Topic 15 Urinary System Physiology

Structures that transport urine

• Renal Pelvis

• Ureter

• Urinary Bladder

• Urethra

Kidney’s Functions

1. Maintain plasma volume → … Maintains Blood Pressure

2. Regulate the [Ion] + [H2O]

3. Acid-base balance

4. Eliminate waste like drugs, hormones

5. Endocrine → - Renin (bp) - Erythropoietin (RBC prod.)

The Nephron

Is a functional unit of the kidney that includes the:

• Renal Corpuscle (glomerus with renal capsule) and Tubules (proximal convoluted, descending and ascending nephron loop, distal convoluted and collecting duct)

Processes in the nephron that lead to urine formation

1. Glomerular Filtration

2. Tubular Reabsorption

3. Tubular Secretion

What happens during Glomerular Filtration

There is a pressure difference across the filtration membrane, which leads to water and small solutes going from: Blood in Glomerus → Bowman’s Capsule.

What happens during Tubular Reabsorption

Substances like water, glucose, and ions, transport from filtrate in the nephron into the bloodstream.

What happens during Tubular Secretion

From here, waste products are transported from peritubular capillaries into the lumen of nephron tubules to be excreted as urine.

Process of Glomerular Filtration→Bowmans Capsule

20% of the plasma is filtered from the glomerus to the bowman’s capsule VIA bulk flow across the filtration membrane.

The filtration membrane which is between the glomerus and bowman’s capsule is composed of?

• Fenestrated endothelium (glomerus)

• Fused basement membranes

• Podocytes (layer of bowman’s capsule) with filtration slits in between.

Filtrate makeup?

• Is identical to plasma minus large proteins

• H2O, glucose, amino acids, vitamins, ions, urea, some small proteins.

• Ph ~7.45

Net Filtration Pressure Components

• Glomerular Hydrostatic P (Favoured)

  • (Blood Pressure)

• Blood Osmotic P

• Capsular Hydostatic P

• Capsular Osmotic P (Favoured)

Glomerular Filtration Rate

~180 L/day filtrate = ~125 mL/min (entire plasma volume filtered ~65 times per day)

How much of filtered volume remains at end of collecting duct?

< 1% (Reabsorption)

Regulation of GFR

Keeping GFR from changing when systematic pressure changes if not - MAP and GFR increase. If GF rate is too high, it filters too quickly and nutrients and water is lost through urine. If GF rate is low, waste will accumulate in the blood w waste.

Regulation of GFR processes.

Intrinsic regulation

Extrinsic regulation

Intrinsic Regulation of GFR

Commonly known as autoregulation - for bp in range of resting to moderate exercise. Two processes:

• Juxtaglomerular complex (apparatus

• Myogenic

Myogenic

Mean Arterial Pressure increases → Aff. Arteriole stretches → Arteriole smooth muscle contracts → prevents the increase of BP in glomerular capillaries. (Vice versa)

Juxtaglomerular Complex

1. The macula densa monitors [NaCl] levels in filtrate

2. High MAP → high GFR → more NaCl → signals afferent arteriole to constrict

3. Low MAP → low NaCl → arteriole dilates

Extrinsic Regulation

When the body outside the kidney takes over the filtration control, usually a “survival mode” when there’s extreme stress. Ex. Heavy exercise.

SNS → Arteriolar vasoconstriction

Afferent → decrease flow into glomerulus

Efferent → blood backs up in glomerulus

Moderate SNS activation → both balance → GFR doesn’t change much.

NFP can change?

1. Blood Osmotic P (proteins)

• ex. Dehydration inc BOP = dec GFR

• Ex. Burns dec BOP = inc GFR

1. Capsular hydrostatic pressure

• ex. Urinary tract obstruction - kidney stones inc CHP = dec GFR

Tubular Reabsorption

99% of filtrate is reabsorbed from the tubules into the peritubular as vasa recta capillaries

Can be passive or active.

Active → requires energy

• Na+, Other ions, Glucose, a

Passive → no energy

• Cl-, H2O, Urea