CVDWK5: Formation of Urine: Filtration and Tubular Secretion

What percentage of cardiac output goes to the kidneys?

~25%

What is the normal Glomerular Filtration Rate (GFR)?

~120 mL/min

How much urine is produced per day?

~1.5 L

What are the three fundamental processes of urine formation?

1. Glomerular Filtration, 2. Tubular Secretion, 3. Tubular Reabsorption

What is the equation for urinary excretion?

Urinary Excretion = Filtration + Secretion - Reabsorption

How is glucose handled by the kidney?

Filtered + completely reabsorbed

How is creatinine handled by the kidney?

Filtered + NOT reabsorbed or secreted

How is penicillin handled by the kidney?

Filtered + secreted

What happens to glucose in the urine if blood glucose exceeds renal threshold?

Glycosuria (glucose in urine) - seen in diabetes

What is the renal threshold for glucose?

~10 mmol/L

What is the first step in urine formation?

Glomerular filtration

What percentage of plasma is filtered in the glomerulus?

10-20%

Which arteriole is smaller - afferent or efferent?

Efferent arteriole (creates resistance for filtration pressure)

What are the three layers of the glomerular filtration barrier?

1. Glomerular capillary wall (fenestrated endothelium), 2. Basement membrane, 3. Inner layer of Bowman's capsule (podocytes)

Which molecular weight is easily filtered?

< 20,000 Daltons

Which molecular weight is NOT filtered?

50,000 Daltons (proteins)

Are protein-bound drugs filtered?

No - they are too large

Are unbound drugs filtered?

Yes - they can pass through the filtration barrier

What is GFR?

Glomerular Filtration Rate - measure of kidney function

What substance is used to measure GFR?

Creatinine (creatinine clearance = GFR)

Why is creatinine used to measure GFR?

It is filtered but NOT reabsorbed or secreted

How does sympathetic stimulation affect GFR?

Vasoconstriction → ↓ GFR

How does parasympathetic stimulation affect GFR?

Vasodilation → ↑ GFR

What is tubular secretion?

Active transport of substances from blood (peritubular capillaries) into the tubular fluid

What transporters are involved in tubular secretion?

OAT (Organic Anion Transporters) and OCT (Organic Cation Transporters)

What is an example of a drug secreted by the kidneys?

Penicillin (rapid elimination)

What drug metabolites are secreted?

Glucuronides (conjugated drugs)

Why is tubular secretion important?

Eliminates drugs and metabolites, excretes substances not filtered, maintains acid-base balance

What happens to GFR with age?

Declines with age (after 40 years)

What happens to GFR in kidney disease?

Declines (e.g., diabetic nephropathy, hypertension)