Internal Environment and Homeostasis

Question 1

What percentage of the body is fluid (approximately)?

Answer 1

60%

Question 2

How is body fluid divided?

Answer 2

Divided into compartments:

• Approximately 70% of body fluid is within cells - ICF
• Approximately 30% of body fluid is outwith cells - ECF
• Compartments are spearated by membranes

Question 3

Describe the basic composition of blood.

Approximately how much blood is there in the body?

Answer 3

Contains both ECF (plasma) and ICF (fluid in RBC).

Approximately 5L (7% of body weight).

Question 4

Describe the composition of ECF.

Approximately how much is there in the body?

Answer 4

Made up of:

• Plasma (in blood) ~3L
• Interstitial fluid (in spaces between cells) ~11L
• Approximately 14L (20% of body weight)

Question 5

What is the difference between plasma and interstitial fluid?

Answer 5

Proteins

Question 6

Why is there a higher conentration of proteins in plasma than interstitial fluid?

Answer 6

Plasma and interstitial fluid exchange substances through highly permeable capillary membranes, but:

• Most proteins are too big to permeate the membrane so they are maintained in a higher concentration in plasma than in interstitial spaces.

Question 7

Describe the composition of ICF.

Approximately how much is there in the body?

Answer 7

• ICF is separated from ECF by membranes highly permeable to water but impermeable to most electrolytes.
• ~28L (40% of body weight)

Question 8

What is the ion concentration of potassium in ICF?

Answer 8

140mM

Question 9

What is the ion concentration of sodium in ICF?

Answer 9

15mM

Question 10

What is the ion concentraion of chloride in ICF?

Answer 10

5mM

Question 11

What is the ion concentration of potassium in ECF?

Answer 11

5mM

Question 12

What is the ion concentration of sodium in ECF?

Answer 12

140mM

Question 13

What is the ion concentration of chloride in ECF?

Answer 13

110mM

Question 14

What determines the rate of osmosis (ie the net movement of water)?

Answer 14

Concentration of dissolved substances in the water

Question 15

Describe osmosis

Answer 15

The process by which molecules of water pass through a semipermeable membrane from a less concentrated solution to a more concentrated one.

Water molecules move randomly, so there is always some water movement in both directions in physiologic systems.

Question 16

What is the osmolality of a fluid?

Answer 16

Osmoles / Kg

A measure of the number of osmotically active particles per kg

Question 17

What is the osmolarity of a fluid?

Answer 17

Particles / L

A measure of the number of osmotically active particles per litre.

Question 18

Describe tonicity

Answer 18

The mechanical effects (eg on a cell) in terms of shrinkage or swelling caused by placing a cell in a solution of impermeant osmotically active particles.

Question 19

Describe isotonic ECF and ICF

Answer 19

Water concentration in ECF and ICF is equal; solutes cannot leave or enter; cell volume stays the same.

Question 20

Describe hypertonic ECF and ICF

Answer 20

A cell in a solution with low water concentration and high concentration of impermeant particles; water diffuses out; the cell shrinks.

Question 21

Describe hypotonic ECF and ICF

Answer 21

A cell in a solution with high water concentration and lower concentration of impermeant particles; water diffuses into the cell; the cell swells (bursts).

Question 22

Some solutes can permeate the cell membrane.

What is an iso-osmotic solution?

Answer 22

Solution with the same osmolarity as the cell, regardless of whether the solute can permeate the cell.

Question 23

Some solutes can permeate the cell membrane.

What is a hyper-osmotic solution?

Answer 23

A solution with a higher osmolarity than usual ECF

Question 24

Some solutes can permeate the cell membrane.

What is a hypo-osmotic solution?

Answer 24

A solution with a lower osmolarity than usual ECF

Question 25

How is homeostasis maintained?

Answer 25

By control systems

• Within cells
  
  • Genetics
• Within / between tissues / organs / systems
  
  • Nervous system - quick response, short duration
  • Endocrine system - longer response, longer duration
• Most control systems use negative feedback

Question 26

Describe an open loop system

Answer 26

• No feedback
• Could result in runaway reactions
• Controller → Controlled component → Output
• The output is not influenced by the resulting conditions

Question 27

Describe a closed loop system

Answer 27

• Output of the system feeds back and influences the controller
• Controller → Controlled component → Output → Sensor → Feedback → Controller
• The output is influenced by the resulting conditions

Question 28

What is negative feedback?

Answer 28

Return to set point

Question 29

What is positive feedback?

Answer 29

Moves away from set point

Question 30

What is feedforward?

Answer 30

A response is generated in anticipation of a change

Question 31

Describe hunting in a negative feedback system

Answer 31

• Negative feedback conrolls the system within limits
• Over time, the process of hunting takes place within a safe limit, around a set point.
• The depth and extent of hunting depends upon the properties of the elements of the feedback system.
• The set point may be altered at times of metabolic or physiologic stress.

Question 32

Describe the body’s response to infection

Answer 32

• Often, temperature increase (fever)
  
  • Results from increase in temperature set point
• Bacteria / waste products phagocytosed by leukocytes (WBC)
• Progens (cytokines) released
• Binds at hypothalamus
• Change in set point

Question 33

Describe the negative feedback control of arterial blood pressure

Answer 33

Question 34

What is gain?

Answer 34

• The degree of effectiveness of the negative feedback control system
  
  • Gain = correction / error