Ch 45 Body Fluid Regulation and Excretion, Part II
Davison BI 102 Lecture Notes 2/20 & 2/25 ‘03
Urinary Systems in Humans
Gross
Anatomy of the Human Urinary System (See fig. 45.5)
Urethra carries urine from the bladder out of
the body
Urinary bladder stores urine
A ureter from each kidney fills the bladder
with urine.
The expanded end of the ureter at the kidney is
called the renal pelvis
Gross Anatomy of the Kidney (see fig. 45.6)
Renal pelvis,
renal medulla and renal cortex.
Microanatomy
of the kidney (see figs. 45.7, 45.8, 45.9)
Nephrons, collecting ducts, and associated capillary
networks (glomerulus and peritubular capillaries)
Nephron structure
Glomerular capsule, proximal tubule, loop w/descending
and ascending limb, distal tubule.
A single layer of cuboidal epithelial cells comprises the walls of
the nephron. Microvilli are
present on these cells enhancing their transport role by providing more
membrane surface to conduct transport (see fig. 45.8).
The nephron is composed of transport epitheilium.
Collecting
duct – larger tube that drains several nephrons output into the
renal pelvis.
Capillary
networks
Glomerulus - a
capillary network surrounding by the glomerular capsule of the nephron.
Peritubular capillaries – capillary network surrounding
the nephron
Physiological Processes
within the Kidney
The Nephron and capillary networks work together in
performing three basic processes aimed at two basic, overall kidney
functions, molecular waste elimination and maintaining water balance.
- Filtration
– occurs in the glomerular capsule.
Much of the blood plasma (water, urea, sugar, etc. –all
molecules whether waste or needed nutrients except for the larger
blood proteins such as clotting factors and albumin) is forced by
blood pressure out of the glomerulus and into the nephron.
On a per day basis, about 180 liters (50 gal.) of filtrate is
produced. Thus, the total
blood plasma volume is filtered through nephrons a total of 70 times
per day!
- Tubular
Reabsorption – water and nutrients are reabsorbed (by the
nephron tubules) and reenter the peritubular capillaries.
Needed nutrients that were filtered out are returned to the
blood stream by the proximal tubule.
Plasma proteins actively transport these molecules out of the
lumen of the nephron and into the lumen of the peritubular
capillaries. As these
solutes are taken out of the nephron and placed into the capillaries,
water follows passively via osmosis; i.e., water also leaves the
nephron and enters the capillaries.
Most (60-70%) of the filtrate is reabsorbed by the proximal
tubule and returned to the capillaries that surround the proximal
tubule.
- Tubular
Secretion – waste molecules not filtered out of the blood by the
glomerulus may be actively secreted into the nephron along the distal
tubule (e.g. ammonia, uric acid, drugs).
Urine is formed by the actions of filtration,
tubular secretion, and reabsorption.
The final form of urine takes shape after the fluid from the
nephrons has drained through the collecting duct into the renal pelvis.
Urine formation and Conservation of Water Require Additional
Reabsorption
Normal urine is a sterile fluid containing water,
urea, salt (NaCl), ammonia, and other metabolic wastes.
Some peculiar contents found in urine depend on the chemical nature
of what has been ingested (red beets & asparagus stories).
The amount of water in urine determines the urine’s
concentration. A concentrated
urine of small volume containing little water is formed when the body is
conserving water. A dilute
urine of larger volume is formed when the body is removing excess water
(as when happens after drinking lots of water).
The final concentration of urine (and thus the amount of urine)
forms as the tubular fluid flows through the collecting duct into the
renal pelvis.
A key to understanding additional water reabsorption
is to know that the inner medulla of the kidney has a very high
solute concentration (it is very hypertonic compared to normal body
fluids). Each time the loop
of the nephron and the collecting ducts pass down through the inner
medulla water is squeezed out of their tubules and reenters the
bloodstream. Consider this
analogy: its like ringing out
a dishrag full of water in which you twist the rag once squeezing out
water and then relaxing the rag so as to grip it again in preparation for
another, harder twist removing even more water.
The first twist of the wet rag represents the descending limb of
the loop of the nephron, the period or re-gripping represents the
ascending limb of the loop of the nephron (which is impermeable to water
movement), and the second twist represents the passage through the
collecting duct. In order to
get another grip to squeeze out more water, NaCl is reabsorbed in the
ascending loop of the nephron (see below for explanation).
Obligatory Reabsorption of Water
The descending limb of the loop of the nephron
will always reabsorb water as this portion of the nephron is always
permeable to water. The high
solute concentration of the inner medulla into which the descending loop
penetrates will always draw water out of the renal tubule.
(The dishrag will always get one good squeeze.)
In fact, about 15-25% of the original filtrate volume is reabsorbed
in the descending limb of the loop of the nephron. At the tip of the loop of the nephron, the fluid within the
nephron is highly concentrated, yet it still contains salts that need to
be reabsorbed and much of the water it contains must also be reabsorbed.
So…
Removal of NaCl from the Ascending Limb of the Nephron Loop
Before any additional water can be reabsorbed from
the fluid inside the nephron , NaCl must be reabsorbed.
The reabsorption of NaCl occurs along the ascending loop of
the nephron. The
removal of NaCl from the nephron has the effect of making the nephron
fluid dilute without adding water.
A dilute fluid can be passed once again through the inner medulla
to force the reabsorption of more water.
This second passage occurs as fluid from the nephron enters the
collecting duct and the collecting duct penetrates through the renal
medulla to the renal pelvis.
Variable Reabsorption of Water by the Collecting Duct
Having passed through the loop of the nephron, only
15% of the original filtrate volume remains in the neprhon tubule to drain
to the collecting duct. This
may not sound like much fluid, but on a daily basis this 15% is about 27
liters! (The total volume of blood is only 5-6 liters).
Obviously we don’t urinate 27 liters per day. Collecting ducts reabsorb much of this volume except for a
good liter or so that is eliminated as urine in the case of a healthy
adult.
As the amount of water in body fluids is maintained
within narrow limits, a variable amount of water reabsorption occurs in
the collecting ducts depending on the body’s need to conserve water or
eliminate excess water (produce a concentrated urine or produce a dilute
urine). Whether one produces
concentrated or dilute urine depends upon several factors:
1) dietary intake of water, 2) water loss through perspiration or
diarrhea, and 3) a hormonal signal from the brain that regulates variable
water reabsorption by the collecting ducts.
Based on the water level detected in body fluids passing through
the brain, variable amounts of antidiuretic hormone (ADH) are released
from the pituitary gland.
Since “diuresis” basically means “more urine,” antidiuretic
hormone will promote less urine formation.
The volume of urine can be less while the amount of metabolic waste
removed will be unaltered.
Antidiuretic hormone released from the brain
(pituitary) travels to the kidneys and enters the filtrate. ADH has does not have any effect until it reaches the walls
of the collecting duct. There
ADH stimulates the opening of water channels which, when opened, speed the
rate of water movement via osmosis out of the collecting duct.
Simply stated, ADH promotes water reabsorption by the collecting
duct. Should the body not
need as much water reabsorbed, the brain will not be sending as much ADH
into the bloodstream. A
dilute urine would result. Should
water need conserving, more ADH is released and a concentrated urine would
result.
A dilute urine may form under the influence of alcohol
which suppresses the release of antidiuretic hormone.
Even though your brain senses that water content in body fluids
should be conserved, the hormonal signal ADH isn’t released in times of
drunkenness. The result is
copious urination that removes precious water from the body.
A hangover is, at least in part, due to the dehydration that
results. |