ideas /key explanation notes · ideas /key point explanation notes positive feedback mechanisms...

Biology Student’s Companion Resource SB025

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CHAPTER 9 : HOMEOSTASIS

SUBTOPIC : 9.1 Concept of homeostasis LEARNING OUTCOMES: Explain the concept of homeostasis and describe the homeostatic control system.

MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

Concept of

homeostasis

What is homeostasis?

✓ A state in which the internal environment is being maintained within a range that cells can tolerate

(Biology: Starr Taggart, 11th edition, pg 484)

✓ Dynamic consistency of the internal environment.

(Biology: Raven 6th edition, pg 1174)

▪ Dynamic is used because the conditions are never absolutely constant but fluctuate

continuously within narrow limits

✓ The existence of a stable internal environment. (Fundamentals of anatomy & physiology, 9th

edition, pg 10)

✓ The consistency of the body's internal environment

(Biology life on earth, 7th edition, pg 536)

review!

Homeostasis is the activity of cells throughout the body to

maintain the physiological state within a narrow range that is

compatible with life. Homeostasis is regulated by negative

feedback loops and, much less frequently, by positive feedback

loops. Both have the same components of a stimulus, sensor,

control center, and effector; however, negative feedback loops

work to prevent an excessive response to the stimulus, whereas

positive feedback loops intensify the response until an end point

is reached.


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MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

What is internal environment?

✓ Internal environment is all the fluid not inside the body’s cell. ▪ It consists of interstitial fluid and blood plasma: extracellular

fluid

Mechanism

Involved in

homeostatic

What is the mechanism involved in homeostasis?

✓ Feedback mechanism ▪ Help control what goes on in cells ▪ They also are major homeostatic controls over how multicell

body functions

▪ There are two types of feedback mechanism ⚫ NEGATIVE FEEDBACK MECHANISM ⚫ POSITIVE FEEDBACK MECHANISM

NEGATIVE feedback mechanisms

✓ Correct the deviations from set point ▪ Some activity changes a specific condition in the internal

environment

▪ If the changes past a certain point, a response reverse the changes


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MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

POSITIVE feedback mechanisms

✓ Controls initiate a chain of events that intensify change from an original

condition

▪ The end result is that change tends to proceed in the same direction as

the initial stimulus

▪ After a limited time, the intensification reverses the change

Homeostatic

control system

How does homeostatic control system operate?

✓ Receptor, control centre and effector are in charge of it

Collectively, they detect, process and respond to

information/stimulus

Hint!

Do you know what disrupted homeostasis? Stimulus and response are both changes in the same variables.


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MAIN

IDEAS /KEY

POINT

EXPLANATION NOTES

STIMULUS

✓ Any disruption that changes a controlled condition ▪ changes in physical or chemical factors

RECEPTOR

✓ A body structure that monitors changes in a controlled condition ▪ stimulus of changes in physical or chemical factors is detected

by receptor

▪ What will receptor do then? ✓ Sends INPUT in the form of nerve impulses or chemical

signals to a control centre

▪ Example of receptor? ✓ Cells of islets of Langerhans

CONTROL CENTRE

✓ Receives and processes the information / input supplied from the receptor

✓ Triggers the action/output that will correct the changes and send to effector

✓ Output from the control center can occur in several forms: ▪ nerve impulses, hormones or other chemical signals

✓ Example of control center: Cells of islets of Langerhans, hypothalamus

EFFECTOR

▪ Functions of effector: ✓ A body structure that receives output from the control

centre

✓ produces a response or effect that changes the controlled condition (through negative feedback mechanisms)

✓ restore condition back to normal ▪ Example of effector?

✓ Liver, muscle

Check this out!

Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, homeostatic imbalance may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death. Diabetes is an example of a disease caused by homeostatic imbalance. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention

can help restore homeostasis and possibly prevent permanent damage to the organism. Normal aging may bring about a reduction in the efficiency of the body’s control systems. This makes the body more susceptible to disease. For example, older people may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop serious heat-induced illnesses such as heat stroke.

