Body Water, Osmolality And pH Flashcards Preview

ESA 1 - Body Logistics > Body Water, Osmolality And pH > Flashcards

Flashcards in Body Water, Osmolality And pH Deck (14):

How much of a human body is made up of water?

(E.g. 42 litres in a 70 kg human - 70 x 0.6 = 42)


What is the distribution of water in the body?

- 2/3 intracellular (28 litres)
- 1/3 extracellular (14 litres)

- Extracellular water is divided into 11 litres of interstitial water and 3 litres plasma.


What is the volume of circulating blood and its composition?

5 litre = 3 litres plasma + 2 litres erythrocytes


What is haematocrit?

Haematocrit = the proportion, by volume, of blood that consists of erythrocytes.
Normal haematocrit is 2 litres in 5 litres so 40%.


Describe the cardiac output at rest.

At rest the heart rate is around 70 beats per minute and the stroke volume of the left ventricle is around 70 ml.
Therefore the cardiac output at rest is 70 x 70 = 4900 mls/min ( 5 litres/min when rounded up).
This means that because the normal circulating blood volume is 5 litres and the normal cardiac output is 5 litres, the total blood volume circulates once per minute.


What is osmosis? How can this describe 2 solutions?

The spontaneous net movement of water molecules across a semi-permeable membrane into a region of higher solute (& protein) concentration.
- Isotonic solutions = equal osmotic concentrations
- Hypertonic solution = solution with the higher concentration of solutes
- Hypotonic solution = solution with the lower concentration of solutes


What is osmolarity and osmolality?

Osmolarity = the concentration of a solution expressed as the total number of solute particles (osmoles) per litre (high solute concentration = high osmolarity).

Osmolality = measure of osmoles per kg of solvent


What is the normal plasma osmolality?

290 mOsmol/kg


What is oncotic pressure?

Form of osmotic pressure exerted by proteins, notably albumin, in a blood vessel's plasma. Usually tends to pull water into the circulatory system - the albumin concentration in interstitial fluid is 1/4 of that in plasma (10 g/l vs 40 g/l).


Cells have high oncotic pressure (are full of proteins) - what stops water from moving into the cell, causing them to burst?

Active situation: K+ is pumped into the cell and Na+ is pumped out, against their electrochemical gradients, via the Na/K ATPase pump. ATPase pumps 3 Na+ out for every 2 K+ in (net loss of osmotically active ions) - uneven charge stops cell from bursting.

Necrotic cells swell and burst due to the failure of the Na/K ATPase pump.


What is pH?

PH = -log [H+]

Represents a logarithmic scale so each 1-unit change in the pH scale corresponds to a 10-fold change in hydrogen ion concentration.


What is the pH of pure water?



What is the normal plasma pH and the pH limits of survival?

Normal plasma pH = 7.36 - 7.44

Limits of survival = 6.8 - 7.8

A change in [H+] by a factor of 2 causes a pH change of 0.3. At pH 7.4 the [H+] is 40 and at pH 7.0 it is 100 - can work out the [H+] at many other pHs.


What are the causes of abnormal plasma pH?

Nearly always results from major organ dysfunction, esp. lungs, kidneys and liver.
Other common causes of low pH is poor tissue perfusion > anaerobic glycolysis > lactic acid production & lactic acidosis > lowered pH > shock.
Shock = state of global cellular and tissue hypoxia due to reduced oxygen delivery, most commonly due to hypoperfusion.