Biology 105
Biology of Exercise
Spring 2003
4/18/03 - Post-class outline
Learning objectives:
How do fluid replacement beverages work?
Diagrams
- Review - why hydration is important
- heart rate during prolonged exercise in the heat tends to increase slowly.
- why?
- progressive fluid loss in sweat - also large blood flow to skin
- increased blood pressure causes increased fluid filtration out of the
capillaries.
- reduction in venous blood pressure
- decrease in stroke volume
- reduction in cardiac output
- reduction in arterial blood pressure
- baroreflex causes increased heart rate.
- at some point, heart rate will not be able to meet demands for increased
cardiac output and fatigue will occur.
- so, hydration seems like a good idea
- Gatorade and other hydration drinks have salt (sodium chloride).
- The Philadelphia Eagles even drank pickle juice before a game last year
(and proceeded to crush the Cowboys in Dallas in 110 F weather).
- what is the relationship between hydration and salt.
- We talked previously about blood pressure regulation in the body -
see diagram - regulation of blood volume is an important component of
this.
- Another factor that is regulated is blood osmolality
- osmolality is the concentration of dissolved substances (solutes).
- sodium is a major component of total osmolality
- normal blood osmolality is 300 mOsm (milliosmolar)
- if a fluid is more concentrated than normal blood, we say it is hyperosmotic
to blood
- if a fluid is less concentrated than normal blood, we say it is hypoosmotic
to blood
- if a fluid is the same concentration as normal blood, we say it is isoosmotic
to blood
- The kidney regulated both blood volume and osmolality
- Chart of what the kidney needs to do in various
situations.
- thus, the kidney needs to be able to transport water and salt independently
- How does the kidney work?
- large amounts of plasma are filtered
- Quantity = 180 L /day
- Plasma volume = less than 3 L
- Plasma volume is filtered more than 65 times/day
- If no water reabsorb, PV would be lost in 30 min.
- most of the salt, water, glucose and other important substances are reabsorbed
(transported back into the blood) in the proximal tubule.
- a large amount of salt is also reabsorbed in the loop of Henle.
This makes the ECF fluid very hyperosmotic.
- regulated salt reabsorption occurs in the distal tubule.
- regulated water reabsorption occurs in the collecting tubule. Water
moves via osmosis because the ECF fluid is hyperosmotic.
- during exercise:
- lose salt and water in sweat
- causes decreased pressure, trigger retention of salt and water
- if water only is ingested.
- this will cause decrease in blood osmolality
- trigger water loss by kidney
- the effect will be to help regulate osmolality, but this will oppose
the hydration effects of the water ingestion.
Boston marathon
(external links)
Boston Athletic Association
Boston.com
