The Urinary (Excretory)
System
Functions of the Urinary System (aka
Functions of Kidneys) (see p. 305)
1. Removal of metabolic wastes;
2. Maintenance of water-salt balance in body
(blood).
3. Maintenance of acid-base balance in body
(blood).
4. Hormonal function (EPO) – described in
Circulatory System; as well as Renin (an
enzyme), described later.
Excretion
• The removal of metabolic wastes (nitrogen-based
(nitrogenous)) from the body.
• The metabolism of amino acids, nucleotides, and creatine
phosphate (all of which are nitrogenous) results in
nitrogenous wastes that need to be ‘disposed’ of by the
body.
• Typical nitrogenous wastes that humans excrete include, in
order of amount: urea, creatinine, uric acid, ammonia
(which tends to buffer H+ present in urine – forming NH4+
(ammonium)). Ammonia is present in trace amounts.
• Feces is not considered to be metabolic waste…it consists
of materials that never actually ‘entered’ the body; the
ridding of feces is known as elimination.
Amino Acid Metabolism
- the deamination of amino acids (ie. the removal of
the amino (-NH2) group) occurs in the liver in order
to convert three amino acids into a glucose.
- Deamination results in the formation of ammonia
(NH3), which is a basic, toxic chemical requiring a
large amount of water for safe dilution and
subsequent removal from any animal.
- Thus, to conserve water for the body, the human
liver converts the majority of the ammonia into
urea, which is far less toxic, and requiring much
less water for disposal, than ammonia.
- Urea is filtered out of the blood into the urine by the
kidneys.
Ammonia reacts with CO2 in the liver to produce
UREA:
2 NH3 + CO2
+ H2O
Nucleotide Metabolism
- if too many DNA/RNA/ADP nucleotides exist
within the body or if they become degraded to a
non-functional state, they might be metabolized in
order to conserve space within cells.
- The metabolism of nucleotides results in the
formation of uric acid, which is a rather insoluble
nitrogenous waste (bird ‘pee’ – the white stuff).
- Uric acid is filtered out of the blood by the kidneys
for excretion.
- If too much uric acid is in the blood, it may
precipitate out, most commonly in joints (known as
gout), less commonly in the urinary tract as a kidney
stone.
Creatine Phosphate Metabolism
- creatine phosphate is a ‘donator’ molecule that
‘donates’ a phosphate group to ADP (adensine
diphosphate), within skeletal muscle cells only, in
order to re-create ATP; this serves to speed up the
production of ATP in muscles (where ATP is needed
quickly and in larger quantities during
exercise/stressful times).
- Once creatine phosphate has given up its phosphate,
it is known as creatinine – a nitrogenous waste that
is filtered out of the blood by the kidneys.
• Urea (and some ammonia), uric acid, and creatinine
are all nitrogenous wastes that exist in urine.
• Urine is produced within the kidneys (the kidneys
essentially filter the blood) for expulsion from the
body.
• The composition of urine is as follows:
– 95% water; 5% solids/dissolved substances
• Solids/Dissolved substances include (per 500 mL urine):
- 10 g urea
- 0.33-0.67 g creatinine
- 0.33 g uric acid
- 8-9 g of the following: Na+, K+, Ca2+, Mg2+, NH4+ (a
combination of H+ and NH3), SO42-, PO43-, HCO3- varying levels of toxins, drugs, antibiotics, histamines.
Anatomy of the Urinary System (fig. 16.1 p. 304)
• All organs and blood vessels of the urinary system
work to maintain a constant internal environment
(ie. they work to maintain homeostasis) with respect
to water-salt balance, pH balance, and nitrogenous
waste management.
• The kidneys are paired, kidney bean-shaped organs
that exist on either side of the spinal cord, just below
the diaphragm; each kidney ~ the size of a fist.
• They are protected from the front and back in the
following manners:
– BACK: they lie against the back muscles in depressions
(‘caves’) within the muscles;
– FRONT: they are protected by the lower ribcage.
• Kidneys are suspended within their back muscle depressions
by connective tissue known as renal fascia, which can
easily be broken by a blow to the back resulting in a
‘floating’ kidney.
