Homeostasis: Active Regulation of the Internal Environment

Lecture 18

Homeostasis

The active process of maintaining internal stability (balance) despite changes in the external environment

What physiological systems does homeostasis regulate?

Acidity, saltiness, water, oxygenation, temperature, energy availability

Thermoregulation

The active process fo closely regulating body temperature around a set value

Why is thermoregulation essential for survival?

• Prevents proteins from denaturing (or unraveling) when too hot

• Prevents chemical reactions from slowing or ice crystals from forming and destroying cells when too cold

Endotherms vs. Exotherms

• Endotherms = generate most of their own heat through internal processes - metabolism and muscular activity

• Exotherms - get most of their heat from the environment

How do ectotherms regulate temperature if they don’t produce heat internally?

Regulates body temperature by behavior only and NOT physiological/internal processes

Advantage vs. disadvantage of endotherms?

• Disadvantage

  • Use a lot of food energy to produce their heat

• Advantage

  • Independence from environmental conditions

  • Improved oxygen use capacity sustains greater muscular activity

Homeostatic mechanisms that regulate temperature, body fluids, and metabolism are primarily

Negative feedback systems

What happens when body temperature deviates from the set point (desired value)?

• Results in compensatory action

• Goal = return to set zone

What detects temperature in the body?

Receptors in the skin, body core, and hypothalamus

What happens after temperature is detected?

Information is transmitted to the spinal cord, brain stem, and hypothalamus

If body temperature is outside the set zone, what behaviors can be initiated to return temperature to the set zone?

Physiological and behavioral

Does the body have one or many physiological systems for the generation of heat & cooling if it gets overheated?

Multiple physiological systems

What brain area is especially important for thermoregulation?

Preoptic area (POA) of the hypothalamus

Two thermoregulatory systems

• Preoptic area of the hypothalamus (POA)

  • Lateral hypothalamus

Preoptic area of the hypothalamus (POA)

Responsible for the physiological responses to cold, such as shivering and constriction of the blood vessels

Lateral hypothalamus

Controls behavioral regulation of temperature, such as turning on heating lamps or cooling fans

What are the 3 behavioral strategies for regulating temperature?

1. Change exposure of the body surface

2. Change external insulation

3. Change surroundings

Why do iguanas move toward heat when infected?

If exposed to bacteria, heat will create a fever by moving toward a heat source —> helps them kill off the virus

Lemur thermoregulation example

Lemurs bask in the sun

• To survive uncharacteristically seasonal and unpredictable climates

Kangaroo thermoregulation example

Kangaroos lick their forearms to cool down

• Promotes evaporation

• The evaporating saliva cools the blood which then circulates throughout the body

How do mammals use behavior to regulate temperature?

To keep heat:

• Curl up

• Build nests

• Huddle

• Increase movement

• Increase food intake

To lose heat:

• Seek shade

• Spread body out

• Pant

• Reduce activity

• Decrease food intake

Why is fluid regulation necessary in the body?

We are constantly losing and gaining water and salts —> NEED physiological and behavioral mechanisms to replace them

Why is fluid balance especially important for the brain?

• Brain is 80% water

• Requires a careful balance of fluids and dissolved salts

Imbalance —> impaired brain function

How does the body differ from food storage vs. water storage?

Unlike with food, we do not store excess water in the body

What are the two main fluid compartments?

• Intracellular compartment = fluid contained within our cells (where most water resides)

• Extracellular compartment = fluid outside our cells

What are the components of the extracellular compartment?

Divided between the

• Interstitial fluid (fluid between cells)

• Blood plasma (protein-rich fluid that carries red and white blood cells)

How does water move in and out of cells

Through aquaporins (specialized protein channels) in cells via osmosis

Osmosis

• Passive movement of a solvent (the liquid)

• Moves across a membrane to equalize solute concentration

Solvent vs. solute

• Solvent: liquid

• Solute: dissolved substances (salt, molecules)

Osmotic pressure

The physical force that pushes or pulls water across the membrane

Osmolality

Number of solute particles per unit volume of solvent

Isotonic salt solution

• Same salt concentration as body fluids (~0.9%)

• No net movement of water

Hypertonic solution

More salt than an isotonic solution —> water moves OUT of cells

Hypotonic solution

Less salt than an isotonic solution —> water moves INTO cells

What happens to cells in hypertonic vs. hypotonic environments?

Hypertonic:

• Cells lose water

• Shrink

Hypotonic:

• Water will push into cells

• Cells swell / may burst

Extreme imbalance can damage or kill cells

Two kinds of thirst

• Osmotic thirst

• Hypovolemic thirst

What triggers osmotic thirst?

High extracellular solute concentration (too much salt / not enough water)

What causes osmotic thirst in daily life?

Losing water through:

• Respiration

• Sweating

• Urination

Eating salty foods

What detects osmotic thirst?

Osmoreceptors in the hypothalamus

• Respond to changes in osmotic pressure

• Detect when water leaves cells

Brain regions involved in osmotic thirst

Hypothalamus

• POA

• Supraoptic nucleus

• Anterior hypothalamus

Circumventricular organs (CVOs)

How do we fix osmotic thirst?

Drinking water —> restores extracellular fluid to isotonic state

What is the first physiological response to osmotic thirst?

Release of aldosterone (steroid hormone from adrenal gland)

What does aldosterone do?

• Causes kidneys to retain Na+ (salt)

• Helps retain water

What are circumventricular organs?

