Overview
This lecture covers the composition of kidney filtrate, the processes of filtration and reabsorption, and how substances like water, ions, and glucose are selectively reclaimed or lost in urine, including factors affecting these processes.
Filtrate Composition and Formation
- Filtrate resembles tissue fluid and lacks proteins and blood cells because they can't pass through the filtration membrane.
- Kidneys initially produce about 180 liters of filtrate daily, far more than the ~2 liters of urine excreted.
- Most filtrate is reabsorbed to avoid fatal dehydration.
Filtration vs. Reabsorption
- Filtration: Non-selective removal of materials from blood to filtrate at Bowman's capsule.
- Reabsorption: Selective process where useful substances (water, nutrients, ions) move from filtrate back into blood.
- Peritubular capillaries surround the nephron and are adapted for reabsorption with low blood pressure and high colloid osmotic pressure.
Mechanisms of Reabsorption
- Most reabsorption occurs in the proximal convoluted tubule, allowing efficient recovery of useful substances.
- Passive transport (diffusion): Substances move from high to low concentration without energy.
- Active transport: Requires ATP to move substances like glucose, amino acids, and some ions against their concentration gradient.
Renal Plasma Threshold and Transport Limitation
- Reabsorption of some substances (like glucose) is limited by the number of transport proteins ("doors").
- Renal plasma threshold: Maximum level at which a substance can be reabsorbed; excess appears in urine.
- In diabetes, lack of insulin leads to glucose in urine because not enough is reabsorbed.
Sodium, Water, and Osmosis
- Sodium reabsorption is active and causes water to follow by osmosis, concentrating the blood and reducing urine volume.
- More sodium reabsorbed means more water is reclaimed from filtrate.
- Sodium reabsorption in the nephron is crucial for maintaining blood volume and preventing excessive urine loss.
Effects of Caffeine
- High caffeine intake inhibits sodium reabsorption, so less water is reclaimed, leading to increased urine output (osmotic diuresis).
- Small amounts of caffeine have minimal effect, but high intake can lead to dehydration.
Key Terms & Definitions
- Filtrate — Fluid filtered from blood in the kidney, similar to plasma but without proteins or blood cells.
- Reabsorption — Selective process of moving substances from filtrate back into the blood.
- Peritubular capillaries — Capillaries surrounding nephron tubules, specialized for reabsorption.
- Passive transport — Movement of substances without energy (diffusion).
- Active transport — Energy-requiring movement of substances against a concentration gradient.
- Renal plasma threshold — Maximum blood concentration of a substance that can be reabsorbed by the kidneys.
- Osmotic diuresis — Increased urination due to unreabsorbed substances (like glucose or with high caffeine) retaining water in filtrate.
Action Items / Next Steps
- Review nephron diagrams and kidney structure to visualize filtration and reabsorption.
- Study mechanisms of active vs. passive transport in renal physiology.
- Be familiar with the effects of caffeine and conditions like diabetes on kidney function.