The Urinary System and Fluid Balance

Comprehensive flashcards covering the anatomy, physiology, and homeostatic mechanisms of the human urinary system and fluid/electrolyte balance.

Ureters

Slender, retroperitoneal tubes that transport urine from the kidneys to the urinary bladder.

Urinary bladder

A muscular sac that serves as a temporary storage reservoir for urine.

Urethra

A muscular tube that drains the urinary bladder and transports urine out of the body.

Erythropoietin

A hormone produced by the kidneys that regulates red blood cell (RBC) production.

Renin

An enzyme produced by the kidneys that regulates blood pressure.

Retroperitoneal

The location of the kidneys, situated in the superior lumbar region between $T12$ and $L5$, behind the peritoneum.

Renal hilum

A vertical cleft on the concave medial surface of the kidney where ureters, blood vessels, lymphatics, and nerves enter and exit.

Renal fascia

The outer anchoring layer of the kidney composed of dense fibrous connective tissue.

Perirenal fat capsule

A fatty cushion surrounding the kidney that helps prevent trauma.

Fibrous capsule

A transparent capsule that prevents the spread of infection to the kidney.

Renal cortex

The granular-appearing superficial region of the internal kidney.

Renal medulla

The internal region of the kidney deep to the cortex, composed of cone-shaped medullary pyramids.

Renal pyramids

Cone-shaped structures in the medulla; their broad bases face the cortex and their papillae point internally.

Renal columns

Inward extensions of cortical tissue that separate the renal pyramids.

Renal pelvis

A funnel-shaped tube continuous with the ureter that collects urine from the major calyces.

Minor calyces

Cup-shaped areas that collect urine draining from the pyramidal papillae.

Major calyces

Areas that collect urine from the minor calyces and empty it into the renal pelvis.

Nephrons

The structural and functional units of the kidney that form urine, with over $1$ million per kidney.

Renal corpuscle

The part of the nephron consisting of the glomerulus and the glomerular capsule.

Glomerulus

A tuft of capillaries composed of fenestrated endothelium, highly porous and specialized for efficient filtrate formation.

Filtrate

Plasma-derived fluid processed by the renal tubules to form urine.

Glomerular capsule (Bowman’s capsule)

A hollow, cup-shaped structure surrounding the glomerulus consisting of a parietal layer and a visceral layer.

Podocytes

Branching epithelial cells in the visceral layer of the glomerular capsule with extensions called foot processes that form filtration slits.

Proximal convoluted tubule (PCT)

Tubule section with cuboidal cells and dense microvilli (brush border) that functions primarily in reabsorption and secretion.

Nephron loop (Loop of Henle)

A u-shaped structure consisting of a descending limb (thin) and an ascending limb (thick).

Distal convoluted tubule (DCT)

Tubule section with cuboidal cells and very few microvilli, functioning more in secretion than reabsorption.

Collecting ducts

Ducts that receive filtrate from many nephrons and fuse together to deliver urine through papillae into minor calyces.

Cortical nephrons

Nephrons that make up $85\%$ of those in the kidney and are located almost entirely within the renal cortex.

Juxtamedullary nephrons

Nephrons with long loops that deeply invade the medulla, essential for the production of concentrated urine.

Peritubular capillaries

Low-pressure, porous capillaries arising from efferent arterioles of cortical nephrons; adapted for absorption of water and solutes.

Vasa recta

Long, thin-walled vessels parallel to the loops of juxtamedullary nephrons that function in forming concentrated urine.

Juxtaglomerular complex (JGC)

A complex involving the distal portion of the ascending limb and the afferent arteriole; regulates filtrate formation and systemic blood pressure.

Macula densa

Tall, closely packed cells of the ascending limb in the JGC that contain chemoreceptors to sense the NaCl content of filtrate.

Granular cells (JG cells)

Enlarged smooth muscle cells of the arteriole in the JGC that act as mechanoreceptors to sense blood pressure and secrete renin.

Extraglomerular mesangial cells

Cells located between arteriole and tubule cells in the JGC that may pass signals between macula densa and granular cells.

Glomerular filtration

A passive, non-metabolic process where hydrostatic pressure forces fluids and solutes through a membrane into the glomerular capsule.

Tubular reabsorption

The process of selectively returning $99\%$ of substances from the filtrate in the renal tubules back into the blood.

Tubular secretion

The selective movement of substances (such as $K^{+}$, $H^{+}$, and creatinine) from the blood into the filtrate.

Net filtration pressure (NFP)

The sum of forces responsible for filtrate formation, calculated as the outward hydrostatic pressure minus the inward osmotic and capsular pressures.

Glomerular filtration rate (GFR)

The volume of filtrate formed per minute by both kidneys, normally $120-125\,ml/min$.

Myogenic mechanism

An intrinsic control where vascular smooth muscle contracts when stretched by high blood pressure, constricting afferent arterioles to protect the glomeruli.

Tubuloglomerular feedback mechanism

An intrinsic, flow-dependent control where macula densa cells respond to high NaCl levels by causing vasoconstriction of the afferent arteriole.

Renin-angiotensin-aldosterone mechanism

The main hormonal extrinsic mechanism for increasing systemic blood pressure by stimulating vasoconstriction and enhancing blood volume.

Aquaporins

Water-filled pores that facilitate the reabsorption of water by osmosis.

Antidiuretic hormone (ADH)

A hormone from the posterior pituitary that inserts aquaporins into the collecting ducts to increase water reabsorption.

Aldosterone

A hormone that targets the DCT and collecting ducts to promote $Na^{+}$ reabsorption and $K^{+}$ secretion, thereby increasing blood pressure.

Urea

The largest solute component of urine, derived from the breakdown of amino acids.

Trigone

The smooth triangular area of the bladder floor outlined by the openings of the ureters and urethra, where infections often persist.

Micturition

The act of urination or voiding, requiring the contraction of bladder smooth muscle and the opening of both urethral sphincters.

Stress incontinence

The leakage of urine caused by increased intra-abdominal pressure from activities such as laughing, coughing, or sneezing.

Intracellular fluid (ICF)

The fluid found inside cells, accounting for $2/3$ of total body fluids.

Extracellular fluid (ECF)

The fluid outside the cells, such as plasma and interstitial fluid, accounting for $1/3$ of total body fluids.

Electrolytes

Chemical substances like salts and acids that dissociate into ions in water and serve to control osmosis and maintain $pH$ balance.

Alkalosis

A homeostatic imbalance where the arterial blood $pH$ rises above $7.45$.

Acidosis

A homeostatic imbalance where the arterial blood $pH$ drops below $7.35$.

Chemical buffer systems

Fast-acting systems consisting of paired weak acids and bases that moderate drastic $pH$ fluctuations by binding or releasing $H^{+}$.