In our body there is a constant balance between acid and base. The following
equations are to be considered:

1) Henderson Hasselbalch equation:
pH = 6.2 + log
[HCO3-]/ 0.03 PCO2
where 6.2 is the pKa (negative logarithm of the acid dissociation constant) for carbonic acid
(H2CO3) and 0.03, the factor which relates PCO2 to the amount of CO2 dissolved in plasma.

2) Kassirer-Bleich equation

A drop or rise in PCO2 will result in a drop or rise in [H+] respectively.

[HCO3-] on the other hand is inversely related to H+ concentration whereby a drop in bicarbonate levels result in an increase in H+ concentration while a rise in bicarbonate levels result in a reduction in H+ concentration.

This buffer system is of physiologic importance because both the pulmonary and renal mechanisms for regulating pH work by adjusting this ratio.

The PCO2 can be modified by changes in alveolar ventilation, while plasma [HCO3-] can be altered by regulating its generation and excretion by the kidneys.

Definitions :- An acid base disorder is a change in the normal value of extracellular pH that may result
when renal or respiratory function is abnormal or when an
acid or base load overwhelms excretory capacity.
Normal acid base values
pH 7.35- 7.45
PCO2 36- 44
HCO3 22-26


Acid base status is defined in terms of the plasma pH.

Acidemia - decrease in the blood pH below normal range of 7.35 -7.45

Alkalemia - Elevation in blood pH above the normal range of 7.35 – 7.45

Clinical disturbances of acid base metabolism classically are defined in terms of the HCO3- /CO2 buffer system.

Acidosis – process that increases [H+] by increasing PCO2 or by
reducing [HCO3-]

Alkalosis – process that reduces [H+] by reducing PCO2 or by
increasing [HCO3-] lightbulb It is important to note that
though acidosis and alkalosis usually leads to acidemia and alkalemia respectively, the exception occurs when there is a mixed acid base disorder. In
that situation, multiple acid base processes coexisting may lead to a normal pH or a mixed picture.

This will be discussed in more detail later.
Since PCO2 is regulated by respiration, abnormalities that primarily alter the PCO2 are referred to as respiratory acidosis (high PCO2) and
respiratory alkalosis (low PCO2).

In contrast, [HCO3-] is regulated primarily by renal processes.
Abnormalities that primarily alter the [HCO3-] are referred to as metabolic acidosis (low [HCO3-])
and metabolic alkalosis (high [HCO3-]).

Simple acid base disorders : Disorders that are either metabolic or respiratory.

Mixed acid base disoders: More than one acid base disturbance present. pH may be normal or
abnormal.

The intracellular and extracellular compartments are separated from one another by the plasma
membrane of the cells. The extracellular
compartments (interstitial/
plasma/lymph) are separated by
a layer of endothelial cells
surrounded by a basement
membrane; the capillaries. To
cross from the plasma to the cells
or vice versa, substances must
either cross both membranes of
the endothelial cells or travel
between the cells and then cross
the basement membrane.

Capillaries come in three main types distinguished largely on the permeability of their walls.

1) Continuous capillaries have a close connection between adjacent cells and will permit only small molecules < 10nm in diameter to cross. Continuous capillaries surround muscle, skin lungs, adipose tissue CNS, retina and mammary glands.

2) Fenestrated capillaries contain
'windows' that offer easy passage to larger molecules(10-100nm) and are found around the kidneys, pancreas,
gallbladder and intestine.

3) Discontinuous have wide gaps between the cells and will allow practically anything (even cells)
across. Discontinuous capillaries surround the liver, spleen,
ovaries and some endocrine glands.

Capillaries act rather like a leaky hosepipe; although the bulk of
the fluid continues along the pipe, the pressure forces some
out of the walls. The fluid and soluble contents of plasma small enough to cross the capillary wall
circulate into the interstitial fluid at the high pressure arterial end of the capillary bed and returns
to the capillary, bringing with it
small soluble waste products from the cells, at the low
pressure venous end of the capillary bed (Oxygen and CO2
being lipid soluble, diffuse from the plasma across the capillaries and to and from the cells as necessary).

