Chapter 26
Urinary System
Urinary System Functions
• Filtering of blood
• Regulation of
–
–
–
–
blood volume
concentration of blood solutes
pH of extracellular fluid
blood cell synthesis
• Synthesis of Vitamin D
Increased PTH promotes Vitamin D formation in the kidneys
1.
Overview
The urinary system is the system responsible for
removing organic wastes from the body in a liquid form.
By doing so in an appropriate manner, the urinary
system contributes to the homeostasis of the body.
They regulate:
Blood volume, blood pressure, plasma electrolytes, pH,
blood glucose and, blood amino acids, etc.
It is also important to note how the urinary system is
isolated from the circulatory system to avoid
contamination.
The manner how the urinary system works is unique. It
first filters out water, electrolytes and small organic
molecules into the capsular space, then one by one
reabsorb those which are needed back from the tubules
and ducts.
If the urinary system fails to properly filter and reabsorb
these vital ions and molecules, they may be wasted into
the urine.
Thus, by observing what you find in the urine you may
be able to judge the health of a subject.
An adult produces about 1.4 L of urine per day. Note
each adult consume about 2L of liquid each day. Where
did the rest go?
Urine Formation
Urinary System Anatomy
Internal Anatomy of Kidneys
• Cortex: Outer area
– Renal columns
• Medulla: Inner area
– Renal pyramids
• Calyces
– Major: Converge to form
pelvis
– Minor: Papillae extend
• Nephron: Functional unit
of kidney
– Juxtamedullary
– Cortical
The Nephron
i.
Overall Functions of the nephron
The basic functional units of the kidney are shown in Fig. 26-6.
The formation of urine starts with the filtrates from arterial blood,
which will be released into the renal corpuscle.
In the renal corpuscle, blood pressure in the renal arterioles
pushes water, electrolytes and small organic molecules into the
capsular space ( Bowman’s space) across the glomerular walls.
Small important substance, which have passed through the
pores of the membrane must be reclaimed, while keeping the
wastes in the collecting duct..
i.
The blood supply to the kidneys
1200 ml/min through the kidneys
Consumes 20 - 25% of the cardiac output!
Study Fig. 26-7b,2 and Fig. 26.6 of Seeley, and follow the
direction of blood vessels.
Note the afferent and efferent arterioles.
Where are the peritubular capillaries
What are vasa recta?
a. The basic principles of urine production
The reason for urine production is to maintain the
homeostasis of the blood.
The urine collects metabolic waste products.
i.
There are three major waste products in
urine: Table 26-2
(a)
Urea form the breakdown of amino acids. 21
g/day.
(b)
Creatinine from the breakdown of creatine
phosphate, recall muscle contraction. 1.8 g/day.
(c )
Uric acid from RNA. 0.48 g/day.
These wastes filter through glomerular membranes with a
large quantity of water and ions. In other words, the
concentrations of these ions and molecules in the in the
filtrate and plasma are the same.
i.
The majority of water must be
reclaimed to avoid dehydration.
Water is first filtered through the pores of glomerular
membrane by the blood pressure into the Bowman’s
space.
Resorption for reclaiming water, electrolytes, small
organic molecules either by simple diffusion or with
the carrier proteins at the proximal tubule starts
immediately. Fig. 26.8 of Seeley
Resorption of water continues at the descending limb
of the loop of Henle. Fig. 26-13b
Urine Formation
Fig. 26-13b
Water uptake
at descending
loop of Henle
Fig. 26-8 The renal corpuscle
Fig. 26-10
Glomerular
filtration
Filtration Pressure
i.
Filtration at the glomerulus
(a) The glomerular filtration pressure (Fig. 26-8, 10, Fig.
26.9)
The driving force at the glomerular is the difference between
the blood pressure and osmotic pressure.
The net filtration pressure is about 7 mmHg and is very small.
Thus, slight change in blood pressure could change the
magnitude of this driving force and can impair the function
of the kidneys.
Note that the muscles of arterioles can easily change the
diameters, thus the regional blood pressure.
(a) The glomerular filtration rate (GFR)
The filtration surface area of each kidney is 6 sq m.
GFR is 125 ml/min for each kidney or 250 ml/min for the both
kidneys.
Since a 1,200 ml/min of blood is supplied to the kidneys, and
about a half of it are fluid, more than 40% of fluid in the blood
are filtered.
360 liters (100 gallons) are filtered through per day.
But 99% of the filtrates are reabsorbed.
The GFR may be regulated with hormone.
(a) The proximal (convoluted) tubule (PCT)
Transports across the nephron cells in this region starts with two
basic driving forces: (Fig. 26-12)
1.
Osmosis: Having higher concentration of water in the
filtrate than in the interstitial fluid.
2.
Active Na+/K= exchange pump at the site of basal
membrane to force Na+ out of the cell to the
interstitial fluid.
Thus, water will go back to the nephron blood vessels, peritublar
capillary, by osmosis.
Na+ gets out from the nephron cells to the interstitial fluid by the
Na+/K+ exchange pump.
Fig. 26-12 Transport at the PCT
Most of the ions, glucose and amino acids from the filtrate
will enter the nephron cells in the form of Na+ cotransport,
since the intracellular Na+ concentration is constantly
being lowered by Na+/K+ exchange pump.
At the site of the basal membrane, they will diffuse out,
sometimes facilitated, according to the concentration
gradients.
Reclaims 60 - 70% of water and most of the glucose
(diabetes mellitus?), amino acids and other organic
substances.
Urea, uric acid and creatinine are not absorbed, thus
increasing their concentrations in the tubule.
(a) The loop of Henle (Fig. 26-13)
The descending portion of the loop of Henle reabsorbs additional
20% of water by osmoais and small amounts of ions may be
returned to the filtrate.
