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152 Nutrition
undertaken regularly. The results also vary from one patient to another and are therefore
specic to the individual involved.
The BMI is more widely adopted as it is easy to calculate at home as well as in the
clinical setting. It estimates body fat by comparison of body weight with height. The BMI
is calculated by the formula:
BMI = body weight in kilograms (kg) ÷ (height in metres)
2
After measuring the person’s body weight in kilograms, and their height in metres, it is just
a matter of multiplying the height by the height (i.e. height
2
), then dividing this result into
the weight. As an example, a man weighing 78 kg is 1.8 metres tall:
height
2
= 1.8 × 1.8 = 3.24 (warning: do not convert the metres to centimetres!)
BMI = 78 ÷ 3.24 = 24 (there are no units)
It can also be done easily on BMI calculators that do the arithmetic for you. They can be
downloaded from the Internet or obtained from healthcare product suppliers.
The results are usually interpreted in this manner:
• BMI below 18.5 = underweight
• BMI 18.5–24.9 = normal weight
• BMI 25–30 = overweight
• BMI more than 30 = obese.
Bioelectrical impedance analysis (BIA)
Bioelectrical impedance analysis (BIA) uses a piece of equipment that passes a safe, low-
voltage electrical current through the body. This current is not enough to be felt by the
subject. Because electrical currents ow faster through water-rich areas of the body (e.g.
muscle), fat tissue (which has low water content) will impede the ow. These variations
in ow rates are measured by the machine, which, together with information on the sub-
ject’s height, weight, age and gender, allows the instrument to calculate the percentage of
body fat (
Haroun et al. 2010).
Total parental nutrition (TPN)
Total parental nutrition (TPN) is a treatment regime that is used to replace oral food
intake in those patients who are unable to take any food or uid by mouth. The nutri-
tion and uids are administered by intravenous cannula directly into a main venous
blood vessel, usually a central vein. Smaller veins are not recommended as they may
get blocked as a result of the uid administered. The nutrition provided must be in the
same form that would be absorbed from the digestive tract, i.e. glucose, fatty acids,
amino acids, vitamins and minerals. It gives the staff the opportunity to measure and
record nutritional and uid intake very accurately and thus to work out the nutritional
and uid balances. Very careful mouth observations and oral management must be
made to prevent dryness and infection since a major part of normal mouth cleanliness
from drinking uids will be missing.
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Nutrition 153
Key points
Energy
• Energy balance is the energy intake compared with the energy used.
• Neutral energy balance means the intake equals the energy used.
• Positive balance (i.e. greater energy input than used) can lead to being overweight.
• Negative balance (i.e. less energy input than used) can lead to weight loss.
Proteins
• Amino acids are the components of proteins; they contain an amino group (NH
2
),
a carboxyl group (COOH) and a radical that differs between the various amino
acids.
• Proteins in the body function as structures (e.g. muscle), hormones, antibodies, plasma
proteins, enzymes and other important elements.
• Proteins are not used for energy except as a last resort.
• Some hospital patients may develop a mild form of kwashiorkor, which should be con-
sidered when assessing patients in hospital.
Fats
• Fats (notably fatty acids) are our secondary energy source.
• It is the healthier option to reduce the intake of saturated fats.
Carbohydrates
• Carbohydrates (notably glucose) are our primary energy source.
Vitamins
• Vitamins are organic nutrients required in small quantities in the diet.
• Vitamins A, D, E and K are fat-soluble, and their presence requires some fat in the diet.
Other vitamins are water-soluble.
Minerals
• Minerals are inorganic chemical elements required in the diet.
• Electrolytes are charged particles in solution.
Fibre
• Fibre remains unchanged in the digestive system; it is not absorbed and provides no
direct nutritional value.
• Fibre is very important for elimination and healthy bowel function.
Nutrition in the young and the elderly
• Adequate nutrition is essential to sustain growth throughout childhood.
• Energy food consumption should be reduced as people get older, particularly fats and
sugars.
• Women after menopause require less iron but more calcium.
