Homeostasis

Homeostasis

The ability of the body to maintain a steady, but not static conditions in the internal environment, necessary for the health of all cells in the body.

How homeostasis is maintained

Biological systems use feedback loops to control homeostatic parameters

Homeostatic Parameters

O2, CO2, Sodium ions, Potassium ions, Calcium ions, Chloride ions, Bicarbonate ions, H+, Glucose, Body Temperature.

Some substances (Potassium/H+) are tightly regulated, while others (Glucose) are not so tightly regulated.

Negative Feedback Loop (Stable)

An initial increase in the variable leads to a subsequent decrease in the variable to bring it back to initial level. Physiologically, there are more of this type of feedback loops.

Ex. Control of Plasma Glucose

Positive Feedback Loop (Unstable)

An initial increase in the variable triggers a further increase in the variable.

Ex. LH secretion

Negative Feedback Loop Example (Glucose)

Ingest meal → increase in plasma glucose → triggers release of insulin → stimulates glucose uptake by cells (skeletal, adipose, and liver cells) & glycogen synthesis → plasma glucose back to basal level

Negative Feedback Loop Example (Water Control)

• Short Term:

  • Increase in volume of water in body → pressure sensors detect change in pressure due to said increase → triggers a decrease in cardiac output and an increase in blood vessel diameter → decreased volume of blood flows out of the heart → decreased blood pressure

• Long Term:

  • Increase in volume of water in body → pressure sensors detect change in pressure due to said increase → triggers an increase in amount of water excreted by kidney → decreased blood pressure

Plasma

Fluid and cells inside the circulation

Interstitial compartment (ISF)

Fluid surrounding all cells of the body

Intracellular compartment (ICF)

Fluid inside cells

Internal Environment / Extracellular Fluid (ECF)

Plasma + Interstitial compartment (ISF)

Barriers of Body Compartments

• ICF separated from ECF by single lipid bilayer

  • Permeable to lipophilic substances (gases, hydrophobic hormones)

  • Impermeable to hydrophilic substances (glucose and ions)

• ISF is separated from plasma by a blood vessel wall

  • Permeable to most small substances

  • Impermeable to large substances (large proteins, cells, and viruses)

Total Body Water (TBW)

ECF + ICF

Breakdown of Water Composition in a Human Body

• TBW = 60%

• ICF = 40%

• ECF = 20%

Methods to Determine Body Compartment Volume

• Estimate

• Measure

• Calculate

Estimation of Body Volumes

TBW = 0.6 x (body weight in Kg)

ICF = 0.4 x (body weight in Kg)

ECF = 0.2 x (body weight in Kg)

Measurement of Compartment Volumes

1. Inject a known amount of tracer (which will diffuse and restrict compartment being measured)

2. Sample is taken from compartment

3. When tracer is equilibrated:

   • Volume = (amount injected) / concentration

   • If some tracer is lost:

     • Volume = (amount injected - amount eliminated) / concentration

Intracellular volume calculation

Total body volume - Extracellular volume

Interstitial fluid volume calculation

Extracellular volume - Plasma volume

Blood volume (Vt) calculation

Plasma volume + Blood cell volume

Major extracellular electrolyte(s)

Na+, Cl-

Major intracellular electrolyte(s)

K+

Moles

Refers to the number of molecules of a given substance (1 mol = 6.022 x 10^23 particles)

Molarity

The concentration of a substance (1 M = 1 mol / L solvent)

Osmoles

The number of particles of a given substance

E. NaCl dissociates in aqueous solutions, yielding two osmoles

Osmolarity

The concentration of particles (1 OsMolar = 1 osmole / L solvent)

Equivalents

Refers to the number of particles of a given substance but takes into account the valence of a substance

Ex.

1 mol Na+ = 1 equivalent

1 mol Ca2+ = 2 equivalents

Molecular weight

The weight in grams of one mole of that substance

Plasma Osmolarity

Na+ is a major determinant. To account for pathologies such as diabetes and renal failure, use the equation [see image]

Percent Solution

The weight of the substance in that solution relative to the weight of water.

Calculating Osmolarity of a Percent Solution

How many Osmoles of NaCl in 2L of a 2.9% solution of NaCl?

1. 2.9% = 2.9 / 100 = 0.029g of NaCl in 1mL

2. 29g of NaCl in 1L

3. 29g x (1 mol/58g) = 0.5 moles of NaCl per 1 L water

4. 2 (# of particles that dissociate) x 0.5 moles = 1 Osmole of NaCl per 1L water

5. For 2L of water: 2 x 1 = 2 Osmoles of NaCl