ch. 19 ch.20

The kidneys play a crucial role in maintaining homeostasis by regulating fluid balance and the composition of body fluids.
They control the excretion or retention of water and ions, affecting concentrations of bodily fluids.

The kidneys adjust the concentration of fluids by excreting or retaining water, which affects the body’s hydration status.
This process is essential for concentration and dilution of urine, maintaining fluid balance in the body.

The kidneys regulate levels of ions (e.g., sodium, potassium) by excreting excess amounts or reabsorbing them when needed.
Regulation of pH levels is achieved by controlling hydrogen ions in the urine.
Calcium levels are also controlled through kidney function.

The kidneys filter out metabolic wastes (e.g., urea from protein breakdown) and xenobiotics (e.g., drugs, foreign substances).

Erythropoietin: Released when blood oxygen levels are low; stimulates red blood cell production in the bone marrow (can speed up production up to 10 times).
Calcitriol: A hormone that enhances calcium absorption in the intestines, regulated by parathyroid hormone.
Renin: An enzyme that activates the renin-angiotensin system, crucial for blood pressure regulation.

Components: Kidneys, ureters, urinary bladder, and urethra.
Kidneys are the most complex part of the urinary system, receiving about 20-25% of cardiac output (1/5 to 1/4 of blood pumped by the heart).

Nephrons: Functional units consisting of:
- Renal Corpuscle: Where filtration occurs (includes glomerulus and glomerular capsule).
- Renal Tubule: Where reabsorption and secretion occur.

Cortical Nephrons: Primarily located in the renal cortex.
Juxtamedullary Nephrons: Located near the border of the medulla; their loops extend deeply into the medulla for efficient water reabsorption.

Renal Corpuscle Components:
- Glomerulus: A ball of capillaries where filtration of blood occurs.
- Glomerular (Bowman's) Capsule: Encases the glomerulus, collects filtrate.
Renal Tubule Sections:
- Proximal Convoluted Tubule: Major site for reabsorption.
- Loop of Henle (Nephron Loop): Reabsorbs water and solutes; splits into descending (permeable to water) and ascending (permeable to ions) limbs.
- Distal Convoluted Tubule: Site for selective secretion and reabsorption, influenced by hormones like aldosterone.
- Collecting Duct: Final processing of urine, influenced by levels of antidiuretic hormone (ADH).

Filtration: Movement of water and small solutes from blood to capsular space, forming filtrate (similar to plasma minus large proteins).
Pathologies: Presence of blood or high levels of proteins in urine indicates potential kidney damage or dysfunction.

Filter, Reabsorb, and Secrete Mechanism:
- Filtration: Non-specific; filters everything below a certain size.
- Reabsorption: Specific substances transported from the renal tubule back into the blood (ex. glucose, ions).
- Secretion: Specific substances actively transported from blood to renal tubule to be excreted.

Glomerular Filtration Rate (GFR): The volume of filtrate produced per minute; average is 125 mL/min or 180 L/day.
Filtration Fraction: Percentage of plasma filtered through glomeruli (approx. 20%).
Net Filtration Pressure: The pressure difference driving filtration across the glomerulus, typically around 10 mmHg.

Blood Pressure in Glomeruli: Higher pressure results in higher GFR; lower pressure decreases GFR.
Osmotic Pressure of Plasma Proteins: Resists filtration; higher osmotic pressure decreases GFR.
Capsular Pressure: Pressure of fluid already in capsule that also resists filtration.

Activated when blood pressure is low or GFR decreases.
Renin Release: Response in juxtaglomerular cells triggers conversion of angiotensinogen to angiotensin I, which is further converted to active angiotensin II.
Effects of Angiotensin II:
- Raises blood pressure by vasoconstriction and increasing blood volume through aldosterone (sodium reabsorption) and ADH (water reabsorption).

The kidneys have a critical role in water retention in response to hydration status via changes in GFR and ADH actions.
ADH: Increases permeability of collecting ducts to water, impacting urine concentration.

Renal Clearance (C): The volume of plasma cleared of a substance per unit time. Calculated using excretion rate of the substance and plasma concentration.
Importance in Medicine: Used to evaluate kidney function and substance handling (e.g., glucose vs. penicillin).

The kidneys maintain homeostasis by filtering, reabsorbing, and secreting substances, controlling blood volume, blood pressure, and electrolyte balance effectively. Understanding these systems is vital in both physiology and clinical settings.