Copyright EMAP Publishing 2017
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Nursing Practice
Systems of life
Renal system
Keywords: Ageing/Respiratory
system/Respiratory tract infections/
This article has been
double-blind peer reviewed
In this article...
● H
ow age affects the normal functioning of the renal system
● Age-related changes to the kidneys, bladder, urethra and prostate
● How declining renal function can affect various conditions in older age
Anatomy and physiology of ageing 4:
the renal system
Key points
1
The renal system
is the most
powerful regulator
of the body’s
internal environment
With advancing
age, renal blood
flow – and therefore
glomerular filtration
rate – are reduced
The mass and
weight of the
kidneys decrease
significantly after
the age of 50 years
The progressive
degradation of
renal function leads
to issues typical of
old age such as
polyuria, nocturia
and incontinence
Most men
experience a
benign enlargement
of the prostate
gland as they get
older, but this
prostatic hyperplasia
can also be a sign of
malignancy
2
3
4
5
Authors Maria Andrade and John Knight are both senior lecturer in biomedical
science, College of Human Health and Science, Swansea University.
Abstract The functions of the renal system include removal of waste products;
regulation of blood volume, blood pressure and red blood cells; and balancing of
electrolytes and blood pH. Renal function starts to gradually decline after the third
decade of life but, in the absence of disease, the renal system is able to fulfil its role
throughout life. In spite of this, many anatomical and physiological changes mean
older people are prone to issues such as polyuria, nocturia and incontinence. This
fourth article in our series on the anatomy and physiology of ageing explains how age
affects the organs of the renal system, leading to a reduction in renal function.
Citation Andrade M, Knight J (2017) Anatomy and physiology of ageing 3: the renal
system. Nursing Times [online]; 113: 5, 46-49.
T
he renal system is the most powerful regulator of the body’s
internal environment. Healthy
kidneys are essential to maintain
homeostasis, ensuring stable conditions
in which all cells can function optimally.
They perform several functions (Montague
et al, 2005), including:
l R
emoval of waste products such as
urea, uric acid, creatinine and toxic
breakdown products of drugs;
● R
egulation of blood volume and pressure;
● E
lectrolyte (salt) balance;
● A
cid-base balance (regulation of
blood pH);
● R
egulation of the number of
erythrocytes (red blood cells);
● S
ynthesis of vitamin D.
In the absence of disease, the kidneys
function optimally into the third decade of
life, after which there is a gradual decline
in renal function (Figs 1 and 2). Around 15%
of people over the age of 70 years have varying degrees of renal disease and dysfunction (Coresh et al, 2007). Factors contributing to the decline in renal function
include hypertension, smoking, exposure
Nursing Times [online] May 2017 / Vol 113 Issue 5
46
to lead, obesity and increased inflammatory mediators in the blood (Weinstein and
Anderson, 2010).
Anatomical changes
Pre-renal changes
The most important pre-renal (occurring
before the kidney) change affecting kidney
function is vascular degeneration. In
young adults, renal blood flow is estimated
to be approximately 600ml/minute; in
older people this is often reduced by half
(Cukuranovic and Vlajkovic, 2005) primarily due to normal age-related changes
in blood vessels (see Part 1 of this series)
and is often exacerbated in people with
atherosclerosis of the renal artery.
Such blood vessel changes usually lead
to ischaemia (reduced oxygenation), particularly in the outer portion of the kidney
(renal cortex). Cells gradually die and are
replaced with scar tissue, giving the outer
surface of aged kidneys a granular or mottled appearance. The arterioles leading to
the glomeruli (filtration membranes) show
deposition of hyaline (clear cartilagelike material) and collagen below the
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Tubule abnormalities
Some kidney tubules gradually degenerate
and are replaced by scar tissue (tubulointerstitial fibrosis). This seems associated
with an increasing number of cells
showing features of senescence (Sturmlechner et al, 2017), which reduces the area
available for the reabsorption of useful
materials such as glucose, amino acids and
salts. The distal convoluted tubules often
shrink and may develop small pouches
(distal diverticula), which can in turn
become fluid-filled cysts, increasing the
risk of kidney infection and pyelonephritis
(Zhou et al, 2008).
