How do you assess clinically the state of hydration?

Examining the fluid chart for the input/output balance

Examining the patient specifically looking for the state of the tissues

Skin turgor

Dry mouth

Sunken eyes

Concentrated urine in the catheter

Possible tachycardia and hypotension

Measure the central venous pressure (CVP), and determine the response to a fluid challenge. If there is ‘underfilling’, the CVP will not increase in response to the challenge

What are the main fluid compartments of the body, and what are their volumes?

The fluid compartments are

Intracellular compartment: 28l

Extracellular compartment: 14l

Plasma: 3l

Interstitium: 10l

Transcellular: 1l

Therefore the total body water is 42l, which makes up ∼60% of the body weight of a 70 kg male and 55–60% for females.

How can the percentage fall of the extracellular fluid volume be calculated?

In the case of loss of extracellular fluid (ECF), the concentration of the plasma albumin increases depending on the amount of water lost during dehydration. The resulting rise in the albumin concentration can be used to calculate the % fall in the ECF volume.

where A1 initial albumin concentration; A2 = albumin concentration following loss of volume.

How can the percentage fall in the plasma volume be calculated?

In situations of loss of plasma, there is a loss of plasma protein, but not of blood. Thus, the haematocrit increases in proportion to the volume of plasma lost. Measurements of the haematocrit are therefore useful in calculating the % fall in the plasma volume.

What is the volume of the cerebrospinal fluid (CSF)?

140–150 ml.

Where is CSF produced, and at what rate?

70% of CSF is produced by the choroid plexus of the lateral, third and fourth ventricles. 30% comes directly from the vessels lining the ventricular walls. It is produced at a rate of 0.35 ml/min, or ∼500 ml/day.

Briefly describe the circulation of CSF.

From the lateral ventricle, the CSF flows into the third ventricle through the interventricular foramen of Monro. From here it enters into the fourth ventricle through the aqueduct of Sylvius. Some continue down into the central canal of the spinal cord, but the majority flow into the sub-arachnoid space of the spinal cord via the central foramen of Magendie, or the two lateral foramina of Luschka. After going around the spinal cord, it enters the cranial cavity through the foramen magnum, and flows around the brain within the sub-arachnoid space.

What are the arachnoid villi composed of?

The arachnoid villi are formed from a fusion of arachnoid membrane and the endothelium of the dural venous sinus that it has bulged into.

Where is the CSF finally absorbed?

80% of CSF is absorbed at the arachnoid villi, and 20% is absorbed at the spinal nerve roots.

What structures form the blood-brain barrier (BBB)?

The BBB, which is a histological and physiological boundary between the blood and the CSF, is formed from two types of special anatomical arrangement

Tight junctions in-between the endothelial cells of the cerebral capillaries

Astrocytic foot processes applied to the basal membranes of the cerebral capillaries

What substances can pass through the BBB?

The BBB, which is a histological and physiological…

What is the distribution of sodium in the body?

Sodium is the major extracellular cation of the body

50% is found in the extracellular fluid

45% is found in the bone

5% in the intracellular compartment

The vast majority (∼70%) is found in the readily exchangeable form.

What are the major physiological roles of sodium?

Because of the content of sodium in the body, it exerts significant osmotic forces, and so important for internal water balance between the intra and extracellular compartments. It also has a role in determining external water balance and the extracellular fluid volume. The other important role of sodium is in generating the action potential of excitable cells.

What is the daily sodium requirement?

The daily requirement is about 1 mmol/kg/day.

What is the normal plasma concentration?

The normal is 135–145 mmol/l.

Give a simple classification of the causes of hyponatraemia.

Water excess

Increased intake: polydipsia, iatrogenic, e.g. TURP syndrome, excess dextrose administration

Retention of water: SIADH

Retention of water and salt: nephrotic syndrome, cardiac and hepatic failure

Water loss (with even greater sodium loss)

Renal losses: diuretics, Addison's disease, relief of chronic urinary obstruction

Gut losses: diarrhoea, vomiting

Pseudohyponatraemia: in the presence of hyperlipidaemia

What is pseudohyponatraemia?

This is hyponatraemia that occurs as a peculiarity of the way in which the sodium concentration of the plasma is measured and expressed. In the presence of hyperlipidaemia or hyper–proteiaemia, the sodium concentration may be falsely low if it is expressed as the total volume of plasma, and not just the aqueous phase (which…