Role of Kidney in the Regulation of Body Fluids Flashcards
What is osmolarity?
The total solute concentration of a solution
1 osmol = 1 mol of solute particle
e.g. 1 M solution of glucose has a concentration of 1 osmol (1 osmol per litre)
Describe the electrolyte composition of the extracellular fluid?
- Na+ is the chief cation
- Cl- is the major anion
Describe the electrolyte composition of intracellular fluid?
- K+ is the chief cation
- Phosphate is the chief anion
- have low sodium and chloride
What are the functions of electrolytes?
- They assist in regulating fluid balance by controlling fluid movements
- Participate in acid-base balance
- Contribute to enzyme reactions
- Secretory activity
- Play an essential role in neuromuscular activity
What are the functions of sodium?
- Regulates osmotic pressure of the ECF and affects the osmotic pressure of the ICF
- Important in neuro-transmitter excitability and responsible for cell membrane depolarisation of excitable cells
What is hyponatremia?
- Reduced sodium concentrations in plasma
- Due to actual loss from body fluids or from excessive gains in water
- affects neuromuscular signals
What is psychogenic polydipsia?
uncommon clinical disorder characterized by excessive water-drinking in the absence of a physiologic stimulus to drink
What is the treatment for psychogenic polydipsia?
- administration of hypotonic saline
- diuretic therapy
- vomiting
Describe how hyponatraemia affects cells in the body?
- low sodium concentration in blood
- low osmotic pressure in extracellular fluids
- water shifts out of blood
- more water shifts into cell from low to high osmotic pressure
- cells swell function decreases and then cell ruptures
What is hypernatremia?
- Increased sodium concentration in plasma
- Due to decreased intake of water, or increased output of water, large ingestion of sodium
What is polyuria?
excessive urination
- Polyuria due to diabetes, excessive water loss from lungs
What hormone regulates sodium?
Reabsorption of sodium in the kidney is largely controlled by aldosterone
Stimuli for aldosterone secretion includes?
- Decrease in extracellular sodium levels
- Increase in extracellular potassium levels
- Angiotensin II
Changes in plasma sodium levels affect what in the body?
plasma volume and blood pressure
Describe the mechanisms that maintain sodium balance?
- sodium too low - aldosterone secreted
2. sodium too high - Atrial Natriuretic Peptide secreted
What is atrial natriuretic peptide?
- ANP promotes natriuresis (loss of sodium)
- Atrial myocytes synthesise, store and release ANP in response to stretch
How does atrial natriuretic peptide work?
- renal vasodilatation
- the blood flow is increased, so is the GFR (glomerular filtration rate)
- more Na+ reaches macula densa and more Na+ is excreted - ANP inhibits actions of renin, and generally opposes effects of angiotensin II
What is the function of potassium?
- Directly affects excitability of nerves and muscles
2. Too little K+ causes hyperpolarization and non-responsiveness
What is hypokalaemia?
- Lack of intake of potassium, diuretics, hyperaldosteronism
- May cause paralysis, muscle flaccidity, cardiac dysrhythmia
What are the mechanisms that maintain potassium balance?
- K+ excretion increases as ECF concentrations rise due to the release of aldosterone
- K+ retention increases when pH falls
(H+ secreted in exchange for reabsorption of K+ in the distal convoluted tubule (DCT)
How is potassium regulated in the body?
- Potassium controls its own ECF concentration via feedback regulation of aldosterone release:
- Increased K+ in the ECF around the adrenal cortex causes release of aldosterone
- Aldosterone stimulates potassium ion
secretion - In cortical collecting ducts, for each Na+ reabsorbed, a K+ is secreted
- When K+ levels are low, the amount of secretion and excretion is kept to a minimum
Ionic calcium in ECF is important for?
- Structural strength essential for bones and teeth
- Blood clotting
- Required for muscle contraction
- Secretory behavior
- Necessary for many metabolic processes and enzyme reactions
Calcium balance is controlled by?
- parathyroid hormone (PTH)
- vitamin D
- calcitonin
What happens when calcium levels are low?
stimulates release of PTH which stimulates:
- Osteoclasts to break down bone matrix
- Intestinal absorption of calcium
What happens when calcium levels are high?
stimulate thyroid to produce calcitonin which stimulates:
- Ca2+ secretion in kidneys
- Ca2+ deposition in bone
Describe the amount of water in the body of an adult?
60% (≈ 42 litres) of body weight in an average 70 kg adult
- Levels vary with sex, age, degree of obesity
What is the amount of water in an infant?
75 to 80% of body weights are water in infants
Why are infants more vulnerable to fluid imbalances than adults?
- Infants have higher metabolic rates and larger surface area
- Infants have more difficulty concentrating urine because of immature kidneys
What is the daily intake of water necessary to maintain life?
- 5 l/day
- Water intake averages 2 l/day
Where is water sourced from?
sourced from food and liquids ingested but also from oxidation of food derivative
How is water lost from the body?
- through the kidney with urine production
- urine production continues even when
water intake is stopped
- urine production continues even when
- through the skin and respiratory tracts
(amounts to 500 ml / m2/ day)
What is obligatory urine output?
the urine output required to eliminate waste products
What are the functions of water?
- as a solvent
- Cushions and acts as a shock absorber
- Used in metabolic processes
- acts as a lubricant
How does water act as a solvent?
- Acts as transport vehicle for nutrients
- Electrolytes
- Blood gases
- Metabolic wastes
- Heat
What metabolic processes does water aid in?
- Aids in food hydrolysis
2. Acts as a medium and reactant for chemical reactions
Describe the body fluid compartments?
- Extracellular fluids
- all fluids outside the cells
- interstitial fluid and plasma
- Intracellular fluid
- inside the cells
What is hypovolemia?
- Extracellular volume depletion
- Water and electrolytes are lost in the same
proportions
What causes hypovulaemia?
Caused by haemorrhage, diarrhoea, vomiting or uncontrolled diabetes mellitus
What is hypervolaemia?
- Extracellular volume excess
- Water and electrolytes are gained together in
equal proportions
What causes hypervolaemia?
- May result from excessive administration of
isotonic solutions or adrenocortical hormone - Also due to hyperaldosteronism and renal
failure
Name the 2 ways in which body water levels are regulated?
- thirst
2. kidneys ability to concentrate urine
What is thirst?
- Conscious desire for water
- Principal regulator of water intake
How does one feel thirsty?
- Thirst centre located in the hypothalamus
- Have receptor cells called osmoreceptors that
respond to changes in the extracellular
osmolarity by either swelling or shrinking
When does one feel thirsty?
- Thirst develops when 0.5% of body water has
been lost
Describe how the kidney maintains body water levels?
- Fluid intake required to match water loss
through lungs, GIT, skin and kidneys - the kidney can form a small volume of
concentrated urine which minimises the intake
of fluid required to maintain homeostasis - the ability of the kidney to form urine that is
more concentrated than plasma is essential
for survival of mammals that live on land
including man
What is the maximal urine concentration the human kidney can produce?
1200 to 1400 mOsm/L
What are the requirements for forming concentrated urine?
- A high level of ADH (anti diuretic hormone)
- increases the permeability of the distal
tubules and collecting ducts to water
thereby allowing the tubules to reabsorb
water
- increases the permeability of the distal
- A high osmolarity of the renal medullary
interstitial fluid- which provides the osmotic gradient
necessary for water reabsorption to occur
in the presence of high levels of ADH
- which provides the osmotic gradient
Describe the countercurrent mechanism in the medulla?
