Biology Chapter 9 - Homeostasis and Hormonal Control

What is Homeostasis?

What is Homeostasis?

The maintenance of a constant internal environment

Eg. Ensures that the conditions of blood and tissue fluid in terms of temperature and water potential are kept within narrow limits

What does homeostasis allow for by ensuring a relatively stable internal environment?

Allows an organism to be independent from changes in the external environment (the environment in which an organism lives in).

Internal environment vs External environment

Internal environment - includes blood and tissue fluids that surrounds the cells in the body

External environment - surroundings / environment in which an organism lives

Examples of homeostasis in humans

1. Regulating body temperature

2. Regulating the water potential of blood and tissue fluid

3. Regulating blood glucose concentration

Why does body temperature have to be kept constant?

Enzymes in body can only works within a certain range of temperatures, and changes in body temperature may result in enzyme inactivation or denaturation.

Why must the composition of blood plasma and tissue fluid have to be kept within very narrow limits?

To ensure that they are kept at a constant water potential as drastic changes can affect cells

Higher WP than normal can cause cells to swell and burst, while lower WP than normal can cause cells to crenate.

Why must blood glucose concentration be kept within narrow limits?

Islets of Langerhans secrete insulin and glucagon (hormones) which are important in the regulation of blood glucose concentration in the body. Body cells need glucose for cellular respiration which provides cells with energy to perform vital activities. Drastic change in BGC can be dangerous.

How much glucose does the blood normally contain and when does it change?

Normally contains 70-90mg per 100cm³ of blood

Glucose levels rise after a sugary meal, and fall during vigorous physical activity or starvation.

What is the negative feedback process?

Occurs when the body reacts to bring about an opposite effect to the changes detected in homeostatic control. If the body (the system) is disturbed, the disturbance sets in motion a sequence of events that tends to restore the body to its original state.

Receptors / Sense organs

Organs or structure in the body that can detect changes in the body condition.

Stimulus

Any change from the normal condition

What is required for negative feedback to take place?

1. Normal level / set point to be maintained

2. Stimulus - change in the internal environment

3. Receptor - to detect stimulus and send signals to the control centre

4. Corrective mechanism - brings about the reverse effect of the stimulus

General stages in negative feedback

1. When condition rises above normal

   • Normal → Stimulus (rise) → Receptor (detects rise) → Control centre → Corrective mechanism → Condition decreases → (back to) Normal

2. When condition decreases below normal

   • Normal → Stimulus (decrease) → Receptor (detects decrease) → Control Centre → Corrective mechanism → Condition rises → (back to) Normal

Negative feedback cycle of homeostatic control of water potential (INCREASE)

Normal water potential in blood → Stimulus: Water potential of blood increases → Receptor: Hypothalamus stimulated → Corrective mechanism: less ADH released by pituitary gland into the bloodstream, less ADH transported to the kidneys, cells in the walls of the collecting ducts become less permeable to water, less water reabsorbed into the bloodstream, more water excreted, urine is more diluted, more urine produced → Water potential of blood decreases to the normal level → (back to) Normal water potential in blood

Negative feedback cycle of homeostatic control of water potential (DECREASE)

Normal water potential in blood → Stimulus: Water potential of blood decreases → Receptor: Hypothalamus stimulated → Corrective mechanism: more ADH released by pituitary glands into the bloodstream, more ADH transported to the kidneys, cells in the walls of the collecting duct become more permeable to water, more water reabsorbed into the bloodstream, less water excreted, urine is more concentrated, less urine is produced → Water potential of blood increases to the normal level → (back to) Normal water potential in blood

Hormone

Chemical substance produced in minute quantities by an endocrine gland, transported in the bloodstream to target organs where it exerts its effects

Functions of hormones

Influence the growth, development and activity of an organism and act as chemical messengers that help various parts of the body to respond, develop and work together smoothly.

Why must hormone production be carefully controlled?

Since hormones help to control and coordinate body activities, production must be controlled to prevent disastrous results (eg. a person will suffer from diabetes if too little of the hormone insulin is produced in the body).

Signs of diabetes

• persistently high blood glucose level

• presence of glucose in the urine

• slow / difficult healing of wounds

• frequent urination

• weight loss

• thirst

• fatigue

What are hormones produced by?

Glands (exocrine or endocrine)

Exocrine gland

Has a duct / tube for carrying away their secretion (eg. salivary gland has a salivary duct to carry saliva to the mouth, sweat gland has a duct for carrying sweat out of the body).

Endocrine gland

Ductless gland, does not have a duct to carry away its secretion, hormone produced by ductless gland is secreted directly into the bloodstream, bloodstream then distributes the hormone around the body.

Pancreas as a gland

Pancreas produce both hormones and other secretions, considered both an endocrine and exocrine gland.

• Produces pancreatic juice, carried by the pancreatic duct to the duodenum

• Contains special groups of cells (islets of Langerhans) which secrete insulin and glucagon (hormones) into the bloodstream

Pituitary gland

Plays an important role as a ‘controller’, secretes a number of hormones which control the secretion of hormones of several other endocrine glands and hence often referred to as the ‘master gland’, excretes antidiuretic hormone (ADH).

Hypothalamus

An endocrine gland that regulates the secretion of some hormones

Ovary (in females)

Secretes oestrogen and progesterone

Testis (in males)

Secretes testosterone

Islets of Langerhans in Pancreas

Secrete insulin and glucagon, increase the secretion of insulin when the concentration of blood glucose increases above normal levels and increase the secretion of glucagon when the concentration of blood glucose decreases below normal levels.

What does insulin do?

Promotes the utilisation of glucose by the cells

Effect of normal amount of insulin secreted

Decreases blood glucose concentration by making cell membranes more permeable to glucose, allowing more glucose to diffuse into the liver and muscle cells, stimulating the liver and muscle cells to convert excess glucose into glycogen for storage, and increasing the use of glucose for respiration.

Effects of lack of insulin secretion

Increases the blood glucose concentration as glucose cannot be stored or utilised by tissue cells and hence blood glucose concentration rises (some glucose subsequently lost in the urine - gives rise to diabetes mellitus). Muscle cells have no reserves of glycogen, body grows weak and continuously loses weight.

Effects of insulin oversecretion

Abnormal decrease in blood glucose concentration such that low blood glucose concentration results in a condition called shock, seizures or fits, collapsing or passing out may follow.

Main target of glucagon

Liver

How does glucagon increase blood glucose concentration?

By stimulating the conversion of glycogen into glucose, and conversion of fats and amino acids into glucose.

What happens when blood glucose concentration rises above the normal level?

Normal → Stimulus: Blood glucose concentration rises above normal → Receptor: Islets of Langerhans in pancreas are stimulated → Corrective mechanism: The islets of Langerhans secrete more insulin into the bloodstream, blood transports the insulin to the liver and muscles where insulin increases the permeability of the cell membrane to glucose and glucose is absorbed more quickly by the cells, the rate of respiration, and causes the liver and muscles to convert the excess glucose to glycogen which is stored in the liver and muscles → Blood glucose concentration decreases, which provides a feedback to the receptor to reduce insulin productions → (back to) Normal

What happens when blood glucose concentration level falls below the normal level?

Normal →