Download urine formation

STUDENT NOTES - URINE FORMATION:

Urine formation occurs in the nephron: Molecules are exchanged between blood
vessels (i.e. the glomerulus and peritubular capillary network that surrounds the
nephron) and nephrons.
Urine formation consists of 3 STEPS:
1. PRESSURE FILTRATION:
Pressure filtration takes place inside Bowman's capsule as molecules are forced
through the glomerulus and into the proximal convoluted tubule.
2. SELECTIVE REABSORPTION:
Occurs in the proximal convoluted tubule (Na+,Cl-, H2O)
3. TUBULAR EXCRETION:
Occurs at the distal convoluted tubule
NEPHRON OVERHEAD: How the Nephon works!
K
Glucose
+
-
HCO3
+
Na
NaCl
H2O
I
n
c
r
e
a
s
I
n
g
S
a
l
t
I
n
e
s
s
Cortex
+
H
K
NH3
H2O
+
+
H
NaCl
H2O
H2O
Outer
Medulla
NaCl
H2O
Urea
Active Transport
Inner
Medulla
Passive Transport
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
1
1. PRESSURE FILTRATION:
High blood pressure in the GLOMERULUS (~60mm Hg) forces SMALL
MOLECULES [such as nitrogenous wastes, *H2O, *nutrients, *ions (salts)] into
BOWMAN'S CAPSULE. The AFFERENT ARTERIOLE supplies the glomerulus
with blood.

*note: we don't want to lose these substances constantly- we would quickly die of
dehydration and starvation. Therefore, these substances must be absorbed back into
the blood.
Large, non-filterable molecules are unable to pass (i.e. blood cells, platelets,
proteins). These remain in the blood and leave the glomerulus via EFFERENT
ARTERIOLE. (Efferent – “E” for “E”xit.)

the small, filterable molecules that are forced into Bowman's capsule form
FILTRATE.

high blood pressure is necessary for filtration. This is accomplished through the
functioning of the regulatory juxtaglomerular apparatus (a special region of afferent
arteriole) and will, if necessary, release RENIN to increase blood pressure. People
with kidney disease often have high blood pressure because their juxtaglomerular
apparatus is constantly releasing renin.
2. SELECTIVE REABSORPTION

if the kidneys only did pressure filtration, we would quickly die from water and
nutrient loss. Once the original filtrate is made, the next task of the kidneys is to
reabsorb molecules in the filtrate that the body cannot afford to lose. (e.g. water,
nutrients, some salts).
i) Reabsorption and Secretion at the Proximal Convoluted Tubule

The process of “SELECTIVE reabsorption” is called “selective” because, at the
proximal convoluted tubule, it is the CARRIER MOLECULES that determine what is
reabsorbed and what passes through the tubule; This is done by ACTIVE transport
(except for H2O, which is reabsorbed by OSMOSIS). (what would happen if there is
more glucose in the filtrate than there are carrier molecules to accommodate?)
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
2

the molecules that are reabsorbed move from the proximal convoluted tubule to the
peritubular capillary network (i.e. back into the blood). This is very efficient. Every
minute about 1200 mL of blood enters the kidneys and 1199 mL of that blood leaves.
About 250 ml become glomerular filtrate - most of which is quickly reabsorbed into
the blood. Only about 1 mL becomes urine.

Most of the glomerular filtrate gets reabsorbed!!

WHAT GETS REABSORBED?: most H2O, nutrients (glucose, amino acids, vitamin
C, potassium ions…) some salts (NaCl)

A balanced salt concentration in the blood must be maintained. The process of
selective reabsorption ensures this by actively reabsorbing sodium ions while chloride
ions follow passively. The reabsorption of salt by the blood does what to the blood in
relation to the filtrate? and causes water from the filtrate to enter the blood! (again, a
water-retaining mechanism).

WHAT DOES NOT GET REABSORBED and therefore SECRETED?: some H2O,
wastes, excess salts.

non-reabsorbed material continues through the Loop of Henle. (What would happen if
100% of the water were reabsorbed?)
ii) Reabsorption and Secretion at the Loop of Henle and the Distal Convoluted
Tubule

Tubular fluid now enters the LOOP OF HENLE and, eventually, the distal convoluted
tubule (indicate on overhead where these are…)

primary role of Loop of Henle and distal convoluted tubule is REABSORPTION OF
WATER. Over 99% of the water in original filtrate is reabsorbed by the nephron
during urine formation. Much of this reabsorption is done by OSMOSIS at the Loop
of Henle.

