Urinary System and Body Fluids

Describe the filtration membrane

The filtration membrane is the barrier between the blood in the glomerular capillaries and the filtrate in Bowman’s capsule. It allows water and small solutes through while restricting blood cells and most plasma proteins.

It has 3 layers:

1. Fenestrated glomerular endothelium

   • Contains pores (fenestrations).

   • Allows water and small molecules through.

   • Prevents blood cells from passing.

2. Basement membrane

   • Lies between the endothelium and podocytes.

   • Acts as an important size and charge barrier.

   • Restricts large molecules and negatively charged proteins such as albumin.

3. Podocytes

   • Specialized cells surrounding the capillaries.

   • Have branching foot processes with gaps called filtration slits.

   • Provide the final filtration barrier.

Explain the forces/factors that promote and oppose filtration

🟢 Promotes filtration

Glomerular hydrostatic pressure (Pᵍᶜ)

• Pressure of blood within the glomerular capillaries.

• Pushes water and small solutes out of the blood and into Bowman’s capsule.

• This is the main force promoting filtration.

• Increased glomerular hydrostatic pressure → increased GFR.

🔴 Opposes filtration

1. Blood colloid osmotic pressure (πᵍᶜ)

• Caused mainly by plasma proteins remaining in the glomerular capillaries.

• Proteins draw water back into the blood.

• Therefore, it opposes filtration.

• Increased plasma protein concentration → ↑ osmotic pressure → ↓ GFR.

2. Bowman’s capsule hydrostatic pressure (Pᴮˢ)

• Pressure of fluid already present in Bowman’s capsule.

• Pushes fluid back toward the glomerular capillaries.

• Therefore, it opposes filtration.

• Increased pressure in Bowman’s capsule → ↓ GFR.

Describe hormonal regulation of glomerular filtration rate

• Angiotensin II

  • Released when blood pressure or blood volume is low.

  • Causes mainly efferent arteriole constriction.

  • Moderate constriction helps maintain glomerular pressure and GFR despite reduced renal blood flow.

  • Severe vasoconstriction can eventually decrease GFR.

• ANP (atrial natriuretic peptide)

  • Released when blood volume/atrial stretch increases.

  • Increases GFR, partly by increasing glomerular capillary pressure.

  • Promotes Na⁺ and water excretion, helping reduce blood volume and pressure.

• ADH

  • Released when plasma osmolarity rises or blood volume falls.

  • Its main renal effect is on the collecting ducts, increasing water reabsorption.

  • It has a smaller/direct role in GFR regulation compared with angiotensin II and ANP.

• Aldosterone

  • Increases Na⁺ reabsorption in the distal nephron.

  • Water follows Na⁺, helping increase blood volume.

  • Therefore, it affects GFR indirectly by helping maintain blood volume and blood pressure.

Outline how the nervous system controls the voiding of urine

Urination (micturition) is controlled by the autonomic nervous system, with voluntary control from the brain.

1. Bladder fills

   • Urine enters the bladder and stretches the bladder wall.

   • Stretch receptors detect the increasing bladder volume.

2. Sensory signals to the CNS

   • Stretch receptors send signals via sensory neurons to the sacral spinal cord.

   • The brain receives information about bladder fullness.

3. Storage phase

   • Sympathetic activity relaxes the detrusor muscle and contracts the internal urethral sphincter → helps retain urine.

   • Somatic motor activity keeps the external urethral sphincter contracted → provides voluntary control.

4. Voiding phase

   • When it is appropriate to urinate, the brain activates the parasympathetic nervous system.

   • Parasympathetic activity contracts the detrusor muscle and relaxes the internal urethral sphincter.

   • The external urethral sphincter is voluntarily relaxed, allowing urine to leave.

Describe tubular reabsorption and how it takes place

• Reabsorption occurs in PCT, loop of Henle, DCT and collecting ducts

• Most reabsorption occurs passively via concentration gradients but the amount entering and exiting is tightly controlled.

• Control is exerted directly by ADH and aldosterone and indirectly by renin. Most water is recovered in PCT, loop of Henle and DCT. About 10% reaches collecting ducts.

• Collecting ducts controlled by ADH can recover almost all water passing through (dehydration) or no water (over-hydration).

• PCT (proximal convoluted tubule) is responsible for most of the water re-absorption (65%). PCT is very long and has many microvilli and mitochondria for active transport. Also reabsorbs many other chemicals compared to other parts of the nephron.

 

What Solutes are Reabsorbed:

1. Sodium - actively transported out of tubule fluid and into the blood

2. Chloride, phosphate, and bicarbonate ions - move passively into the blood because of an imbalance of charge

3. Glucose, amino acids - move passively out of tubule fluid by sodium cotransport mechanisms

4. Urea - aprox 50% moves passively out of tubule

5. Water - movement of ions into the blood causes a sodium imbalance, causing water to move via osmosis into the blo

Describe tubular secretion and state solutes that are secreted.

Moves metabolic by-products, drugs, other molecules not normally produced by the body from the blood into the tubule of the nephron.

• Can be active or passive transport

• Example of ammonia which is produced by epithelial cells of the nephron

• Potassium, penicillin, frusemide is actively secreted into the nephron.

 

Secretion of PCT and loop of Helene:

• Waste removal -  - urea, creatinine, uric acid, bile salts, ammonia, catecholamines, many drugs

• Acid-base balance -

• When blood pH decreases: t secretion of H+ and t reabsorption of bicarbonate ions = T pH of body fluids

• When blood pH increases: secretion of and reabsorption of HC03- pH of body fluids

 

Secretion of DCT and Collecting Duct:

• DCT and collecting duct secrete K+, H+ and ammonium ions.

• Secretion of H+ increases with increased blood H+ concentration (decreased pH).