Kidney Structure and Filtration Process

This section delves into the intricate structure of the kidney and the filtration process, crucial for understanding osmoregulation in A Level Biology.

The kidney's filtration system is designed for efficiency:

1. Blood enters the glomerulus through the afferent arteriole, which has a wider diameter than the efferent arteriole, creating high pressure.

2. Three layers separate blood in the glomerulus from the Bowman's capsule:

   • Capillary wall with fenestrae (pores)
   • Basement membrane acting as a molecular sieve
   • Bowman's capsule wall made of podocytes with filtration slits

Definition: Ultrafiltration is the process of filtration under high pressure, which occurs in the glomerulus.

The glomerular filtrate contains:

• Water
• Glucose
• Salts
• Urea
• Amino acids

Highlight: The glomerular filtrate resembles plasma but lacks large proteins, blood cells, and platelets.

The glomerular filtration rate (GFR) is determined by the difference in water potential between the glomerular capillaries and the Bowman's capsule.

Vocabulary: Selective reabsorption is the uptake of specific molecules and ions from the glomerular filtrate back into the bloodstream.

The proximal convoluted tubule (PCT) is crucial for selective reabsorption:

• It's the longest and widest part of the nephron
• Has adaptations like microvilli and basal channels for increased surface area
• Contains numerous mitochondria for ATP production

Example: In the PCT, 70% of salts are reabsorbed into the blood, with most requiring active transport via membrane pumps.

Homeostatic Mechanisms and Kidney Disease

This section explores the intricate homeostatic mechanisms and kidney diseases, essential knowledge for Eduqas A Level Biology students.

Homeostatic mechanisms in the kidney are complex and interconnected:

1. Glomerular filtration rate (GFR) regulation:

   • Myogenic mechanism: Smooth muscle in afferent arteriole responds to blood pressure changes
   • Tubuloglomerular feedback: Macula densa cells detect NaCl concentration in distal tubule

2. Renin-Angiotensin-Aldosterone System (RAAS):

   • Responds to low blood pressure or low blood volume
   • Renin release triggers a cascade leading to increased blood pressure and water retention

Example: When blood pressure drops, the RAAS activates, causing vasoconstriction and increased sodium and water reabsorption in the kidney.

Kidney diseases can severely impact homeostasis:

1. Chronic Kidney Disease (CKD):

   • Progressive loss of kidney function
   • Can lead to fluid and electrolyte imbalances, anemia, and bone disorders

2. Glomerulonephritis:

   • Inflammation of the glomeruli
   • Can impair filtration and lead to proteinuria

Highlight: Understanding kidney diseases is crucial for appreciating the importance of renal function in maintaining overall homeostasis.

The kidney's role in acid-base balance:

1. Bicarbonate reabsorption in the PCT
2. Hydrogen ion secretion in the collecting duct
3. Ammonia production to buffer excess hydrogen ions

Vocabulary: Acidosis is a condition where blood pH falls below the normal range, which the kidneys help to correct through increased acid excretion.

Dialysis and kidney transplantation:

• Hemodialysis: Blood is filtered externally to remove waste products
• Peritoneal dialysis: Uses the peritoneum as a natural filter
• Kidney transplantation: Provides a long-term solution for end-stage renal disease

Definition: Dialysis is a medical procedure that artificially performs the function of the kidneys in filtering blood and removing waste products.

Homeostasis Fundamentals and Kidney Overview

This section introduces the core concepts of homeostasis and kidney function, essential for understanding Eduqas A Level Biology.

Definition: Homeostasis is the maintenance of a constant internal environment, crucial for optimal cellular function.

The importance of homeostasis lies in its ability to:

1. Keep body fluid concentrations constant
2. Protect cells from external environmental changes
3. Ensure reactions occur at appropriate rates
4. Allow normal cell function

Negative feedback, a key mechanism in homeostasis, involves a system change that triggers a reversal of that change. The process includes:

1. Set point establishment
2. Receptor detection of deviations
3. Coordinator communication
4. Effector response
5. Return to normal levels

Example: Glucose concentration regulation in plasma demonstrates negative feedback. When levels rise, insulin secretion increases, promoting glucose conversion to glycogen and increasing respiration rate.

The kidney serves two primary functions:

1. Excretion - removing nitrogenous metabolic waste
2. Osmoregulation - controlling body fluid water potential

Vocabulary: Deamination is the removal of an amine group from a molecule, a crucial step in urea production.

Kidney structure includes:

• Renal capsule covering
• Blood supply via renal arteries and veins
• Filtration occurring in the outer cortex
• Medulla containing loops of Henle and collecting ducts

Highlight: The kidney's complex structure is optimized for its dual roles in excretion and osmoregulation, making it a critical organ for maintaining homeostasis.