Ch 19 Kidneys Exam 3

What is the principle of mass balance and how does the urinary system use it?

Mass balance means input equals output to maintain homeostasis. The urinary system applies this by regulating water, ions, and waste through filtration, reabsorption, secretion, and excretion.

What are the main functions of the kidneys?

• Regulate ECF volume and blood pressure

• Regulate osmolarity

• Maintain ion balance

• Regulate pH

• Excrete wastes and foreign substances

• Participate in endocrine pathways (e.g., EPO, renin)

What are the major components of the urinary system?

Two kidneys, two ureters, one bladder, one urethra.

What is a nephron?

The functional unit of the kidney. Each kidney has ~1 million. It filters blood and forms urine.

Describe the flow of blood through the kidney.

Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries → Vasa recta (if juxtamedullary nephron).

Describe the flow of filtrate through the nephron.

Bowman’s capsule → Proximal tubule → Loop of Henle → Distal tubule → Collecting duct → Renal pelvis → Ureter → Bladder → Urethra.

What is glomerular filtration?

The process of filtering plasma from glomerular capillaries into Bowman’s capsule.

What is the filtration fraction?

The percentage of total plasma volume that enters the nephron (~20%).

What are podocytes and their function?

Specialized epithelial cells in Bowman’s capsule that form filtration slits to allow selective filtration of plasma.

What three barriers must a solute cross during filtration?

1. Capillary endothelium
2. Basement membrane
3. Podocyte filtration slit membrane

What is the net filtration pressure (NFP) in the glomerulus?

NFP = Glomerular hydrostatic pressure (55 mm Hg) – Colloid osmotic pressure (30 mm Hg) – Capsule hydrostatic pressure (15 mm Hg) = +10 mm Hg

What is glomerular filtration rate (GFR)?

The volume of filtrate formed per minute (~125 mL/min or 180 L/day).

How is GFR autoregulated?

• Myogenic response: Smooth muscle in afferent arteriole constricts in response to high BP.

• Tubuloglomerular feedback: Macula densa senses increased flow and signals afferent arteriole to constrict.

What systemic mechanisms regulate GFR?

Hormonal (e.g., renin-angiotensin-aldosterone system) and autonomic nervous system (sympathetic control).

Where does most reabsorption occur?

In the proximal tubule (~70% of filtrate).

What is transepithelial transport?

Movement of substances across tubule epithelial cells from lumen to interstitial fluid.

What is the paracellular pathway?

Reabsorption of solutes or water between adjacent tubule cells via tight junctions.

What drives passive reabsorption of solutes like urea?

Concentration gradients created by active transport of Na⁺.

How is glucose reabsorbed in the nephron?

Through secondary active transport via Na⁺-glucose symporters in the proximal tubule.

What is the transport maximum (Tm)?

The maximum rate at which a solute can be reabsorbed; beyond this, the solute appears in the urine.

What is the renal threshold?

The plasma concentration of a solute at which it first begins to appear in the urine.

What is tubular secretion?

The movement of substances from peritubular capillaries into the nephron tubule.

Which substances are commonly secreted?

K⁺, H⁺, and various drugs and toxins.

How do molecular competitors affect secretion?

They slow secretion by competing for transporter proteins.

What is the formula for excretion?

Excretion = Filtration – Reabsorption + Secretion

What is clearance?

The volume of plasma cleared of a substance per unit time.

Why is inulin used to measure GFR?

It is freely filtered, not reabsorbed or secreted, so its clearance equals GFR.

What is creatinine clearance used for clinically?

As an estimate of GFR; it slightly overestimates GFR because a small amount is secreted.

What controls the external urethral sphincter?

It is skeletal muscle under voluntary control by the somatic nervous system.

Describe the micturition reflex.

Stretch receptors in the bladder activate → Parasympathetic neurons cause detrusor muscle contraction → Internal sphincter relaxes → Somatic motor inhibition relaxes external sphincter → Urination.