Fig 1. Age-related changes to the renal system
Increased collagen deposition, reduced
renal blood flow and glomerular filtration
rate, loss of nephrons and abnormal
nephron structure
Increased risk of
urinary incontinence
due to:
l Reduced bladder
elasticity
l Loss of sphincter
tone
l Poor bladder
control
Shortened urethra in some
women may increase risks
of urinary tract infection. Prostatic
hyperplasia in men may interfere
with urine flow
CATHERINE HOLLICK
endothelium; this reduces the diameter of
the vessels, thereby restricting blood flow.
The smallest blood vessels in the kidney,
including the capillaries that form the glomeruli, also progressively degenerate and
are replaced with fibrous scar tissue
(Musso and Oreopoulos, 2011).
Aged blood vessels experience a general
reduction in the synthesis of the potent
vasodilator nitric oxide (see Part 1 of this
series) and this contributes to reducing
blood flow to the kidneys (Weinstein and
Anderson, 2010).
Reduced glomerular filtration rate
The glomerular filtration rate (GFR) is a
measure of the rate of fluid filtration
through the glomerular capillaries into
Bowman’s capsule. It is expressed in millilitres per minute, and is routinely used to
measure the progression of kidney disease
(Bit.ly/RAStagesCKD). GFR peaks in the
third decade of life, where it may be as high
as 140ml/min/1.73m2. Blood vessel changes
progressively reduce renal blood flow and
GFR: in normal ageing, it drops by around
8ml/min per decade after the age of 30. The
GFR of people in their 80s may be only
60-70% of what it was when they were
young adults; at 90 years of age it has typically fallen to around 65ml/min/1.73m2.
Reduced GFR means reduced clearance
of waste products. However, age-related
decline in GFR is not observed in all people;
indeed many maintain a stable GFR
throughout life, which suggests that variables other than ageing contribute to the
decline (Zhou et al, 2008).
Renal changes
Both kidney mass and weight decrease significantly after the age of 50 (Zhou et al,
2008). The kidneys of people in their 20s
weigh 250-270g each; in 90-year-olds this has
dropped to 180-200g. It has been estimated
that, between the ages of 40 and 80, approximately 20% of kidney mass is lost (Choudhury et al, 2016); only 3% of people in their
90s have histologically normal kidney tissue.
There is a gradual increase in collagen
deposition, leading to progressive kidney
fibrosis. In old age, whole nephrons (the
functional units in the kidneys) are
replaced by fatty material or scar tissue; on
average, 70-year-olds have lost 30-50% of
their nephrons. Aged nephrons often show
a variety of physical defects (Fig 2).
Glomerular abnormalities
The number of damaged glomeruli (glomerulosclerosis) increases, typically
leading to progressive capillary collapse.
Fewer than 5% of glomeruli show sclerosis
in people in their 20s but, by their 80s, this
will have risen to around 30% (Weinstein
and Anderson, 2010).
Filtration membrane abnormalities
Some nephrons display a progressive
thickening and wrinkling of the filtration
membrane in the glomerulus and Bowman’s capsule, decreasing the renal filtering surface area. The filtration membrane
also
becomes
increasingly
permeable, allowing large molecules such
as proteins to collect in the filtrate and
appear in the urine (proteinuria).
Nursing Times [online] May 2017 / Vol 113 Issue 5
47
Impaired renal repair
In young adult kidneys, around 1% of renal
cells have the ability to divide and proliferate. This declines with age, reducing the
kidneys’ ability to repair. The chemical signalling pathways that coordinate cell division and repair in the kidneys also become
impaired with age (Bolignano et al, 2014).
Diet and renal ageing
Age-related changes in renal structure and
function are thought to occur as a result of
both genetic and environmental factors
(Bolignano et al, 2014). One factor that
appears to play a role is exposure to oxidative stress, which tends to lead to the
release of pro-inflammatory mediators.
While most oxidative stress is linked to
free radicals produced during cellular
metabolism, some of it comes from diet.
Foods cooked at high temperature (particularly fried or roasted) are high in prooxidants; it has been suggested that limiting their intake could reduce oxidative
and inflammatory stress on the kidneys
(Vlassara et al, 2009).
Gender differences in renal ageing
Although this is still poorly understood,
oestrogens such as 17 beta-estradiol appear
to protect the renal system in women
from the effects of ageing, while androgens such as testosterone increase the risk
of renal dysfunction in men. One hypothesis is that androgens promote fibrosis
in the kidney; this may partially explain
why chronic kidney disease progresses
more quickly in men (Weinstein and
Anderson, 2010).