Hydrostatic (blood)
pressure is not the only force acting to cause fluid movement
in and out of the capillaries. The plasma proteins that cannot
cross the capillary walls exert an osmotic pressure to draw water back into the capillaries which
outweighs the hydrostatic pressure at the venous end of
the capillaries. The balance of hydrostatic and osmotic forces causing movement out and into the capillaries are known as Starling forces.

Cell is a structural and functional unit of life, as being
a free-floating and self-replicating
molecule, it would appear to offer
survival advantages, after all there are lots of cells around now and very few non-cellular life forms. Although it might
be argued that bacteria represent the most successful
cellular lifeform, because there
are more of them then anything
else, being a multicellular
organism does seem to have some things going for it. The
simplest possible multicellular
organism is a sponge. A sponge
is a collection of identical cells
that exist as a colony. It is
possible to disperse a sponge
through a sieve, if you leave the
pieces in a bucket for a few days
they will reform back into a
colony with apparently no ill
effects. However, the real
advantages to being multicellular
aren't apparent until component
cells start to show specialisation.
A flatworm is a good example of
the next stage of development.
Flatworms do not have digestive
or circulatory systems; most of
the animal is a flattened cylinder
of cells. Nevertheless, the cells on
the outside of the cylinder are
specialised and act to hold the
animal together, to protect the
inner layers and to absorb food
from the environment. In short
the outer layer of the flatworm
creates a comfortable internal
environment for all the other cells
to live in. Homeostasis may now
occur at both the level of the
single cell and at the level of the
whole organism. One of the main
disadvantages of being a
flatworm is that you have to be
flat to allow oxygen to diffuse
into the innermost layers of cells
(and also to let CO2 diffuse out).
Any arrangement other than
being flat puts the innermost
cells to far away from the outside
to survive. Everything more
advanced than a flatworm has
discovered the way around this
is to have a circulatory system to
reach the parts that diffusion
cannot reach. Once past this
hurdle, all sorts of specialisation
amongst the cells that comprise
an organism become possible.
The digestive system is a way of
extending the surface area for
digestion whilst simultaneously
protecting it by internalising it,
thus the same cells don't have to
try and combine protective and
digestive functions. The lungs are
essentially the result of the same
principle applied to the problem
of gaseous exchange. The liver,
the brain, the muscles etc. etc.
etc. have all evolved to play their
own part in the homeostasis of
the whole organism. The part of
the body that has charge of
maintaining the ionic
composition of the organism, a
task analogous to that of the
plasma membrane in a single cell,
is the kidney.
In the human body (for example)
there are several major fluid
compartments each of which is
subject to homeostatic
regulation.
The largest compartment is the
intracellular compartment. Any
fluid not contained inside a cell
therefore comprises the
extracellular compartment. The
extracellular compartment may
be divided into an interstitial
compartment (means literally 'in
the spaces', in this case, the
spaces between the cells) and a
circulating compartment (the
blood plasma and the lymph
fluid). A 70 kg man (the figures
are slightly different for women)
contains about 40-45 litres of
water.


Total 45 litre

Intracellular 27

Extracellular 18
interstitial 13

plasma 3.5

lymph 1.5

If all the water were removed
from a human body some 30 kg
of assorted salts would remain.
Despite the impression given by
Start Trek or Red Dwarf where
dried people are represented by
a tiny pile of salt crystals, 30 kg is
a lot of material. Imagine 15 bags
of sugar poured all over the floor.

The normal composition of the
major body fluid compartments
is approximately as follows
(mmol/l, except Ca2+)

Constituent (1st)
Plasma (2nd)
Interstitial fluid (3rd)
Intracellular fluid (4th)

Na+ 142 139 14

K+ 4.2 4 140

Ca2+ 1.3 1.2 0

Mg2+ 0.8 0.7 20

Cl- 108 108 4

HCO3- 24 28.3 10

Protein 1.2 0.2 4

Glucose 5.6 5.6 absent

Amino acid 2 2 8

Urea 4 4 4

* The ionic Ca2+(free) is very low
inside cells, total calcium may be
much higher (1-2 mmol/l)

In the above overall osmolarity of all three
compartments is identical at
about 300 mOsmol/l.