Kidneys: Structure, Function, and Urine Formation

Kidneys

Overview

The kidneys are vital organs responsible for maintaining homeostasis by regulating the composition and volume of blood. They perform essential functions in waste removal, fluid and electrolyte balance, and blood pressure regulation.

Kidney Functions

Main Functions

• Removes wastes: Filters metabolic wastes (e.g., urea, creatinine) from the blood for excretion in urine.

• Regulates blood pH: Adjusts hydrogen ion and bicarbonate levels to maintain acid-base balance.

• Regulates blood pressure: Controls blood volume and releases hormones (e.g., renin) that influence vascular resistance.

• Aids in the formation of red blood cells: Secretes erythropoietin, stimulating erythrocyte production in bone marrow.

Urine Formation Steps

Three Main Processes

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

These processes occur in the nephron, the kidney's functional unit.

Glomerular Filtration

Process of Filtration

• Volume filtered: Approximately 48 gallons (180 liters) of fluid are filtered from the blood into the nephrons each day.

• Mechanism: Movement of fluids and dissolved substances from blood into the nephron, driven by blood pressure.

Filtration Steps

1. Blood enters the glomerulus (a capillary network within the nephron).

2. High glomerular blood pressure forces plasma (minus proteins and cells) through the capillary wall into Bowman’s capsule.

3. Red blood cells and proteins remain in the blood due to the size and charge selectivity of the filtration barrier.

Renal Corpuscle

• Glomerulus: Large network of capillaries providing a large surface area for filtration.

• Podocytes: Specialized cells that wrap around capillaries and form filtration slits.

• Arterioles:

  • Afferent arteriole: Brings blood into the glomerulus (larger diameter).

  • Efferent arteriole: Carries blood away (smaller diameter), maintaining high pressure in the glomerulus.

Filtration Pressures

• Glomerular blood pressure: +55 mm Hg (drives filtration out of glomerulus)

• Blood osmotic pressure: -30 mm Hg (pulls fluid back into capillaries)

• Capsular hydrostatic pressure: -15 mm Hg (opposes filtration)

• Net filtration pressure: +10 mm Hg

Equation:

$\text{Net Filtration Pressure} = \text{Glomerular Blood Pressure} - (\text{Blood Osmotic Pressure} + \text{Capsular Hydrostatic Pressure})$

$\text{Net Filtration Pressure} = 55 - (30 + 15) = 10\ \text{mm Hg}$

Factors Which Alter Filtration Pressure

• Kidney disease: Increased permeability of glomerulus, loss of plasma proteins into urine.

• Blood pressure drops: Hemorrhage or shock reduces filtration pressure, possibly causing anuria (no urine output).

• Stress: Sympathetic stimulation constricts afferent arteriole, reducing filtrate and urine volume.

• Kidney stones: Blockage increases pressure in nephron, reducing filtration.

Tubular Reabsorption

Overview

About 99% of the filtrate is reabsorbed, with only about 0.5 gallons excreted as urine daily. Reabsorption occurs throughout the renal tubule and involves active transport, facilitated diffusion, and osmosis. Fluids and solutes move from the nephron back into the peritubular capillaries.

Substances Reabsorbed

• Water

• Salts (Na+, Cl-)

• Urea

• Glucose

• Amino acids

• Vitamins

Mechanisms of Reabsorption

• Na+: Actively transported from proximal convoluted tubule, distal tubule, and ascending limb.

• Water: Follows Na+ by osmosis (about 80% reabsorbed in proximal tubule).

• Cl-: Follows electrical gradient established by Na+ movement.

• Urea: Concentration increases as water leaves; about 50% is returned to blood by diffusion.

• Glucose: Normally all filtered glucose is reabsorbed by active transport up to the tubular maximum (220 mg/dL); excess is excreted in urine (glycosuria).

• Amino acids and vitamins: Reabsorbed by active transport, mainly in the proximal convoluted tubule.

Reabsorption of H2O

• For water to be reabsorbed from the nephron, the surrounding tissues must be more salty (hypertonic) than the nephron fluid.

• Water moves by osmosis towards the higher salt concentration.

Salt Gradient and Tissue-Salt Concentration

Salt Gradient Creation

• Vasa recta: Region of peritubular capillaries that recycles salts.

• Loop of Henle: Pumps salts out of the nephron, establishing a gradient.

Tissue-Salt Concentration

• Cortex of kidney: ~300 mOsm

• Inner medulla of kidney: ~1200 mOsm

This gradient is essential for water reabsorption and urine concentration.

Vasa Recta Function

• Descending limb: NaC(sodium cholidel low, absorbs NaCl from tissues.

• Ascending limb: NaCl high, returns solutes to tissues.

• Helps concentrate solutes in the medulla.

Loop of Henle

Role in Urine Concentration

• Major role in reabsorption of H2O and formation of hypertonic (concentrated) urine.

• Ascending limb: Not permeable to H2O; Na+ and Cl- actively transported out.

• Descending limb: Freely permeable to H2O.

• Collecting tubule: Freely permeable to H2O (regulated by ADH).

As fluid ascends, it becomes dilute; the surrounding tissue becomes salty, especially in the inner medulla (up to 1200 mOsm/liter).

• Water is drawn out of the collecting tubule by osmosis due to the high salt concentration, allowing further water reabsorption and conservation.

Antidiuretic Hormone (ADH)

Role in Water Reabsorption

• Regulates the amount of water reabsorbed from the collecting tubule.

• If ADH is present: More H2O is reabsorbed, resulting in concentrated urine and increased blood pressure.

• If ADH is absent: Less H2O is reabsorbed, resulting in dilute urine and decreased blood pressure.

Factors Affecting ADH

Stimulation	Inhibition
Dehydration Reduced blood volume Pain	Alcohol Caffeine Ingestion of water

Regulation of ADH

• Hypothalamus: Osmoreceptors detect changes in blood osmolarity.

• ADH is produced in the hypothalamus and released via the neurohypophysis (posterior pituitary).

Summary Table: Key Processes in Urine Formation

Process	Main Location	Mechanism	Key Substances
Glomerular Filtration	Renal corpuscle (glomerulus & Bowman’s capsule)	Blood pressure-driven filtration	Water, electrolytes, glucose, amino acids, urea
Tubular Reabsorption	Proximal & distal tubules, loop of Henle, collecting duct	Active transport, diffusion, osmosis	Water, Na+, Cl-, glucose, amino acids, vitamins
Tubular Secretion	Distal tubule, collecting duct	Active transport	H+, K+, drugs, toxins

Additional info: Tubular secretion is the process by which substances are actively transported from the blood into the nephron for excretion, helping regulate pH and eliminate drugs or toxins.