• Kidneys serve to produce urine, whereas other urinary
structures serve to transport, store, and eliminate (excrete)
urine.
• Blood carrying dissolved wastes enters each kidney on its
concave side (called the hilium) via the renal artery; even
though the kidneys comprise only 1% of the mass of a
human body, they receive about 20% of the blood pumped
with each heartbeat.
• The blood is essentially filtered by the kidneys as it flows
through a series of arterioles, capillaries, and venules.
• Blood eventually exits the kidneys via the renal vein.
• Urine, the product of the filtering by the kidneys (ie. the
waste fluid), exits the organs through paired ducts known as
ureters.
• Ureters are narrow, muscular tubes that carry urine to its
storage area, the urinary bladder, by peristalsis.
• The urinary bladder is a muscular organ that expands as
urine enters it; it can store up to 600 mL of urine due to its
distensible (‘stretchy’) walls.
• At ~250 mL, one gets the urge to urinate; at ~500 mL, it
becomes uncomfortable; eventually, the urge can be lost if
the bladder is stretched too much.
• The urinary bladder also houses special stretch receptors
that send sensory impulses to the spinal cord (reflex), which
sends motor impulses back to the bladder signaling it to
contract.
• So why don’t we ‘pee our pants’ when this occurs?
• The brain plays a role as well – there is a voluntary
‘holding’ of your urine that may occur (not in babies
as they have not developed the ability to do so).
• Urine that leaves the bladder enters the urethra,
which is the final tube it passes through before
leaving the body; the passage of urine from the
bladder to the urethra is mitigated by two musclecontrolled sphincters.
• The first sphincter is under parasympathetic,
involuntary control (spinal cord/reflex control) and
is relaxed when the bladder contracts (ie. we have
no choice in the matter).
• The second sphincter is under the conscious,
voluntary, somatic control of the brain; this is how
one can ‘hold their pee’.
• See fig. 16.2 p. 305
• The urethra extends from the bladder to the exterior
opening known as the external urethral orifice.
• The urethra is ~4 cm long in females, and ~20 cm
long in males (this is why females are more
susceptible to urinary tract infections (bacterial)).
Read ‘Health Focus’ on p. 306.
• The prostate gland encircles the urethra in men and
can cause a restriction of urination if it becomes
enlarged.
FEMALE URETHRA
MALE URETHRA
Effects of Aging upon Urinary System
• Reduced blood supply to kidneys – less filtering of
blood.
• On top of that, the kidney become less efficient with
respect to the filtering process.
• Poorer salt-water/pH balance – dehydration more
evident in the elderly.
• Bladder control adversely affected.
• In men, the prostate may enlarge and apply pressure
to the urethra making urination difficult (surgery
required).
Kidneys
• When a kidney is sliced lengthwise, it is evident that the
renal artery, the renal vein, and the ureter all enter/exit the
kidney at the hilium (fig. 16.3a p.307).
• The longitudinal section also provides one with a glimpse
into the three major regions (layers) of a kidney: the renal
cortex, renal medulla, and renal pelvis (fig. 16.3b p. 307).
• The renal cortex is the outer, overlying, protective layer that
houses an ECF with a relatively low osmotic pressure.
• The renal medulla is the inner, ‘fan-shaped’ layer that
houses an ECF with a relatively high osmotic pressure; this
region consists of multiple cone-shaped tissue masses called
renal pyramids, which serve to collect and drain urine into
the third region, the renal pelvis – the innermost cavity that
collects urine and allows it to drain into a ureter
• On a ‘micro’ scale, a kidney is composed of ~ one
million nephrons, which are urine-producing
‘factories’ in and of themselves.
• Each nephron serves to produce a tiny bit of urine,
which eventually drains into the renal pelvis.
• Each nephron possesses structures that exist in the
renal cortex or the renal medulla. In fact, the
collecting ducts of nephrons (the ‘final’ tubules of
nephrons) are responsible for the ‘fan-shaped’
appearance of the renal pyramids in the medullar
region of the kidneys.
Nephron Structure (fig. 16.4 p. 308)
TWO MAJOR PARTS: Blood Vessels and Urinary Tract; keep
in mind, as well, the existence of ECF between these two
major parts).
