Homeostasis: Proximal Convoluted Tubule Reabsorption

Homeostasis: Selective Reabsorption in the Proximal Convoluted Tubule

• The proximal convoluted tubule is the longest nephron section, reabsorbing a significant portion of the filtrate.
• Tubule walls are a single cell thick, rich in mitochondria for energy-intensive active transport.
• A brush border membrane with microvilli increases surface area for reabsorption.

Reabsorption Mechanisms

• Active Transport: Sugars and amino acids are actively transported across the cell membrane by carrier proteins. Hydrogen ions diffuse into the cell, driving the uptake of sugars against their concentration gradient.
• Water Reabsorption: Large amounts of water are reabsorbed because the reabsorption of ions, glucose, and amino acids lowers the water potential of the blood. Water then diffuses into the blood via osmosis.
• Other Metabolites: Other essential metabolites and mineral ions are absorbed via active transport and facilitated diffusion. Urea and proteins are also moved.

Na+-K+ Pumps

• Na+-K+ pumps in the basal membrane utilize ATP to maintain low sodium levels in the cytoplasm.
• Folded basal membranes increase surface area for carrier proteins.
• Blood quickly removes absorbed sodium, glucose, and amino acids, promoting further uptake from the tubule lumen.

Loop of Henle

• The loop of Henle's structure comprises ascending and descending limbs with a parallel blood supply (vasa recta).
• The loop creates and maintains an osmotic gradient.
• This gradient allows water to move out of the collecting ducts if necessary, which is crucial for water regulating.
• Countercurrent multiplier creates the osmotic gradient. Fluids flow in opposite directions in two systems, facilitating exchange and gradient establishment.
• Ascending limb is impermeable to water but permeable to salts, increasing salt concentration within the loop.
• Descending limb is permeable to water but only slightly permeable to salts, allowing water to leave the filtrate.
• Vasa recta helps regulate the osmotic gradient by absorbing absorbed water/salts and carrying them out of the medulla.

Distal Convoluted Tubule (DCT)

• Blood proteins act as buffers, regulating pH variations.
• H+ ions are secreted, and bicarbonate ions are reabsorbed, maintaining blood pH at a balanced level (7.35-7.45).
• K+ ion concentration is regulated by secretion to adjust levels that enter the filtrate.
• Na+ ion reabsorption is adjusted to regulate concentration in filtrate.

Collecting Duct

• Osmoregulation controls blood water content.
• The hypothalamus monitors blood composition and receives data from sensory receptors.
• The hypothalamus controls the pituitary gland which produces Antidiuretic Hormone (ADH).
• ADH increases/decreases the collecting duct's permeability to water.
• ADH increases permeability to water when water levels are low, promoting water retention.
• When water levels are high, ADH release is reduced, causing less water reabsorption, and production of dilute urine.
• Aquaporins are channels formed due to ADH that regulate water permeability.

Blood Glucose Regulation

• Normal blood glucose levels in 100cm3 of blood are 80-120mg.
• Low glucose levels occur during fasting or prolonged exercise.
• Elevated glucose levels occur after carbohydrate-rich meals.
• The body stores glycogen for converting to glucose during exercise.
• Glucose is the main substrate for brain cells and this process does not occur in the absence of glycogen
• Hypoglycemia: Low blood glucose, potentially life-threatening.
• Hyperglycemia: High blood glucose, leading to health complications.
• Islets of Langerhans in the pancreas sense blood glucose levels. Beta cells release insulin in response to elevated glucose.
• Insulin promotes glucose uptake by cells and conversion to glycogen.
• Alpha cells release glucagon in response to low glucose levels; promoting glycogen breakdown in the liver and production of glucose.

Other

• Endocrine glands secrete hormones directly into the bloodstream, rather than through ducts.
• Glucose is reabsorbed in the proximal convoluted tubule, absent in healthy urine.
• Dipsticks use glucose oxidase and peroxidase to measure glucose concentrations in urine.
• Glucose biosensors use immobilized glucose oxidase to detect glucose in blood samples.

This quiz explores the mechanisms of selective reabsorption occurring in the proximal convoluted tubule. Understand how active transport, water reabsorption, and metabolite absorption contribute to homeostasis. Test your knowledge on the processes that maintain fluid balance in the body.