Homeostasis

The maintenance of a constant environment within

a living organism

Homeostatic control systems

Blood glucose levels Thermoregulation Osmoregulation by the kidney

What would happen if the body did not regulate the above?

The homeostatic process

A system needs to operate at a certain level called the set point

Involves a detector that detects a stimulus (a change in a level)

The detector informs a coordinator which triggers an appropriate method of correcting the deviation

The coordinator communicates with an effector which carries out the corrective procedure

Once the correction is made and the factor returns to normal, information is feed back to the detector which ‘switches off’

This is called negative feedback

Negative feedback response

The set point

Negative feedback loop restores the set point. Once this has been adjusted then the corrective mechanism is switched off.

The control of blood glucose

Osmoregulation by the kidney

The Kidney

The structure and function of the kidney

The kidney is an organ of excretion

Excretion is the removal of the waste products of metabolism (not digestion)

Carbon dioxide, urea and bile The kidneys have two roles:

• They deal with the removal of urea (nitrogenous waste)

• Osmoregulation (maintaining a balance of water and dissolved salts in the body)

The production of urea

Urea is made by the liver by the breakdown of unwanted and excess amino acids

The amino acids are deaminated (the NH2 group removed) and ammonia is made

Ammonia is very toxic therefore the body converts the ammonia into urea which is carried around the body in the blood and removed by the kidneys

Deamination of amino acids

The remaining keto group can be used in respiration either converted to acetyl CoA, pyruvic acid or a krebs intermediate

•The liver removes the amino group (deamination) and produces ammonia which is toxic and needs to excreted as urea

• Carbon dioxide reacts with ammonia as part of the ornithine cycle to produce urea

ammonia +carbon dioxide=urea and water

2NH3+CO2=CO(NH2)2+H20

Conversion of ammonia to urea

Kidneys are approx. 11cm by 6cm by 2.5cm and surrounded by fat for protection. They have filtered all the blood in the body every 22 minutes.

Fibrous capsule

Position of the nephron

The nephron is the functional unit of the kidney

There are about a million in each kidney.

The nephron undertakes several processes

The removal of waste includes:• Ultra filtration in the Bowman’s capsule

• Selective re absorption in the proximal convoluted tubule

Osmoregulation includes:• Concentration of salts by the loop of Henle in

the medulla of the kidney

• Hormonal control (ADH) of the permeability of the collecting duct cells

The process of ultra filtration Each nephron has a rich blood supply Each Bowman’s capsule is supplied with blood by an afferent

arteriole This branches inside the Bowman’s capsule to form the

glomerulus Taking blood away from the capsule is the efferent arteriole. The afferent arteriole is much wider than the efferent arteriole So there is more blood carried to the glomerulus than is carried

away, and pressure in the glomerulus is high. Small molecules such as water, glucose, urea and salts are

filtered out of the blood under high pressure and move into the Bowman’s capsule

This is called glomerular filtrate

Ultra filtration in the Bowman’s capsule

Filtration takes place in the pores of the capillary endothelium. Only molecules of a certain size can pass through.

The podocyte cells are special cells lining the Bowman’s capsule. They leave a gap for the filtrate to pass through easily.

Ultra filtration

Molecule or ion Approx. concentrations / g dm-3

Plasma Filtrate

water

protein

glucose

amino acids

urea

inorganic ions

900.0

80.0

1.0

0.5

0.3

7.2

900.0

0.0

1.0

0.5

0.3

7.2

Selective re absorption in the proximal convoluted tubule Water and solutes are removed from the plasma as it passes

through the glomerulus and forms glomerular filtrate These solutes include some useful substances such as amino

acids, glucose and water, these must be reabsorbed back into the body.

This process is called selective re absorption. All the glucose, amino acids, vitamins and many Na+ and Cl-

ions reabsorbed. 85% of the water is reabsorbed Re absorption of glucose and salts is by active transport and

diffusion, water is reabsorbed by osmosis

Adaptations of the proximal tubule cells

The cells possess microvilli to provide a large surface area for absorption

Numerous mitochondria are present to provide ATP for active transport.

Research the mechanism of active transport in the PCT

Osmoregulation by the kidney The body needs to control the water potential of the blood So far the kidney has filtered the blood and selectively re

absorbed 85% of the water back into the blood There has been no mechanism so far to finely tune the

water content of the blood The osmoregulatory function of the kidney undertakes

this role We will consider:

• The action of the loop of Henle

• The homeostatic mechanism

• The action of the hormone ADH (antidiuretic hormone) on the distal convoluted tubule and the collecting duct

The Loop of Henle

Its function is to create an area of high solute concentration deep in the medulla. (low water potential)

The collecting ducts of each nephron pass through this area and so a lot of water can potentially be reabsorbed from the ducts by osmosis

The permeability of the collecting ducts is controlled by a hormone ADH (anti diuretic hormone)

Diagram showing the solute concentration in the medulla of the kidney(milliosmoles per kilogram (mOsm/Kg)-thousandths of a mole of molecules in 1kg of solution

The homeostatic mechanism

The set point

Negative feedback loop restores the normal water potential of the blood. Once this has been adjusted then the corrective mechanism is switched off.

Osmoregulation in the collecting ducts of the kidney This acts as a process of negative feedback The receptors for detecting changes in the water

potential of the blood are in the hypothalamus of the brain (osmoreceptors)

The hypothalamus sends a nerve impulse to the pituitary gland

The posterior lobe of the pituitary acts as the co ordinator (releases hormone anti diuretic hormone ADH)

This hormone travels in the blood to the cells lining the collecting duct of the kidney

The cells of the collecting ducts of the kidney act as the effector (may or may not become permeable)

Control of water re absorption by ADH

The permeability of the walls of the distal convoluted tubule and collecting duct is controlled by ADH

ADH makes the walls of the these more permeable so that water moves out and is reabsorbed into the blood

With ADH

Without ADH

BloodCells of the collecting duct

Lumen of the collecting duct

normal water potential

of blood

receptors in

hypothalamus

second convoluted tubule

and collecting ducts

less water

reabsorbed into blood

blood water potential becomes more negative

becomes less negative

less ADHproduced

becomes more negative

more ADH released

more water

reabsorbed into blood

blood water potential becomes less negative

Negative feedback mechanism in action

Describe how the body copes with a drop in the water potential of the blood.

Describe how the body copes with a rise in the water potential of the blood

A fall in water potential of the blood can be caused by:

Reduced water intake Sweating Keeping exchange surfaces moist Loss in faeces

Environmental adaptations in animals

Aquatic animals produce ammonia rather than urea since it is highly soluble in water. The ammonia diffuses out across the gills

Birds and insects secrete uric acid. This takes lots of energy to produce but very little water is needed for excretion. This conserves water and allows them to live in dry environments.

Mammals excrete urea which is less toxic than ammonia. Mammals have adapted the loop of Henle to absorb more water in arid climates.

The kangaroo rat also reduce water loss by living in burrows during the daywhich are cool and humid reducing water loss of evaporation.

Comparison of lengths of the Loop of Henle

Desert animals

The kangaroo rate lives in the desert where water in scarce

They live on water produced from metabolic processes

They have an extra long loop of Henle to increase the solute concentration in the medulla and re absorb more water producing more concentrated urine