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154 Nutrition
Disorders of nutrition
• Cachexia is a moribund state of critical malnutrition caused by a metabolic malfunction.
• Starvation is a lack of foods that, if it continues, results in weight loss, muscle wasting
and possibly death.
• Anorexia is starvation due to an eating disorder, more often seen in young women.
• Bulimia is an eating disorder where eating binges are followed by periods of guilt dur-
ing which the patients try to retrieve the food by vomiting and the use of laxatives.
• Obesity causes multiple long-term complications to health and shortens the lifespan.
Observations of the nutritional state
• Simple observations of nutritional status include the dietary history, observing the
patient eat and ensuring they are able to eat adequately.
• Other observations include body weight, the body mass index (BMI) and bioelectrical
impedance analysis (BIA) to measure fat percentage.
Notes
1 Don’t get amylose and amylase muddled. Any chemical word ending in ‘ose’ is usually a sugar,
and the word ending in ‘ase’ is the enzyme acting on that sugar.
2 Inorganic means not derived from a living source, i.e. originally derived from a geological source.
References
Bell J. (2007) Nutritional screening during hospital admission: 1. Nursing Times, 103(37): 30–31.
Blows W.T. (2011) The physiology of food intake regulation and eating disorders. Journal of Gastro-
intestinal Nursing, 9(6): 40–45.
Blows W.T. (2022) The Biological Basis of Mental Health Nursing (4th edition). Routledge, Abingdon
and New York.
Haroun D., Taylor S.J., Viner R.M., Hayward R.S., Darch T.S., Eaton S., Cole T.J. and Wells J.C.
(2010) Validation of bioelectrical impedance analysis in adolescents across different ethnic
groups. Obesity, 18(6): 1252–1259.
Perry L. (2009) Using nutritional screening tools to identify malnourished patients. Nursing Times.
www.nursingtimes.net/clinical-archive/nutrition/using-nutritional-screening-tools-to-identify-
malnourished-patients-06-01-2009/ (Accessed 19th February 2023).
Wilson A. (2011) How to provide effective oral care. Nursing Times, 107(6): 14–21.
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8 Elimination (I)
Urinary observations
• Introduction
• Urine formation
• The urinary system in youth
• Urinary observations and tests
• Urinalysis
• Other urine tests
• When to test urine
• Drugs active on the urinary system
• Key points
• Notes
• References
Introduction
The eliminatory systems, the bowel and kidneys, are the principal excretory pathways for
many surplus and toxic substances produced by the tissues. Their functions maintain the
body’s tissues and blood at permanently optimum levels. The systems also excrete prod-
ucts from disease, i.e. the urine offers insights into many pathologies. Accurate observa-
tions of urine can reveal much about the individual at that time, both the factors affecting
the system and the underlying metabolism.
The renal system
The renal system consists of two kidneys attached to the posterior abdominal wall, one
each side of the lumbar vertebrae (Figure 8.1). Each kidney consists of around one million
nephrons which lter water and other substances (ltrate) from the blood. This will be
acted on by the nephron until it becomes urine. Urine drains from the kidneys down a
ureter on each side, to the bladder where it is temporarily stored bladder. The nal pas-
sageway to the outside world is via the single urethra (Figure 8.1).
DOI: 10.4324/9781003389118-8
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156 Elimination (I)
Adrenal
gland
Left
kidney
Right
kidney
Abdminal
aorta
Left ureter
Bladder
Urethra
Ureteral
opening
Right
ureter
Inferior
vena cava
Figure 8.1 The renal system consists of a left and right kidney with ureters, the urinary bladder
within the bony pelvis below and a single urethra.
The kidney, when seen in cross section, shows an outer cortex and an inner medulla
(Figure 8.2). The cortex is the site where nephrons are housed, and the medulla is made
of renal collecting ducts. These ducts empty into the renal pelvis from where urine drains
down the ureters to the bladder (Figure 8.2).
