Osmoregulation 1

What are the main fluid compartments in the body, and what percentage of body water do they contain?

Intracellular fluid (~60% of total water, approximately 25L), extracellular fluid (~25%, approximately 14L), which includes plasma (~3L)

Extracellular fluid can be broken down into

1. Interstitial fluids, which is surrounding and bathe cells

2. Blood plasma, which is within the vessels of organisms with closed circulatory systems

What are the components that make up extracellular fluid?

Extracellular fluid is made of water, electrolytes (which dissociate into ions such as Na+ and Cl-), and nonelectrolytes (organic molecules that do not dissociate like sugars).

Why is it necessary to maintain extracellular fluid composition?

To ensure cell function by maintaining electrical gradients across membranes (allowing for action potentials in heart and neurons, Na+ outside cell, K+ inside cell)

To maintain cell structure (water/ions affect cell volume, think osmosis and red blood cells), and hydrostatic pressure maintains in tissues and organs in place

To serve as a vehicle for nutrients and chemicals, amount of fluid (volume) and concentration of solutes are both important to maintain

What are the differences between conforms and regulators?

Conformers

• No homeostasis

• Cells are iso-osmotic with the environment

• Stable environments (typically marine, invertebrates)

Regulators

• Homeostatis

• Tissues maintain relatively stable internal conditions (energetically costly)

• Unstable environments (Estuarine/brackish)

• Constantly inhospitable habitats (Salty or terrestrial)

If organisms are regulators, what parameters do they need to regulate?

• Volume of water in the ECF (volume regulation)

• Concentration of ions available in the ECF (ionic regulation)

• Osmotic pressure (concentration) of the ECF (osmotic regulation)

What are the major cation and anion in extracellular fluid?

The major cation is sodium (Na+), and the major anion is chloride (Cl-).

How does extracellular fluid composition influence osmosis and cell function?

It affects osmosis, which impacts cell volume, and maintains electrical gradients necessary for cellular activity.

What are the two strategies organisms utilize to regulate extracellular fluid composition across environments?

Regulators (organisms that actively maintain constant internal conditions) and conformers (organisms that allow internal conditions to vary with the environment).

What adaptations do regulators have for freshwater and seawater environments?

Freshwater: Tissues are hypertonic to environment; water enters by osmosis; ions diffuse out; they produce diluted urine and uptake Na+ and Cl- via gills.

Seawater: Tissues are hypotonic to environment; water exits tissues by osmosis; ions diffuse in; they produce concentrated urine and secrete Na+ and Cl- via gills.

What problems do marine fish face and how do they adapt?

They face desiccation and inward salt diffusion; they actively secrete Cl- through gills, Na+ follows, and they excrete hypoosmotic tissues to cope with 1,000 mOsm seawater. Opposed by urea and TMAO

Describe the regulation of water and salt in extracellular fluids

• 40% water loss caused fatigue and dizziness

• 10% water loss can cause health deterioration [renal & heart failure; neurological problems (seizure)]

• >15% can be fatal

Triggers of hypertonic dehydration and hypovolemia (isotonic dehydration)

Hypertonic Dehydration

• Intracellular thirst

• Trigger: High electrolyte levels (often hypernatremic dehydration)

Hypovolemia (Isotonic Dehydration)

• Extracellular thirst

• Trigger: Loss of fluid, without changing concentration (decrease in blood plasma volume)

What causes intracellular or osmotic thirst?

Triggered by high electrolyte levels

Leads to an increase in plasma osmolarity (caused by salt consumption)

Activates osmoreceptors in the hypothalamus

Water leaves the osmoreceptors, leading to dehydration and shrinkage

More action potentials leading to intracellular thirst

Solution: Drink water

How does extracellular or hypovolemic thirst occur?

Triggered by loss of fluid

A decrease in plasma volume activates baroreceptors

Decreased arterial pressure reduces stretch in arterial walls

Decreasing stretch-activated receptor activity, leading to fewer action potentials and triggering thirst.

