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.

This section focuses on the processes of selective reabsorption and osmoregulation, key topics in Eduqas A Level Biology.

Selective reabsorption in the proximal convoluted tubule $PCT$ is a highly efficient process:

1. 70% of salts are reabsorbed into the blood Some through passive diffusion Most using active transport via membrane pumps
2. All glucose and amino acids are reabsorbed This occurs through secondary active transport Sodium ions are actively pumped out of the PCT cells Creates a concentration gradient for glucose and amino acids to follow

Highlight: The PCT's ability to reabsorb all glucose and amino acids demonstrates the kidney's crucial role in conserving essential nutrients.

The loop of Henle plays a vital role in osmoregulation:

1. Descending limb: Permeable to water but not to salts Water leaves by osmosis, concentrating the filtrate
2. Ascending limb: Impermeable to water Actively pumps out sodium and chloride ions

Vocabulary: Countercurrent multiplication refers to the process where the loop of Henle creates a concentration gradient in the medulla.

The distal convoluted tubule $DCT$ and collecting duct are key sites for hormone-controlled osmoregulation:

1. Antidiuretic hormone $ADH$: Increases water permeability of the DCT and collecting duct Promotes water reabsorption in low water conditions
2. Aldosterone: Increases sodium reabsorption in the DCT Indirectly increases water reabsorption

Example: In dehydration, ADH levels increase, leading to more concentrated urine as more water is reabsorbed in the DCT and collecting duct.

The role of the kidney in osmoregulation is multifaceted:

• Filtration of blood in the glomerulus
• Selective reabsorption in the PCT
• Concentration of filtrate in the loop of Henle
• Hormone-controlled water and salt balance in the DCT and collecting duct

Definition: Osmoregulation is the control of water and solute concentrations in the body's fluids, a critical function of the kidney in maintaining homeostasis.

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.

This final section covers advanced topics in renal physiology, crucial for Eduqas A Level Biology Component 3 and higher-level understanding.

Renal blood flow regulation:

1. Autoregulation: Maintains constant GFR despite fluctuations in systemic blood pressure Involves myogenic mechanism and tubuloglomerular feedback
2. Neural regulation: Sympathetic nervous system can reduce renal blood flow in stress situations

Example: During exercise, sympathetic stimulation reduces renal blood flow, redirecting blood to skeletal muscles.

Hormonal influences on kidney function:

1. Antidiuretic Hormone $ADH$: Increases water reabsorption in collecting ducts Release controlled by hypothalamus in response to blood osmolarity
2. Atrial Natriuretic Peptide $ANP$: Released by heart atria in response to increased blood volume Increases sodium and water excretion

Vocabulary: Natriuresis refers to the increased excretion of sodium in urine, a key effect of ANP.

Renal handling of different substances:

1. Glucose: Completely reabsorbed under normal conditions Appears in urine only when blood glucose exceeds renal threshold
2. Protein: Large proteins not filtered; small amounts reabsorbed in PCT Presence in urine $proteinuria$ indicates kidney damage
3. Urea: Freely filtered, partially reabsorbed Concentration in urine varies with hydration status

Highlight: The kidney's ability to selectively handle different substances demonstrates its crucial role in maintaining body homeostasis.

Countercurrent systems in the kidney:

1. Countercurrent multiplier: Loop of Henle creates and maintains medullary osmotic gradient
2. Countercurrent exchanger: Vasa recta preserves medullary concentration gradient

Definition: Countercurrent systems in the kidney allow for the concentration of urine and efficient water conservation, crucial for osmoregulation in A Level Biology.

Understanding these advanced concepts provides a comprehensive view of renal physiology, essential for mastering Eduqas A Level Biology and preparing for higher education in biological sciences.

This section covers different types of dialysis treatments for kidney failure.

Definition: Haemodialysis is the process of filtering blood through selective permeable membranes.

Example: Continuous ambulatory peritoneal dialysis $CAPD$ uses the peritoneum as a natural filter.

Highlight: Counter-current flow enhances the efficiency of dialysis.

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.