lecture 19

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