Kidney Function: Hydration and Urea Concentration

• Hydration levels influence urea concentration and excretion due to the kidney’s role in water balance and urea filtration.

Low Hydration (Dehydration)

• Water is conserved to prevent further loss.
• Kidneys reabsorb more water in the collecting ducts via antidiuretic hormone (ADH).
• Less water available to dilute urea results in higher blood urea concentration (BUN), less urine output (dark yellow urine), and more concentrated urea in urine.

High Hydration (Overhydration)

• The body has excess water, reducing need for conservation.
• Kidneys excrete more water, and ADH secretion decreases.
• Results in lower blood urea concentration (BUN decreases), increased urine output (light-colored or clear urine), and diluted urea in urine.

Ultrafiltration in the Kidney

• Ultrafiltration is the first step in urine formation and occurs in the glomerulus of the nephron.
• Blood plasma is filtered under high pressure, allowing small molecules into the Bowman's capsule while retaining larger molecules in the bloodstream.

Steps of Ultrafiltration

• Blood enters the glomerulus through the afferent arteriole, which has a wider diameter than the efferent arteriole
• The diameter difference between the afferent and efferent arterioles creates high hydrostatic pressure in the glomerulus, forcing plasma components out.
• The filtration barrier consists of endothelium of the glomerular capillaries that contain pores to permit fluid and small solutes to pass through while preventing blood cells from escaping.
• The basement membrane is a negatively charged layer acting as a sieve, preventing large plasma proteins from passing.
• Podocytes have filtration slits regulating what enters the filtrate.
• The glomerular filtrate contains water, glucose, amino acids, ions (Na⁺, K⁺, Cl⁻), and nitrogenous wastes (urea, creatinine).
• Large molecules like proteins and red blood cells remain in the blood.
• The filtrate enters the proximal convoluted tubule (PCT), where essential substances are reabsorbed back into the bloodstream.

Significance of Ultrafiltration

• It removes metabolic wastes while retaining essential molecules.
• It maintains blood pressure and fluid balance.
• It regulates the composition of body fluids, contributing to osmoregulation and homeostasis.
• It initiates urine formation and allows the kidney to effectively filter and regulate the blood.

Role of the Proximal Convoluted Tubule (PCT) in Reabsorption

• The PCT reabsorbs essential substances from the glomerular filtrate back into the bloodstream.
• It ensures nutrients and ions are retained and waste products continue through the nephron for excretion.

Major Functions of the PCT in Reabsorption

Reabsorption of Nutrients

• Nearly 100% of glucose and amino acids are reabsorbed in the PCT via active transport using sodium-dependent cotransporters.
• This prevents valuable nutrients from being lost in urine.

Reabsorption of Ions and Salts

• Sodium (Na⁺) ions are actively transported from the filtrate into the blood, then water follows by osmosis, aiding fluid balance.
• Potassium (K⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻) ions are reabsorbed to regulate electrolyte balance and blood pH.
• Bicarbonate maintains blood pH by neutralizing excess hydrogen ions.

Water Reabsorption

• 65% of filtered water is reabsorbed in the PCT via osmosis, driven by the movement of sodium and other solutes.

Reabsorption of Proteins and Peptides

• Small proteins entering the filtrate are endocytosed by PCT cells, broken down into amino acids, and returned to the blood.

Secretion of Waste Products and Toxins

• The PCT secretes hydrogen ions (H⁺), ammonia (NH₃), creatinine, and certain drugs (e.g., penicillin) into the filtrate for elimination.

Importance of the PCT in Homeostasis:

• Prevents nutrient loss and ensures waste removal.
• Maintains electrolyte balance for nerve and muscle function.
• Regulates blood pH by reabsorbing bicarbonate and secreting hydrogen ions.
• Contributes to osmoregulation by adjusting water and salt levels.
• It selectively reabsorbs valuable substances and maintains internal stability.

