Dehydration

What happens to the kidneys with age?

Reduced:

• kidney mass

• nephron number

• kidney blood flow

• GFR

What is dehydration?

A decrease in total body water

Water loss dehydration: caused by insufficient fluid intake, produces decreased plasma volume with increased plasma osmolarity

Salt loss dehydration: deficit in both water and salt caused by excessive fluid loss, produces decreased plasma volume with normal plasma osmolarity

How do we respond to dehydration?

Thirst - increases water intake

Vasopressin - decreases water excretion

How does vasopressin release?

1. An increase in plasma osmolarity OR decrease in ECF/blood pressure

2. Hypothalamic osmoreceptors OR left atrial volume receptors sense

3. Activates hypothalamic neurons

4. Increases thirst and releases vasopressin

5. Stimulates insertion of aquaporins in distal tubule and collecting duct

6. Increases water reabsorption

Why are elderly people predisposed to dehydration?

Decreased total body water

Decreased sense of thirst

Decrease in urine concentration ability

Decrease in vasopressin effectivenesse

What are the effects of age on thirst?

The elderly are sensitive to changes in plasma osmolarity but less sensitive to changes in plasma volume

Why does urine output increase in the elderly?

The elderly are less able to concentrate urine because vasopressin receptors may be less sensitive and diurnal cycle in vasopressin may be dampened

What are some other factors causing dehydration in elderly?

Swallowing issues (dysphagia - up to 50% of nursing home residents)

Reduced appetite

Acute pathology (vomiting)

Medications

Cognitive problems

What are the types of dysphagia?

Oropharyngeal dysphagia - in mouth

Oesophagael dysphagia - food ‘sticking’ in throat or chest