Adjustment and Control: Homeostasis + Hormonal Control

Homeostasis

Homeostasis

The maintenance of constant conditions in the internal environment

Homeostatic mechanisms

These makes organisms independent of the environment, in which it is changing continuously. Homeostatic mechanisms can be: anatomical, physiological and behavioral

Control systems

organisms make use of these to achieve homeostasis. The basic components include:
Input→ Detector→ Regulator→ Set point→ Effect→ Output

Input

A change in the environment that can be detected by an organism

Detector

Detects a change in some variable of the animal’s internal environment

Regulator

processes information it receives from the detector. It compares the information with the set point

Set point

the desired value, amount or concentration

Effector

brings about an appropriate response according to the detected change

Output

the actual response observed in the body i.e. a physiological or behavioural change

Feedback

refers to the interdependence of input and output into the system. There are two types: the negative and the positive

Negative Feedback

here a change in the internal environment leads to a response, which counteracts further change in the same direction. Therefore the response restores the system to its original state. Example compare to a thermostat regulating temperature.

Positive feedback

Here a change produces a response that intensifies the original change. Thus, a disturbance leads to events that amplify (increase) the disturbance even further. This response leads the system to an extreme state. Example: During childbirth, the pressure of the baby’s head against sensors near the opening of the uterus stimulates uterine contractions. These cause greater pressure against the uterine opening, heightening the contractions, which causes still greater pressure. Positive feedback brings childbirth to completion

Thyroxine

a hormone released by the thyroid gland, which regulates the basal metabolic rate, growth and development of an organism

Nervous Control

• Fast responses

• impulses pass along neurons

• Responses precise and localised

• rapid, short-lived responses

• Make use of chemical transmitters

Hormonal Control

• Slower responses

• hormones carries by the blood

• responses diffuse and widespread

• slower responses which continue over a long period

• make use of chemical transmitters

Dynamic Equilibrium

The internal state of an animal body. This implies that internal conditions are never absolutely constant, but they fluctuate continuously within narrow limits. However, the body compensates for them through its control mechanisms. The net results of this activity is that physical and chemical parameters are kept within the narrow range that cells need to function

The endocrine system

This consists of a number of glands that secrete hormones. It is adapted to carry information from one source to another to bring about long lasting responses.

Gland

It is a structure, which secretes a specific chemical substance or substances.

Exocrine glands

produce secretions that are not hormones, into a duct

Sweat glands

secretes sweat into sweat ducts leading to the surface of the skin.

Salivary glands

release saliva into the mouth

Endocrine glands

secretes chemicals called hormones. Has no duct and so the hormone is secreted directly into the bloodstream. Has a rich supply of blood.

Hormones

It is a regulatory chemical messenger, secreted by a gland (endocrine gland) that is released into the blood plasma, and is transported to a target organ on which it exerts its effects.

Characteristics of hormones

• A small soluble organic molecule

• Produces in a gland but acts at another site (target). Have no effect on the glans itself.

• Transported in blood

• Specific to the target. Hormone fits to its receptors by means of a lock and key arrangement.

• Effective in low concentrations

Mechanisms controlling hormone release

1.       Presence of a metabolite in the blood example glucose stimulates release of insulin

2.       Nervous stimulation example adrenaline secretion

3.       Presence of another hormone example many glands release hormones if pituitary hormones are present.

Chemical nature of hormones

Molecular structure must be sufficiently complex to convey regulatory information. Simple molecules are not good enough. They must also be stable enough to resist destruction prior to reaching target cells. However, they must not persist in blood for too long.

Amines

simple molecules derived from single amino acid molecules. Example adrenaline and noradrenaline are derived from tyrosine.

Peptides

example insulin, glucagon, antidiuretic hormone and growth hormone

Steroids

example sex hormones such as oestrogen, progesterone and testosterone

Fatty acids

consists of 2 fatty acid carbon chains attached to a five-carbon ring. Example prostaglandins

Glycoproteins

example follicle stimulating hormone (FSH), luteinizing hormone (LH) and thyroid stimulating hormone

The Steroid hormone

• Diffuses through the plasma membrane.

• Binds to a receptor protein present only in the target cells.

• Hormone receptor complex enters the nucleus.

• Binds to specific regulatory sites

• Stimulates the transcription of specific genes to mRNA. The hormone-receptor complex may also suppress the expression of other genes.

• mRNA is translated to specific proteins.

• The proteins that result often have a regulatory function in pathway of hormone release. For example, when thyroid hormone binds to a receptor in the anterior pituitary gland, it inhibits the expression of the gene for thyrotropin. This is a negative feedback mechanism

Control of hormones: Negative feedback

When blood glucose level rises, the pancreas produces insulin, which causes the liver to store glucose. The stimulus for the production of insulin has thus been dampened, and insulin production stops.

