Objective Two: Explain the Role of Nephrons in Urine Formation. Discuss Regulation of GFR

Nephron

Structural and functional unit of the kidney; filters blood and produces urine.

Three processes of urine formation

Glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular filtration

Hydrostatic pressure moves water and small dissolved substances from glomerular blood into Bowman's capsule.

Normal glomerular filtrate

Contains water, electrolytes, glucose, amino acids, and wastes such as urea and creatinine; normally excludes blood cells and most plasma proteins.

Tubular reabsorption

Moves needed substances and water from the tubular lumen into peritubular capillaries and back to the bloodstream.

Substances reabsorbed by the kidneys

Glucose, amino acids, sodium, bicarbonate, and water.

Tubular secretion

Moves substances from peritubular capillaries into the tubular lumen for elimination.

Substances eliminated by tubular secretion

Additional wastes, medications, excess hydrogen ions, and other substances requiring removal.

Glomerular filtration rate (GFR)

Volume of fluid filtered from the blood by both kidneys per minute.

Average healthy adult GFR

Approximately 120–125 mL/minute.

Renal autoregulation

Kidney mechanisms that maintain a relatively constant GFR despite changes in systemic blood pressure.

Myogenic mechanism

Increased blood pressure stretches the afferent arteriole, causing smooth-muscle contraction and vasoconstriction to limit glomerular blood flow and GFR.

Afferent arteriole response to decreased pressure

Relaxes and dilates to help maintain renal blood flow and filtration.

Tubuloglomerular feedback

Macula densa cells detect NaCl delivery in distal tubular fluid and adjust afferent arteriolar tone and GFR.

High NaCl delivery at macula densa

Signals afferent arteriole constriction, decreasing glomerular blood flow and GFR.

Low NaCl delivery at macula densa

Promotes afferent arteriole dilation, increasing glomerular blood flow and helping raise GFR.

Angiotensin II and GFR

Constriction of renal arterioles helps regulate glomerular pressure and renal blood flow.

Atrial natriuretic peptide (ANP) and GFR

Dilates the afferent arteriole, which can increase glomerular blood flow and filtration.

Sympathetic stimulation and GFR

Strong stimulation constricts afferent arterioles, reducing renal blood flow and GFR to redirect blood toward vital organs.

What is the nephron and what does it do?

The kidney's structural and functional unit; it filters blood, regulates substances, and produces urine.

What are the three processes of urine formation?

Glomerular filtration, tubular reabsorption, and tubular secretion.

How does glomerular filtration occur?

Hydrostatic pressure pushes water and small dissolved substances from glomerular capillaries across the filtration membrane into Bowman's capsule.

Which substances normally enter filtrate, and which generally remain in blood?

Water and small solutes enter; blood cells and most plasma proteins normally remain in the bloodstream.

How does tubular reabsorption differ from secretion?

Reabsorption moves needed substances from tubular fluid back to blood; secretion moves substances from blood into tubular fluid for elimination.

What is GFR, and what is the average healthy adult value?

Volume filtered by both kidneys per minute; approximately 120–125 mL/minute.

Why is GFR an important measure?

It indicates how effectively the kidneys filter blood; low GFR can allow wastes and fluid to accumulate.

How does the myogenic mechanism respond to increased blood pressure?

The afferent arteriole stretches and contracts, causing vasoconstriction that limits glomerular blood flow and GFR.

How does the afferent arteriole respond when blood pressure falls?

It relaxes and dilates to help maintain renal blood flow and filtration.

How does tubuloglomerular feedback regulate GFR?

Macula densa cells sense distal tubular NaCl delivery and signal changes in afferent arteriolar diameter.

What happens when the macula densa detects increased NaCl delivery?

It signals afferent arteriole constriction, decreasing glomerular blood flow and GFR.

What happens when the macula densa detects decreased NaCl delivery?

The afferent arteriole dilates, allowing more blood into the glomerulus and helping increase GFR.

How do angiotensin II and ANP affect renal filtration?

Angiotensin II constricts renal arterioles; ANP dilates the afferent arteriole and can increase filtration.

What does strong sympathetic stimulation do to GFR?

Constriction of afferent arterioles decreases renal blood flow and GFR, redirecting blood toward vital organs.