Urine formation
Urine is formed by nephron (Figure 8.3), of which there are about one million in each
kidney. Urine production takes place in three distinct steps.
Step 1 is ltration under pressure within the glomerulus, which is surrounded by
the glomerular (Bowman’s) capsule (Figure 8.4). Blood plasma passing through the glo-
merular arterioles is subject to hydrostatic pressure, i.e. blood pressure as it occurs in the
glomerular arterioles, causing ltration by forcing water and other substances through the
glomerular membrane. The resulting ltrate is collected in the capsule at a rate known
as the glomerular ltration rate (GFR), which is about 120 ml per minute, measured
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Elimination (I) 157
Renal
Artery
Calyx
Pyrmaids
Vein
Cortex
Ureter
Medulla
Urine to
pelvis
bladder
Figure 8.2 The kidney in cross section. The cortex is mostly nephrons and the medulla is mostly
straight collecting ducts set in pyramids. These pyramids drain urine into the calyces and
the renal pelvis. Urine then passes to the ureters and onto the bladder.
as a total of all 2 million nephrons. Water and other small molecules may pass through
the membrane but large molecules, such as most proteins and blood cells, cannot pass
through the normal membrane and remain in the blood.
Step 2 is the reabsorption of required substances, including much of the water, back
into the circulation from the proximal convoluted tubule (Figure 8.5). Water returns to the
blood at a rate of about 105 ml per minute, measured as a total of all 2 million nephrons.
Other substances returned include some sodium and all of the glucose.
The remaining ltrate passes into the loop of Henle, which has the effect of causing
an osmotic gradient across the renal cortex that becomes important to water reabsorption
from the straight collecting ducts (Figure 8.6).
Filtrate then passes into the distal convoluted tubule, where further reabsorption of
substances takes place, some of which is under hormonal control. The reabsorption of
water is controlled by antidiuretic hormone (ADH) and that of sodium by aldosterone
(Figure 8.5). ADH is also active along the straight collecting ducts controlling water reab-
sorption in response to the osmotic gradient.
Step 3 involves the addition of certain substances, notably hydrogen ions (H
+
) (see page 173),
ammonia (NH ) (see page 76) and potassium ions (K
+
3
) into the ltrate from the blood by
means of tubular secretion, which takes place in the straight collecting ducts. The resultant
ltrate is now urine, which then passes rst to the renal pelvis and then into the bladder.
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Proximal
convoluted tubule
Glomerulus
Straight
collecting
duct
Distal
convoluted
tubule
To renal
pelvis
Afferent
arteriole
Efferent
arteriole
Urine
Bowman's
capsule
Descending
limb of loop
of Henle
Ascending limb
of loop of Henle
158 Elimination (I)
Figure 8.3 T he renal nephron. The glomerulus, inside the Bowman’s capsule, is formed from a tuft
of arterioles. The blood enters via the afferent arteriole and leaves via the efferent arteri-
ole. Filtrate ows along the proximal convoluted tubule to the loop of Henle. The distal
convoluted tubule conveys ltrate to the straight collecting duct, where, as urine, it ows
to the renal pelvis and to the bladder.
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Elimination (I) 159
Capsular
hydrostatic
pressure
Glomerular
osmotic
pressure
Glomerular
filtration rate
= 120 ml/min
Bowman's
capsule
Efferent
arteriole
Glomerulus
Afferent
arteriole
Glomerular
hydrostatic
pressure
Figure 8.4 T he glomerulus and Bowman’s capsule. Water is pushed out of the blood and into the
Bowman’s capsule by the glomerular hydrostatic pressure (GHP). Two other forces are
returning water to the blood. The capsular hydrostatic pressure is produced by the uid
within the capsule, and the glomerular osmotic pressure is the force exerted by the pro-
teins within the blood to attract water back. The GHP is greater than the sum of the other
two, creating a glomerular ltration rate of approximately 120 ml/min, measured as a
sum total for all 2 million nephrons.
