Biology Chapter 9 - Homeostasis and Hormonal Control

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.

What does the IOL secrete and why must blood glucose concentration be kept within narrow limits + what is glucose used for?

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 → Stimulus: Blood glucose concentration falls below normal → Receptor: Islets of Langerhans in the pancreas are stimulated → Corrective mechanism: Islets of Langerhans secrete more glucagon into the bloodstream, blood transport the glucagon to the liver and muscles where the glucagon causes the conversion of stored glycogen back to glucose and from the liver, glucose enters the bloodstream → Blood glucose concentration increases, which provides a feedback to the receptor to reduce glucagon production → (back to) Normal

What is diabetes mellitus?

A disease in which the body is unable to control its blood glucose concentration in order for it to remain within normal limits, and hence as a result the BGC can rise to a level that exceeds the kidney’s ability to completely reabsorb all the glucose while the glucose that is not reabsorbed is excreted in the urine.

Type 1 diabetes

Develops early in a person’s life, known as juvenile or early-onset diabetes. Islets of Langerhans are unable to produce or secrete sufficient insulin. Inherited.

Type 2 diabetes

Occurs later in a person’s life, known as late-onset diabetes, more likely to be developed by overweight people. Develops when the target cells (eg. muscle cells) don’t respond well to insulin.

What do people with diabetes have to do?

Measure their BGC and test their urine regularly, watch their diet carefully and make sure they don’t take in too much carbohydrates. Need to take medication that increases the uptake of glucose from bloodstream into cells.

People with type 1 diabetes have to inject insulin regularly into the fat tissue under the skin (eg. into abdomen). If they use too much insulin, exercise too much / eat too little, BGC can drop too low and in some cases, low BGC may cause them to go into a coma, hence need to be ready with sugary food like glucose sweets.

Insulin resistance

Occurs in type 2 diabetes when the liver and muscle cells do not respond well to insulin and are unable to take in excess glucose in the blood, causing blood glucose level to remain high.

Risk factors of type 2 diabetes

1. Obesity - main risk

2. Age - risk is higher as you get older

3. Family history - more likely to develop if family members have diabetes

4. Blood lipid levels - risk is increased if you have high levels of ‘bad’ cholesterol or DH and low levels of ‘good’ cholesterol or HDL in blood

5. Sedentary lifestyle - risk is higher the less active you are as physical activities help to control weight and use up excess glucose to provide energy for muscular concentrations and make the liver and muscle cells more sensitive to insulin.

How to reduce the risk of getting type 2 diabetes

Leading a healthy lifestyle by eating healthily (consuming foods that are low in calories and high in fibre - more fruit and vegetables), engaging in more active physical activities, avoiding being inactive for long periods of time (sitting for too long), maintaining a healthy body weight based on age and height.

What does skin do?

Forms a protective covering over the body surface and plays a role in the regulation of body temperature and excretion.

How do the arterioles help regulate body temperature?

Through contraction and dilation

Vasodilation

Dilation of the arterioles. When the skin arterioles dilate, more blood is sent to blood capillaries in the skin (shown when we blush / become flushed after vigorous activities). Skin turns red because numerous blood vessels in the skin dilate.

Vasoconstriction

The constriction of the arterioles in the skin which reduced the amount of blood flowing through the capillaries in the skin and causes us to become pale.

What is secreted sweat and where does it flow?

Secreted sweat is made up of water, contains dissolved salts (mainly sodium chloride) and small amounts of metabolic waste products like urea, and secreted / flows continuously through a sweat duct to a sweat pore that opens at skin surface.

Difference in appearance between small quantities and large quantities of sweat

Sweat produced in small quantities evaporate almost immediately, while sweat produced in large quantities appears as droplets on the skin or as running streaks of liquid.

What factors cause the amount of sweat produced to vary?

external and internal envrionmental conditions

How does the skin regulate body temperature?

Through sweat, to remove excess heat from the body

What are sensory receptors and what do they do?

Sensory receptors / organs are structures in the body that detect changes in the environment (i.e. stimuli). The nerve endings in the skin are sensory receptors, enable you to sense pain, pressure and temperature changes in the external environment.