SUBTOPIC : 9.2 Negative feedback mechanism


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LEARNING OUTCOMES: Explain the negative feedback mechanism in controlling blood glucose level.

MAIN IDEAS

/KEY POINT EXPLANATION NOTES

Negative

feedback

mechanism

➢ A form of regulation in which accumulation of an end product

of a process slows the process;

➢ In physiology, a primary mechanism of homeostasis, whereby a

change in a variable triggers a response that counteracts the

initial change.

(Campbell 11th edition)

Regulation of blood glucose level.

➢ Involves negative feedback mechanism but; both receptor and control centre are cells of islets of Langerhans (in pancreas)


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MAIN IDEAS


When blood glucose level increase

✓ Receptor: β-cells of Langerhans in pancreas are stimulated

• to secrete insulin ✓ Insulin triggers uptake of glucose from the blood ✓ by stimulate the effector (liver, muscle cells and other

tissues) to store glucose as glycogen (in liver & muscle

cell) or increase the metabolic rate

✓ so blood glucose level return (back) to normal level

When blood glucose level decrease,

✓ Receptor: α-cells of Langerhans in the pancreas are stimulated

• to secrete glucagon ✓ Glucagon stimulates effector: the liver cells

• to convert glycogen to glucose ✓ Blood glucose level return to normal

Check this out!

Glycogen is mainly stored in the liver (where it makes up as much as 10% of liver weight and can be released back into the blood stream) and muscle (where it can be converted back to glucose but only used by the muscle). Therefore, excess glucose is removed from the blood stream and stored.


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SUBTOPIC : 9.3 Human homeostatic organ: Structure and functions of kidney

LEARNING OUTCOMES:

a) Describe the structure of nephron

b) Analyse the processes in urine formation:

i. Ultrafiltration

ii. Reabsorption

iii. Secretion

c) Describe the counter current multiplier mechanism in urine formation.

d) Relate the regulation of blood water content with ADH

MAIN IDEAS


Kidney

➢ A major excretory and osmoregulatory organ ➢ A pair bean-shaped organ ➢ About 10 cm in length

➢ Consists of: • Renal cortex • Renal medulla • Renal pelvis

Structure of

nephron

➢ Nephron: basic structural and functional unit of the kidney ➢ Each kidney consists about a million nephrons ➢ Total (nephron) tubule length: approximately 80 km ➢ Enormous surface area for the exchange of materials


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MAIN IDEAS


Glomerulus

- A spherical cluster of blood capillaries - Located in the cortex

Bowman’s capsule

- A double-walled, cup-shaped swelling capsule - Blind end of the tubule - Located in the cortex

Proximal convoluted tubule

- Lumen is continuous with the Bowman’s capsule - Highly coiled - Located in the cortex

Loop of Henle

- Hair-pin shaped - Have descending limb & ascending limb - Located in the medulla

Distal convoluted tubule

- Highly coiled - Located in the cortex

Collecting duct

- End of kidney - Eventually drain into the pelvis of the kidney (from where the urine flows into the ureter) - Located in the medulla

Nephron and its

blood supply

Main blood vessels:

1. Renal artery: - supplies oxygenated blood from aorta to kidney

2. Renal veins: - carries deoxygenated blood from kidney to posterior vena cava


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MAIN IDEAS


An individual nephron and its blood supply:

Renal artery Afferent arteriole Glomerulus

Efferent arteriole Vasa recta Renal vein

Urine formation

Involved 3

processes:

1. Ultrafiltration 2. Reabsorption 3. Secretion

1.Ultrafiltration

➢ Takes place between the glomerulus and the Bowman’s capsule ➢ Occurs due to the hydrostatic pressure caused by the blood pressure ➢ Blood enters the glomerulus via afferent arteriole (larger diameter)

and leaves via efferent arteriole (smaller diameter)

• Produce high hydrostatic pressure - Forces small molecules (glucose, amino acids, sodium,

potassium, chloride, bicarbonate, other salts, water and urea);

except red blood cell, plasma proteins and platelets

• through the walls of capillaries and Bowman’s capsule into the capsular space


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MAIN IDEAS


✓ The structure of Bowman’s capsule also contribute to the efficient ultrafiltration process

