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)
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1
Q

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

A

60%

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

2
Q

What is the distribution of water in the body?

A
  • 2/3 intracellular (28 litres)
  • 1/3 extracellular (14 litres)
  • Extracellular water is divided into 11 litres of interstitial water and 3 litres plasma.
3
Q

What is the volume of circulating blood and its composition?

A

5 litre = 3 litres plasma + 2 litres erythrocytes

4
Q

What is haematocrit?

A

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

5
Q

Describe the cardiac output at rest.

A

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.

6
Q

What is osmosis? How can this describe 2 solutions?

A

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
7
Q

What is osmolarity and osmolality?

A

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

8
Q

What is the normal plasma osmolality?

A

290 mOsmol/kg

9
Q

What is oncotic pressure?

A

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).

10
Q

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

A

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.

11
Q

What is pH?

A

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.

12
Q

What is the pH of pure water?

A

7.0

13
Q

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

A

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.

14
Q

What are the causes of abnormal plasma pH?

A

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.