lecture 19
DESCRIPTION
physioTRANSCRIPT
Water and Electrolytes
Key concepts involving water
Body water is contained in two major body compartments (intracellular fluid compartment and extracellular fluid compartment)
Fluid balance is maintained when input and output are equal
The primary source of intake is water ingestion (eating and drinking) …60% from drinking
In addition, digestion and metabolism of carbohydrates, proteins, and fats provides another source of intake
Key concepts involving water
Compartments
Intracellular fluid (ICF) represents the fluid inside the cells and is the largest compartment (2/3 of body water)
Extracellular fluid (ECF) represents the fluid outside the cells and is 1/3 of total.
ECF is further divided into interstitial (ISF) and is the plasma.
ECF divisions
ISF occupies the space between cells and consists of 15% of total body fluid or ¾ of ECF
Plasma is the fluid portion of the blood and is 4% of total body fluid or ¼ of ECF
BODY FLUIDS
ICF ECF
40% TBW 20% TBW
P IS
Distribution of Body Fluids
60-kg manTBW = 0.6 x 60 kg = 36 L
ICF = 0.4 x 60 kg = 24 L
ECF = 12 L
3L 9L
Water balance
In a healthy person, the fluid ingested balances the fluids excreted
If you have a daily intake of 2500 ml, you should have an output of 2500 ml
Water regulation is associated with sodium regulation
Primary regulator is thirst-- alerts body to a fluid deficit
Water loss
Regulation of water loss is by ADH which increases the permeability of the distal convoluted tubule and collecting duct to water
Water loss from kidneys is primary source (urine 60%)
evaporation through skin and respiratory system (also called insensible is 28%), sweat-6%, gastrointestinal (feces 6%)
diarrhea, vomiting are additional sources occasionally
Disturbances of water homeostasis
Water disturbances involve1. Gain or loss extracellular fluid volume2. Gain or loss of solute Four examples of water disturbances1. Hypervolemia2. Overhydration3. Hypovolemia4. Dehydration
Hypervolemia occurs when too much water and solute taken at the same time. Extracellular fluid volume increases and normal plasma osmolarity
Overhydration occurs when too much water taken without solute. Volume increases and osmolarity decreases
Hypovolemia occurs when water and solute lost at the same time. Loss of plasma volume. Osmolarity remain normal
Dehydration occurs when water is lost, volume decreases and plasma osmolarity increases
Mechanisms of fluid balance
1. Antidiuretic hormone (ADH)2. Thirst3. Aldosterone4. Sympathetic nervous system
ADH (Antidiuretic Hormone)
Made in hypothalamus; water conservation hormone
Stored in posterior pituitary gland Acts on renal collecting tubule to
regulate re-absorption or elimination of water
If blood volume decreases, then ADH is released & water is reabsorbed by kidney. Urine output will be lower
Effect of ADH
Losing water through sweating results increase solute concentration in the plasma and thus blood become more osmotic and this results in tissue osmotic pressure increase
Then ADH is released & water is reabsorbed by kidney.
Urine output will be lower but concentration will be increased.
THIRST Mechanism
Major factor that determines fluid intake
1. Saliva production decreases. Impulses from the mouth and throat to the thirst centre in the hypothalamus.
2. Increase in osmotic pressure of body fluids stimulate the osmoreceptor in the thirst centre of hypothalamus
3. Decrease blood volume stimulate the release of renin which stimulate thirst center
Effect of Aldosterone
Reduce blood volume leads to reduced blood pressure
This stimulate release of renin which leads to release Aldosterone
Aldosterone can be also released when K concentration in the blood is high
Presence of Aldosterone, Na reabsorbed in the plasma and K secreted in the urine
This leads to increase blood pressure
Sympathetic nervous system
When blood pressure is low baroreceptor in the heart, aortic arch and carodit arteries send sensory information to the medulla, which increase the sympathetic impulses to the kidney
Smooth muscle cell constrict causing a decrease in blood flow into glomerulus