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Option H.1 – Hormonal Control

H.1.1 – State that hormones are chemical messengers secreted by endocrine glands into

the blood and transported to specific target cells

Hormones are used in the form of long-distance communication of the endocrine system.

The endocrine glands, such as the pancreas, produce hormones and secrete them into the

bloodstream. The blood carries the hormones through the body. When the hormone

reaches the cells that have the right receptor, it will bind to the target cell and produce a

response.

H.1.2 – State that hormones can be steroids, proteins and tyrosine derivatives, with one

example of each

Steroid Hormones

Steroids are a type of molecule made up of four organic rings.

An example of a steroid hormone is cortisol, a stress hormone,

shown right. The sex hormones œstradiol and testosterone are

also steroids.

Protein Hormones

These hormones are made up of polypeptides.

An example of a protein hormone is insulin, which is used in the control

of blood glucose concentration.

Tyrosine Derivative Hormones

Tyrosine is one of the amino acids. Some hormones are derived from it.

An example is the thyroid hormone, thyroxine (T4), which helps to

regulate metabolism.

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H.1.3 – Distinguish between the mode of action of steroid hormones and protein

hormones

Steroid Hormones

Since steroid hormones are non-polar molecules, they are

able to move through the lipid bilayer and into the cell. It

then bonds to an intracellular receptor protein to form a

hormone-receptor complex, which enters the nucleus.

It affects the DNA by causing expression of genes and

consequently protein synthesis.

This process is used by steroid hormones, such as the sex

hormones, to regulate growth and development.

Protein Hormones

These hormones tend to be larger and are polar molecules. Hence, they cannot cross the

lipid bilayer. Instead, they attach to receptor proteins on the outer cell membrane. The

binding triggers the action of a second messenger in the cytoplasm. This activates the

cellular response.

H.1.4 – Outline the relationship between the hypothalamus and the pituitary gland

Within the human body, there are many endocrine glands. The main ones that control

homeostasis are the hypothalamus and the pituitary gland.

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Hypothalamus

The hypothalamus integrates the nervous and endocrine systems. It contains

neurosecretory cells that perform the endocrine functions including synthesising some

hormones such as oxytocin and ADH. It also produces releasing hormones, which cause the

secretion of other hormones.

Underneath the hypothalamus is the pituitary gland, which is made up of two lobes.

Posterior Pituitary

This is an extension of the hypothalamus, connected by the neurosecretory cells. The

hormones that are produced in the hypothalamus travel along the axons of the

neurosecretory cells. When the nerve impulses move along the axons, it stimulates the

release of the hormones into the bloodstream at the posterior pituitary to be stored for

release.

Anterior Pituitary

This is regulated by hormones that are released by the hypothalamus into portal blood

vessels, or the portal vein. It synthesises and releases TSH and PRL (prolactin).

H.1.5 – Explain the control of ADH (vasopressin) secretion by negative feedback

Negative feedback means that the response reduces the initial stimulus. This is used to

prevent excessive pathway activity.

Vasopressin, or Anti-Diuretic Hormone, is produced in the hypothalamus and released from

the posterior pituitary. It increases levels of urination. The neurosecretory cells in the

hypothalamus synthesis and transport ADH along the axons of nerves for storage in the

synaptic ends.

When the body loses water, the osmolarity of the blood increases. Osmoreceptors of the

hypothalamus detect the change in the water content of blood as it passes through the

hypothalamus. The osmoreceptors send an action potential through the neurosecretory

cells to the posterior pituitary is the water contant is low, stimulating the release of

vasopressin into the bloodstream.

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The target cells for ADH are mainly in the distal tubules and collecting duct of the kidney. It

causes the epithelium to become more permeable to

water, increasing renal reabsorption of water and

reducing urine volume. Consequently, the urine is more

concentrated. This is achieved by acting upon the

aquaporins.

Once the osmolarity is returned to the homeostatic set

point, a negative feedback loop is used to signal to the

hypothalamus to reduce vasopressin production.

Conversely, if there is excessive water consumption, then

ADH secretion is reduced so that a greater volume of

urine is produced.