⁺ and Cl⁻ are pumped out into the medulla, which is what makes the medulla salty in the first place, and the filtrate dilutes as it rises. The two limbs run in opposite directions side by side, so the salt pumped out of one is what draws the water out of the other: the countercurrent arrangement. Osmosis, water following salt, is the only physics in it.
5. The distal tubule and collecting duct. The adjustable part. This is where ADH and aldosterone act (next section), and where H⁺ and K⁺ are secreted into the tubule to tune the blood's pH and potassium.
6. Urine. About 1 to 2 litres a day, call it 1.5, flows from the collecting ducts into the renal pelvis, down the ureter to the bladder.

The computed line: about 180 L in and about 1.5 L out means (180 − 1.5) ÷ 180 = 178.5 ÷ 180 = 0.992, so about 99 % of the filtrate is reabsorbed. Keep "about" on both volumes; they vary with the person, the day and how much you drink.

ADH vs aldosterone, placed on the nephron

ADH, antidiuretic hormone, is released from the posterior pituitary when osmoreceptors in the hypothalamus read the blood as too concentrated, or when blood volume is low. It acts on the collecting duct, making its cells insert water channels (aquaporins) into their membranes, so more water leaves the duct into the salty medulla and back to the blood. Urine becomes small and concentrated. When you have drunk a lot, ADH falls, the channels are withdrawn, and urine is plentiful and dilute.

Aldosterone is released from the adrenal cortex, at the end of the renin–angiotensin pathway, which the kidney starts when blood pressure or blood sodium is low. It acts on the distal tubule and collecting duct, making them reabsorb more Na⁺; water follows the salt by osmosis, so blood volume and pressure rise.

The one-line test: ADH is the water hormone, aldosterone the salt hormone. Both end with more water in the blood, by different doors: ADH opens water channels; aldosterone pumps sodium and lets the water follow.

The two things students swap

ADH and aldosterone. Water channels against sodium pumps; the pituitary against the adrenal cortex; blood concentration against blood pressure and sodium as the trigger. The one-line test above sorts them.

The direction of secretion and reabsorption. Secretion adds to the filtrate (blood to tubule, by transport). Reabsorption takes from it (tubule to blood). A student who writes "glucose is secreted back into the blood" has the word backwards.

A third: thinking glucose in the urine means the kidney filtered badly. Glucose is always filtered, every drop of it, and normally all of it is reabsorbed in the proximal tubule. Glucose in the urine means the transporters there were overwhelmed by too much glucose in the blood, which is the sign the kidney gives of diabetes; the filter did its job.

Check: the nephron cold, then one sibling

With the page closed, draw the nephron as a line with its six regions and write under each what is reabsorbed or secreted there and which hormone acts there. Write the glucose loop in six lines (meal, insulin, uptake and storage, fall; fast, glucagon, breakdown, rise). Write the ADH and aldosterone one-line test. Then open the page and mark it; Karpicke and Blunt (2011) found that recalling and checking produced more learning than re-studying the diagram, and how to study biology has the redraw-from-memory routine for a whole unit.

Then a sibling, cold: on a day when the filtrate is about 150 litres and the urine about 1.5 litres, what fraction of the filtrate was reabsorbed? And after a long run with no water, is ADH high or low, and is the urine concentrated or dilute?

Answer. (150 − 1.5) ÷ 150 = 148.5 ÷ 150 = 0.99, so about 99 %, the same share as before: the kidney keeps the fraction, not the litres. After the run the blood is concentrated, so ADH is high, water channels are in the collecting duct, more water is reabsorbed, and the urine is small and concentrated.

Where this sits in the course

These are ideas 24 and 25 of the 27 in the SBI4U course map: The endocrine system and hormonal control, and The kidney, osmoregulation and excretion, the close of strand E. Before them comes idea 23, The nervous system and the action potential, the fast half of the control story. After them idea 26, Population growth models and carrying capacity, opens strand F, the ecology.

The endocrine system and the nephron on one screen

• A hormone acts on cells with its receptor: surface receptor and second messenger for water-soluble hormones, a receptor inside for steroids; negative feedback switches the gland off.
• The table in six lines: hypothalamus and pituitary (releasing hormones, ADH, growth hormone); thyroid (thyroxine); adrenal cortex (aldosterone, cortisol); adrenal medulla (adrenaline); pancreas (insulin, glucagon); gonads.
• Glucose: insulin lowers, glucagon raises; an antagonistic pair around one set point.
• The nephron's six regions: glomerulus and capsule (filter), proximal tubule (reabsorb most), descending limb (water out), ascending limb (salt out), distal tubule and collecting duct (adjust and secrete), urine.
• The volumes: about 180 L filtered, about 1.5 L out, about 99 % reabsorbed.
• ADH water, aldosterone salt. Secretion adds to the filtrate; reabsorption takes from it.