Homeostasis: Proximal Convoluted Tubule Reabsorption

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Questions and Answers

What two types of cells are found in the Islets of Langerhans, and which hormone does each secrete?

Alpha cells secrete glucagon, and beta cells secrete insulin.

Besides the liver, what other process is stimulated by insulin in cells of the body to reduce blood glucose levels?

Insulin stimulates the uptake of glucose by cells.

What is the immediate effect of glucagon binding to its receptor on liver cells? Name the enzyme.

It activates G protein which stimulates the enzyme adenylyl cyclase.

What is the final product of the enzyme cascade initiated by glucagon?

The breakdown of glycogen into glucose. Signup and view all the answers

What type of membrane protein allows glucose to enter cells by facilitated diffusion, and how is this protein moved to the surface of the cell?

GLUT proteins are used and insulin signals vesicles containing GLUT to fuse with the cell membrane. Signup and view all the answers

What is the role of the liver with respect to importing and exporting glucose?

The liver imports glucose when blood glucose levels are high, and it exports glucose when they are low. Signup and view all the answers

What molecule is produced by adenylyl cyclase in response to glucagon binding to its receptor?

cAMP. Signup and view all the answers

What three processes are directly stimulated when the hormone glucagon is released?

Gluconeogenesis, glycogenolysis, and an increase in the rate of respiration. Signup and view all the answers

What is the primary function of the countercurrent multiplier in the nephron?

To create a high salt concentration in the medulla. Signup and view all the answers

What is the permeability of the ascending limb of the loop of Henle to water and salt?

Impermeable to water, permeable to salts. Signup and view all the answers

What is the function of the vasa recta in the kidney?

To supply the cells in the loop of Henle with oxygen and removes carbon dioxide, whilst also absorbing water from the medulla. Signup and view all the answers

How does blood pH regulation occur in the distal convoluted tubule (DCT)?

By the secretion of H ions and reabsorption of $HCO_3$ ions. Signup and view all the answers

Besides hydrogen ions, what other ion concentration is controlled in the distal convoluted tubule?

Potassium ($K^+$) and Sodium ($Na^+$) ions. Signup and view all the answers

What role does the hypothalamus play in osmoregulation?

The hypothalamus is responsible for monitoring blood composition. Signup and view all the answers

How does Antidiuretic hormone (ADH) affect the collecting duct?

ADH increases the permeability of the collecting duct to water. Signup and view all the answers

Where is ADH produced and stored before being released into the blood stream?

Produced in the hypothalamus and stored in the posterior pituitary gland. Signup and view all the answers

What is the primary function of the mitochondria in the cells of the proximal convoluted tubule?

To produce energy (ATP) for active transport. Signup and view all the answers

How do microvilli on the cell surface membrane of the proximal convoluted tubule aid in reabsorption?

They increase the surface area for reabsorption. Signup and view all the answers

Explain the process of co-transport in the proximal convoluted tubule using hydrogen ions.

Carrier proteins use the diffusion of hydrogen ions down their gradient into the cell to drive the uptake of molecules such as glucose. Signup and view all the answers

What is the effect of selective reabsorption in the proximal convoluted tubule on the blood's water potential?

It lowers the blood's water potential. Signup and view all the answers

Describe one method by which sodium ions are moved out of the cell at the basal membrane of the proximal convoluted tubule cells.

By using Na-K pumps which require ATP to actively transport them from the cell. Signup and view all the answers

What is the role of the blood plasma in the reabsorption process?

It rapidly removes the absorbed Na, Cl, glucose and amino acids. Signup and view all the answers

What is the vasa recta?

The vasa recta is a parallel blood supply that surrounds the Loop of Henle. Signup and view all the answers

How does the loop of Henle help in water conservation in the kidneys?

It creates and maintains an osmotic gradient in the medulla that allows the collecting ducts to reabsorb water when necessary. Signup and view all the answers

Study Notes

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.

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Description

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.