Hormones, homeostasis and reproduction
(VI. 6)

Blood glucose concentration

They also contain β cells that produce insulin

Glucagon stimulates the breakdown of glycogen into glucose in the liver and its release in the blood (higher conc.)

The islets contain α cells that produce glucagon

Insulin stimulates the uptake of glucose in skeletal muscle and liver, where it stimulates the conversion into glycogen.

The pancreas is made of exocrine tissue (secretes digestive enzymes) and endocrine tissus (islets of Langerhans that secrete hormones)

Insulin is broken down by the receiving cells, so it must be produced continuously

If the glucose concentration shifts, pancreatic cells mediate the production of insulin or glucagon

Hormones

Leptin

Melatonin

Thyroxin

Threfore, thyroxin regulates metabolic rate and controls body temperature

Hypothyroidism results in: fatugue, depression, weight gain, feeling cold, constipation, brain underdevelopment in children

Higher metabolic rates cause body heat, therefore cooling triggers the secretion of thyroxin (body temperature rises)

Most cells in the body are affected by thyroxin, since it regulates metabolic rates

Secreted by the thyroid gland in the neck and it contains four atoms of iodine

If adipose tissue increases, blood leptin concentrations rise, causing appetite inhibition

It targets cells in the hypothalamus of the brain that contribute to the control of appetite

Leptin is a protein hormone released by adipose cells

Melatonin secretion increases in the evening and drops to a low level at dawn

Melatonin promotes sleep and its lack promotes wakefullness

It depends on two groups of cells in the hypothalamus called the suprachiasmatic nuclei (SCN). They control melatonin secrection in the pineal gland

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Cyrcadian rhythm is a process that regulates the sleep–wake cycle and repeats every 24 hours

A ganglion cell in the retina of the eye detects light (460–480 nm) and passes impulses to cells in the SCN

Sex determination in males

Sex determination in females

The testes produce testosterone up the the 15th week

Testosterone production increases again in puberty, causing the development of second sexual characteristics and sperm production

The embrios that do not have Y chromosomes will develop ovaries

It codes for a DNA-binding protein called TDF (testis
determining factor). It stimulates the expression of genes that cause testis development

If the gene SRY (Y chromosome) is present, the embryonic gonads develop into testes (8th week of pregnancy)

The absence of testosterone and the presence of female hormones trigger development

During puberty, production of estrogen and progesterone increases again, leading to second sexual characteristics

First they are released by the mother and later by the placenta

Estrogen and progesterone are always present in the pregnancy

If the gene SRY is not present, the embryonic gonad develop as ovaries

Menstrual cycle

In the second half (luteal phase) the wall of the egg-releasing follicle become the corpus luteum

If fertilisation doesn't occur, the corpus luteum and the endometrium break down, causing menstruation

The endometrium is repaired and thickens, the
most developed follicle releases its egg into the oviduct

In the first half of the cycle (follicular phase) a group of follicles is developing in the ovary, in each follicle an egg is stimulated to grow

Hormones

Estrogen peaks towards the end of the follicular phase. It stimulates the thickening of the endometrium and an increase in FSH receptors (more estogen production)

LH peaks towards the end of the follicular phase, it stimulates the completion of meiosis in the oocyte and partial digestion of the follicle (causes it to burst)

FSH peaks towards the end of the cycle and stimulates the development of follicles, it also stimulates secretion of estrogen by the follicle wall

When it reaches high levels, estrogen inhibits the release of FSH and increases that of FH

Estrogen and progesterone are ovarian hormones, produced by the wall of the follicle and corpus luteum

LH also stimulates the formation of the corpus luteum (that secretes estrogen and progesterone)

FH and FSH are protein hormones released by the pituitary gland that bind to receptors in the membranes of follicle cells

Progesterone levels rise at the start of the luteal phase, peak and then drop back to a low level by the end of this phase. It promotes thickening and maintainance of the endometrium. It also inhibits FSH and FH production