Thermoreceptors

Receptors that detect temperature changes

How does the body maintain a constant internal temperature?

By balancing heat release and heat loss as the heat is released in your body and lost from your body at all times.

How is heat gained and lost from the body?

Heat is released within the body as a result of metabolic activities such as cellular respiration, and high levels of cellular respiration take place in the muscles and liver, hence a large amount of heat is released in these organs which is then distributed to the rest of the body via the bloodstream. Body also gains heat through vigorous muscle exercises, consumption of hot food and being in warm environments.

Heat is lost through the skin, by evaporation of water in sweat form from the surface of the skin, in the faeces and urine, and in the air that is exhaled.

Amount of blood flowing through the skin capillaries affects heat loss through the skin surface.

When skin arteriole dilates, blood flow to the skin increases and more heat is carried to the skin surface through the arteriole. When more blood flows to the skin surface, more heat is lost. Similarly, when the skin surface constricts, blood flow to the skin is decreased and less heat is carried to the surface, and when less blood flows to the skin surface, less heat is lost.

Why does excess heat need to be removed from the body?

One could die from overheating due to excess heat in the body.

Hypothalamus

A part of the brain that monitors and regulates the body temperature and receives information about temperature changes from two types of thermoreceptors.

What two sources does the hypothalamus receive information from?

Thermoreceptors in the skin which detect temperature from the environment and thermoreceptors in the hypothalamus which detect the temperature of blood.

What happens when rate of heat loss decreases?

On a warm day and when we perform vigorous muscular activities, large amount of heat released internally accumulates in the body which causes a rise in blood temperature. Hypothalamus detects this rise and sends out nerve impulses to the relevant parts of the body to bring about the necessary changes within the body which will then cause a decrease in the body temperature.

What happens when the surrounding temperature is higher than our body temperature?

The thermoreceptors in the skin detect the rise in the external temperature and immediately send nerve impulses to the hypothalamus. The absorbed heat is mainly lost by the evaporation of sweat from the body.

Why is extra heat removed from the body?

To ensure body temperature is maintained within a normal range and kept constant.

Homeostatic control of body temperature through negative feedback loop when the temperature rises

Normal body temperature → Body temperature rises above normal → Thermoreceptors in the skin and the hypothalamus are stimulated → The arterioles in the skin dilates which allows more blood to flow through blood capillaries in the skin and hence more heat will be lost through the skin, sweat glands become more active and produce more sweat and as more water in the sweat evaporates from the surface of the skin, more heat is lost from the body and is an efficient means of losing heat, and metabolic heat is decreased which reduces the amount of heat released within the body → Body temperature decreases → (back to) Normal body temperature

What happens when rate of heat loss from the body increases?

A drop in temperature at the skin surface stimulates the thermoreceptors in the skin to rapidly send nerve impulses to the hypothalamus, and a drop in temperature of the blood is also detected by the hypothalamus. In both cases, the hypothalamus will promptly send nerve impulses to the relevant parts of the body to bring about certain changes within the body which will cause an increase in the body temperature.

Homeostatic control of body temperature through negative feedback loop when the temperature falls

Normal body temperature → Body temperature falls below normal → Thermoreceptors in skin are stimulated and impulses are transmitted to the hypothalamus in the brain → The arterioles in the skin constrict which allows less blood to flow through blood capillaries in the skin and hence, less heat will be lost through the skin. Sweat glands also become less active and produce less sweat, and as less water in the sweat evaporates from the surface of the skin, less heat is lost from the body. Metabolic rate is increased, which increases the amount of heat released within the body. When the above responses are not sufficient to prevent a drop in body temperature, shivering (a reflex contraction of the body muscles) occurs. This rapid contraction and relaxation of the skeletal muscles increases the amount of heat released → Body temperature increases → (back to) Normal body temperature

What happens in conditions of prolonged exposure to extremely cold temperatures?

Frostbite may occur, fingers may lose their sensation, ice crystals form around the cells causing death and destruction of the cells in the skin.