Physiological changes
Older people experience a significant
reduction in renal function. Even in the
absence of disease, some people over the
age of 65 only possess 60% of the renal
function of young adults (Razzaque, 2007).
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This gradual decline has major health
implications, particularly when long-term
conditions and circulatory problems are
present.
Long-term conditions that lead to fluid
overload and oedema – such as heart
failure, liver disease and chronic kidney
diseases (for example, diabetic and hypertensive nephropathies) – are exacerbated
by poorly functioning aged kidneys.
Patients with these conditions (commonly
managed via diuretics) need their treatment regimens monitored and adjusted to
compensate for age-related renal decline.
Numerous studies have indicated that obesity, diabetes, high blood pressure, ethnicity and genetics can all contribute to
the onset of renal disease in older people
(Kazancioglu et al, 2013).
Common age-related physiological
changes in renal function are described
below.
Electrolyte imbalance
Reduced renal blood flow and GFR,
together with the gradual loss of nephrons,
reduce the kidneys’ ability to keep electrolytes (sodium, potassium, calcium and
chloride) within optimal ranges. As these
play a major role in maintaining blood
pressure and generating nerve impulses,
older people may experience hyper- or
hypotension and confusion.
Older people are also less efficient at
clearing salt, which may be due to reduced
GFR (less sodium-laden blood is filtered),
tubular degeneration, and reduced sensitivity to hormones such as aldosterone
(Musso and Oreopoulos, 2011).
CATHERINE HOLLICK
Reduced acid-base balance
While the lungs play a role in regulating
blood pH, only the kidneys can excrete
acidic or basic molecules directly, so they
are the ultimate regulators of acid-base
balance. With age, they become less efficient at clearing acidic/basic metabolites/
ions due to tubule degeneration. This is
problematic in older people with diabetes,
as acidic molecules such as ketones may
accumulate in the blood, leading to lifethreatening ketoacidosis.
Reduced creatinine clearance
Creatinine is a molecule that is continually
generated by skeletal muscles; serum
levels usually remain constant because the
kidneys clear it from the blood at the same
rate as it is produced. By the age of 80,
clearance is reduced by around 30%
(Choudhury et al, 2016); however, serum
creatinine levels remain fairly constant
Fig 2. D
efects found in aged nephrons and their consequences
Glomerulus and Bowman’s
capsule
l Thickened
filtration
membrane
lR
educed
glomerular blood flow
l Reduced filtration
l Glomerular
degeneration
Loop of Henle
Reduced control over salt
reabsorption potentially
leading to electrolyte
imbalance
because of the gradual reduction in skeletal muscle mass.
Polyuria and nocturia
The progressive loss of nephrons makes
the kidneys less efficient at concentrating
urine, therefore a greater volume of water
is required to excrete toxic waste products.
Additionally, the effect of antidiuretic hormone on the renal tubules is blunted in
older people, leading to a larger volume of
diluted urine. The result is a gradual
increase in urine volume leading to polyuria (frequent urination).
In older patients, it is essential to monitor fluid status for signs of dehydration,
and ensure they have free and convenient
access to water. Fluids drunk during the
evening take longer to be processed,
potentially leading to nocturia (nocturnal
urination), which is experienced by
80-90% of people aged over 80 years
(Kujubu, 2009). Ideally, patients should be
reminded to pass urine before going to bed
to minimise the problem.
Reduced clearance of toxic metabolites
Special care must be taken with drugs that
are excreted/eliminated in the urine. Dosages of water-soluble drugs – such as certain antibiotics, amphetamines and digitalis – may need adjustment according to
renal function to avoid toxic accumulation. Indeed, overestimation of the GFR
can lead to unexpected drug toxicity in
older patients (Zhou et al, 2008). However,
other organs and tissues, particularly the
Nursing Times [online] May 2017 / Vol 113 Issue 5
48
Proximal tubule
Reduced reabsorption
of amino acids, glucose
and hormones
Distal tubule
l Reduced size and
development of distal
diverticulae may lead
to cyst formation
l Reduced sensitivity to
antidiuretic hormone
l Reduced clearance of
drugs and toxic
metabolites
liver, play a role in drug metabolism and
clearance, so their ageing also increases
the risk of drug toxicity.