Extracellular thirst

Drink water and eat salt

Baroreceptors: Detect changes in stretch

Osmoreceptors vs Baroreceptors

Osmoreceptors

• Detect high blood solute concentration

• High osmolarity/saltiness

• Too much salt/too little water

• Causes cells to shrink, signaling the need for water

• Triggers ADH release, signaling the brain to initiate a desire for water to dilute the blood

• Drives pure water intake

Baroreceptors

• Detect low blood pressure and volume (hypovolemia)

• Hypovolemia: Critical reduction in blood plasma or body fluid volume

• Drive sodium appetite to restore volume

Intracellular or Osmotic Thirst

Due to an increase in plasma osmolarity (salt consumption)

• Increase in extracellular fluid, osmolarity

• Leads to

  • Osmoreceptors in hypothalamus being activated

  • Saliva production decrease > dry mouth

• Hypothalamic thirst center

Extracellular or Hypovolemic Thirst

Due to a decrease in blood volume

Presents a risk for heart function (heart increases its rate to maintain cardiac output)

Induces blood vessel constriction (vasopressin also named ADH)

Induces thirst for water AND craving for salt

• ECF volume decrease (5%), leads to…

• Blood pressure decrease, leads to…

  • Activation of baroreceptors

  • Decrease in renal perfusion > Angiotensin II

• Hypothalamic thirst center (and 3rd ventricle in brain)

Would an organism that is constantly in a hypertonic environment likely be an osmoregulator or an osmoconformer?

Osmoregulator, because it would need to prevent water from leaving its body to remain alive

Movement of salt from the surrounding water to the blood of a freshwater fish requires __ because __

Expenditure of ATP; the salt is moved against its concentration gradient, from low concentration (fresh water) to high concentration (blood)

How does salt intake affect blood pressure (BP), and what steps are involved?

Eating salty food raises plasma osmotic pressure

Leads to dehydration

Triggers thirst, increases water intake

Expands plasma volume, raises blood pressure, and stresses the cardiovascular system, potentially contributing to hypertension.

Which scenario describes what occurs when we eat salty popcorn at the movie theater?

Intacellular thirst - increase in plasma osmolarity detected by osmoreceptors which trigger thirst

Donating a pint (half liter) of blood will stimulate:

Baroreceptors to generate extracellular thirst

In response to hypovolemic dehydration, the following structures/cells contribute to thirst

Cells of the kidney

The salt challenge for terrestial animals

Carnivores: Obtain salt from food (ECF’s rich in salt)

Herbivores: Eat plants, plants have low need for sodium and actually limit sodium uptake by roots, plants are deficient in sodium so herbivores are often deficient in sodium

Animals have an intense salt hunger

Roles of Salt (Part of ECF)

• Helps our cells uptake nutrients & water

• Helps our muscles contract

• Helps our nerves carry messages between the brain and the body

• Balances our bodies fluids and regulates blood pressure

Human Adaptation to Salt

• Hunger was adaptive as salt was scarce for vegetarian primate ancestors

• Salt was a currency for exchange

• Now, salt is everywhere and people crave it

The Salty 6

Bread: 1 piece = 15%

Pizza: 1 slice = 50%

Nuggets: 3 ounces > 50%

Soup: 1 cup = 100%

Sandwich: 150%

Deli: Packaged turkey = 100%

How can fluid retention from high salt intake be counteracted?

Drinking more water restores osmolarity, stimulates diuresis, and helps eliminate excess water and sodium, reducing puffiness.

Why do people look “puffy” after eating salty food?

Excess salt increases osmotic pressure in plasma, pulling water out of cells into interstitial tissues, causing water retention and edema.

Summarize the process from salty food to potential hypertension

1. Salt intake increases plasma osmotic pressure.

2. Dehydration triggers thirst and drinking of water, expanding plasma volume.

3. Kidneys reduce water excretion, retaining water.

4. Excess water moves to interstitial tissue, causing edema.

5. Elevated blood volume increases blood pressure, potentially leading to hypertension.