The Kidneys and HomeostasisThe Kidneys and Homeostasis

Homeostasis is the ability to control the Homeostasis is the ability to control the internal environment to enable organisms to internal environment to enable organisms to be independent of the external environmentbe independent of the external environment

Cells are bathed in tissue fluid in which Cells are bathed in tissue fluid in which conditions are maintained at a constant conditions are maintained at a constant level (e.g. temperature, pH, salt content)level (e.g. temperature, pH, salt content)

The internal environment is maintained by The internal environment is maintained by physiological mechanisms which are physiological mechanisms which are controlled by feedback mechanismscontrolled by feedback mechanisms

Control MechanismsControl Mechanisms Self-regulating by means of feedback Self-regulating by means of feedback

response:response:1.1. Reference pointReference point: set level of operation: set level of operation

2.2. DetectorDetector: signals deviation from reference point: signals deviation from reference point

3.3. ControllerController: co-ordinates info from detectors, : co-ordinates info from detectors,

sends out instructionssends out instructions

4.4. EffectorEffector: brings about necessary change: brings about necessary change

5.5. Feedback LoopFeedback Loop: informs detector of any change : informs detector of any change in system as a result of action by effectorin system as a result of action by effector

Feedback is usually NEGATIVEFeedback is usually NEGATIVE E.g. temperature controlE.g. temperature control

– Temp detectors in skin relay info on changes in Temp detectors in skin relay info on changes in external temp. to hypothalamus in brain external temp. to hypothalamus in brain (controller). Message sent to skin and blood (controller). Message sent to skin and blood vessels (effectors) to start corrective responses. As vessels (effectors) to start corrective responses. As temp adjusts then negative feedback to temp adjusts then negative feedback to hypothalamus to stop corrective measureshypothalamus to stop corrective measures

Rare occasions when feedback is POSITIVERare occasions when feedback is POSITIVE– E.g Oxytocin during labour stimulates uterine E.g Oxytocin during labour stimulates uterine

contractions. A positive feedback as more is contractions. A positive feedback as more is released as pressure on cervix continuesreleased as pressure on cervix continues

The kidneys have several homeostatic functions; The kidneys have several homeostatic functions; primarily regulating the composition of body primarily regulating the composition of body fluids by OSMOREGULATIONfluids by OSMOREGULATION

They control the They control the water and salt content water and salt content of the body and the of the body and the pH of the bloodpH of the blood

They excrete nitrogenous products in solution They excrete nitrogenous products in solution through the ureter into the bladder where it is through the ureter into the bladder where it is stored.stored.

The exit of the bladder is controlled by a The exit of the bladder is controlled by a voluntarily controlled sphincter.voluntarily controlled sphincter.

Urea is formed in the liver from deamination of Urea is formed in the liver from deamination of excess amino acids (removal of the NHexcess amino acids (removal of the NH22), this ), this

is combined with COis combined with CO22 to form urea in a to form urea in a

sequence of reactions known as the ornithine sequence of reactions known as the ornithine cycle. cycle.

Each kidney is supplied with oxygenated blood Each kidney is supplied with oxygenated blood by the RENAL ARTERY, and deoxygenated by the RENAL ARTERY, and deoxygenated blood returns via the RENAL VEIN.blood returns via the RENAL VEIN.

Each kidney contains around 1.2 million Each kidney contains around 1.2 million NEPHRONS; each of which have blood vessels NEPHRONS; each of which have blood vessels closely associated with itclosely associated with it

The AFFERENT ARTERIOLE brings blood The AFFERENT ARTERIOLE brings blood into each renal capsule, dividing to form a into each renal capsule, dividing to form a network of capillaries called the network of capillaries called the GLOMERULUSGLOMERULUS

Blood leaves the capillaries in the EFFERENT Blood leaves the capillaries in the EFFERENT ARTERIOLE ARTERIOLE

The NephronThe Nephron The functional unit of The functional unit of

the kidneythe kidney Carry out the process Carry out the process

of osmoregulation of osmoregulation and excretionand excretion

Made up of the Made up of the following partsfollowing parts

– Bowman’s CapsuleBowman’s Capsule– GlomerulusGlomerulus– Proximal convoluted Proximal convoluted

tubuletubule– Loop of HenleLoop of Henle– Distal convoluted Distal convoluted

tubuletubule– Collecting ductCollecting duct

The Bowmans CapsuleThe Bowmans Capsule The The ultrafiltrationultrafiltration unit unit It is a single layer of It is a single layer of

flattened epithelial cells flattened epithelial cells which contains the which contains the glomerulus (a tightly coiled glomerulus (a tightly coiled network of capillaries)network of capillaries)

The inner layer of the The inner layer of the Capsule has “slit pores” Capsule has “slit pores” which enable ultrafiltrationwhich enable ultrafiltration

It filters large particles (stay It filters large particles (stay in blood) from small ones in blood) from small ones (pass into nephron)(pass into nephron)

Afferent arteriole has larger diameter than the Afferent arteriole has larger diameter than the efferent arteriole which leads to a high efferent arteriole which leads to a high pressure in the glomerulus capillariespressure in the glomerulus capillaries

This pressure forces substances through This pressure forces substances through pores in the capillary wallspores in the capillary walls

Glomerular filtrate contains glucose, amino Glomerular filtrate contains glucose, amino acids, vitamins, hormones, urea, ions and acids, vitamins, hormones, urea, ions and waterwater

All cells, platelets and proteins remain in the All cells, platelets and proteins remain in the bloodblood

