Hormones are secretions of ductless or endocrine glands which are poured or secreted directly into the bloodstream by diffusion and they affect organs that respond to their actions. Examples of animal hormones are thyroxin, adrenaline, insulin and sex hormones.
The system consists of a collection of hormones producing glands, many regulated by trophic hormones secreted by the pituitary gland. The pituitary gland is itself influenced by hormones secreted by the hypothalamus in the brain.
This gland secretes insulin and glucagon which controls the body’s utilization of glucose.
When stimulated by ACTH, the adrenal cortex produces hydrocarbon which has widespread effects on metabolism. It also produces androgen hormones and also aldosterone hormone which maintains blood pressure and the body’s salt balance.
These produce the hormones oestrogen and progesterone which influence multiple aspects of female physiology. These processes are controlled by gonadotrophic hormones secreted by the pituitary.
The pituitary gland secretes hormones that stimulates the adrenals, thyroid pigment producing skin cells and gonads. It also secretes growth antidiuretic hormones, prolactin and oxytocin.
This gland produces the hormone thyroxin, triodothyronine, and calcitonin which stimulates metabolism, body heat production and bone growth. Thyroid activity is controlled by TSH, which is secreted by the pituitary.
These glands secretes parathyroid hormones, which maintain the calcium level in the Blood.
The testes produce testosterone in response to gonadotropins secreted by the pituitary. A combination of gonadotropins and testosterone stimulates sperm production and the development of the other male characteristics.
Plant hormones includes auxins, gibberellins, kinins, ethylene, and abscissic acid.
Auxins are produced at the apical regions of roots and shoots.
TYPES OF AUXINS
These include 2-4-Dichloro phenoxy ethanoic acid (2, 4-D) 2 Indole acetic acid (IAA).
FUNCTION OF AUXINS
1. Cause stem elongation hence growth
2. Cause lateral bud development.
3. Control leaf fall in plant.
4. Initiate adventitious roots formation.
5. Help fruit development.
6. 2-4-Dichlorophenoxy ethanoic acid is a normal weed killer at high concentration.
7. Cause apical dominance growth movement or tropism in plants.
8. Cause ripening of fruits.
This is found in roots and embryos of germinating seeds
1. Promote stem elongation.
2. Break dominancy in buds and seeds.
3. Promote flowering in plants.
4. Promote germination of seeds.
5. Retard leaf abscission.
6. Induce enzyme’s synthesis in seeds.
These are found in roots and embryos of germinating seeds.
1. Stimulation of mitosis or cell division in a root and stem or in meristematic tissues.
2. Break seed dormancy.
3. Promote the growth of lateral buds and fruits.
4. Control the growth level in shoots.
Note that kinins occur in nature as cytokinins.
THE MAMMALIAN SKIN
The mammalian skin is the outer covering of the mammalian body. This acts as a protective covering over the internal organs of the body.
The mammalian skin consists of an outer layer called the Epidermis and an internal layer, the Dermis.
1. EPIDERMIS: It is the upper part of the skin and its made up of three layers; the cornified layer, the granular layer, and the malpighian layer.
a. Cornified layer: This layer is made up of dead flattened cells which are constantly peeling off. It protects the skin from microbial invasion, mechanical injury, and water loss.
b. Granular layer: it is made up of living cells. It produces and replaces cells in the cornified layer.
c. Malpighian layer: it is made up of meristematic cells and contains melanin pigment. Melanin is responsible for skin coloration and also protects the skin from ultra violet rays. Vitamin D is also produced in this layer.
2. DERMIS: the dermis s the inner layer of the skin. It consists of blood vessels, nerves and sweat glands.
a. Sweat gland: it is a colloid tubular gland opening into the epidermis. It excretes sweat. Evaporation of sweat on hot day cools the body.
b. Hair or fur: hair is a solid rod of keratinized cells enclosed in a follicle. It has a hair papilla at its base which contains blood vessels. It insulates the body from losing heat. Papilla supplies materials for hair construction.
c. Erector muscles: it is attached to the hair follicle. Its contraction makes the hair to stand erect thus enhancing heat insulation.
d. Sebaceous gland: it is a sac of glandular cells which opens into the hair follicle. It secretes an oily substance which lubricates the hair and water proofs the skin.
e. Blood capillaries: they supply materials to the skin. They remove wastes from skin cells and carry out temperature regulation.
f. Nerve endings: nerve endings called Receptors are sensitive to heat, cold, pressure, pain and touch.
FUNCTIONS OF THE MAMMALIAN SKIN
1. The cornified layer protects the body against bacterial invasion, mechanical injury, and desiccation.
2. The skin helps to keep the temperature of the body constant. This is carried out by the sweat glands, fatty layer, hairs and by dilation or constriction of the blood vessels of the skin. This is a homeostatic control.
3. The nerve endings also known as receptors are sensitive to heat, cold, pressure, and touch.
4. The sweat glands excrete sweat which consists of water, urea, salts, and carbon (IV) Oxide. Each sweat gland absorbs the waste products such as water, urea, salts, and carbon (IV) Oxide from blood flowing in a network of blood capillaries surrounding it. These substances are excreted at the surface of the skin through the sweat ducts.