Samenvatting van het vak De Nier, gebasseerd op de slides van de colleges. Dit vak wordt (meestal) gegeven in het derde jaar van de studie Medische Wetenschappen en Technologie, of voor de studenten met vakkenpakket Basic Tissue Engineering op de TU/e.
College 1: introduction
Silverthorn, chapter 5 – osmosis and tonicity
Boron & Boulpaep, chapter 33
Homeostase
- Regulation of the body’s internal environment
- Keeping internal environment stable
- Interaction between outside world extracellular fluid intracellular fluid
Homeostasis - definitions
- External or internal change
- Loss of homeostasis - sensed by organism
- Successful compensation homeostasis reestablished
- Study of compensation is physiology
- Failure to compensate disease
- Study of failure to compensate is pathophysiology
Mass balance in an open system
Input (metabolic production) body load output (metabolism to a new substance)
Law of mass balance: mass balance = existing body load + intake or metabolic production - excretion
or metabolic removal
Intake 2.2 L/day + metabolic production 0.3 L/day - Output 2.5 L/day = 0
Diffusion: properties
- Passive process
- High concentration to low concentration
- Chemical gradient
- Net movement until concentration is equal
- Equilibrium
- Rapid over short distances
- Directly related to temperature
- Inversely related to molecular weight and size
- In open system or across a partition (a membrane)
Fick’s law of diffusion
Rate of diffusion = surface area x concentration gradient x membrane permeability
Membrane permeability = lipid solubility/molecular size
Changing the composition of the lipid layer can increase or decrease membrane permeability.
Cell membrane is negatively charged because of the phosphate groups of the phospholipids.
Factors affecting rate of diffusion through a cell membrane:
- Lipid solubility
- Molecular size
- Concentration gradient
- Membrane surface area
- Composition of lipid layer
Osmose:
1
,Stofje dat niet door het membraan kan diffunderen, daar diffundeert water naar binnen.
Stofje dat wel door het membraan kan diffunderen, daar diffunderen de stofjes uit.
Body fluid compartments
The body fluids in two compartments: the extracellular fluid (ECF) and intracellular fluid (ICF).
The ECF and ICF are in osmotic equilibrium but have very different chemical composition.
ICF is 2/3 of the total body water volume. Material must cross the cell membrane.
ECF includes all fluid outside the cells. The ECF is 1/3 of the body fluid volume. The ECF consists of:
- Interstitial fluid (IF), which lies between the circulatory system and the cells, is 75% of the
ECF volume.
- Plasma, the liquid matrix of blood, is 25% of the ECF volume. Substances moving between
the plasma and interstitial fluid must cross the leaky exchange epithelium of the capillary
wall.
Water distribution and composition
Homeostase does not mean equilibrium
- Osmotic equilibrium
- Chemical disequilibrium
- Electrical disequilibrium
Two compartments are separated by a membrane that is permeable to water
but not glucose, where solution B is more concentrated than solution A.
Water moves by osmosis into the more concentrated solution.
Osmotic pressure is the pressure that must be applied to B to oppose
osmosis.
Osmolarity: the concentration of a solution expressed as the total number of solute particles per
liter.
1 M glucose = 1 OsM
1 M NaCl = 2 OsM
2
,Hyposmotic: smaller OsM
Isosmotic: same OsM
Hyperosmotic: bigger OsM
Tonicity: the ability of an extracellular solution to make water move into or out a cell by osmosis.
Solution is hypotonic cell in solution swells
Solution is isotonic cell in solution doesn’t change in size
Solution is hypertonic cell in solution shrinks
Rules for osmolarity and tonicity:
1. Assume that all intracellular solutes are nonpenetrating
2. Compare osmolarities before the cell is exposed to the solution. (at equilibrium, the cell and
solution are always isosmotic)
3. Tonicity of a solution describes the volume change of a cell at equilibrium
4. Determine tonicity by comparing nonpenetrating solute concentrations in the cell and the
solution. Net water movement is into the compartment with the higher concentration of
nonpenetrating solutes.
5. Hyposmotic solutions are always hypotonic.
Buffers
Buffer of blood : pCO2-bicarbonate-system
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-
The pH of blood is directly dependent of the ratio of HCO3 and CO2 Henderson-Hasselbach
equation:
[𝐻𝐶𝑂 ]3
pH = pKa + log10 0.03 ∙ 𝑝𝐶𝑂
2
Buffer systems
Extracellular:
- Bicarbonate CO2 (~25 mmol/L)
- Phosphate (~1 mmol/L)(H2PO4- ↔ H+ + HPO42-)
- Protein
Intracellular:
- Proteins (erythrocytes: hemoglobin)
- Organic and inorganic phosphates
- Bone
Acidosis or alkalosis? Metabolic or respiratory?
Acids and alkalis (bases)
3 mechanisms to buffer pH changes:
- Chemical buffering (seconds)
- CO2 (volatile acid) exhaled via lungs
(minutes to hours)
- Non-volatile acid (sulfate and
phosphate) and H+ excretion or base
retention via kidney (hours to days)
4
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