L33

Renal System Overview

• The renal system is responsible for filtering blood and producing urine.
• Learning outcomes include understanding renal tubule functions, reabsorption, secretion, concentration, and acidification.
• Mechanisms of reabsorption across different sections of the nephron (PCT, descending and ascending loops, collecting ducts) are key.

Kidney Structure and Function

• Kidneys have cortex and medulla regions.
• Cortical and juxtamedullary nephrons are present; juxtamedullary nephrons are deeper in the medulla.
• Structures such as glomeruli, renal tubules, and collecting ducts are crucial for urine production.
• The renal artery delivers blood and renal vein removes blood.
• Pressure differences throughout the nephron play a key role.

Urine Formation

• Glomerular filtration creates a filtrate of blood
• Tubular reabsorption removes useful solutes, returns them to the blood.
• Tubular secretion removes additional wastes.
• Water conservation removes water from urine and returns it to blood.

Tubular Transport Mechanisms

• Active transport uses energy (e.g., Na-K-pump, H+-pump).
• Secondary active transport uses an electrochemical gradient.
• Passive transport includes simple diffusion, facilitated diffusion, and osmosis.
• Specific molecules are transported through different nephron segments using different mechanisms.

Primary Active Transport

• This transport uses ATP directly.
• Examples include the sodium-potassium pump and hydrogen pumps.
• Active transport plays a crucial role in establishing gradients.

Secondary Active Transport

• This transport relies on an existing electrochemical gradient.
• An example is glucose uptake coupled with sodium transport.

Osmosis

• Solute transport through the nephron creates concentration differences, causing osmosis of water.
• Water reabsorption is largely coupled to sodium.
• Permeability varies throughout the nephron.

Renal Tubular Reabsorption

• In the proximal convoluted tubule (PCT), 65-70% of water and sodium are reabsorbed along with 90% bicarbonate, calcium, and potassium along with 100% of glucose and amino acids.
• Secretion occurs here of organic acids and bases, bile salts, oxalate, urate, and catecholamines plus certain drugs.

Loop of Henle

• The loop of Henle is responsible for producing concentrated urine.
• It creates a concentration gradient in the medulla of the kidney.
• Water reabsorption occurs in the descending limb, while Na-Cl reabsorption occurs in the ascending limb.
• Counter-current multiplier mechanism is important for urine concentration.

Distal Convoluted Tubule and Collecting Ducts

• Hormonal control (aldosterone, ADH, parathyroid hormone) is critical.
• Fine adjustment of tubular filtrate occurs here according to the body needs.
• The diluting segment has characteristics similar to the ascending limb of Henle.

Late Distal Tubule and Collecting Tubules

• Principal cells absorb Na+ and H2O and secrete K+, interstitial/ intercalated cells absorb K+ and secrete H+.
• Tubules are impermeable to urea, water permeability under ADH, with K+ secretion and Na+ reabsorption being controlled by aldosterone.

Medullary Collecting Ducts

• Reabsorbs <10% of sodium and water, and is the final site for urine processing.
• Highly permeable to urea, secretes H+ for acid-base balance.

Na+ Absorption

• Na and Cl reabsorption is vital for electrolyte and water balance.
• Coupling of Na+ transport with other molecules, such as glucose, amino acids.
• Multiple mechanisms play roles in different parts of the nephron.

Glucose Handling

• Glucose absorption relies on a sodium gradient and Na-glucose cotransport (SGLT).
• Most glucose reabsorption occurs in the proximal tubule.

Water Reabsorption

• Proximal tubule reabsorbs a significant portion of water (65%).
• Loop of Henle's descending limb is permeable to water, and the ascending limb is largely impermeable.
• Distal tubules and collecting tubules are regulated by ADH (antidiuretic hormone).

Secondary Active Secretion

• This transport uses a sodium gradient to move substances against their concentration gradient.
• Hydrogen ions are a prevalent example of this transport mechanism across membranes and through tubules.

K+ Handling

• Potassium balance is critical for cell function.
• K+ is reabsorbed in the proximal tubule and secreted in the distal tubule.
• Secretion in the distal tubule is vital for maintaining potassium balance.