9.3 - Urinary system part 3

What are the 3 processes involved in urine formation?

Filtration (out of blood into nephron), selective reabsorption (from nephron into blood), tubular secretion (from blood into nephron).

What determines urine composition?

The exchange of substances between the nephron and renal capillaries.

What are the sources of water input?

Food, drink, and metabolic water from cellular respiration.

What are the sources of water output?

Urine, expired air from lungs, faeces, and sweat.

What is the minimum urine output required to excrete waste?

Approximately 500 mL/day.

How much water is filtered daily by the kidneys?

Approximately 180 L/day.

How much water is reabsorbed daily?

Approximately 179 L/day.

What is the final daily urine output?

Approximately 1–1.5 L/day.

What is obligatory reabsorption?

Automatic water reabsorption by osmosis following solute movement.

What is facultative reabsorption?

Hormone-controlled water reabsorption, mainly regulated by ADH.

What are the 3 mechanisms regulating water balance?

Osmotic gradient, counter-current multiplication, and hormonal regulation.

How is the osmotic gradient created in the kidney?

Active transport of sodium out of the ascending loop of Henle via Na+/K+ ATPase.

Why can water not leave the ascending loop of Henle?

Because it has tight junctions and no aquaporins, making it impermeable to water.

Why can water leave the descending loop of Henle?

Because it contains aquaporins and leaky junctions.

What is the purpose of the counter-current mechanism?

To concentrate urine while conserving water.

How concentrated can the renal medulla become?

Approximately 1200–1400 mOsm/L.

Why is sodium important in water balance?

It is the main extracellular cation and drives osmotic movement of water.

What is ADH?

Anti-diuretic hormone (vasopressin), the principal regulator of water balance.

What triggers ADH release?

High plasma osmolarity (dehydration/concentrated blood).

Where is ADH released from?

The posterior pituitary gland.

What detects changes in osmolarity?

Osmoreceptors in the hypothalamus.

What does ADH do?

Increases water permeability of distal tubules and collecting ducts.

How does ADH work?

It inserts aquaporin channels into collecting duct membranes.

What is the effect of high ADH?

More water reabsorbed, concentrated urine, lower urine volume.

What is the effect of low ADH?

Dilute urine and increased urine volume.

What is diabetes insipidus?

A condition caused by low ADH resulting in inability to concentrate urine and excessive urination.

Why does diabetes mellitus also cause polyuria?

Glucose in urine pulls water out by osmosis.

Why is urination reduced at night?

ADH levels rise overnight due to circadian rhythm.

Where is aldosterone produced?

The adrenal cortex.

What does aldosterone do?

Increases sodium reabsorption, potassium secretion, and water follows sodium.

What is the effect of high aldosterone?

Water retention and concentrated urine.

What is ANP?

Atrial natriuretic peptide.

What triggers ANP release?

Stretch of the atria due to increased blood volume.

What does ANP do?

Reduces sodium and water reabsorption, producing copious dilute urine.

Which hormones does ANP inhibit?

ADH and aldosterone.

What is diuresis?

Increased urine production.

What substances can cause diuresis?

Caffeine, alcohol, cold weather, cortisol, and furosemide.

How does alcohol increase urination?

By inhibiting ADH secretion.

How does caffeine increase urination?

By inhibiting sodium reabsorption.

How does the sympathetic nervous system affect kidneys?

It conserves water.

How does the sympathetic nervous system conserve water?

By constricting the afferent arteriole to reduce GFR and stimulating renin release.

What are the main regulators of electrolyte balance?

RAAS and ANP.

What is the main extracellular cation?

Sodium (Na+).

What is the main intracellular cation?

Potassium (K+).

What does RAAS regulate?

Electrolyte balance and blood pressure.

What triggers RAAS?

Low sodium, low blood pressure, low blood volume, or sympathetic stimulation.

What is the RAAS sequence?

Low Na+/BP → renin → angiotensin I → ACE → angiotensin II → aldosterone.

Where is renin produced?

The kidney.

Where is ACE produced?

Mainly in the lungs.

Where is aldosterone produced?

The adrenal cortex.

What does angiotensin II do?

Causes vasoconstriction and stimulates aldosterone release.

What does aldosterone do in the nephron?

Inserts sodium channels, potassium channels, and Na+/K+ pumps.

How much sodium is reabsorbed?

Approximately 99%.

Where is most sodium reabsorbed?

Proximal convoluted tubule.

What happens when sodium in the distal tubule is low?

Renin is released.

What causes potassium secretion?

Aldosterone, high dietary potassium, and luminal anions.

What causes hypokalaemia?

Diarrhoea, diuretics, and excess aldosterone.

What causes hyperkalaemia?

Acidosis due to H+/K+ exchange.

What is normal blood pH?

7.35–7.45.

What are the main acid-base regulators?

Blood buffers, respiratory system, and renal system.

What are the 3 main blood buffer systems?

Protein, phosphate, and bicarbonate.

What is the main extracellular buffer?

Bicarbonate buffer system.

How does the respiratory system regulate pH?

By controlling CO2 excretion.

What happens in acidosis?

Hyperventilation removes CO2 and reduces H+.

What happens in alkalosis?

Hypoventilation retains CO2 and increases H+.

How do kidneys regulate pH?

By secreting H+ and reabsorbing bicarbonate.

Where is H+ secreted in the nephron?

Proximal tubule and collecting duct.

How much bicarbonate is reabsorbed?

Approximately 99%.

How is acidic urine buffered?

H+ combines with ammonia to form NH4+ and phosphate to form acidic phosphate salts.

What is acidosis?

Blood pH below 7.35.

What causes respiratory acidosis?

Hypoventilation causing CO2 retention.

What causes metabolic acidosis?

Renal failure causing H+ accumulation.

What is alkalosis?

Blood pH above 7.45.

What causes respiratory alkalosis?

Hyperventilation.

What causes metabolic alkalosis?

Vomiting, diuretics, or endocrine disorders.

What hormones are produced by the kidney?

Renin and erythropoietin.

What triggers erythropoietin release?

Hypoxia (low oxygen).

What does erythropoietin do?

Stimulates red blood cell production in bone marrow.