Homeostasis

Homeostasis

Homeostasis, read pg 817, SOLOMON TEXTBOOK

Temperature Blood pressure Osmotic pressure

Homeostasis “steady state”

pH value [sugar] [salt]

A process of maintaining constant physical and chemical factor within an internal body environment

WHY? Why is homeostasis important? • Biochemical reactions are controlled by enzymes. – Changes in pH and temperatures – affect the rate of enzymes-controlled reactions, denature the enzymes and proteins

• Water moves in and out of cells by osmosis – Maintaining constant water potential in the interstitial fluid, avoid cellular

Mechanism of Homeostasis • All homeostatic mechanisms – use negative feedback to maintain a constant value (set point)

• Feedback system (feedback loop) – A cycle of events, status of a body condition is monitored, evaluated, changed, remonitored, reevaluated

• Negative feedback – Whenever a change occurs in a system, the change“Regulatory trigger a corrective mechanism mechanism maintainto start, brings the system back negative to normal homeostasis through feedback loops”

Negative feedback • Applies to biological system, electronic circuits (e.g. central heating system, oven, air-conditioner system)

– When your oven gets too hot, heating switches off; allows the oven to cool down. – Eventually it will get cold, the heating will switch back in, so raising the temperature once again.

• In a system controlled by negative feedback, the set level is never perfectly maintained, but constantly fluctuate/ oscillates within the set point. Constant value

Homeostasis • Internal conditions vary, but always within relatively narrow limits. • Hormone-controlled homeostatic mechanism, significant time-lag before corrective mechanism can be activated. – It takes times for protein synthesis to commence, the hormone to diffuse into the blood stream and for it to circulate around the body and take effect.

3 components in homeostatic control mechanism Detect 1. Receptors

2. Control centre

3. Effectors

changes

Define changes Triggers appropriate corrective actions

Execute the changes

Homeostasis is achieved by: Negative feedback

Types of feedback (1) Negative feedback (2) Positive feedback

Negative feedback For example, – If the human hypothalamus detects a high blood temperature – it send nerve impulses to sweat glands, which increase sweat output and cause evaporative cooling. – When the body temperature returns to normal, no additional signals are sent. Receptors (Hypothalamus, skin) > Control center (Hypothalamus) > Effectors (muscles/sweat glands/BV)

• Positive feedback – Occurs when a change in some variable causes a reaction which increases that change. – pushes changes further in the same way, strengthen or reinforce a change in the body’s controlled conditions For example :Uterine contraction During childbirth, the baby’s head press against receptors near the opening of the uterus, stimulates uterine contractions which cause greater pressure against the uterine opening, heightening the contraction, which causes still greater pressure. Positive feedback brings childbirth to completion.

Receptors (stretch-sensitive nerve cell in cervix) > Control center (brain, release oxytocin) > Effectors (muscles in wall of uterus contract more forcefully)

Examples of homeostasis control system – Temperature homeostasis (Thermoregulation) – Blood glucose homeostasis – Blood water homeostasis (osmotic conc. of blood; osmoregulation) >Will discusses in Chapter: Excretion

Temperature homeostasis Thermoregulation > Is the regulation of body temperature References: pg 822

Thermoregulation • Classification based on the source of heat in determining body temperature – use metabolic heat to regulate body temperature: Endotherms – use environmental energy and behavioral adaptations to regulate its body temperature: Ectotherms • Behavioral adaptation: hibernation • E.g. lying in the sun when cold, moving into shade when hot

Ectotherms and Endotherms • Ectotherms

(Ecto-: outside)

– Absorb heat from their surrounding

– most invertebrates, fishes, amphibians, and reptiles

• Endotherms

(Endo-:

inside/within) – Derive heat from metabolic proceses

– birds and mammals – Have homeostatic mechanism

Honeybees are endotherms By adjusting metabolic rates in their flight muscles, honeybees are able

Benefits of homeostatic control (Birds and mammals) • Become less dependent on the external environment – able to control changes in its internal environment to compensate for changes in external conditions.

