Chapter 20 — The Kidneys

Regulate extracellular fluid volume and blood pressure; regulate osmolarity; maintain ion balance; regulate pH; excrete wastes; and produce hormones.

Approximately 300 mOsm/L.

Approximately 300 mOsm/L, similar to plasma.

The outer cortex and inner medulla.

The functional unit of the kidney that filters blood and modifies filtrate to produce urine.

About 80% are cortical nephrons in the cortex; about 20% are juxtamedullary nephrons that extend deep into the medulla.

In the renal corpuscle, which consists of the glomerulus and Bowman's capsule.

Along the nephron tubules, with substances returning to the blood through the peritubular capillaries or vasa recta.

Along the nephron tubules, as substances move from the blood into the tubular fluid.

The renal corpuscle, proximal tubule, loop of Henle, and distal nephron.

Filtration of mostly protein-free plasma from glomerular capillaries into Bowman's capsule.

Isosmotic reabsorption of organic nutrients, ions, and water; it also secretes metabolites and xenobiotics.

Reabsorbs ions in excess of water, diluting the tubular fluid and helping create the concentrated medullary interstitial fluid.

Regulated reabsorption of ions and water for salt balance, water balance, and pH homeostasis.

The distal tubule and collecting duct.

Approximately 180 liters per day.

Approximately 1.5 liters per day.

It is reabsorbed into the blood; only a small fraction leaves the body as urine.

Approximately 70%, leaving about 54 liters per day.

Approximately 18 liters per day.

Approximately 1.5 liters per day, depending on hydration.

Excretion = filtration − reabsorption + secretion.

Approximately 20%.

Less than 1%.

More than 99%.

High filtration allows the kidneys to rapidly remove wastes and regulate plasma composition; selective reabsorption returns needed water and solutes to the blood.

Fenestrated glomerular capillary endothelium with its glycocalyx, the glomerular basement membrane, and podocyte filtration slits.

A carbohydrate-rich coating on the glomerular capillary endothelium that contributes to the filtration barrier.

A specialized layer between the capillary endothelium and podocytes that restricts the passage of large molecules and proteins.

Specialized cells of Bowman's capsule whose foot processes surround glomerular capillaries and form filtration slits.

Cells between glomerular capillaries that can contract and alter capillary surface area and blood flow.

Glomerular capillary hydrostatic pressure, plasma colloid osmotic pressure, and Bowman's capsule fluid hydrostatic pressure.

Glomerular capillary hydrostatic pressure.

Plasma colloid osmotic pressure and Bowman's capsule fluid pressure.

Net filtration pressure = glomerular hydrostatic pressure − plasma colloid osmotic pressure − capsule fluid pressure.

55 mmHg − 30 mmHg − 15 mmHg = 10 mmHg.

The volume of fluid filtered by the glomeruli per unit time.

Net filtration pressure and the filtration coefficient.

The available surface area of the filtration barrier and its permeability.

Changes in glomerular capillary surface area and changes in filtration-barrier permeability.

Renal autoregulation keeps GFR relatively constant when mean arterial pressure is approximately 80–180 mmHg.

Primarily by adjusting resistance in the afferent arteriole through the myogenic response and tubuloglomerular feedback.

When blood pressure rises, afferent arteriolar smooth muscle stretches and contracts, increasing resistance and limiting the rise in GFR; when pressure falls, the arteriole relaxes.

The macula densa senses NaCl delivery in tubular fluid and signals the afferent arteriole and juxtaglomerular cells to adjust blood flow and GFR.

It signals afferent arteriolar constriction, reducing glomerular pressure and GFR.

It promotes afferent arteriolar dilation and renin release, helping restore GFR and blood pressure.

Renal blood flow, glomerular capillary pressure, and GFR decrease.

Renal blood flow, glomerular capillary pressure, and GFR increase.

Glomerular capillary pressure and GFR initially increase, while renal blood flow decreases.

Glomerular capillary pressure and GFR decrease, while renal blood flow increases.

Peritubular capillaries have relatively low hydrostatic pressure and high colloid osmotic pressure, favoring movement of fluid from the interstitial space into the blood.

Movement of substances from the nephron lumen back into the blood.

Movement of substances from the blood into the nephron lumen.

Removal of substances from the body in urine.

The fluid that enters Bowman's capsule after filtration of blood plasma.

Movement of a substance through a tubular epithelial cell, crossing its apical and basolateral membranes.

Movement of a substance between adjacent epithelial cells through their junctions.

The membrane of a tubule cell facing the nephron lumen.

The membrane of a tubule cell facing the interstitial fluid and blood.

Movement of a substance against its electrochemical gradient using energy, usually from ATP.

The Na⁺/K⁺ ATPase, which uses ATP to move sodium out of the cell and potassium into it.

Transport in which one substance moves down its electrochemical gradient and provides the energy to move another substance against its gradient.

Sodium-glucose cotransport, in which sodium moving into the cell drives glucose uptake.

Movement of substances down their concentration or electrochemical gradients without direct energy input.

Water moving by osmosis, or certain solutes moving down their concentration gradients.

A process in which tubular cells take up filtered substances in vesicles, such as small amounts of filtered proteins.

Direct active transport uses ATP at the transporter; indirect active transport uses an ion gradient established by ATP-powered transport.

Transport driven by a gradient that was created by a secondary active transport process, rather than directly by ATP.

It keeps intracellular sodium low, creating the sodium gradient that drives secondary active transport.

A transport process reaches its maximum capacity because all available transport proteins are occupied.

Some glucose remains in the tubular fluid and is excreted in urine.

The presence of glucose in the urine.

The filtered glucose load can exceed the transport maximum of glucose transporters.

Specificity means a transporter recognizes particular substances; competition occurs when similar substances compete for the same transporter.

Substances move from peritubular capillary blood into the interstitial fluid and then across tubular cells into the lumen.

Hydrogen ions, potassium ions, metabolic wastes, drugs, and other foreign compounds.

Excretion rate = urine concentration × urine flow rate.

The volume of plasma from which a substance is completely removed per unit time.

Clearance = (urine concentration × urine flow rate) ÷ plasma concentration.

Inulin clearance is used to measure GFR because inulin is filtered but not reabsorbed or secreted.

Excretion is the amount of a substance removed per unit time; clearance is the volume of plasma cleared of that substance per unit time.

The tube that carries urine from a kidney to the urinary bladder.

A muscular organ that stores urine until micturition.

The tube that carries urine from the bladder to the outside of the body.

Located behind the peritoneum; the kidneys are retroperitoneal.

A specialized region where the distal nephron passes near the afferent and efferent arterioles and helps regulate GFR and renin release.

A group of specialized tubular cells that senses NaCl delivery and participates in tubuloglomerular feedback.

Modified smooth muscle cells in the afferent arteriole that release renin.

The internal and external urethral sphincters maintain continence while the detrusor muscle remains relaxed.

The process of emptying the urinary bladder.

Bladder stretch activates sensory signals to the spinal cord, parasympathetic output contracts the detrusor muscle, and sphincter relaxation allows urine to leave.

The external urethral sphincter is skeletal muscle and can be voluntarily contracted or relaxed.

As the bladder fills, stretch signals and the micturition reflex increase the urge to urinate; sustained sphincter control has limits.