On the contrary, the ascending portion of the loop of Henle, which
is impermeable to water, reabsorbs 25% of sodium and chloride
ions. by the Na+/K+ exchange pump locate in the basal
membrane - similar to the proximal tubule.
In fact, the sodium and chloride ions absorbed back into the
interstitial fluid contribute to extract more water from the
descending portion of the loop.
NOTE: Pumping of ions across a cell
Fig. 26-13 The loop of Henle
Fig. 26-13b,c
We have already seen that cell membrane may have a sodium pump which is
energized with ATP. If the cell membrane is uniform over the entire cell, sodium
may simply pumped out from the cell, but the sodium ion cannot run across the
cell.
To transport Na+ across a cell, requires a cell with asymmetirc membrane.
Proximal tubules and ascending loop of Henle are surrounded with such cells.
In the membrane towards the interstitial space, sodium ion is actively
transported out from the cell into the interstitial space.
While on the tubular side of the membrane, in which no active sodium pump is
found, sodium ion, along with the others, passively enters into the cell.
The over all movement of sodium ion is to actively transport from the tubular to
interstitial fluid across the cell.
The results of active pumping of the salts in the ascending loop of Henle
are in two fold:
(1) The salt concentrations in the upper end of the tubule go down.
(2) The salt concentrations in the upper region of medulla will go up due
to the released salts.
The consequence of increased salt concentrations in medullar will result
in more effective osmotic release of water from the tubule.
By the time the urine leaves the loop of Henle, it has much lower
concentrations of Na+ and Cl-.
By this time 80% of water and 85% of the solutes have been reabsorbed.
The waste products are not reabsorbed.
(a) The distal convoluted tubule (DCT)
and the collecting system
The DCT and collecting duct are impermeable to
solutes.
Requires active reabsorption or secretion for transport.
Active reabsorption of Na+ is exchanged for K+ or H+
in response to aldosterone. Fig. 26-14, 15
Fig. 26-14 The DCT
Fig. 26-14c
The DCT
The water reabsorption is controlled by antiduretic
hormone (ADH). (Fig. 26.17of Seeley) ADH activate
ADH receptor and in turn activates G-proteins for cAMP
production, which opens up the water channel in the
membrane, thus removes water out of the urine.
At the end of the distal collecting duct, the solution
osmolarity could increase from 100 mOSM at the
entrance to 1200 mOSM
Effect of ADH on Nephron
a. The control of kidney function
By adjusting the diameters of the afferent and efferent
arterioles.
Activities of the sympathetic division of the ANS.
Via hormonal control.
i.
The local regulation of kidney function
By automatic changes in the diameters of the arterioles and the
glomerular capillaries - change in the blood pressure.
ii.
Sympathetic activation and kidney function
By adjusting the flow of blood to the kidneys.
Sympathetic activation results in constricting the afferent arterioles reduced blood flow to the glomerular capillaries.
Sympathetic activation of the vasomotor center changes the
regional pattern of blood circulation - reduced GFR.
ADH:
The function of ADH has been discussed earlier. (Fig.
26.17 of Seeley) Insufficient release of ADH may
result in diabetes insipidus, large quantity of clear urine
accompanied with dehydration and abnormal
electrolyte balance.
In contrast, diabetes mellitus may result in large
quantity of urine with high concentration of glucose.
Renin -Angiotensin II- Aldosterone system for Na+,
Cl- and K+ balance. (Fig. 26.18 of Seeley)
Decrease in the concentration of Na+ in the
interstitial fluids increase the rate of aldosterone
secretion initiating the release of Na+ from the
distal tubule.
Aldosterone Effect on Distal Tubule
Fig. 26-16 Summary
Fig. 26-16b summary
Table 26-6 Urine composition
(Review)
Fig. 26-19c Urinary Bladder
Micturition Reflex
1.
Urine transport, storage and elimination
c. The micturition reflex and urination
The process of urination is coordinated by the
micturition reflex. Fig. 26.20
Note the stretch receptor - sensory fiber parasympathetic motor neurons etc.
200 ml of urine in the bladder sends the urge starting
from the stretch receptors.
Contraction and relaxation of the internal and external
sphincters decide the release of urine.
CHAPT 27: Water, Electrolytes and acid base
balance
Extensive discussion on fluid, electrolytes
and acid-base balance are presented in this
chapter. We will focus only on acid base
balance based on carbonic acid.
1.
Acid and base
Most commonl definition of acid is its ability to produce H+.
There are strong and weak acids.
Strong acids dissociate completely to ions and produce H+
and the counter ions. Examples are
HCl -- H+ + ClH2SO4 –- 2H+ + SO4=
On the other hand, weak acids dissociate partially and
establish equilibrium. Examples are acetic acid and
carbonic acid.
2.
Buffers and acid base balance
Buffers are substances which resist the change of pH a
solution and a weak acid is a good candidate.
In human body, proteins and carbonic acid take the major role
as buffers.
The buffering action of proteins depend ionization of groups,
which may be ionized at physiological range of pH, 7.2 - 7.4.
Such compounds are the alpha amino group and histidine of
proteins.
R-NH3+ = R-NH2 + H+
Carbonic acid may be formed when carbon dioxide is released
in solution slowly or quickly with the aid of an enzyme, carbonic
anhydrase and reacts with water. It further dissociates to ionic
forms, proton and bicarbonate.
CO2 + H20 = H2CO3 = H+ + HCO3As we have seen the production of bicarbonate during
respiration, release of protons into the tubular lumen also
involves the above reaction.
Knowing that the membrane at the side of tubular lumen is
impermeable to bicarbonate , but permeable to carbon dioxide,
the transport of proton is possible to tubular lumen.
The acid forming proton is now neutralized by reaction with
bicarbonate.