Overview
This lecture covers the process of glomerular filtration in the kidneys, emphasizing how filtration works, the factors influencing it, and the balance between urine production and blood retention.
Glomerular Filtration Basics
- Glomerular filtration is the first step in urine formation, occurring in the Bowman's capsule.
- The filtration barrier consists of two layers: the capillary wall and podocyte cell layer.
- The main purpose is to prevent large molecules (like blood cells and proteins) from leaving the bloodstream.
- Filtration is non-selective, allowing small substances (waste, sugar, water) to pass through.
Filtration Pressure and Blood Flow
- Filtration is driven by hydrostatic (blood) pressure in the glomerular capillaries.
- Afferent arteriole brings blood in; efferent arteriole takes blood out of the glomerulus.
- Dilation of the afferent arteriole increases filtration and urine output.
- Constriction of the afferent arteriole decreases filtration and urine output.
- Dilation of the efferent arteriole decreases filtration, while constriction increases it.
- Systemic blood pressure affects filtration rate: higher pressure increases filtration.
Regulation and "Tug-of-War"
- The cardiovascular system wants to retain water, while the urinary system wants to remove it.
- Filtration is opposed by factors that slow water leaving the bloodstream.
Opposing Filtration Forces
- Colloid osmotic pressure (from plasma proteins like albumin) attracts water to stay in the blood.
- Capsular hydrostatic pressure increases as Bowman's capsule fills with filtrate, reducing filtration.
- Increased colloid osmotic pressure (e.g., dehydration) reduces urine output; decreased pressure (e.g., starvation) increases it.
- Blockages (e.g., kidney stones) can raise capsular pressure, further slowing filtration.
Homeostasis and Kidney Adaptation
- Kidneys constantly adjust filtration to match changing water needs due to temperature, activity, and diet.
- The balance between filtration forces prevents excessive urine loss or retention, maintaining fluid homeostasis.
Key Terms & Definitions
- Glomerular Filtration — process of filtering blood through the glomerulus into Bowman's capsule to form urine.
- Podocyte — specialized cells forming the second layer of the filtration barrier in the kidney.
- Hydrostatic Pressure — pressure exerted by fluids, driving filtration out of capillaries.
- Colloid Osmotic Pressure — osmotic pressure from plasma proteins pulling water into blood vessels.
- Bowman's Capsule — cup-shaped structure in the kidney where filtered substances collect.
- Afferent/Efferent Arteriole — blood vessels that bring blood to (afferent) and from (efferent) the glomerulus.
Action Items / Next Steps
- Review the factors affecting glomerular filtration and how arteriole diameter and protein levels impact urine output.
- Understand the interaction between hydrostatic and osmotic pressures in filtration.
- Prepare for questions on filtration dynamics and kidney homeostasis for the next class.