Brunzel Chapter 3 - The Kidney Key Terms

Question 1

Active transport

Answer 1

The movement of a substance across a cell membrane and against a gradient, requiring the expenditure of energy.

Question 2

Afferent arteriole

Answer 2

A small branch of an interlobular renal artery that becomes the capillary tuft within the glomerulus.

Question 3

Antidiuretic hormone

Answer 3

A hormone produced in the hypothalamus and released from the posterior pituitary that regulates the reabsorption of water by the collecting tubules. Without adequate arginine vasopressin present, water is not reabsorbed.

Question 4

Basement membrane

Answer 4

A trilayer structure located within the glomerulus along the base of epithelium (podocytes) of the urinary (Bowman’s) space. With the overlaying slit diaphragm, the basement membrane is the size discriminating component of the glomerular filtration barrier, limiting the passage of substances to those with an effective molecular radius less than 4 nm.

Question 5

Collecting duct

Answer 5

The portion of a renal nephron that follows the distal convoluted tubule. Many distal tubules empty into a single collecting duct. The collecting duct traverses the renal cortex and the medulla and is the site of final urine concentration. The collecting ducts terminate at the renal papilla, conveying the urine formed into the renal calyces of the kidney.

Question 6

Countercurrent Exchange Mechanism

Answer 6

A passive exchange by diffusion of reabsorbed solutes and water from the medullary interstitium of the nephron into the blood of its vascular blood supply (i.e., the vasa recta). A requirement of this process is that the flow of blood within the ascending and descending vessels of the U- shaped vasa recta must be in opposite directions, hence the term countercurrent. The countercurrent exchange mechanism simultaneously supplies nutrients to the medulla and removes solutes and water reabsorbed into the blood. As a result, the mechanism assists in maintaining medullary hypertonicity.

Question 7

Countercurrent Multiplier Mechanism

Answer 7

A process occurring in the loop
of Henle of each nephron that establishes and maintains the osmotic gradient within the medullary interstitium. The medullary osmolality gradient ranges from being isosmotic (≈300 mOsm/kg) at its border with the cortex to approximately 1400 mOsm/kg at the inner medulla or papilla. A requirement of this process is that the flow of the ultrafiltrate in the descending and ascending limbs must be in opposite directions, hence the name countercurrent. In addition, active sodium and chloride reabsorption in the ascending limb combined with passive water reabsorption in the descending limb is an essential component of this process. The countercurrent multiplier mechanism accounts for approximately 50% of the solutes concentrated in the renal medulla.

Question 8

Distal Convoluted Tubule

Answer 8

The portion of a renal nephron immediately following the loop of Henle. The tubule begins at the juxtaglomerular apparatus with the macula densa, a specialized group of cells located at the vascular pole. The distal tubule is convoluted and after two to three loops becomes the collecting tubule (or duct).

Question 9

Efferent Arteriole

Answer 9

The arteriole exiting a glomerulus; the efferent arteriole is formed by rejoining of the anastomosing capillary network within the glomerulus.

Question 10

Glomerular Filtration Barrier

Answer 10

The structure within the glomerulus that determines the composition of the plasma ultrafiltrate formed in the urinary space by regulating the passage of solutes. The glomerular filtration barrier consists of the capillary endothelium, the basement membrane, and the epithelial podocytes, each coated with a “shield of negativity.” Solute selectivity by the barrier is based on the molecular size and the electrical charge of the solute

Question 11

Glomerulus

Answer 11

A tuft or network of capillaries encircled by and intimately related with the proximal end of a renal tubule (i.e., Bowman’s capsule). The glomerulus is composed of four distinct structural components: the capillary endothelial cells, the epithelial cells (podocytes), the mesangium, and the basement membrane.

Question 12

Isosmotic

Answer 12

Term describing a solution or fluid that has the same concentration of osmotically active solutes as the blood plasma.

Question 13

Juxtaglomerular Apparatus

Answer 13

A specialized area located at the vascular pole of a nephron. The apparatus is composed of cells from the afferent and efferent arterioles, the macula densa of the distal tubule, and the extraglomerular mesangium. The juxtaglomerular apparatus is actually an endocrine organ and the primary producer of renin.

Question 14

Kidneys

Answer 14

The organs of the urinary system that produce urine. Normally, each individual has two kidneys. The primary function of the kidneys is to filter the blood, removing waste products and regulating electrolytes, water, acid-base balance, and blood pressure.

Question 15

Loop of Henle

Answer 15

The tubular portion of a nephron immediately following and continuous with the proximal tubule. Located in the renal medulla, the loop of Henle is composed of a thin descending limb, a U-shaped segment (also called a hairpin turn), and thin and thick ascending limbs. The thick ascending limb of the loop of Henle (sometimes called the straight portion of the distal tubule) ends as the tubule enters the vascular pole of the glomerulus.

