Class notes Human anatomy and physiology

Week 7 Part 1 Study Guide

4 Components of Renal Process of Plasma
1. Filtration:
- Process:
- Plasma flows from afferent arteriole into glomerular capillaries, where hydrostatic
pressure forces plasma and dissolved solutes through filtration membrane.
- Filtered substances: electrolytes, glucose, amino acids, hormones, and drugs
- Not filtered: large proteins and blood cells
- Resulting fluid in Bowman’s capsule called glomerular filtrate
- Not all plasma is filtered – continues into efferent arteriole and peritubular capillaries.
2. Reabsorption
- Location: along renal tubule
- Process:
- Molecules in filtrate are reabsorbed back into bloodstream through specific transporters
in tubular cells.
- Examples:
- Glucose: 100% reabsorbed under normal conditions; if blood glucose is high, transporters
become saturated -> glucose appears in urine.
- Electrolytes: sodium, calcium bicarbonate, potassium
- Hormonal Control:
- Aldosterone increases sodium reabsorption when BP or plasma sodium is low
- Non-reabsorbed:
- Creatinine is never reabsorbed – completely excreted
- Elevated blood creatinine = poor kidney filtration
3. Secretion:
- Location: mostly in distal tubule and collecting duct
- Process:
- Substances move from peritubular capillaries to tubular cells to tubular lumen.
- Allows nephron to eliminate additional unwanted molecules not filtered initially.
- Examples:
- Hydrogen ions to acid-base balance
- Ammonia
- Certain drugs
- Mechanism:
- Uses specific transporters and can be hormone-regulated
- Similar transport processes as reabsorption, but reverse direction
4. Excretion:
- Location: collecting ducts to renal pelvis to ureters to bladder to urethra
- Process:

, - Final urine consists of all substances that were filtered and not reabsorbed, plus all
secreted substances.
- Represents end result of filtration, reabsorption, and secretion
Role of Plasma Osmolarity in Water Balance
1. Osmoreceptors and Plasma Osmolarity:
- Location: anterior hypothalamus
- Function: continuously monitor osmolarity of plasma
- High osmolarity is too many solutes, not enough water = dehydration
- Low osmolarity is too much water, diluted solutes = overhydration
2. Response to High Plasma Osmolarity:
a. ADH Secretion
- Source: posterior pituitary gland
- Called: Vasopressin or arginine Vasopressin (AVP)
- Function: prevents water loss
- Increasing permeability of distal tubules and collecting ducts in nephron
- Allowing more water reabsorption from filtrate back in bloodstream
- Result: less urine output; conservation of body water to osmolarity returns toward
normal.
b. Stimulation of Thirst
- The hypothalamus triggers thirst response, encouraging fluid intake
- ADH release and increased thirst restore plasma osmolarity and volume
3. Response to Low Plasma Osmolarity:
- ADH release is inhibited
- Distal tubules and collecting ducts become less permeable to water
- Less water is reabsorbed where more water stays in filtrate
- Result: high-volume, dilute urine
- Thirst is suppressed, reducing additional fluid intake.
4. Additional Insight:
- Alcohol inhibits ADH release
- Less water reabsorption and increased urine output, which why people frequently urinate
after drinking alcohol.
Renin Angiotensin System (RAS)
1. Site of Renin Synthesis and Storage:
- Location: Juxtaglomerular (JG) cells
- Found in walls of afferent arterioles of nephron
- Role: JG cells act as both baroreceptors and endocrine cells
2. Stimulus for Renin Release:
- Primary Trigger: decreased arterial pressure in afferent arteriole.
- Occurs when systemic BP falls or during renal ischemia
- Reduced pressure signals JG cells to release renin into bloodstream
3. Function of Renin:
- Renin = enzyme
- Main function: converts angiotensinogen to angiotensin 1



2