Human Body Systems

Endocrine System

MCAT Anatomy > Human Body Systems > Endocrine System


The endocrine system can be divided up into the autocrine, the paracrine and the endocrine systems

MCAT Endocrine System - Endocrine System

Autocrine System

See Autocrine System

The Autocrine System is characterized when the cell that is stimulated works on other cells. For example, T cells release interleukin-2 that will bind to other T-cells to increase their functionality and efficiency. Click here to learn more about how the immune and endocrine systems are related.

MCAT Endocrine System - Helper T Cell Activation

Paracrine System

See Paracrine System

The Paracrine System is categorized when cells act over a very short distance together such as with two neurons or the hypothalamus and pituitary.

MCAT Endocrine System - Cell Signaling

Endocrine System

The endocrine system involves action of a cell over a great distance. For example FSH, acts on the gonads and yet is from anterior pituitary

MCAT Endocrine System - Endocrine Gland

Endocrine Glands

See Endocrine Glands

Glands produce peptide and steroid hormones.

Glands in the Head

Glands in the Body

Mechanisms of Action


See Peptides

(1) Peptides (amino acids) such as ADH and Insulin are derived from larger precursor polypeptides that are cleaved after post-translational modifications. The smaller units are transported to the Golgi apparatus for modification. These modifications activate and direct the hormone where it is released via exocytosis in a vesicle.

Peptide hormones are charged so they cannot cross the phospholipid bi-layer, so they act as first messengers and bind to receptors. They then stimulate the release of second messengers like cAMP (cyclic AMP), which is catalyzed by adenylatecyclase. cAMP can then bind to intracellular targets such as protein and DNA for the full effect. This full effect is called the signaling cascade and has the possibility of an amplification of effect. cAMP is also determined by phospodiesterase and the effects do not last without constant stimulation.


See Steroids

(2) Steroids are derived from cholesterol and non-polar which means they can easily cross the cell membrane and have intra and extracellular receptors. Upon binding they can dimerize which means they pair up with a receptor/hormone complex. The dimer can then bond to DNA and alter transcription by either increasing or decreasing rates. The effects are longer lived but it takes longer to see the effects because transcription takes time.

Amino Acid Derivatives

See Amino Acid Derivatives

(3) Amino Acid Derivatives include epinephrine, norepinephrine and thyroxine and are derived from 1-2 amino acids with post-translational modifications.

Depending on the polarity of the molecule, they make work via second messenger systems such as peptides and epinephrine do, or may enter a cell and act as a steroid hormone like thyroxine does.


See Testes

The testes are the sites of spermatogenesis.

(1) Follicle stimulating hormone (FSH) stimulates the sertoli cells for sperm maturation.

(2) Luteinizing hormone (LH) causes the interstitial cells to produce testosterone, which is a major androgen and is important for spermatogenesis, male embryonic differentiation, male sexual development at puberty, and the maintenance of secondary sex characteristics such as auxiliary and pubic hair.


See Ovaries

The ovaries are under control of the hormones FSH and LH, which are secreted by the anterior pituitary and are under control of the hormone GnRH from the hypothalamus. The ovaries produce estrogen and progesterone.

MCAT Endocrine System - Ovulation Hormonal Control


See Estrogen

MCAT Endocrine System - Estrogen

(1) Estrogen is secreted in response to elevated FSH and LH by the ovarian follicles and corpus luteum. Those hormones are responsible for the secondary female characteristics that lead to a thickening of the endometrium in the menstrual cycle in preparation for the zygote. In the embryo, the estrogen stimulates the development of the female reproductive tract. It has both a positive and a negative feedback loop.


See Progesterone

MCAT Endocrine System - Progesterone

(2) Progesterone is secreted in response to LH with stimulation from the anterior pituitary gland. It is released from the corpus luteum also and is responsible for the development and maintenance of the endometrium layer. The placenta eventually releases it at the end of the first trimester when the corpus luteum atrophies and dies.

Menstrual Cycle

See Menstrual Cycle

Endocrine System

Endocrine System Links

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