This CONCENTRATES THE URINE, making it HYPERTONIC to plasma (blood).
However, the ascending loop of Henle is IMPERMEABLE to water, therefore, the
hypertonic filtrate does not absorb water from the capillary network! (see textbook).
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
3

Also, the loop of Henle secretes NaCl into the surrounding tissue (the renal medulla)
to ensure that the neighbouring tissue is hypertonic to the filtrate. By doing this, the
loop of Henle creates an osmotic gradient.

Na+ ions are actively reabsorbed as their uptake is associated with water retention
(remember, the body does not want to lose or waste water!)

this also further CONCENTRATES THE URINE, making it HYPERTONIC to
plasma (blood)
3. TUBULAR EXCRETION (=TUBULAR SECRETION)
Although urine formation occurs primarily by the selective reabsorption, a supporting
mechanism, called tubular secretion, is also involved.
Although urine formation occurs primarily by selective reabsorption, a supporting
mechanism, called tubular secretion, is also involved.

This is an ACTIVE PROCESS by which other non-filterable wastes (i.e. those wastes
that cannot be added to the filtrate at Bowman’s capsule) can be added to the tubular
fluid so that these wastes will also be excreted in the urine.

Tubular excretion occurs along the distal convoluted tubule: Actively secreted
substances include some chemicals (e.g. penicillin, histamine) H+ ions, NH3.

fluid now enters the COLLECTING DUCT (mark this on your diagram):

in the cortex (label on the diagram the region of the cortex), fluid in duct is
ISOTONIC to the surrounding cells (therefore, there is no net movement of water)

in the medulla (indicate on the diagram the area of the medulla), fluid is
HYPOTONIC to cells of medulla. Remember, the loop of Henle helps produce an
osmotic gradient to ensure that the filtrate is hypotonic relative to the surrounding
cells.
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
4
THE FILTRATE IS HYPOTONIC AT THIS POINT BECAUSE…
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Also, both H+ and K+ ions are secreted INTO the filtrate exchanging it with Na+.

Tubular excretion is important in maintaining the pH of blood, as we will see in more
detail next class.

The tubular fluid, which we can now “OFFICIALLY” call URINE passes from the
collecting duct into the pelvis of kidney, and enters the ureter for transport to the
bladder. [The filtrate becomes, technically, “urine” when it is in the distal convoluted
tubule].

A comparison of the compositions of plasma, filtrate, and urine (each in g/100 ml
of fluid). (The values for salts are especially variable, depending on salt and water
intake). This shows how efficient a healthy kidney is in reclaiming what is useful
for the body.
Component
Urea
Plasma
0.03
Filtrate
0.03
Urine
1.8
Concentration
60X
% Reclaimed
50%
Uric acid
0.004
0.004
0.05
12X
91%
Glucose
0.10
0.10
None
-
100%
Amino acids
0.05
0.05
None
-
100%
Total inorganic salts
0.9
0.9
<0.9–
3.6
<1–4X
99.5%
Proteins and other
macromolecules
8.0
None
None
-
-
WRAP-UP QUESTIONS:
1. Tubular excretion occurs at WHICH part of the nephron?
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__________________________________________________
__________________________________________________
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
5
2. What is the purpose of tubular excretion?
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__________________________________________________
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3. What is absolutely required for pressure filtration to take place?
__________________________________________________
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4. Do glucose and water enter the filtrate?
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5. What is the name of the region that secretes renin?
__________________________________________________
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6. What does renin do?
__________________________________________________
__________________________________________________
__________________________________________________
7. Where is the juxtaglomerular apparatus located in the nephron?
__________________________________________________
__________________________________________________
__________________________________________________
8. Where does most of the water and glucose from the blood get reabsorbed?
__________________________________________________
__________________________________________________
__________________________________________________
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
6
Biology 12 - Human Biology - Urinary System
Paul Plut, Jessica Wan
7