• Able to live in a wider range of habitats or in areas with variable conditions. • Can increase or decrease the metabolic rate of its body according to its requirements. • A controlled internal environment enables

Thermoregulation • Our response in encountering hotter and colder condition is voluntary. – Too hot, switch on air-conditioner or move into shade. – Too cold, put extra clothes on or turn on the heater.

• When these response are not enough, the thermoregulatory centre is stimulated, involves autonomic nervous system (A.N.S.), responses are all involuntary.

Thermoregulation • In humans, body temperature is controlled by the thermoregulatory centre: Hypothalamus • Hypothalamus receives input from two sets of receptors: – Receptors in the hypothalamus (monitor the temperature of the blood as it passes through the brain). – Receptors in the skin (monitor the external temperature).

Thermoregulation • When body temperature rises, – The erector muscles relax, hairs lie flat against the skin, no longer trapping air, allowing more heat to be lost by radiation. (radiation: heat transfer from body to air)

– The dermal blood vessels dilate and the sweat glands are stimulated into vigorous secretory activity. – Evaporation of sweat from skin surface dissipates body heat and cools thecooling body, Heat transfer from the body to the surroundings, evaporative thus preventing overheating.

• When the external environment is cold, – Impulses to the erector muscle attached to your hair follicles contract, which made your hairs stand, trapping air. – dermal blood vessels constricted. This causes the warm blood to bypass the skin temporarily and allows skin temperature to drop to that of the external environment. – Shivering: involuntary shuddering contractions of the skeletal muscles, effective in increasing body temperature, muscle activity produces large amounts of heat. – Enhance thyroxine (thyroid gland) and adrenaline (adrenal gland) release: increases the metabolic rate.

Capillary

Epidermis

Nerve endings

Openings of sweat glands

Stratum corneum Stratum basale

Melanocyte (pigment cell) Hair erector muscle Hair shaft Sensory receptor (Pacinian corpuscle)

Dermis Subcutaneous tissue Artery Vein

Sweat gland

Hair follicle Sebaceous gland

• In mammals, the integumentary system – Acts as insulating material Fig. 39-1, p. 829

(a) Erector muscle contract

(b) Erector muscle relax

Vasodilation and vasoconstriction • Vasodilation of cutaneous blood vessels – Blood flow in the skin increases, facilitating heat loss.

• Vasoconstriction of cutaneous blood vessels – Blood flow in the skin decreases, lowering heat loss. – Blood is restricted to deep body areas and largely bypasses the skin. – The skin is separated from deeper organs by a Restriction flow to the skin for a brief period(fatty) is not a tissue, problem , layerblood of insulating subcutaneous but if prolonged exposure to very cold weather, skin cells deprived of heat loss reduced. oxygen and nutrients begin to die. This extremely serious condition is called frostbite.

Sweat glands secrete sweat that evaporates, cooling the body.

• In humans, the hypothalamus

Thermostat in hypothalamus activates cooling mechanisms.

(underside part of the vertebrate forebrain), – Contains a group of nerve cells that function as a thermostat

Figure 40.21: The thermostat function of the hypothalamus in human thermoregulation

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Increased body temperature (such as when exercising or in hot surroundings)

Blood vessels in skin dilate: capillaries fill with warm blood; heat radiates from skin surface.

Body temperature decreases; thermostat shuts off cooling mechanisms.

Homeostasis: Internal body temperature 36–38°C Body temperature Blood vessels in Decreased body increases; temperature skin constrict, thermostat (cold shuts off warming diverting blood from skin to surroundings) mechanisms.

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deeper tissues and reducing heat loss from skin surface.

Skeletal muscles rapidly contract, causing shivering, which generates heat.

Thermostat in hypothalamus activates warming mechanisms.

