Homeostasis & Fluid Compartments

By Sokha Kheam

Learning Objectives• Explain the basic organization of the body,• Define the fluid compartment of the body,• Explain how solutes distribute in the body,• Explain homeostasis.

Body Components• Differentiated Cells: Specialized function• Tissues: Groups of cells with related function muscle, nervous, connective

and epithelium. • Organ: Function unit• Organ system: several organs act together to perform specific function• Skin: Barrier• Entry: respiratory (O2 & CO2) & digestive system• Transport = Diffusion• Exit = Digestive system

Fluid Compartments• The body can be divided into two fluid compartments:• Intracellular fluid (ICF) is the cytoplasm within cell.• Extracellular fluid (ECF) surrounds the cells and serves as a buffer.

• interstitial fluid (ISF) that bathes the outside of the cells,• ¾

• intravascular fluid (IVF) (i.e., plasma, lymph, and cerebral spinal fluid).• ¼

• The main difference is that the IVF has higher protein content. • However, the composition of the ICF and ECF differ due to the

hydrophobic nature of the cell membrane which prevents free exchange of ions and proteins.

Fluid Compartments• ICF is a reducing environment that has a high concentration of K+,

but low concentrations of Na+ and free Ca++. • Additionally, the concentrations of phosphates and proteins in the ICF

are greater than in the ECF.

• ECF is an oxidizing environment that has low concentration of K+ but high concentrations of Na+ and free Ca++

Fluid Compartments• This leak is matched by pumping K+ back into the cell via the Na+ -K+

ATPase, an integral membrane protein. • The movement (pumping) of K+ back into the cells requires energy

(ATP). • During each cycle of the ATPase, two K+ are exchanged for 3 Na+ and

one molecule of ATP is hydrolyzed to ADP.

• Sodium ions can enter cells through special protein channels.

Fluid Compartments• Approximately 60% of body weight is water• Total Body Water (TBW) = ----/70kg• ICF = cytoplasm ?• ECF = Surrounds cells?• interstitial fluid (ISF)• intravascular fluid (IVF)

Self-Regulating Mechanisms• Equilibrium: Equal amount of substance• No net transfer of substance or energy• No barrier to movement• No energy expenditure to maintain

• Steady State: Constant amount of substance in compartments• Input = output• Requires energy to maintain

What is Homeostasis?• Maintenance of extracellular fluid (ECF) constituents as relatively

constant.

• Central theme of physiology

Homeostasis Vs. Illness

Homeostatic Control & Reflex Loops

Summary• The human body is an interdependent set of self-regulating systems

whose primary function is to maintain an internal environment compatible with living cell and tissues (homeostasis).

• Stability of internal variables is achieved by balancing inputs and outputs to the body and among organs systems.

Problem• Identify the components of the reflex loop in the following scenario.• You have finished the marathon in just under three hours. You are

tired, sweating profusely, and start to drink Gatorade. After a few minutes you are still tired but no longer sweating or thirsty.