➢ The wall of Bowman’s capsule consists of a specialized epithelial cell called podocytes (pod:foot)

➢ Podocytes: cover most of the capillaries ➢ Foot processes of adjacent podocytes are separated by narrow

gaps called filtration slits

➢ the perforated walls of the capillaries and the podocytes form a filtration membrane

- Permits fluid and small solutes to pass


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MAIN IDEAS


2.Reabsorption ➢ The process of absorbing useful substances (glucose, amino acids, vitamins, most of the water, sodium and chloride ions) from the

tubule into capillaries which wrapped around tubule

➢ Occurs in: 1. Proximal convoluted tubule 2. Loop of Henle 3. Distal tubule 4. Collecting duct


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MAIN IDEAS


Structure of

Proximal

Convoluted

tubule

➢ The epithelial luminal surface is covered with densely packed microvilli

• to increase the surface area, facilitating their reabsorptive function

➢ The cytoplasm of the cells is densely packed with mitochondria • to supply energy for active transport of sodium ions out of the

proximal tubule.

• Water passively follows the sodium out along its concentration gradient.

Reabsorption at

proximal

convoluted

tubule

Proximal convoluted tubule

Most reabsorption occurs : over 80 %

• All glucose, amino acids, vitamins and hormones, 85% of NaCl and other ions (by

active transport)

• 40-50% of urea by diffusion • 85% of water as concentration of ions

increases in plasma. Water moves out by

osmosis

Reabsorption at

Loop of Henle

Function:

To create a water potential gradient

- Between the filtrate and the interstitial fluid in the medulla The longer loop of Henle → urine produced is more concentrated

Structure of

Loop of Henle

Divided into 2:

1. Descending limb • Thin walls • Highly permeable to water • Impermeable to NaCl and other solutes

2. Ascending limb • Impermeable to water but permeable to NaCl and urea

- Thin segment (transport NaCl passively) - Thick segment (transport NaCl actively)


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MAIN IDEAS


Counter-current

multiplier

✓ Reabsorption occurs in loop of Henle is contribute by counter-current multiplier reaction

✓ Counter current multiplier: The interaction between the flow of filtrate through the ascending and descending limbs of loop of Henle

and the flow of blood in the vasa recta

Function:

▪ To establish and maintain a high salt concentration in the loop of Henle extending from the cortex through the medulla

▪ Enable water to be reabsorb into the vasa recta to conserve water.

➢ In the descending limb :


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MAIN IDEAS


• As the filtrate flow from cortex to medulla, water is drawn out by osmosis

• NaCl become more concentrated in the tubule, increasing the osmolarity of the filtrate

• Filtrate concentration is highest at the bottom of the loop of Henle (hairpin loop of Henle)

➢ At the ascending limb: • Filtrate concentration is high in the tubule • As the filtrate move through the thin segment, NaCl passively

diffuse out into the interstitial fluid

• and at the thick segment, NaCl is actively pumped out into interstitial fluid

• Produce a high concentration of NaCl around the descending limb

• Water potential in the interstitial fluid is lower • Causes water in the descending limb drawn out by osmosis • Creating a concentration gradient in the medulla • The water and NaCl moves into the vasa recta surrounding the

loop of Henle

Reabsorption at

distal

convoluted

tubule

➢ Distal convoluted tubule receives a hypotonic (high water concentration) filtrate from the ascending limb

➢ Distal convoluted tubule impermeable to water but (the permeability to water) depends on hormonal control

• ADH (antidiuretic hormone) - under regulation of ADH, distal convoluted tubule became

more permeable towards water

• aldosterone - under regulation of aldosterone, distal convoluted tubule

became more permeable towards NaCl and water

(secondary effect)


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MAIN IDEAS


Reabsorption at

collecting duct

➢ The collecting duct drains the filtrate from cortex to medulla to the renal pelvis

➢ Permeability to water and urea is under hormonal control (anti-diuretic hormone (ADH)).