Question 16

Maximal Tubular Reaborptive Capacity

Answer 16

Denoted Tm, the maximal rate of reabsorption of a solute by the tubular epithelium per minute (milligrams per minute). Reabsorptive capacity varies with each solute and depends on the glomerular filtration rate.

Question 17

Maximal Tubular Secretory Capacity

Answer 17

Also denoted Tm; the maximal rate of secretion of a solute by the tubular epithelium per minute (milligrams per minute). This rate differs for each solute.

Question 18

Mesangium

Answer 18

The cells that form the
structural core tissue of a glomerulus. The mesangium lies between the glomerular capillaries (endothelium) and the podocytes (tubular epithelium). The mesangial cells derive from smooth muscle and have contractility characteristics and the ability to phagocytize and pinocytize.

Question 19

Nephron

Answer 19

The functional unit of the kidney. Each kidney contains approximately 1.3 million nephrons. A nephron is composed of five distinct areas: the glomerulus, the proximal tubule, the loop of Henle, the distal tubule, and the collecting tubule or duct. Each region of the nephron is specialized and plays a role in the formation and final composition of urine.

Question 20

Osmosis

Answer 20

The movement of water across a semipermeable membrane in an attempt to achieve an osmotic equilibrium between two compartments or solutions of differing osmolality (i.e., an osmotic gradient). This mechanism is passive, that is, it requires no energy.

Question 21

Passive Transport

Answer 21

The movement of a substance (e.g., ion, solute) across a cell membrane along a gradient (e.g., concentration, charge). Passive transport does not require energy.

Question 22

Peritubular Capillaries

Answer 22

The network of capillaries (or plexus) that forms from the efferent arteriole and surrounds the tubules of the nephron in the renal cortex.

Question 23

Podocytes

Answer 23

The epithelial cells that line the urinary (Bowman’s) space of the glomerulus. These cells completely cover the glomerular capillaries with large finger-like processes that interdigitate to form a filtration slit. The term podo, which is Greek for “foot,” relates to the footlike appearance of the podocyte when viewed in cross-section. Collectively, the podocytes constitute the glomerular epithelium that forms Bowman’s capsule.

Question 24

Proximal Tubule

Answer 24

The tubular part of a nephron immediately following the glomerulus. The proximal tubule has a convoluted portion and a straight portion, the latter becoming the loop of Henle after entering the renal medulla.

Question 25

Renal Threshold Level

Answer 25

The plasma concentration of a solute above which the amount of solute present in the ultrafiltrate exceeds the maximal tubular reabsorptive capacity. Once the renal threshold level has been reached, increased amounts of solute are excreted (i.e., lost) in the urine.

Question 26

Renin

Answer 26

A proteolytic enzyme produced and stored by the cells of the juxtaglomerular apparatus of the renal nephrons. Secretion of renin results in the formation of angiotensin and the secretion of aldosterone; thus renin plays an important role in controlling blood pressure and fluid balance.

Question 27

Shield of Negativity

Answer 27

A term that describes the impediment produced by negatively charged components (e.g., proteoglycans) of the glomerular filtration barrier. Present on both sides of and throughout the filtration barrier, these negatively charged components effectively limit the filtration of negatively charged substances from the blood (e.g., albumin) into the urinary space.

Question 28

Titratable Acids

Answer 28

A term that represents H+ ions (acid) excreted in the urine as monobasic phosphate (e.g., NaH2PO4). Urinary excretion of these acids results in the elimination of H+ ions and the reabsorption of sodium and bicarbonate. Titration of urine to a pH of 7.4 (normal plasma pH) using a standard base (e.g., NaOH) will quantitate the number of H+ ions excreted in this form, hence the name titratable acids.

Question 29

Tubular Reabsorption

Answer 29

The movement of substances (by active or passive transport) from the tubular ultrafiltrate into the peritubular blood or the interstitium by the renal tubular cells.

Question 30

Tubular Secretion

Answer 30

The movement of substances (by active or passive transport) from the peritubular blood or the interstitium into the
tubular ultrafiltrate by the renal tubular cells.

Question 31

Urea Cycle

Answer 31

A passive process
occurring throughout the nephron that establishes and maintains a high concentration of urea in the renal medulla. This process accounts for approximately 50% of the solutes concentrated in the medulla. With the countercurrent exchange mechanism, the urea cycle helps establish and maintain the high medullary osmotic gradient. Because urea can passively diffuse into the interstitium and back into the lumen fluid, the selectivity of the tubular epithelium in each portion of the nephron plays an integral part in the urea cycling process.

Question 32

Vasa recta

Answer 32

The vascular network of long, U-shaped capillaries that forms from the peritubular capillaries and surrounds the loops of Henle in the renal medulla.