SUMMARY

Mechanism of body temperature • Heat-loss mechanisms

– Skin hairs lowered – Vasodilation of cutaneous blood vessels (skin arterioles dilate) – Enhanced sweating

• Heat-promoting mechanisms

– Skin hair raised, trapping air – Vasoconstriction of cutaneous blood vessels (skin arterioles constrict) – Shivering – Glands secreting adrenaline and thyroxine

Increase in blood temperature

NORM

Detected by hypothalamus

EFFECT: Increase sweating, Vasodilation, Hairs lay flat

Control of body temperature

(Blood temperature)

Decrease in blood temperature

Detected by hypothalamus

EFFECT: Decrease sweating, Vasoconstriction, Hairs raised, Shivering

NORM

Blood glucose homeostasis Read pg 1043

Blood glucose homeostasis • Glucose is a major fuel for cells • Its metabolism, regulated by hormone action

Glucose regulation

pg 955

1 When blood glucose

level rises, the pancreas secretes insulin, a hormone, into the blood. 2

Insulin enhances the uptake of glucose in body cells and stimulates the liver and muscle cells to store glucose as glycogen. As a result, blood glucose level drops.

β-cells Hyperglycemic STIMULUS: Blood glucose level rises after eating.

4 Glucagon promotes

the breakdown of glycogen in the liver and the release of glucose into the blood, increasing blood glucose level.

Homeostasis: Blood glucose level 90 mg/100 mL

Hypoglycemic STIMULUS: Blood glucose level drops below set point.

α-cells Figure 41.3

3

When blood glucose level drops, the pancreas secretes the hormone glucagon into the blood.

Increase in blood glucose

Detected by β cells (islets of Langerhans in the pancreas)

Increase in insulin secretion

NORM (Blood glucose)

EFFECT: √ Enhances the uptake of glucose in body cells. √ Stimulates the liver and muscle cells to store glucose as glycogen.

NORM

Control of blood glucose concentration EFFECT:

Decrease in blood glucose

Detected by α cells of pancreas

Increase in glucagon secretion

Glucagon promotes the breakdown of glycogen in the liver and the release of glucose into the blood

Control of blood glucose concentration INSULIN GLUCAGON • A hormone produced by the β cells of the islets of Langerhans in the pancreas.

• A hormone produced by the α cells of the islets of Langerhans in the pancreas.

• Secretion is stimulated by the rise in blood glucose.

• Secretion is stimulated by the fall in blood glucose.

• Speeds up the rate at which glucose is taken into liver and muscle cells from the blood, stored as glycogen.

• breakdown of glycogen in the liver. • Stimulates the formation of glucose from other molecules

THREE sources of blood glucose • Digestion of carbohydrates in the diet • Breakdown of glycogen (glycogenesis) • Conversion of non-carbohydrate compounds (gluconeogenesis)

The pancreas secretes insulin and glucagon

• Blood glucose concentration, controlled by the pancreas • Pancreas has – Glucose receptor cells, which monitor the concentration of glucose in the blood – Endocrine cells (called the Islets of Langerhans), which secrete hormones. ∀ α-cells: secrete the hormone, glucagon ∀ β-cells: secrete the hormone, insulin

Insulin and glucagon: control of blood glucose • Insulin stimulates – The uptake of glucose by body cells and – The conversion of glucose to glycogen in liver and muscle cells. – Therefore, decrease blood glucose

• Glucagon stimulates – The breakdown of glycogen to glucose in the liver (glycogenolysis) – In extreme case, stimulate the synthesis of glucose from pyruvate – Therefore, increases blood glucose Antagonistic effects of glucagon and insulin are vital to glucose homeostasis, management of both fuel storage and fuel consumption by body cells.

What happen the mechanism ……….go awry?

Diabetes mellitus • Diabetes is a disease caused by a failure of glucose homeostasis – Type I diabetes – Type II diabetes

Type I diabetes Appears during childhood, disability to produce insulin

• Insulin-dependent diabetes or early onset diabetes • Due to an autoimmune disorder, killing off the β-cells • Treatment: Insulin injection

Type II diabetes

Mostly occurs after age of 40 years

• Non insulin-dependent diabetes or lateonset diabetes • Most type II diabetics produce insulin, but the amount is inadequate or • The insulin receptors are unable to respond to insulin, a phenomenon called “insulin resistance” • Cause: excess body weight, high sugar diet, lack of exercise

Symptoms of diabetes • Excessive thirst – Due to osmosis of water from cells to the blood, which has a low water potential

