Homeostasis


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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

Substance

Percentage of Body Fluids/%

Water 60
Proteins 18
Lipids 15
Minerals 7

Transport Mechanisms:

Are of two types, Passive and Regulated.

Passive

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).

Regulated

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

Example

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

Type

Examples

Detail

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

Definition:
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

Level

Components

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.

Feedback

Types:

  • 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