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Water Balance

How do those kidneys dunnit?

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We don’t pee the same everyday…

• Close your eyes and imagine the following scenario…

• So what happens when you drink too much water?

• What happens when you do not drink a lot of water?

• Your kidneys aren’t alone in regulating the urine output – it’s a tag team effort between the nervous and endocrine system

• What is the nervous system? Endocrine system?

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Before we continue…

• Osmolarity – the concentration of solute in an aqueous solution

• Osmotic pressure - Pressure generated by water moving by osmosis into or out of a cell

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ADH

• Antidiuretic hormone that travels through the bloodstream

• Small polypeptide of 9 amino acids• Produced by the pituitary gland in the brain (which is

located in and regulated by the hypothalamus)• The nervous system and brain (particularly the

hypothalamus) monitor the osmolarity in the blood• Increases the amount of water that is retained in the

body (more reabsorption)• This results in a lower volume of water in the urine• Less water with the same amount of salt & other solutes

makes the urine more concentrated• Negative or positive feedback?

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What do we mean by concentrated urine?

• How do we calculate concentration?

More Water in Urine(more/less ADH?)

Less Water in Urine(more/less ADH?)

Which one is more

concentrated?

Same amount of solute

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ADH

• When cells shrink, they transmit nerve impulses to the osmoreceptors (detect changes in osmolarity) in the hypothalamus which respond by releasing ADH into the bloodstream

• When ADH reaches the kidneys, they bind to receptors on the surface of the cells lining the collecting duct thereby increasing water permeability

• The brain also triggers a behavioural response: increases your thirst drive

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ADH

• The greater the volume of water consumed, the lower the osmotic pressure of the blood

• When a good level of osmolarity is reached, ADH release diminishes

• Can you come up with a rule for when ADH is released in relation to osmolarity of the blood?– ADH release is essentially triggered by

increases in osmolarity of the blood

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Where does ADH do its noble work?

• About 85% of the water in the nephron is reabsorbed in the proximal tubule since its membrane is permeable to water

• The rest of the nephron is mostly impermeable to water

• ADH thus targets the distal tubule and the collecting duct

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With ADH Action

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Without ADH Action

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What’s the Diuretic with you?

• Diuretics are chemicals (usually drugs) that stimulate the production of urine to regulate blood pressure

• Reduces the amount of ADH produced thereby increasing the amount of filtrate that leaves the kidney

• A prime example: caffeine• At first, because there is less water

reabsorbed in to the capillaries, blood pressure decreases (less stress on the heart)

• However, when you have too much it makes the blood hypertonic and water must leave the interstitial fluid in the tissues around the body resulting in higher blood pressure

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Kidneys and Blood Pressure

• Aldosterone is a hormone produced by glands in the kidney called the adrenal glands (on the top section of the kidney)

• Aldosterone stimulates the reabsorption of sodium ions (Na+)

• Targets the distal tubule in the nephron• Water follows Na+ out of the nephron by osmosis so

the blood volume and therefore the blood pressure increases

• Aldosterone production is increased by drop in blood pressure or a deficiency of Na+ (usually occurs when there is injury or severe diarrhea)

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Angiotensin

• Is also produced in response to low blood pressure

• An enzyme in the kidney is released which converts angiotensinogen (a plasma protein produced by the liver) into angiotensin

• Two vital functions:– Constricts blood vessels which increases

blood pressure– Also stimulates the release of aldosterone

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pH Balance

• During cellular respiration, carbon dioxide (CO2) is produced which dissolves in our blood to make carbonic acid (H2CO3)

• Carbonic acid then breaks down to the bicarbonate ion (HCO3

-) and releases hydrogen protons (H+)

• How is pH maintained? Read p355

CO2               H2CO3               HCO3-  +   H+

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Homework

• Read section 7.6 on p353

• Copy figure 1 and 2 on pages 353 and 354 respectively into your notes

• Do questions #1-10 on page 356