• Copious urine – Huge urine output due to excess water in blood

• Poor vision – Due to osmotic loss of water from the eye lens

• Tiredness – Due to loss of glucose in urine and poor uptakes of glucose by liver and muscle cells

• Ketosis – Abnormal condition of excess ketone bodies (fatty acid metabolites) production, break down of lipids to supply energy

• Muscle wasting

THE END

Blood water homeostasis (Osmoregulation) Homeostasis of blood volume and osmolality

Blood water homeostasis (Osmoregulation) • The water potential of the blood must be regulated to prevent loss or gain of water from cells. • Osmoregulation

– Regulates solute concentrations and balances the gain and loss of water

• Osmoregulation is based largely on controlled movement of solutes

– Between internal fluids and the external environment

• Blood water homeostasis (osmoregulation) is controlled by hypothalamus, which

1

Osmoreceptors in hypothalamus

Thirst

Hypothalamus

ADH

Drinking reduces blood osmolarity to set point Increased permeability

Pituitary gland Distal tubule

STIMULUS: The release of ADH is triggered when osmoreceptor cells in the hypothalamus detect an increase in the osmolarity of the blood

H2O reabsorption helps prevent further osmolarity increase

Collecting duct

Homeostasis: Blood osmolarity

When body becomes dehydrated, the osmotic [ c ] of the blood ↑ Posterior lobe of the pituitary glandADH Figure 44.16a: Antidiuretic hormone (ADH) enhances fluid retention by making the kidneys reclaim more water

Low blood water potential (too little water) • Hypothalamus controls the sensation of thirst and it also secretes the hormone ADH (antidiuretic hormone; a.k.a vasopressin). • ADH is stored in pituitary gland, – and its target cells are the distal tubules and collecting ducts of the kidney nephrons.

• ADH increases the permeability of the epithelium to water. • Increased water reabsorption, reduces urine volume.

Low blood water potential (too little water, high osmotic concentration) • Osmolarity of the blood subside, – reduces the activity of osmoreceptor cells in the hypothalamus – and less ADH is secreted

Homeostasis: Blood pressure, volume

Increased Na+ and H2O reabsorption in distal tubules

Aldosterone Arteriole constriction

STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (such as due to dehydration or loss of blood)

Adrenal gland

Angiotensin II

Distal tubule

Angiotensinogen JGA Renin production

Renin

Figure 44.16b

(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase in blood volume and pressure.

In response to low blood pressure OR blood vol., READ pg 936

Increase retention Aldosteroneof Na+ by the kidneys, greater fluid retention, increases blood vol.

Renin Angiotensinoge n (plasma protein)

Angiotensin II Vasoconstriction, increase blood pressure

High blood water potential (low osmotic concentration) > low blood pressure • A second regulatory mechanism involves juxtaglomerular apparatus (JGA), located near the afferent arteriole that supplies blood to the glomerulus.

• When the blood pressure/blood volume in the afferent arteriole drops, enzyme renin initiates the conversion of angiotensinogen (a plasma protein) to angiotensin II (a peptide).

High blood water potential (low osmotic concentration) > low blood pressure

• Angiotensin II

– raises blood pressure by constricting arterioles, decreasing blood flow to many capillaries (including those in the kidney) – Stimulates the proximal tubules of the nephrons to reabsorb more NaCl and water. – Stimulates the adrenal glands to release aldosterone (hormone), that acts on the nephrons distal tubule, reabsorb more sodium and water

• This reduces the amount of salt and water excreted in the urine and consequently raises blood volume and pressure

• The renin-angiotensin-aldosterone system (RAAS) – Is part of a complex feedback circuit that functions in homeostasis

Juxtamedullary nephron

Cortical nephron

Afferent arteriole from renal artery

Glomerulus Bowman’s capsule

Renal cortex

Proximal tubule Peritubular capillaries

Collecting duct

20 µm Renal medulla

To renal pelvis

SEM

Efferent arteriole from glomerulus

Distal tubule

Collecting duct

Branch of renal vein

Loop of Henle

Descending limb Ascending limb Vasa recta

(c) Nephron

(d) Filtrate and blood flow

THE END