Blood Vessels
- The renal arteries are the vessels carrying blood to the
kidneys (to be filtered and to provide O2 and nutrients to the
kidney cells).
- A renal artery splits into ~ one million afferent arterioles,
each of which carry blood to a nephron.
- An afferent arteriole carries blood to the first nephronal
capillary bed known as the glomerulus, a ‘knot-like’ ball of
capillary vessels located in the Bowman’s (Glomerular)
Capsule of the nephron.
• Some filtering occurs here, then blood leaves the
glomerulus and enters an efferent arteriole, which
carries blood to the second nephronal capillary bed
called the peritubular capillary network (the
efferent arteriole is another example of a portal
system as it connects two capillary beds).
• This network surrounds the majority of the nephron
like a spider’s web – more filtering occurs here.
• Blood then flows into a renal venule, and eventually
the renal vein, where its composition is quite
different from what it was in the renal artery. In
fact, blood in the renal vein is hypotonic compared
to blood in the renal artery (due to the filtering that
occurs between the glomerulus/peritubular network
and the ECF/Urinary tract).
The Urinary Tract
- The nephron is really a tube (with a lumen) that acts to
receive and process material from the blood.
- It consists of several parts: Bowman’s (Glomerular)
Capsule, Proximal Convoluted Tubule, Loop of Henle
(Loop of the Nephron), Distal Convoluted Tubule, and
Collecting Duct.
- Bowman’s Capsule:
- a ‘goblet-like’ structure found at the blind end of the
nephron.
- Outer layer comprised of protective epithelial cells; inner
layer comprised of special cells called podocytes, which
integrate with the glomerulus to form pores (‘slits’) that
allow for simple size-based filtration to occur between the
glomerulus and Bowman’s Capsule.
Proximal Convoluted Tubule (PCT):
- Comprised of special epithelial cells that possess
microvilli, which form a ‘brush border’ that serves to
increase the SA for the tubular reabsorption that
occurs here (microvilli collect materials from lumen
of tubule and allow entrance to epithelial cells,
which perform tubular reabsorption). Fig. 16.5 p. 309
- This reabsorption process (described later) is an
active one requiring ATP and carrier proteins. Thus,
a plethora of mitochondria exist within each
epithelial cell of the proximal convoluted tubule.
*both the Bowman’s Capsule and the PCT exist in the
renal cortex of the kidney.
Loop of Henle:
- Begins in the renal cortex, dips deep into the renal
medulla, and returns (due to a hairpin turn at its
base) to the cortex.
- Thus, the loop is discussed in two parts, its
descending arm and its ascending arm as they both
have distinctly different structures/functions.
- Both ‘arms’ possess a thin-walled and a thick-walled
portion whose effects will be discussed during urine
formation (fig. 16.7 p. 312).
Distal Convoluted Tubule (DCT):
- Lined by epithelial cells (this time, no microvilli)
that possess many mitochondria for active transport
during a process known as tubular secretion.
- Here, substances tend to actively move from the
blood into the DCT for excretion.
- More tubular reabsorption occurs here as well.
- Exists solely within the renal cortex.
- Unlike most diagrams of the nephron, the DCT
actually neighbours the afferent arteriole (ie. they
touch) at a region known as the juxtaglomerular
apparatus (fig. 16.8 p. 313).
Such a ‘twisting back’ of the nephron allows each nephron to
be skinnier so that a million can exist within a kidney.
Collecting Duct:
- The nephron’s last urinary structure is the collecting duct,
which dives back into the renal medulla region of the
kidney.
- Comprised of epithelial cells that are permeable to water but
not to salts.
- Several DCTs drain into one collecting duct (~ 200-300
collecting ducts per kidney – why you see ‘branches’
coming off of the coll. duct in pictures).
- Each collecting duct drains ‘collected’ urine into the renal
pelvis for eventual expulsion from the body.
- The collecting duct does not have the capillary network as
close to it as the other nephron structures do. The material
that it releases enter the ECF and contribute to the higher
than normal osmotic pressure that exists in the renal
medulla.