Transcript The Kidneys and Homeostasis

The Kidneys and Homeostasis
Homeostasis is the ability to control the
internal environment to enable organisms to
be independent of the external environment
 Cells are bathed in tissue fluid in which
conditions are maintained at a constant level
(e.g. temperature, pH, salt content)
 The internal environment is maintained by
physiological mechanisms which are
controlled by feedback mechanisms
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Control Mechanisms
Self-regulating by means of feedback
response:
1. Reference point: set level of operation
2. Detector: signals deviation from reference point
3. Controller: co-ordinates info from detectors,
sends out instructions
4. Effector: brings about necessary change
5. Feedback Loop: informs detector of any change
in system as a result of action by effector
Feedback is usually NEGATIVE
 E.g. temperature control
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– Temp detectors in skin relay info on changes in
external temp. to hypothalamus in brain
(controller). Message sent to skin and blood
vessels (effectors) to start corrective responses. As
temp adjusts then negative feedback to
hypothalamus to stop corrective measures
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Rare occasions when feedback is POSITIVE
– E.g Oxytocin during labour stimulates uterine
contractions. A positive feedback as more is
released as pressure on cervix continues
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The kidneys have several homeostatic functions;
primarily regulating the composition of body
fluids by OSMOREGULATION
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They control
the water and
salt content of
the body and
the pH of the
blood
They excrete nitrogenous products in solution
through the ureter into the bladder where it is
stored.
 The exit of the bladder is controlled by a
voluntarily controlled sphincter.
 Urea is formed in the liver from deamination of
excess amino acids (removal of the NH2), this
is combined with CO2 to form urea in a
sequence of reactions known as the ornithine
cycle.
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Each kidney is supplied with oxygenated blood
by the RENAL ARTERY, and deoxygenated
blood returns via the RENAL VEIN.
 Each kidney contains around 1.2 million
NEPHRONS; each of which have blood vessels
closely associated with it
 The AFFERENT ARTERIOLE brings blood
into each renal capsule, dividing to form a
network of capillaries called the
GLOMERULUS
 Blood leaves the capillaries in the EFFERENT
ARTERIOLE
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The Nephron
The functional unit
of the kidney
Carry out the process
of osmoregulation
and excretion
Made up of the
following parts
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Bowman’s Capsule
Glomerulus
Proximal convoluted
tubule
Loop of Henle
Distal convoluted
tubule
Collecting duct
The Bowmans Capsule
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The ultrafiltration unit
It is a single layer of
flattened epithelial cells
which contains the
glomerulus (a tightly coiled
network of capillaries)
The inner layer of the
Capsule has “slit pores”
which enable ultrafiltration
It filters large particles (stay
in blood) from small ones
(pass into nephron)
Afferent arteriole has larger diameter than the
efferent arteriole which leads to a high
pressure in the glomerulus capillaries
 This pressure forces substances through pores
in the capillary walls
 Glomerular filtrate contains glucose, amino
acids, vitamins, hormones, urea, ions and
water
 All cells, platelets and proteins remain in the
blood
 Most of the filtrate produced will be
reabsorbed as it passes through the renal
tubules
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Proximal Convoluted Tubule
The longest region of the nephron for
Reabsorption
 It is formed by a layer of cuboidal epithelial
cells which have microvilli projecting into the
lumen of the tubule
 They contain a large number of mitochondria
due to their high energy requirement
 The volume of the filtrate is reduced by 7580% in the proximal tubule
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Amino acids, glucose and ions diffuse into cells of
proximal tubule and are then actively transported into
the surrounding where they diffuse into the nearby
capillaries
The microvilli of the epithelial cells greatly increase
the SA for uptake of ions and other solutes
Ion uptake
– Na+ ions are actively pumped out of the tubule, as these
leave an electrochemical gradient is formed, meaning
negative ions such as Cl- and PO43- follow
– The ion concentration so increases in the capillaries so an
osmotic potential formed so water moves out of the tubule
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Glucose uptake
– This involves a specific type of active transport;
Sodium Cotransport
– This involves a sodium ion and glucose binding
to the same carrier molecule in the surface
membrane
– Normally all glucose is reabsorbed and non is
lost in the urine
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Urea uptake
– Urea is in relatively high concentrations in the
remaining tubule fluid so diffuses into the
blood
The Loop of Henle
Has an descending limb which starts in the
cortex and a ascending limb which returns
to the cortex
 This reabsorbs water by osmosis which
enables concentrated urine production
 It functions thanks to a Countercurrent
exchange mechanism with the surrounding
capillaries
 The mechanism involves exchange between
2 liquids moving in opposite directions e.g
the descending limb and the ascending limb
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The loop of Henle is a countercurrent multiplier due to
the fact it actively secretes solutes into the surroundings
to produce a very high conc. gradient
The ascending limb actively secretes Cl- ions out as so
Na+ ions follow.
The descending limb is impermeable to solutes but
permeable to water
The water moves out of the descending limb due to
osmosis into the capillaries and the contents becomes
very conc.
As a consequence the fluid entering the ascending limb
contains high conc. of ions so more pumped out, so more
water out of descending limb and so on..
Distal Convoluted Tubule +
Collecting Duct
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Controls the amount of water reabsorbed into the blood;
Osmoregulation
In times of water shortage mammals can produce
Hypertonic urine (a higher conc. of solutes than blood)
In times of over hydration urine can be very dilute
(little water reabsorption in kidney)
The amount of water reabsorbed from the distal tubule
and collecting duct depends on their permeability to
water, which is controlled by negative feedback
involving ADH (antidiuretic hormone)
ADH is released by the posterior pituitary gland
 It acts on cells in the tubule and collecting duct
to increase the permeability to water
 The more ADH the more water is reabsorbed, so
the more conc. the urine is
 The mechanism is controlled by Osmoreceptors
in the hypothalamus.
 If the conc. of solutes in blood is raised then
osmoreceptors increase activity and ADH
secretions increase, so more water reabsorbed
 If the conc. decreases ADH release is decreased
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