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 literally means ‘cell eating’ and is particularly important in the cells of the immune system which are actively engaged in removing pathogenic material such as bacteria, fungal cells and viral particles. The process of phagocytosis utilises the fluid nature of the plasma membrane. When a pathogen such as a bacterium is encountered the plasma membrane flows around it, engulfing it and enclosing it in a membrane-bound vesicle. Once internalised within the cytoplasm, the lysosomes produced by the Golgi fuse with the vesicle and discharge their enzymes into its interior, killing the pathogen and initiating intracellular digestion.
Exocytosis
Following the intracellular digestion of solid particulates such as bacteria, waste materials such as components of bacterial cell walls are released from cells by exocytosis (Figure 1.7). This process of exocytosis is also the mechanism by which cells of endocrine glands release their hormones into the blood and neurons release their neurotransmitters into synapses (Chapters 5 and 6).
Pinocytosis
This form of endocytosis allows cells to take up small droplets of fluid from the extracellular environment. The term pinocytosis literally means ‘cell drinking’, with most cells capable of taking up droplets of the interstitial fluid which surrounds them.
Histology
Histology is the study of biological tissues. A tissue can be defined as a collection of one or more cell types that work together for a common purpose. In the human body tissues can be thought of as the building blocks of organs. As highlighted in Josie’s case study at the beginning of this chapter, tissue samples can be collected via biopsy to screen for disease.
Although the human body is incredibly complex, only four major categories of tissue are present: epithelial tissues, connective tissues (e.g. bone, cartilage, blood, adipose tissue and fat), muscle (skeletal muscle, cardiac muscle and smooth muscle) and nervous tissue (neurons and neuroglial cells).
As we move through this list from epithelial tissue through to nervous tissue, there is a gradual increase in complexity, with epithelial tissues regarded as the simplest human tissues and nervous tissue the most highly organised and complex. In this chapter we will only examine the nature of the epithelial tissues since the other tissue types are explored throughout the book.
The nature of epithelial tissue
Epithelial tissues are found throughout the body and have many diverse roles that are typically associated with absorption, protection and secretion. Epithelial tissues are recognisable since they rest on a thin delicate basement membrane (Figure 1.8).
Figure 1.8 Some of the major epithelial tissues
Epithelial tissues can be broadly split into the simple epithelia, which consist of a single layer of cells, and the stratified epithelia, which consist of multiple layers of cells stacked one on top of the other like bricks in a wall.
The simple epithelia
Simple squamous epithelium
The term squamous means resembling the scale of a fish, therefore squamous cells are described as being thin and flat in appearance (Figure 1.8A). This tissue is found in multiple locations within the body including the alveolar walls of the lung, endothelial lining of blood vessels and capillary walls, and lining the major serous membranes and serous layers of organs.
Since the cells of squamous epithelium are so thin and flat, they are perfectly adapted