The parathyroid, the thymus and the gonads · Biology… | StemPy Lab
Course 18.2·Lesson 3 of 3
The parathyroid, the thymus and the gonads
The four parathyroid glands make parathyroid hormone, which raises the calcium of the blood. The thymus makes thymosins that help the T lymphocytes to mature. The testis makes androgens, and the ovary makes estrogen and progesterone.
about 30 min1 simulation1 practice question
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The parathyroid glands
On the back of the thyroid gland lie four small glands, one pair in each lobe of the thyroid. They are the parathyroid glands. They make a peptide hormone, parathyroid hormone (PTH).
The release of PTH is controlled by the level of calcium ions in the blood. A fall in the calcium makes the glands release more PTH.
PTH raises the level of calcium in the blood. So it is a hypercalcaemic hormone. It acts in three places.
On the bones, where it stimulates bone resorption, the dissolving of bone, which releases calcium.
On the renal tubules, where it stimulates the reabsorption of calcium.
On the gut, where it increases the absorption of calcium from the digested food.
A loop for the calcium of the blood. A box for the blood calcium has two arrows. When the calcium is low, the parathyroid glands release PTH, which acts on three targets: the bones, where bone resorption releases calcium, the kidney tubules, which reabsorb more calcium, and the gut, where more calcium is absorbed from the food. All three raise the blood calcium. When the calcium is high, the thyroid releases thyrocalcitonin, TCT, which helps to bring the calcium down.
Along with thyrocalcitonin (TCT) of the thyroid, PTH has a major part in the balance of calcium in the body. The normal calcium of the blood is about 8.5 to 10.5 mg/dL.
When the parathyroid goes wrong. A low level of calcium makes the nerves and muscles too excitable. The muscles twitch and go into spasm. This is tetany. It can follow a loss of the parathyroid glands, for example in an operation on the neck. Too much PTH takes too much calcium from the bones and weakens them. A-Level and IB books add that it can cause kidney stones.
You should find that a very low PTH gives a calcium of about 6.5 mg/dL, far below the normal range. A high PTH raises the calcium above the normal range, but the curve flattens, because the real control has limits. This is a teaching model.
The thymus
The thymus is a lobed gland that lies between the lungs, behind the breast bone, on the front side of the aorta. It makes peptide hormones called thymosins.
Thymosins play a major part in the differentiation of T lymphocytes, which give cell-mediated immunity.
They also promote the making of antibodies, which give humoral immunity.
The thymus shrinks in old age, so less thymosin is made. The immune responses of old people become weaker.
This part is not in the NEET syllabus text for this chapter, but NCERT, A-Level and IB teach it. You meet immunity again in the chapter on human health and disease.
The testis
A man has a pair of testes in the scrotal sac. They are both reproductive organs and endocrine glands. Between the tubes where the sperm form, the interstitial cells, also called Leydig cells, make a group of hormones called androgens. The main androgen is testosterone.
Two panels. The left panel shows a cut piece of the testis. Several round seminiferous tubules, where sperm form, lie close together. Between them, in the space around the tubules, are the interstitial cells, also called Leydig cells, which make androgens, mainly testosterone. The right panel shows the ovary as a cycle. A growing follicle makes estrogen. A mature follicle releases the egg at ovulation. The empty follicle turns into the corpus luteum, which makes mainly progesterone.
Androgens.
Regulate the development, the maturation and the work of the male accessory sex organs: the epididymis, the vas deferens, the seminal vesicles, the prostate gland and the urethra.
Stimulate muscle growth, the growth of facial and armpit hair, and aggressiveness. They give the low pitch of the voice.
Play a stimulating part in spermatogenesis.
Act on the central nervous system and influence male sexual behaviour (libido).
Have an anabolic effect, which means a building effect, on the metabolism of proteins and carbohydrates.
The ovary
A woman has a pair of ovaries in the lower abdomen. Each month an ovary makes one ovum. The ovary is made of ovarian follicles and stromal tissue. It makes two groups of steroid hormones, estrogen and progesterone.
Estrogen is made mainly by the growing follicles. It stimulates the growth and the work of the female secondary sex organs and the growth of the follicles. It gives the female secondary sex characters, such as the high pitch of the voice. It stimulates the development of the mammary glands, and it regulates female sexual behaviour.
After ovulation, the burst follicle turns into the corpus luteum. It makes mainly progesterone. Progesterone supports pregnancy. It also acts on the mammary glands and stimulates the forming of the alveoli, the sacs that store milk, and the secretion of milk.
Aldosterone: adrenal cortex. Na+ and water are kept.
Insulin: beta cells. Glucose falls.
Glucagon: alpha cells. Glucose rises.
PTH: parathyroid. Calcium rises.
Thymosin: thymus. T lymphocytes mature.
Androgens: testis. Male characters.
Estrogen: ovarian follicle. Female characters.
Progesterone: corpus luteum. Supports pregnancy.
Try it in Python
The program uses the model of the simulation to say whether the calcium is in the normal range.
Practise with new numbers
Common mistakes
Saying that PTH lowers the calcium. PTH raises it. TCT is the one that helps to bring it down.
Confusing the parathyroid with the thyroid. The parathyroid glands are four small glands on the back of the thyroid.
Saying that estrogen is made mainly by the corpus luteum. Estrogen comes from the growing follicles. The corpus luteum makes mainly progesterone.
Saying that the Leydig cells are the cells that form sperm. They lie between the tubes and make androgens.
Saying that tetany is caused by too much calcium. It comes from too little calcium in the blood.
Remember these
Parathyroid: four glands on the back of the thyroid. PTH raises blood calcium by acting on bone, kidney and gut.
Thymus: thymosins, differentiation of T lymphocytes. It shrinks in old age.
Testis: Leydig cells make androgens. Ovary: follicles make estrogen, the corpus luteum makes progesterone.
A low level of calcium causes tetany.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
How many parathyroid glands are there, and where do they lie?
Name the three places where PTH acts to raise the calcium of the blood.
What is the work of the thymosins, and what happens to the thymus in old age?
Which cells of the testis make androgens? Name two actions of androgens.
Which hormone is made mainly by the corpus luteum, and what is its work?
References
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on parathyroid hormone, calcitonin and calcium metabolism, and on the reproductive and hormonal functions of the male.
NCERT (2023). Biology, Textbook for Class XI, chapter 19.
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