Urine production

Renal Physiology II: Urine Production

• Urine production involves solute and water transport along the nephron.
• The nephron is the basic functional unit of the kidney.
• There are approximately one million nephrons in each kidney.
• Nephrons are highly organized structures involved in filtering blood.
• The kidney has two main types of nephrons: cortical and juxtamedullary.
• Cortical nephrons have shorter loops of Henle.
• Juxtamedullary nephrons have longer loops of Henle.

Learning Outcomes

• Students should understand the role of nephron segments.
• The process of reabsorption and secretion across renal epithelial membranes should be described.
• Students are expected to explain how and why the loop of Henle generates a hyperosmotic medulla.

Nephron Overview

• The nephron is the fundamental functional unit of the kidney.
• It filters blood, controlling water and solute reabsorption.
• The nephron includes a large number of specialized structures.
• It processes about 180 liters of filtrate per day.

Nephron: Cell Types

• Epithelial cells form the tubule.
• Podocytes are glomerular epithelial cells involved in filtration.
• Brush border cells are in the proximal tubule and are responsible for absorption.
• Cells in the renal tubules express different channels and transporters that carry out reabsorption and secretion.
• Different regions of the nephron have unique structural and functional characteristics.

Nephron: Three Basic Processes

• The nephron segment by segment processes filtered substances through filtration, reabsorption and secretion.
• Filtration, the initial step, removes substances from the blood.
• Reabsorption recovers essential materials from the filtrate.
• Secretion actively moves substances from the blood to the filtrate.
• These processes regulate fluid balance, pH, and blood composition

Glomerular Filtration

• The glomerular filtration membrane filters blood without cells or proteins.
• It contains water, glucose, amino acids, and ions.
• Glomerular filtration rate (GFR) is usually constant despite blood pressure changes.
• The glomerular filtration membrane is intrinsic and regulated by several factors such as myogenic activity and the tubuloglomerular feedback mechanism (JGA), hormonal and nervous control.

Tubule Organisation

• Filtration, reabsorption, and secretion are major functions of the nephron.
• Filtration involves the movement of fluids and solutes into the nephron from the blood.
• Reabsorption involves materials moving from the nephron back into the blood.
• Secretion is the movement of substances from the blood into the nephron.

Proximal Reabsorption

• The proximal tubule is highly efficient at reabsorbing substances.
• It reabsorbs about 65% of filtered water and a varying amount of solutes that are important.
• Sodium reabsorption drives the reabsorption of glucose and amino acids.
• Reabsorption in the Proximal tubules, surface area is increased due to abundant microvilli.
• Cells contain abundant mitochondria, the main energy source for the active transport processes.

Loop of Henle

• The loop of Henle is a critical segment in the nephron for water and sodium reabsorption.
• This creates an osmotic gradient and concentration of urine.
• The loop has a descending limb (permeable to water) and an ascending limb (impermeable to water).
• The descending limb concentrates solutes because water moves out into the surrounding tissues.
• The ascending limb actively transports salt ions out of the tubule and concentrates solutes in the medulla

Countercurrent Multiplier

• The loop of Henle creates a hypertonic environment by actively transporting salts.
• This mechanism, called the countercurrent multiplier, maintains a high concentration of solutes in the renal medulla.
• The flow of filtrate in the descending and ascending limbs is in opposite directions, maintaining the concentration gradient.
• Water reabsorption is stimulated by the ADH hormone.

Counter Current Multiplier: The Vasa Recta

• Blood flows in the opposite direction to the filtrate in the loop of Henle.
• The vasa recta capillaries help to maintain the established concentration gradient by reabsorbing water.
• The kidney capillaries do not remove the urea and sodium needed for the counter current system.

Urea Recycling

• Urea recycling is the process of urea reabsorption.
• It's a major contributor to osmolarity in the renal medulla, a critical process for concentrating urine.
• About 40–50% of the filtered urea is reabsorbed in the collecting tubule.

Distal Convoluted Tubule and Collecting Duct

• Final adjustments of urine composition, including water and salt reabsorption, occur in the distal tubule and collecting duct.
• Fine tuning urine composition is controlled by different hormones.

Urine Composition

• Urine contains water, ions, and metabolic waste products.
• The composition of the urine varies depending on the body fluid balance.