Endocrine System and Homeostasis, Grade 12: Negative Feedback and Thermoregulation

Grade 12 material lists capillaries among the blood vessels that dilate when you are too hot. Capillaries have no muscle in their walls. They cannot dilate, and they cannot constrict.

It is the arterioles that do the work, and the distinction is not pedantry: it is the reason a flushed face goes red in patches rather than all at once, and it is examinable.

This page covers the endocrine system, negative feedback, homeostasis and thermoregulation as one piece, because that is how they are examined. It also corrects five things the printed material gets wrong, including a heading that belongs to a different unit.

Where this fits in the curriculum

Subject Life Sciences
Grade 12
Term 3
Topic Topics 7 and 8, the human endocrine system and homeostasis
Status Carries a 50-mark formally assessed task, which is a written project, not a practical
Marks 50 for the endocrine disorder project. Our worksheet is 70

This page will not sell you anything

There is no apparatus in this topic and we are not going to invent any. The assessed task is a 400-word research report with a bibliography. Thermoregulation needs a thermometer at most, and every school already owns one.

We would rather say that plainly than dress a reading topic up as a practical. If you want the one thing that genuinely helps, it is a wall chart of the endocrine glands, and even that is optional.

The assessed task is a project, not a practical

The contents page lists it as a "Practical task". The page it sits on heads it "Formal assessment: Project".

It is 50 marks of written research: choose an endocrine disorder, write about 400 words, name the gland and the hormone, define the disorder, give causes, effects and current treatments, and attach a bibliography. There is no apparatus and no method.

This matters for your year plan. A teacher working from the contents page will timetable a laboratory session that does not exist, and will discover it in the week the task is due.

Part of the task Marks
Select at least one disorder from the list 2
Which gland secretes the hormone 2
Where that gland is in the body 2
Define the disorder 14
Suggest possible causes 6
Describe the effects on the body 16
Current treatments 4
Bibliography 4
Total 50

Thirty of the fifty marks are for defining the disorder and describing its effects. Learners who spend their time on causes and treatments are working on ten marks.

Negative feedback, which is the spine of both topics

A change is detected, a control centre responds, and the response pushes the factor back the other way. That is the whole mechanism, and it runs the thyroid, blood sugar, salt balance and body temperature.

Loop Too high Too low
Thyroxin Inhibits the pituitary, so it secretes less TSH, so less thyroxin Stimulates the pituitary, so more TSH, so more thyroxin
Blood glucose Pancreas secretes insulin, glucose is taken into cells, level drops Pancreas secretes glucagon, liver converts glycogen to glucose, level rises
Blood sodium Less aldosterone, so less sodium is reabsorbed by the nephron Adrenal gland releases aldosterone, nephron reabsorbs more sodium
Body temperature Vasodilation and sweating Vasoconstriction, shivering, hair erection

Insulin and glucagon are the pair most often swapped. Insulin is the one that puts glucose in to the cells. Both words start the same way, and that is where the mark goes.

Negative feedback is a dial, not a switch

The printed flow diagram for salt balance says that when there is too much sodium, "no aldosterone" is released and there is "no reabsorption of sodium" in the nephron.

Neither is true, and the second one is not survivable. The nephron reabsorbs the great majority of filtered sodium in the proximal tubule no matter what aldosterone does. What falls is the extra reabsorption in the distal tubule and collecting duct, and aldosterone secretion is reduced, not switched off.

The wording matters because it teaches the wrong model of the whole mechanism. Homeostasis holds a value near a norm by making graded adjustments. A system that switched fully on and fully off would oscillate, not stabilise.

Thermoregulation

Body temperature is held between about 36 and 37,5 degrees Celsius, with roughly 36,9 as the optimum, because that is where human enzymes work best.

Too hot Too cold
Vasodilation of the arterioles supplying the skin, so more blood flows near the surface Vasoconstriction of those arterioles, so less blood reaches the surface
Sweating. Evaporation takes heat energy from the blood in the surface capillaries Sweating stops
Hairs lie flat Hairs stand up, trapping a layer of still air
Behavioural: shade, cool drinks, less clothing Shivering, and behavioural: clothes, huddling, movement, hot drinks

Capillaries cannot dilate

You will read that in warm conditions "the surface blood vessels, arteries, capillaries and veins, dilate, or open".

Vessel Can it change diameter?
Arteriole Yes. This is where vasodilation and vasoconstriction happen, because the wall carries a ring of smooth muscle
Artery Yes, but the control of skin blood flow is done at the arterioles
Capillary No. A capillary wall is one cell thick and has no muscle at all. It cannot dilate and it cannot constrict
Vein Some, and in the skin blood is shunted between deep and surface veins

What actually changes is how much blood reaches the capillary beds near the surface, and that is decided upstream by the arterioles. The capillaries are passive. A learner who writes that capillaries dilate has described something anatomically impossible, and in a question about the structure of blood vessels that is a lost mark.

The skin, with its layers put back in the right order

Layer What is actually in it
Epidermis The stratum germinativum, which divides to make new cells, and the melanocytes that make melanin. The outer cells die and form a waterproof layer
Dermis Sweat glands, blood vessels, nerves, temperature receptors, oil glands and hair follicles
Hypodermis Fat, which insulates, and the attachment to the organs below

Printed material lists oil glands and hairs as structures of the epidermis. Both are in the dermis. Sebaceous glands and hair follicles sit in the dermis and open through the epidermis, which is probably where the confusion starts. The shaft of a hair passes through the epidermis; the follicle that grows it does not.

The test that settles it: the epidermis has no blood supply. Anything that needs blood, and a hair follicle very much does, cannot be living in it.

Three more things to correct

What is printed What is correct
A "Unit 1: Key questions" box on the page headed "Unit 2: Thermoregulation" The box belongs to Unit 2. A learner revising by unit will answer the wrong questions, and it is the kind of error that survives because nobody reads a key questions box twice
Heat is lost through the skin "by radiation of heat energy and evaporation of sweat" Conduction and convection are missing. You lose heat to the air touching your skin, and to the air moving past it, which is the entire reason a breeze cools you and a still hot room does not
"You studied examples of on negative feedback", and a figure labelled "less thryoxin secreted" Two typographic slips in the part of the course with the most precise vocabulary. Thyroxin is worth spelling correctly, because learners copy figure labels into answers

What a class actually needs

  • Nothing, for the assessed project. Internet or library access, and the 400-word limit enforced
  • A thermometer, if you want to make thermoregulation concrete. Take a reading before and after two minutes of exercise and the class sees a controlled variable being defended
  • A wall chart of the endocrine glands, which is the one purchase this topic can justify, and it is optional

Thermometers are in our thermometer range and wall charts in educational wall charts. If you already own both, this topic costs you nothing, and that is the honest answer.

If the project does not go well

What happens What caused it
Reports come in at 1 500 words The 400-word limit was not enforced. Thirty of the fifty marks are for two sections, and length is not one of them
Every learner picks diabetes Allocate from the list, or allow it and mark the depth rather than the choice
No bibliography Four marks lost for something that takes two minutes. Say so before they start
Learners confuse the gland with the hormone Thyroid is the gland, thyroxin is the hormone. Pancreas is the gland, insulin and glucagon are the hormones

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 70 marks, covering the glands and their hormones, negative feedback, homeostasis, thermoregulation and the structure of the skin
  • Marking memorandum, with the project mark split and the five answers that look right and score nothing

Related practicals