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Body Fluid Compartmentalization

Fluid Volume test Percentage of fluid from total body water/% Volume of fluid/L
Intracellular Antipyrin – D2O 40 28
Extracellular Plasma Evans blue – 131J 5 3.5
Interstitial fluid Inulin – Manitol – Saccharose 15 10.5
  • Distribution volume of an administered substance (VD)/its plasma concentration: Amount of the given compound – amount of the excreted compound.
  • Circulation-involved organs: GIT, skin, liver, and kidney.

Body Composition


Percentage of Body Fluids/%

Water 60
Proteins 18
Lipids 15
Minerals 7

Transport Mechanisms:

Are of two types, Passive and Regulated.


Transport Mode

Factor it Depends on

Diffusion Solubility of the substance in water/ in lipids.
Osmosis The number of dissolved particles.
Filtration: The movement of a substance due to osmotic and hydrostatic pressure – controlling interstitial fluid’s resorption and production. Whether oncotic pressure difference exceeds the hydrostatic pressure difference (no filtration) or vice versa (ultrafiltration; water moving out of the capillary).


Transport Mode Features Examples
Facilitated diffusion A generally selective carrier with limited specificity is required; leading to a limited capacity. Amino acids Phosphate
Cotransport: Symport Both substances move in the same direction Na+ and Glucose; with glucose using Na+’s concentration gradient as a driving force into the cell.
Antiport The substance move in opposite directions Ca2+ and H+

Active: ATP is uncleaved > Closed channel. Phosphorylation (following splitting of ATP) > Open channel.

Mediators: Na/K – Ca/H – K/H – & Na/H ATPase.

Ionic Channels

  • They contain filter selectivity filters inside aqueous pores.
  • Transport capacity: 6×104 particles per event.

Gating Mechanism


Voltage-gated Na+ channel
Extracellular-ligand-gated ACh nicotinic receptor
Intracellular-ligand-gated Glucose transporter
Mechanically gated Oxytocin receptor

Na+ Super-channel

Characterized by an extremely rapid movement between states of: Active (open) > depolarized membrane – Deactivated (closed) > depolarized membrane – Elicitable (closed) > polarized membrane.

Intercellular Communication




Mechanical Adhering incl. desmosomes/occluding junctions For the mechanical stability of and adhesion between cells.
Electrical Gap junctions Through connexons.

In e.g. intercalated discs of the myocardium.

Humoral Regulation Signaling modes: autocrine – paracrine – juxtacrine – endocrine – neurocrine Through a receptor, a ligand, possibly a second messenger.
Nervous Regulation Inducing responses in excitable cells Overlaps with humoral regulation.

Meeting point of humoral and nervous regulation:

  • Synapses.
  • The adrenal medulla.
  • The hypothalamal-hypophysial axis.

Features of Homeostasis

The maintenance of constant conditions within an internal environment. Said internal environment may be e.g. the total body fluid, or that in specific compartments e.g blood or organelles.

Parameters maintained:

Blood pressure – muscle tension level.

Parameters which can be regulated:

  • Temperature
  • pO2
  • pH
  • pCO2
  • osmotic pressure
  • body fluids’ volume
  • ion concentration
  • glycaemia.

Levels of Regulation



Local/ Metabolic Chemicals: pO2, pCO2, pH.

Local hormones: Prostaglandins.

Autoregulation In muscles: Myogenic; as the blood flow in them is maintained at a near-constant level despite changes in perfusion pressure.

In the heart:

  • Homeometric: Meaning the restoration of normal stroke volume is possible following the increase in afterload (higher pressure in the left ventricle, against which the myocardium must pump blood).
  • Heterometric: Meaning the increase in stroke volume upon an increase in the volume entering the heart; the Frank-Starling law. This is a result of the stretch forces elicited on the myocardial fibres during diastole with a greater blood volume, leading to an increase in the contraction force.
Systemic Humoral regulation. Nervous regulation.

Regulatory Time Definition

The time between the start of a regulatory event and the return of the target-parameter to its original/resting value.



  • Direct and indirect negative feedback
  • Positive feedback
  • Feedback in series and in parallel (in the same-direction process or on an event of another process).

Positive feedback

Example of its detrimental effect when non-physiological: Hemorrhage > decrease in the amount of blood returning to the heart > decrease in contractility and cardiac output > decrease in blood pressure > decrease in coronary flow >: Death results in the case that no negative feedback mechanisms take over.

Notes by: Lina El Rifaie