Renal Physiology: Ultrafiltration, Loop of Henle, and Osmoregulation

Where does ultrafiltration occur?

In Bowman's capsule (Renal Corpuscle).

What creates the high hydrostatic pressure in the glomerulus?

The afferent arteriole is wider than the efferent arteriole.

What is forced out of the glomerular capillaries during ultrafiltration?

Fluid is forced through the fenestrations in the capillary walls and the basement membrane.

What does the filtrate contain?

Small molecules like water, glucose, amino acids, ions (urea, Na^+), and urea.

What is excluded from the filtrate during ultrafiltration?

Blood cells and large proteins (e.g., albumin) are too large to pass through the basement membrane.

Where does selective reabsorption take place?

In the Proximal Convoluted Tubule (PCT).

What adaptations in the PCT enhance reabsorption efficiency?

Microvilli for large surface area, mitochondria for ATP production, and an extensive capillary network.

What is reabsorbed 100% into the blood in the PCT?

Glucose and amino acids via co-transporter proteins (secondary active transport with Na^+).

What is the function of the Loop of Henle?

To create a very low water potential (high salt concentration) in the medulla for water conservation.

What is the permeability of the ascending limb of the Loop of Henle?

Impermeable to water; sodium and chloride ions are actively transported out.

What is the permeability of the descending limb of the Loop of Henle?

Permeable to water but impermeable to ions; water leaves by osmosis.

What is the counter-current multiplier?

The opposite flow in the two limbs maintains a concentration gradient, increasing the efficiency of water removal.

What is osmoregulation?

The maintenance of a constant internal environment, such as blood water potential.

How do osmoreceptors in the hypothalamus respond to dehydration?

They detect a decrease in blood water potential.

What hormone is released by the posterior pituitary gland in response to dehydration?

Antidiuretic Hormone (ADH).

What is the target of ADH in the kidneys?

Cells of the collecting duct.

What is the mechanism of action of ADH?

It causes vesicles containing aquaporins to fuse with the cell membrane, increasing permeability.

What is the result of ADH action on the collecting duct?

More water is reabsorbed by osmosis, resulting in a small volume of concentrated (hypertonic) urine.

What is negative feedback in the context of osmoregulation?

When water potential returns to normal, ADH release is inhibited.

Why is constant water potential important for cells?

It prevents cells from shrinking (crenation) or bursting (lysis) due to osmosis.

What is the first messenger in the general mechanism of hormone action?

The hormone (e.g., ADH) that binds to a specific receptor on the cell surface membrane.

What is the role of adenyl cyclase in the hormone signaling pathway?

Adenyl cyclase converts ATP into cyclic AMP (cAMP) after the hormone-receptor complex is activated.

What is the function of cyclic AMP (cAMP) in cellular signaling?

cAMP acts as the second messenger, diffusing through the cytoplasm to activate other enzymes.

What is the outcome of enzyme activation by cAMP?

These enzymes trigger specific cellular changes, such as glycogen breakdown or aquaporin insertion.

How does ADH affect the collecting duct cells in the kidneys?

ADH binds to receptors on the basal membrane of the collecting duct cells, activating adenyl cyclase.

What happens to vesicles containing aquaporins when cAMP is produced?

cAMP triggers vesicles to move toward the luminal membrane and fuse with it, inserting aquaporins.

What is the result of aquaporin insertion in the collecting duct?

The permeability to water increases, allowing water to move out of the filtrate and into the blood by osmosis.

How does adrenaline prepare the body for exercise?

Adrenaline binds to receptors on liver cells, leading to the production of cAMP and activation of enzymes that break down glycogen into glucose.

What is the benefit of glycogen breakdown during exercise?

It increases blood glucose concentration, providing energy for aerobic respiration in muscles.