65% of
filtered
H
2
O
Proximal convoluted tubule
50% of
filtered
urea
67% of
filtered
Na
+
100% of all
filtered
glucose,
amino acids
and K
+
Glomerulus
Bowman's
capsule
Loop of
Henle
Figure 8.5 The proximal convoluted tubule, between the Bowman’s capsule and the loop of Henle,
reabsorbs water, urea, sodium and other substances. The percentage shown is that of the
amount ltered by the glomerulus.
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160 Elimination (I)
H
y
p
e
r
t
o
n
i
c
300
400
600
900
1,200
300
Straight
collecting
duct
100
Distal
tubule
400
600
900
300
H
2
O
(isotonic)
300
Proximal
tubule
300400
400
200
700
400
600
NaCL
900
1,200
Ascending limb of
loop of Henle
Descending limb of
loop of Henle
All figures in mosmol/l
(milliosmoles per litre)
1
2
0
0
NaCL
NaCL
,
Figure 8.6 T he loop of Henle passes through a sodium concentration gradient outside the loop
caused by blood capillaries called the vasa recta, which follow the loop closely (not
shown). The concentration gradient is shown from 300 to 1,200 mosmol/l. Water
can pass out of the descending limb but not the ascending limb, and sodium is
unable to pass out of the descending limb but is actively transported out of the
ascending limb. The ltrate passing down the descending limb is isotonic to begin
with but becomes more concentrated in sodium (i.e. hypertonic) as water is lost. As
the ltrate returns up the ascending limb, sodium is lost and the ltrate becomes
less concentrated. The resultant dilution of the ltrate in the distal convoluted tubule
(100 mosmol/l) allows the kidney to excrete urine more diluted than body uids
(hypotonic) when necessary. The gradient along the straight collecting duct allows
the kidney to reclaim water (under antidiuretic hormone control) and therefore
excrete urine more concentrated than body uids (hypertonic) when necessary (Fig-
ures 8.7–8.9).
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Elimination (I) 161
H
2
O
H
2
O
1 ml/min
Na
+
trace
Na
+
Aldosterone
Adrenal
cortex
Loop of
Henle
ADH
Cl
–
HCO
3
–
Hypothalamus
Figure 8.7 The distal convoluted tubule. After the loop of Henle, the distal tubule absorbs chlorine
(Cl
−
) and bicarbonate (HCO
−
3
). Sodium (Na
+
) is reabsorbed under aldosterone control
from the adrenal cortex, and water is reabsorbed under antidiuretic hormone (ADH)
control from the hypothalamus. The amount of water returned to the blood allows an
average of 1 ml per minute as urine production.
The urinary system in youth
After birth, the kidneys are anatomically complete but remain underdeveloped and imma-
ture and only reach maturity of function by 2 years of age. Normally, the conscious con-
trol of urination arises somewhere about 2 or 3 years of age, and by 12 years micturition
reaches adult standards. In the teenage years, the risk of urinary tract infections is higher
for girls than for boys. Young women aged over 15 years old are at the highest risk, with 1
in 5 becoming affected. This is largely due to the short urethra in girls which allows quick
and easy access for bacteria to reach the bladder. The urethral opening is not far from the
anus, the major source of the bowel commensal Escherichia coli (E. coli), which causes
70–80% of infections. Staphylococcus saprophyticus is the second most common cause
of urinary tract infections in young women.
Urinary observations and tests
Urinary observations are made on the volume, colour, smell, deposits and specic gravity of
the urine, and there are chemical tests, in the form of reagent pads on a strip, testing for glu-
cose, pH, protein, blood, bilirubin, urobilinogen, ketones, nitrite, leucocytes and vitamin C.
Collecting urinary specimens and the use of reagent testing strips are described by Simerville
et al. (2005). The reagent test strips are calibrated to perform accurately within precise limits,
and they will give false results if used wrongly. These limits include the need to use the strips
before the expiry dates, store them in a dry environment and to read the results at the speci-
ed time limit (usually 60 seconds). Testing freshly voided urine is important since constituents
change with time when left standing, especially when exposed to the air.
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