Topic 11 Human Health
11.3 The Kidney
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2009-2010
11.3.1 Define excretion.
Excretion is the removal from the body of
the waste products of metabolic
pathways
Nitrogen wastes from digestion of
proteins
 Nitrogen wastes are toxic
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 Ammonia (fish – H2O can dilute)
 Urea (Humans, most other mammals)
 Uric Acid (insoluble – birds, reptiles)
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11.3.2 Diagram of the kidney.
Two kidneys commonly referred to as left and right kidney.
 Each has an arterial blood supply
 (the left and right renal arteries)
 (branches of the aorta)
 Each has a vein
 (left and right Renal Vein)
 returns filtered blood to the Vena Cava
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The urine produced is transported by each ureter to the
bladder.
Kidney Structure
 The photograph is of a large pig kidney. There are three distinct
regions based on the distribution of the different sections of the
nephron. The human kidney contains approx 106 nephrons.
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Three Regions of Kidney
 Cortex: Lighter brown colour
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contains the Malpighian bodies
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the capsules that contains Bowman's
capsule and a glomerulus at the
expanded end of a nephron.
 also the proximal and distal
convoluted tubules and the upper
sections of collecting ducts.
 Medulla: darker, redder region
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composed of loops of henle
lower sections of the collecting ducts.
seems to form triangular regions
which are called the pyramids.
 Pelvis:
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a cavity which collects the urine from
the open ends of the collecting ducts.
The nephrons open on the margin of
the pyramids and pelvis.
The white tissue forms a funnel called
the ureter which conducts the urine to
the bladder.
11.2.3 Glomerulus structure.
a) Afferent arteriole a branch of the renal artery.
(b)The Malpighian Body.
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The Bowman's capsule (renal capsule)
The glomerulus is a multiple branching of the afferent arteriole
before rejoining to the efferent arteriole.
Together the Bowman's capsule and the glomerulus are known as
the Malpighian body.
(c) Efferent Arteriole (narrower than afferent)
join together to form the renal vein.
(d) Proximal Convoluted Tubule
 (14mm long / diameter 60um)
 longest section of the nephron.
(e) Loop of Henle.
(f) Distal Convoluted Tubule.
(g) Collecting Duct
 opens into the Pelvic region.
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11.2.4 Ultrafiltration.
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Ultrafiltration, Selective Reabsorption and Urine formation
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The labels of the processes on the left side correlate with the a region of the nephron on
the right. Note however that selective reabsorption of substances into the blood takes place
along the entire length of the nephron.
Ultrafiltration: formation of kidney filtrate
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This structure is called the malpighian
body ( structure( b) in the diagram
above) and is the location of
Ultrafiltration.
The glomerulus increases blood
pressure by forming narrow branches
(also an increase in surface area for
filtration).
The pressure is maintained by the
narrower efferent arteriole which
restricts the outflow of blood from the
glomerulus.
The expanded end of the nephron
forms an invaginations to form a cup
that accommodates the glomerulus
The efferent blood vessel associated
itself with the other regions of the
nephrons for selective reabsorption.
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Glomerulus
 High Pressure is generated in the glomerular knot.
 Fenestrations (gaps) between the cells that form the
glomerula blood vessel create a path of low resistance
out of the glomerulus.
 The basement membrane is the filtration barrier. Cells
and large plasma protein macromolecules cannot pass
through this structure.
 Podocytes for the inner membrane of the Bowman's
capsule. The interdigitation of the podocyte extension
creates gaps for the filtrate to pass between the cells
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Alternative diagram of the podocyte/
arteriole structure:
 The podocytes
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have many fine arm-like projections
which wrap around the arterioles.
Although the fenestration's of the
arteriole allow large molecules to
leave the blood vessel
large molecules are largely prevented
from further movement by the small
spaces between the podocyte
extensions.
fine mesh work of the basement
membrane (lamina) that will prevent
any large molecules such as proteins
from leaving the blood.
11.2.5 Define Osmoregulation.
 Osmoregulation is the control of the water balance of the blood,
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tissue or cytoplasm of a living cell.
The water content of body fluids has to be controlled such that the
movement of water to and from cells can changes be controlled.
The body experiences external and internal changes such as drinking
water availability, sweating and the accumulation of salts that require
adjustments in the water content of blood, tissues and cytoplasm.
Osmoregulation is under the control of receptors in the
hypothalamus.