• Brain regions that lie on the walls of the ventricles

• Can directly monitor:

  • Salt levels

  • Hormones in blood

What happens if physiological responses aren’t enough

• Brain activates behavioral process (via circumventricular organs) —> gets you to drink water

What triggers hypovolemic thirst?

Reduced extracellular volume

What causes hypovolemic thirst?

Loss of fluid volume (e.g. blood loss, vomiting, diarrhea) —> NO concentration change

What detects hypovolemic thirst?

Baroreceptors in major blood vessels and the heart —> detect drop in blood pressure

What does hypovolemic thirst trigger

• Thirst

• Salt hunger

• Hormone responses

What must be consumed in hypovolemic thirst?

Water AND salts

What hormone is released during hypovolemic thirst?

Vasopressin (ADH) from posterior pituitary

What does vasopressin (ADH) do?

& Induces blood vessel constriction

Reduces water flow to the bladder

Renin-angiotensin system

• Kidneys release renin (enzyme)

• Triggers hormonal cascade resulting in circulation of angiotensin II

Angiotensin II conserves water by

1. Constricting blood vessels

2. Increasing blood pressure

3. Releasing vasopressin and aldosterone

4. Acts at the circumventricular organs to stimulate drinking

What is atrial natriuretic peptic (ANP) and how does it change in hypovolemia?

Normally:

• Promotes water excretion

During hypovolemia:

• ANP is reduced —> increased blood pressure, stimulates drinking, inhibits excretion of water

Why do we need nutrients (beyond energy)?

Required for:

• Growth

• Maintenance

• Repair of the body

What controls digestion?

The nervous system —> anticipates future energy needs

Why do we have many mechanisms that trigger eating?

It is critical to have enough nutrients

What non-biological factors influence eating?

Social and cultural factors

Glucose

Principal sugar used for energy

Glycogen

• Complex carbohydrate made of glucose molecules

• Stored in liver and muscles

• Used for short-term energy storage

Glycogenesis

• Process of converting glucose —> glycogen

• Controlled by insulin

• Released by beta cells in pancreas

Glucagon

• Hormone released by alpha cells in the pancreas

• Mediates glycogenolysis

Glycogenolysis

• Conversion of glycogen back into glucose

• Occurs when blood glucose levels drop

Lipids

• Fats for long-term energy storage

• Stored in adipose tissue

What is gluconeogenesis?

• Converts fat and proteins to glucose and ketones (which can also be used by the body and brain)

• Occurs during prolonged food deprivation

Glucose transporters

Span the cell membrane and interact with insulin to bring glucose into the cell

Does the brain need insulin for glucose uptake?

No —> brain can take in glucose without insulin

What are the 3 phases of insulin release

• Cephalic phase

• Digestive phase

• Absorptive phase

Cephalic phase

Sensory stimulus of food evokes insulin release in anticipation of glucose

Digestive phase

Food causes gut hormone release which stimulates the pancrease to secrete insulin

Absorptive phase

Glucodetectors in the blood and liver detect glucose and signal the pancreas to release insulin

Diabetes mellitus

Caused by failure of insulin to induce glucose absorption

Type I diabetes (juvenile-onset) diabetes mellitus

Pancreas doesn’t produce insulin

Type II (adult-onset) diabetes

Consequence of reduced sensitivity to insulin

Satiety vs. Hunger

• Satiety = feeling of fulfillment or satisfaction

• Hunger = internal state of an animal seeking food

How does the brain decide whether to eat?

Integrates insulin signals, glucose signals, and hormones

What brain region is most important for hunger regulation?

Hypothalamus

No single brain region has control of appetite

Two main “appetite centers” in the hypothalamus

• Ventromedial Hypothalamus (VMH) —> satiety center

• Lateral Hypothalamus (LH) —> hunger center

Arcuate nucleus of the hypothalamus

Integrates peptide hormone signals from the body

Shorter-term energy balance is reported by

Hormones from the digestive organs

Two hormones important for appetite control

• Ghrelin

• Cholecystokinin (CCK)

Ghrelin

• Synthesized and released by endocrine cells of the stomahc

• Reaches high levels before eating and drops off after eating

• Works as an appetite stimulant

Cholecystokinin (CCK)

• Released by intestinal cells

• Reaches high levels after eating

• Works as an appetite suppressant

Organization of appetite control relies on two types of neurons in the arcuate nucleus

• POMC neurons

• NPY neurons

POMC neurons

• Satiety neurons

• Decrease appetite and increase metabolism

NPY neurons

• Hunger neurons

• Increase appetite and reduce metabolism

How do ghrelin and PYY3-36 affect hunger neurons?

• Ghrelin —> stimulates NPY neurons & increases appetite

• PYY3-36 —> inhibits NPY neurons & decreases appetite

What is the nucleus of the solitary tract (NST)?

• Located in the brainstem

• Where appetite signals converge

• Common pathway for feeding behavior

How does CCK reduce appetite?

• Peptide released by the gut after feeding

• Acts on vagus nerve to inhibit appetite

Leptin

• Hormone released by fat cells into the blood stream

• Signals to the brain about long-term energy reserves

How does leptin affect appetite?

• Activates POMC —> satiety neurons

• Inhibits NPY —> hunger neurons

Suppresses hunger

What happens if leptin is defective?

Brain thinks “no fat stored” —> overeating —> obesity

Why was one mouse smaller in the leptin experiment if neither produce petin?

Given EXOGENOUS leptin —> reduces eating behavior