Biology 105
Biology of Exercise
Spring 2003

4/16/03 - Pre-class outline

Learning objectives:

How do fluid replacement beverages work?

Diagrams

Capillary function
• Kidney function in different conditions - answers
• Kidney function when making hyperosmotic urine
• Kidney function when making hypoosmotic urine
• Osmoreflex

• 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.