In responses to changes the hypothalamus controls the sensation of
thirst and also the endocrine secretion of anti-diuretic
hormone.(ADH).
ADH is secreted from the pituitary and causes the opening of cell
membrane pores called aquaporins which allows water reabsorption
into the blood.
This control mechanism is covered in more detail in Option H
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Selective Reabsorption
 The process of control and regulation in the kidney
begins with a non discriminating filtration (ultrafiltration)
that removes just as many useful substances as harmful
ones from the blood to make filtrate.
 The kidney then takes back from the filtrate to the blood
those substances that it still requires in the
blood.(Selective Reabsorption)
 The beauty of the way the kidney works is that it is able
to control how much of a substance it reabsorbs back
into the blood (Regulation)
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11.2.6 Proximal convoluted tubule.
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Note that the PCT has a microvilli cell border to
increase the SA for absorption from filtrate.
There are also a large number of mitochondria
which produce the extra ATP required for active
transport.
1. All glucose, all amino acids and 85% of
mineral ions are reabsorbed by
active transport from the filtrate to the tissue
fluid. They then diffuse into the
blood capillaries.
2. Small proteins are reabsorbed by pinocytosis,
digested, and the amino acids diffuse into the
blood.
3. 80% of the water is reabsorbed to the blood
by osmosis.
4. As urea molecules are so small and carry no
charge that they diffuse passively through the
cell membrane. In part this explains why not all
urea is excreted as blood passes through the
kidney.
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11.2.7 Loop of Henle
 Function
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create a salt bath concentration in the
surrounding medullary fluid.
results in water reabsorption in the
collecting duct
Results inreduction in the filtrate
volume.
Loop of Henle
 Mechanism:
1. There is a concentrated gradient
down through the medullary fluid (a).
2. The descending limb is permeable to
water but not to salt.
3. Filtrate enters the loop hypotonic to
the medullary fluid so water is lost(b).
4. The concentration difference between
medullary fluid and the filtrate is small.
5. The amount of water lost at each
stage is small but accumulates on
descent.
6. The water is lost but immediately
taken up by the blood.
7.. Filtrate volume reduces and filtrate
salt concentration increases.
8. The base of the loop is impermeable
(c)
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Fluid turns the impermeable loop.
 1.The filtrate moves up the ascending limb.
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2. The ascending limb is permeable to salt.
3.The ascending limb is impermeable to water.
4. The filtrate is slightly more concentrated than
the surrounding fluid.
5. There is a small but accumulating loss of salt
( Na+and Cl-)at each level.
6. The concentration of the filtrate is gradually
reduced.
6. The medullary gradient is maintained through
exchange with the surrounding blood vessels
Note that this has resulted in:
1. Filtrate entering and leaving the loop of henle
are approx isotonic
2. Reduced volume of the filtrate
3. Creation and Maintenance of the medullary
salt bath gradient
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 The concentration gradient of the
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medullary fluid brings about the removal
of water from the collecting duct by
osmosis.
The permeability of both Distal Convoluted
Tubule (DCT) and the Collecting tube(CT)
can be increased by the hormone ADH
(Vasopressin).
The cell membranes of these tubules do
not allow the movement of water by simple
diffusion. Rather pores called Aquaporin
can be opened the action of ADH.
The DCT is involved in other homeostatic
functions such as the secretion of H+in pH
regulation or K+ in salt regulation.
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11.2.8 Comparison of glomerular
filtrate with urine.
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Urea
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Uric acid
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100% reclaimed by selective reabsorption.
The presence of glucose in the urine would be an indication of diabetes.
Amino acids
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fairly toxic molecule (main nitrogenous excretion in birds)
largely removed from blood and tissue fluids.
Glucose
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collecting duct is permeable to both water which as the filtrate descends this
collecting duct is removed concentrating the filtrate (urine).
collecting duct also leaks some urea which to the kidney interstitial fluid.
Some of this lost urea is reabsorbed by the ascending limb of the loop of henle
but not all, hence the 50% reabsorption.
This cycling of urea is an important feature of the kidneys ability to produce a
concentration gradient through the medulla.
all selectively reabsorbed in the nephron and then undergo deamination in the
liver (urea excretion).
Proteins and other macromolecules
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should not be filtered in the Bowman's capsule
any presence in urine is usually regarded as an indicator of high blood pressure
and damage to the basement membrane (nephritis) of the bowman's capsule.