Most of the filtrate produced will be Most of the filtrate produced will be reabsorbed as it passes through the renal reabsorbed as it passes through the renal tubulestubules

Proximal Convoluted TubuleProximal Convoluted Tubule The longest region of the nephron for The longest region of the nephron for

ReabsorptionReabsorption It is formed by a layer of cuboidal epithelial It is formed by a layer of cuboidal epithelial

cells which have microvilli projecting into the cells which have microvilli projecting into the lumen of the tubulelumen of the tubule

They contain a large number of mitochondria They contain a large number of mitochondria due to their high energy requirementdue to their high energy requirement

The volume of the filtrate is reduced by 75-The volume of the filtrate is reduced by 75-80% in the proximal tubule80% in the proximal tubule

Amino acids, glucose and ions diffuse into cells of Amino acids, glucose and ions diffuse into cells of proximal tubule and are then actively transported into proximal tubule and are then actively transported into the surrounding where they diffuse into the nearby the surrounding where they diffuse into the nearby capillariescapillaries

The microvilli of the epithelial cells greatly increase The microvilli of the epithelial cells greatly increase the SA for uptake of ions and other solutesthe SA for uptake of ions and other solutes

Ion uptakeIon uptake– NaNa++ ions are actively pumped out of the tubule, as these ions are actively pumped out of the tubule, as these

leave an electrochemical gradient is formed, meaning leave an electrochemical gradient is formed, meaning negative ions such as Clnegative ions such as Cl-- and PO and PO44

3-3- follow follow

– The ion concentration so increases in the capillaries so an The ion concentration so increases in the capillaries so an osmotic potential formed so water moves out of the tubuleosmotic potential formed so water moves out of the tubule

Glucose uptakeGlucose uptake– This involves a specific type of active transport; This involves a specific type of active transport;

Sodium CotransportSodium Cotransport– This involves a sodium ion and glucose binding This involves a sodium ion and glucose binding

to the same carrier molecule in the surface to the same carrier molecule in the surface membrane membrane

– Normally all glucose is reabsorbed and non is Normally all glucose is reabsorbed and non is lost in the urinelost in the urine

Urea uptakeUrea uptake– Urea is in relatively high concentrations in the Urea is in relatively high concentrations in the

remaining tubule fluid so diffuses into the remaining tubule fluid so diffuses into the bloodblood

The Loop of HenleThe Loop of Henle Has an descending limb which starts in the Has an descending limb which starts in the

cortex and a ascending limb which returns cortex and a ascending limb which returns to the cortexto the cortex

This This reabsorbs waterreabsorbs water by osmosis which by osmosis which enables concentrated urine productionenables concentrated urine production

It functions thanks to a It functions thanks to a Countercurrent Countercurrent exchange mechanismexchange mechanism with the surrounding with the surrounding capillariescapillaries

The mechanism involves exchange between The mechanism involves exchange between 2 liquids moving in opposite directions e.g 2 liquids moving in opposite directions e.g the descending limb and the ascending limbthe descending limb and the ascending limb

The loop of Henle is a The loop of Henle is a countercurrent multiplier countercurrent multiplier due to due to the fact it actively secretes solutes into the surroundings the fact it actively secretes solutes into the surroundings to produce a very high conc. gradientto produce a very high conc. gradient

The ascending limb actively secretes ClThe ascending limb actively secretes Cl-- ions out as so ions out as so NaNa++ ions follow. ions follow.

The descending limb is impermeable to solutes but The descending limb is impermeable to solutes but permeable to waterpermeable to water

The water moves out of the descending limb due to The water moves out of the descending limb due to osmosis into the capillaries and the contents becomes osmosis into the capillaries and the contents becomes very conc.very conc.

As a consequence the fluid entering the ascending limb As a consequence the fluid entering the ascending limb contains high conc. of ions so more pumped out, so more contains high conc. of ions so more pumped out, so more water out of descending limb and so on..water out of descending limb and so on..

Distal Convoluted Tubule + Distal Convoluted Tubule + Collecting DuctCollecting Duct

Controls the amount of water reabsorbed into the blood; Controls the amount of water reabsorbed into the blood; OsmoregulationOsmoregulation

In times of water shortage mammals can produce In times of water shortage mammals can produce Hypertonic urine (a higher conc. of solutes than blood)Hypertonic urine (a higher conc. of solutes than blood)

In times of over hydration urine can be very dilute In times of over hydration urine can be very dilute (little water reabsorption in kidney)(little water reabsorption in kidney)

The amount of water reabsorbed from the distal tubule The amount of water reabsorbed from the distal tubule and collecting duct depends on their permeability to and collecting duct depends on their permeability to water, which is controlled by negative feedback water, which is controlled by negative feedback involving ADH (involving ADH (antidiuretic hormoneantidiuretic hormone))

ADH is released by the posterior pituitary glandADH is released by the posterior pituitary gland It acts on cells in the tubule and collecting duct It acts on cells in the tubule and collecting duct

to increase the permeability to waterto increase the permeability to water The more ADH the more water is reabsorbed, so The more ADH the more water is reabsorbed, so

the more conc. the urine isthe more conc. the urine is The mechanism is controlled by The mechanism is controlled by OsmoreceptorsOsmoreceptors

in the hypothalamus.in the hypothalamus. If the conc. of solutes in blood is raised then If the conc. of solutes in blood is raised then

osmoreceptors increase activity and ADH osmoreceptors increase activity and ADH secretions increase, so more water reabsorbedsecretions increase, so more water reabsorbed

If the conc. decreases ADH release is decreasedIf the conc. decreases ADH release is decreased