Reduced insulin clearance
The kidneys remove around 50% of
secreted insulin from the peripheral blood.
Although reduced GFR along with degeneration and loss of nephrons significantly
diminish older people’s ability to clear it,
response to insulin is gradually blunted
with age, which offsets the reduced clearance (Zhou et al, 2008).
Changes in erythropoietin and vitamin
D biosynthesis
The cells that form the tubules of the
nephron, along with the peritubular cells,
produce erythropoietin (EPO) and play a
role in vitamin D biosynthesis. Tubular
degeneration often leads to reductions in
EPO, which can lead, in turn, to reduced
erythrocyte production and to anaemia.
However, some studies have demonstrated
increases in EPO secretion – these may
be driven by age-related resistance to the
effects of this hormone (Bolignano et
al, 2014).
Reduced vitamin D biosynthesis
impairs the absorption of calcium and
phosphate in the gut, which can contribute to osteoporosis (Zhou et al, 2008).
Post-renal changes
With age, the bladder gradually loses its
elasticity due to an increase in collagen
fibres in its wall. Loss in elasticity and
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fibrosis of the bladder can contribute to
incomplete emptying during micturition
(urination), particularly in men with prostate enlargement. There is conflicting evidence as to whether bladder volume
changes with age: recent research suggests
it rarely changes (Pfisterer et al, 2006).
Ageing and urinary incontinence
The urinary sphincter often weakens with
age and this may lead to urinary incontinence – a problem often compounded by
age-related changes in the nervous system.
However, urinary incontinence is not a
normal consequence of ageing, as many
individuals never experience it, even in
extreme old age.
Research indicates that around 11.6% of
people aged 65-80 years experience incontinence; this rises to around 35% in those
over 85, and to 69% in over-85s living in
nursing homes. Women seem to be at
greater risk of incontinence than men
(26.6-35.0% compared with 12.6-24.0% in
those aged 85 years or older); this may be
due to weakened pelvic floor muscles as a
result of childbirth.
Incontinence can have a major negative
impact on psychological wellbeing and
quality of life (Ranson and Saffrey, 2015).
Ageing of the urethra and prostate
Reports on age-related changes to the
female urethra are contradictory: some
indicate that the urethra shrinks and its
walls become thinner and atrophied (Jaipaul, 2017), while others report no evidence of change in its length (Pfisterer et
al, 2006). Age-related changes in vaginal
pH can encourage abnormal microbial
growth, increasing the risk of urinary tract
infections.
Most middle-aged and older men experience a benign enlargement of the prostate gland (prostatic hyperplasia) (Fig 3);
Anatomy and physiology of
ageing – updated series
PETER LAMB
ArticleDate
Part 1: the cardiovascular system
Part 2: the respiratory system
Part 3: the digestive system
Part 4: the renal system
Part 5: the nervous system
Part 6: the eyes and ears
Part 7: the endocrine system
Part 8: the reproductive system
Part 9: the immune system
Part 10: the musculoskeletal system
Part 11: the skin
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Fig 3. Age cycle of the prostate
Age 20–25
walnut
Adrenal gland
Age 35–40
golf ball
Kidney
Ureter
Bladder
Prostate
gland
this results in a gradual compression of
the urethra, making micturition more difficult. The onset of prostate cancer is also
marked by an increase in prostate size and
causes a similar compression of the urethra and reduction in strength of the urine
stream. Tests such as prostate-specific
antigen, together with physical examination of the prostate, are often necessary to
differentiate between malignancy and agerelated prostatic hyperplasia.
Conclusion
Although it appears that little can be done
to slow age-related changes to the renal
system, research indicates that a highprotein diet can normalise GFR in some
older people (Musso and Oreopoulos,
2011), potentially improving kidney function. Renal function must be assessed
before making any dietary changes, as preexisting renal disease can preclude high
protein intake. In recent years a number of
studies have demonstrated the anti-aging
effects of calorie-restricted diets. Longterm calorific restriction appears to be
effective at reducing the effects of renal
ageing, with vascular damage, glomerulosclerosis and tubular fibrosis all reduced in
animal models (McKiernan et al, 2007).
Although age-related decline in renal
function is inevitable, the kidneys have a
built-in redundancy – the renal reserve –
and, in the absence of disease, will function adequately throughout life. NT
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lemon
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cricket ball
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lBlood tests and age-related changes
in older people
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