Role of the Loop of Henle and Collecting Ducts in Water Reabsorption

• Water reabsorption in the nephron is crucial for maintaining fluid balance and dehydration prevention.
• The Loop of Henle and the collecting ducts use countercurrent multiplication and hormonal regulation.

Loop of Henle–Establishing an Osmotic Gradient

• The Loop of Henle is a U-shaped structure.
• The descending limb is permeable to water but impermeable to salts.
• The ascending limb is impermeable to water but actively pumps out salts.

How the Loop of Henle Contributes to Water Reabsorption

• Water is passively reabsorbed down the descending limb by osmosis due to the increasing salt concentration in the medulla.
• The deeper medullary region has high osmolarity created by the ascending limb.
• Filtrate becomes more concentrated down the descending limb.
• Sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) are actively pumped out into the medulla.
• Osmotic gradient increases, making the interstitial fluid hypertonic.
• The filtrate becomes more dilute up the ascending limb as water cannot follow the salt.
• Countercurrent multiplication ensures high solute concentration in the medulla for water reabsorption.

Collecting Duct–Final Water Reabsorption

• The collecting duct fine-tunes water reabsorption based on the body's hydration by passing through the hypertonic medulla.

Role of Hormones in Water Reabsorption

• Antidiuretic Hormone (ADH) / Vasopressin is released from the pituitary gland when the body is dehydrated.
• ADH increases the collecting duct's permeability by inserting aquaporins, allowing more water to be reabsorbed into the bloodstream.
• Concentrated urine (dark yellow) is produced.
• When ADH Levels Are Low, the collecting duct remains impermeable to water, leading to dilute urine (light-colored) in response to excess water.

Importance of the Loop of Henle and Collecting Ducts

• The Loop of Henle creates the concentration gradient for water reabsorption.
• The Collecting Duct adjusts water reabsorption based on hydration.
• Together, they ensure water conservation or excretion.
• This system maintains homeostasis, preventing dehydration and regulating blood osmolarity.

Role of ADH in Water Reabsorption

• ADH controls the collecting ducts' permeability to water.
• ADH is released by the posterior pituitary gland in response to signals from the hypothalamus.

When the Body is Dehydrated (Water Deficiency, High Blood Osmolarity)

• Osmoreceptors in the hypothalamus detect an increase in blood osmolarity (higher solute concentration due to low water levels).
• The posterior pituitary gland releases ADH into the bloodstream.
• ADH binds to receptors on the collecting duct cells, stimulating the insertion of aquaporins.
• Water permeability increases, allowing more water to be reabsorbed into the bloodstream by osmosis.
• Urine becomes more concentrated (dark yellow) to conserve water.

When the Body is Overhydrated (Excess Water, Low Blood Osmolarity)

• Osmoreceptors detect a decrease in blood osmolarity (lower solute concentration due to excess water).
• The posterior pituitary gland reduces or stops ADH secretion.
• Collecting ducts remain impermeable to water without ADH.
• Water remains in the filtrate, leading to the excretion of large volumes of dilute urine (light yellow or clear).
• This removes excess water and restores osmotic balance.

Interaction with the Loop of Henle and Medullary Gradient

• The Loop of Henle establishes a hypertonic medullary environment, which provides the osmotic gradient for water reabsorption in the collecting ducts.
• A longer Loop of Henle increases the ability to concentrate urine.

Importance of Osmoregulation in Homeostasis

• Prevents dehydration by conserving water when needed.
• It eliminates excess water when hydration levels are too high.
• It maintains blood pressure and volume by regulating fluid balance.
• Ensures stable osmolarity, crucial for proper cellular function.
• The body effectively regulates water levels, preventing dehydration and overhydration, ensuring overall homeostasis through modification of water reabsorption in the collecting ducts.

Explore the impact of hydration levels on urea concentration and excretion. Learn how the kidneys regulate water balance and urea filtration. Understand ultrafiltration in the kidney and its role in urine formation.