➢ When the filtrate pass along the collecting duct, • water moves out by osmosis to the interstitial fluid • Some urea will also diffuse out along with NaCl, contributes to

the high concentration of solute (lower water potential) in the

interstitial fluid (aids the water reabsorption in descending limb

of loop of Henle)

• This urea is recycled by diffusion into the ascending limb of loop of Henle

3. Secretion ➢ Substances from blood capillaries were secreted into the tubule ➢ Occurs in the distal convoluted tubule (mainly) and the proximal

convoluted tubule

➢ In distal tubule; H+ and NH3 secreted from the blood into the filtrate to maintain blood pH level.

➢ Secretion of K+ occurs under hormonal control by aldosterone - Proximal & distal tubule also actively secretes harmful or toxic

substances (Example: drugs such as penicillin and caffeine) into

the filtrate

- to be removed (from human blood) by urine


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MAIN IDEAS


SUMMARY :

Urine formation

Regulation of

blood water

content

➢ Control the normal blood osmotic pressure (normal water potential of blood plasma) controlled by antidiuretic hormone (ADH)

➢ Produced in hypothalamus ➢ Released by posterior pituitary gland ➢ Target tissue

- Distal convoluted tubule and collecting ducts ➢ Actions

- Increases permeability to water and urea


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MAIN IDEAS


review!

The most common disease of man and animals related to antidiuretic hormone is diabetes insipidus. This condition can arise from either of two situations:

• Hypothalamic ("central") diabetes insipidus results from a deficiency in secretion of antidiuretic hormone from the posterior pituitary. Causes of this disease include head trauma, and infections or tumors involving the hypothalamus.

• Nephrogenic diabetes insipidus occurs when the kidney is unable to respond to antidiuretic hormone. Most commonly, this results from some type of renal disease, but mutations in the ADH receptor gene or in the gene encoding aquaporin-2 have also been demonstrated in affected humans.

The major sign of either type of diabetes insipidus is excessive urine production. Some human patients produce as much as 16 liters of urine per day! If adequate water is available for consumption, the disease is rarely life-threatening, but withholding water can be very dangerous. Hypothalamic diabetes insipidus can be treated with exogenous antidiuretic hormone.

Dehydration/

blood osmotic

pressure high/

water potential

low

Normal blood osmotic pressure(normal water potential

of the blood plasma)

DehydrationBOP

Blood water potential

Detected by osmoreceptorsin the hypothalamus

Stimulated posterior pituitary to release ADH

Distal tubule andCollecting duct

Permeability towardswater and urea increase

Greater water and urea absorption

Small volume ofconcentrated urine produced

During low water intake,high salt intake or lots

of sweating

• Less intake of water caused an increased in blood osmotic pressure (BOP)/ osmolarity/ low blood volume

• (This condition) is detected by osmoreceptor in hypothalamus • Leads to the stimulation of the posterior pituitary gland to

secrete antidiuretic hormone (ADH)

• ADH acts on nephron tubule: - increases water permeability of distal tubule and

collecting duct

• More water is reabsorbed back (into the blood vessel) by osmosis • Blood osmotic pressure/ blood volume returns back to normal

level

• This regulation involved negative feedback mechanism


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MAIN IDEAS


High water

intake/ blood

osmotic

pressure low/

water potential

high

Normal blood osmotic pressure(normal water potential

of the blood plasma)

BOP Blood water potential

Detected by osmoreceptorsin the hypothalamus

Inhibit the release of ADHfrom posterior pituitary

Distal tubule andCollecting duct

Permeability towardsWater and urea decrease

Less water and urea absorption

Large volume ofdiluted urine produced

During high water intake,low salt intake or little

of sweating

• High intake of water caused a decreased in blood osmotic

pressure/ osmolarity/ high blood volume

• (this condition) Detected by osmoreceptor in hypothalamus • Inhibits the posterior pituitary gland to secrete antidiuretic

hormone (ADH)

- reduce water permeability of distal tubule and collecting duct

• Less water is reabsorbed (into the blood vessel) by osmosis • Blood osmotic pressure/ blood volume returns back to normal

level

• Involved negative feedback mechanism

ideas /key explanation notes · ideas /key point explanation notes positive feedback mechanisms...

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