Control of hormones: Antagonism

contrary hormones oppose each other’s actions. Example: Thyroid gland releases calcitonin to lower the blood calcium level, whilst the parathyroid glands release parathyroid hormone (PTH) to raise the blood calcium level. Insulin and glucagon are also antagonistic hormones

The Cascade effect

it is a mechanism that enables the effect of the release of a small amount of the initial hormone to become amplified at each stage of a pathway involving several hormones.

The Anterior lobe-adenohypophysis

It has a glandular origin and is functionally connected to the brain by the circulatory system. It consists of endocrine cells that synthesize and secrete several hormones directly into blood. Neurosecretory cells in the hypothalamus control the anterior pituitary by secreting 2 kinds of hormones into the blood the releasing hormones and the inhibiting hormones

Releasing hormones

induce the anterior pituitary to secrete its hormones

Inhibiting hormones

prevent the anterior pituitary from secreting its hormones

The Posterior lobe- neurohypophysis

As the posterior lobe receives ends of neurosecretory cells from the hypothalamus, it is considered as an extension of the hypothalamus itself. It releases only two hormones: antidiuretic hormone and oxytocin

Antidiuretic hormone (ADH)

prevents dehydration due to excessive loss of water with urine

Oxytocin

causes contraction of uterus during childbirth and ejection of milk from mammary glands

Insulin

compensates for high levels of glucose

·       Small protein consisting of 2-polypeptide chains

·       Secreted by beta cells in the Islets of Langerhans in the pancreas

·       Insulin release increases when blood glucose concentration rises above set point

·       Transported in blood plasma bound to beta globulin.

Glucagon

Compensates for low levels of glucose

• Protein molecule consisting of a single polypeptide chain.

• Alpha cells in the Islets of Langerhans

• Secreted when blood glucose concentration falls below set point.

Insulin’ s mode of action

It binds to a glycoprotein receptor on the cell surface. This leads to changes in the cell membrane permeability and the activity of enzyme systems within the cell.It reduces in the amount of glucose in blood due to: An increase in the uptake of glucose into cells especially skeletal muscles, An increase in the rate of cellular respiration and the use of glucose as a respiratory substrate, An increase in the conversion of glucose to fat in adipose cells, An increase in the rate of uptake of amino acids into cells and the rate of protein synthesis, An increase in the rate of conversion of glucose to glycogen in liver and muscle cells (glycogenesis), Decrease in the formation of glucose (gluconeogenesis).
Insulin appears to affect all cells, but muscle, fat and liver cells are its main targets.

Regulation of Insulin Production

It is regulated by a negative feedback mechanism. An increase in blood glucose level is detected by the β cells in the pancreas, which produce more insulin. As insulin levels rise, glucose is removed from the blood. As blood glucose level decreases, the β cells reduce output of insulin

Effects of Insulin Deficiency

• Blood glucose level above set point (Hyperglycaemia)

• Breakdown of muscle tissue

• loss of wieght

• tiredness

• damage to blood vessels, kidney and nerve supply to lower limbs

Effect of Insulin Excess

• Blood glucose level below set point (hyperglycaemia)

• Hunger

• sweating

• irritability

• palpitations

Mode of action of Glucagon

When glucagon binds to receptors in liver cells:

• It activates adenyl cyclase to form cyclic AMP. This activates phosphorylase enzymes that catalyse the breakdown of glycogen to glucose (glycogenolysis).

• It increases the conversion of amino acids and glycerol into glucose - 6 - phosphate.

• It increases gluconeogenesis i.e. The synthesis of glucose from non-carbohydrate sources

Diabetes Mellitus

A metabolic disorder caused by a lack of insulin or a loss of responsiveness to insulin. May result from incomplete development of, damage to, or disease of the Islets of Langerhans.

Type I (Insulin dependnet)

The pancreas does not produce insulin. This is possibly due to an autoimmune disorder that destroys the Islets of Langerhans. Generally, appears before the age of 40, often diagnosed in childhood. If condition is untreated it may result in a lethal diabetic coma. Generally, type I diabetes is treated through a combination of planned diet, exercise, glucose monitoring and a daily insulin injection. The main injection sites are the thighs (green), arms (yellow), abdomen (orange) and buttocks (blue).

Type II (non-insulin dependent)

This condition arises when the pancreas produces a low amount of insulin, or the body cells do not respond to insulin. Symptoms often show above the age of 40. Best controlled through a low-fat diet and regular exercise. If this fails, oral drugs may be used to make cells more sensitive to the effects of insulin or stimulate pancreas to secrete more insulin