Therapeutic clinical application
Patients with renal failure may undergo peritoneal dialysis in which a catheter is implanted into the abdomen and glucose-rich fluid (known as the dialysate) is infused via the catheter into the peritoneal cavity. The peritoneum acts as a selectively permeable membrane through which excess fluid (and electrolytes and waste products) are drawn out of the blood and into the dialysis fluid. Peritoneal dialysis may be continuous ambulatory peritoneal dialysis (CAPD) in which the dialysate is infused into the abdomen and retained there for approximately eight hours before being allowed to drain. The process is then repeated two or three times a day. Alternatively, automated peritoneal dialysis (APD) may be used in which a machine is used to cycle the fluid into and out of the abdomen. This is usually done overnight.
Isotonic, hypertonic and hypotonic
The term isotonic has become more familiar to the general public with the introduction of isotonic sports drinks. Isotonic solutions are at the same or close to the same concentration as the fluid found in human cells. Nurses routinely use isotonic saline solutions to help keep patients hydrated, particularly when they are confined to bed, unconscious or unable to drink fluids normally. To help you understand the nature and composition of isotonic saline when you are on your next hospital placement, attempt Activity 1.3.
Activity 1.3 Evidence-based practice and research
On your next placement take a few minutes to examine the saline drip bags at the side of your patients’ beds; pay close attention to the chemical composition specified.
What do you notice?
Now that you have an understanding of the composition of isotonic saline, we can explore why these are routinely used in clinical practice.
Human cells are stable in isotonic solutions because the concentrations of dissolved materials (solutes) are equal on both sides of the plasma membrane and so no net movement of water is occurring.
In health, the blood is a near-perfect isotonic medium kept at the same concentration as the cytosol of our cells by a multitude of homeostatic mechanisms. However, in certain diseases human cells can be taken out of their isotonic comfort zone, which can cause damage and in some circumstances become life-threatening.
Dehydration
In patients with diabetes the presence of large amounts of sugar (hyperglycaemia) results in the blood becoming too concentrated. Highly concentrated blood is referred to as being hypertonic (too concentrated) to human cells. In hypertonic solutions water will leave the cells of the body by osmosis and move into the blood, and this can lead to progressive dehydration which is a common presenting symptom in patients with undiagnosed or poorly controlled diabetes.
Dehydration caused by not drinking enough fluids or by severe vomiting or diarrhoea will similarly lead to hypertonic blood and loss of water from cells. As cells lose water, their cell membranes become loose and flaccid and may take on a crinkled appearance; this phenomenon is referred to as crenation. Progressive loss of water from the intracellular compartments can lead to tissues of the body such as the skin becoming noticeably looser, and this can be detected in patients using skin-pinch tests. As well as being a sign of shock, prolonged capillary refill time may also indicate dehydration.
Dehydration is also characterised by the mucous membranes of the body drying out which is why many people wake up with a dry mouth after drinking too much alcohol, which is known to cause dehydration by promoting diuresis (increased urination). A common cause of dehydration in hospital patients is infection with norovirus. To help develop your knowledge of this problem, attempt Activity 1.4.
Activity 1.4 Evidence-based practice and research
An outbreak of norovirus is confirmed on your placement. List the most effective infection prevention and control measures that should be applied to minimise the spread of infection.
Dehydration, such as that experienced in patients following norovirus infection, is very common. More rarely, nurses will encounter patients who have too much water in their body.
Water intoxication (water toxaemia)
Water intoxication can be thought of as the opposite of dehydration and is caused by consuming too much water. It is frequently seen in endurance athletes such as cyclists and marathon runners who may routinely consume large quantities of water at drinking stations along the routes of their races. Occasionally it occurs following the use of recreational drugs such as ecstasy (MDMA), which can induce thirst and also upset the normal water balance of the body by reducing urine output. Young babies who are fed on formula milk may also be at risk, particularly in poorer households where the milk powder may be over-diluted to make it last longer.
Consuming large quantities of water dilutes the blood, in effect making it hypotonic and at a lower concentration to the cytosol within cells. The dilution of blood in these patients will also lead to hyponatraemia (low blood sodium). Water will gradually move from the blood into the cells by osmosis, causing the cells to swell. Since all the tissues of the body are composed of cells, during water intoxication all the soft tissues will begin to swell and internal organs will enlarge. Early signs of water intoxication will include headache, nausea and vomiting. In more serious cases, the patient may experience confusion, visual disturbances, drowsiness, breathing difficulties, muscle weakness and cramping.
Since the brain is enclosed within the cranium of the skull, there is minimal space available to accommodate cerebral enlargement, and the intracranial pressure will increase restricting blood flow and reduce cerebral perfusion. As a result, the patient will gradually lose consciousness, commonly slipping into a coma and, unless quickly treated, they will suffer permanent brain damage and may die.
Treatment will be determined by the cause and severity of water intoxication. Firstly, the amount of fluid taken on board must be reduced and excess water expelled. This can be achieved by the administration of diuretics to increase urine output. If the condition has been caused by medication, the patient’s medication must be reviewed and the drug causing the problem should be discontinued.
The importance of following the manufacturer’s instructions when mixing infant formula must be reinforced and where financial hardship is a contributing factor, parents should be advised on appropriate support networks and benefits that might be available. Sodium levels should be corrected by careful administration of intravenous fluids with a relatively high concentration of sodium. Diuretics will also help increase sodium levels as excess fluid is excreted; however, these have to be used with care as some can cause significant loss of potassium, leading to hypokalaemia (low blood potassium).
Other forms of membrane transport
In addition to allowing the passage of single molecules into and out of the cell, the plasma membrane can allow larger groups of molecules and even solid materials/fluids to enter the cell via endocytosis or leave the cell via exocytosis (Figure 1.7).
Figure 1.7 Endocytosis and exocytosis
Phagocytosis
This is the form of endocytosis by which cells can take up solid particulate materials. The term phagocytosis
