Urinary System Lecture

Flashcards testing foundational anatomy, histology, vascular architecture, nephron physiology, filtration pressures, autoregulation, and micturition mechanics of the urinary system.

Excretion

The separation of wastes from body fluids and their elimination outside the body.

Erythropoietin (EPO)

A hormone released by the kidneys that stimulates erythrocyte (red blood cell) formation in the bone marrow.

Renin

An enzyme secreted by the kidneys that regulates blood pressure via the activation of the angiotensin-aldosterone system.

Retroperitoneal

The anatomical position of the kidneys, located between the parietal peritoneum and the posterior muscular wall of the abdomen.

Renal Hilum

The concave medial border of the kidney through which blood vessels, lymphatic vessels, nerves, and the ureter enter and exit.

Renal Fascia

The outermost anchoring layer of dense fibrous connective tissue that surrounds the kidney and binds it to surrounding structures.

Perirenal Fat Capsule (Adipose Capsule)

A middle layer of adipose tissue that forms a fatty cushion to protect and hold the kidney in place.

Fibrous Capsule (Renal Capsule)

A thin layer of dense irregular connective tissue directly adhering to the kidney's surface, serving as the outermost barrier of the kidney organ itself.

Renal Cortex

The superficial, outer region of the kidney parenchyma situated immediately deep to the renal capsule.

Renal Medulla

The deep, inner region of the kidney parenchyma consisting of cone-shaped renal pyramids separated by extensions called renal columns.

Renal Papilla

The apex or tip of a renal pyramid that points toward the renal sinus and empties urine into a minor calyx.

Renal Sinus

The internal cavity within the kidney that houses adipose tissue, blood vessels, nerves, and the initial urine drainage structures (calyces and renal pelvis).

Nephron

The microscopic structural and functional unit of the kidney (approximately $1\text{ million}$ per kidney) that filters blood and produces urine.

Renal Corpuscle

The portion of a nephron located in the renal cortex that is responsible for blood plasma filtration, consisting of a glomerulus and a glomerular (Bowman's) capsule.

Glomerulus

A specialized tuft of fenestrated capillaries supplied by an afferent arteriole and drained by an efferent arteriole, serving as the site of filtration without gas exchange.

Podocytes

Modified epithelial cells forming the visceral layer of Bowman's capsule whose extensions (pedicels) interdigitate to create filtration slits around glomerular capillaries.

Glomerular Filtration

The passive, nonselective process in which blood hydrostatic pressure forces water and small dissolved solutes across the filtration membrane into the capsular space, producing a cell- and protein-free filtrate.

Tubular Reabsorption

The selective process of moving substances—including water, electrolytes, and nutrients—from the tubular filtrate back into the peritubular capillaries or vasa recta, reclaiming $99\%$ of filtered volume.

Tubular Secretion

The selective transport of substances—such as $\text{H}^+$, $\text{NH}_4^+$, and certain medications—from the blood in peritubular capillaries directly into the filtrate for elimination in urine.

Glomerular Hydrostatic Pressure (GHP)

The blood pressure within glomerular capillaries (normally $50\,\text{mm Hg}$) that forces fluid and solutes across the filtration membrane into the capsular space.

Glomerular Colloid Osmotic Pressure (GCOP)

The osmotic force exerted by non-filterable proteins remaining in the glomerular blood (normally $30\,\text{mm Hg}$) that opposes filtration by drawing water back into capillaries.

Capsular Hydrostatic Pressure (CHP)

The hydrostatic pressure exerted by accumulating filtrate inside the capsular space (normally $10\,\text{mm Hg}$) that pushes against the glomerular capillaries to oppose filtration.

Net Filtration Pressure (NFP)

The overall driving pressure moving fluid across the filtration membrane, determined by: $\text{NFP} = \text{GHP} - (\text{CHP} + \text{GCOP}) = 50 - (10 + 30) = 10\,\text{mm Hg}$.

Glomerular Filtration Rate (GFR)

The total volume of filtrate formed collectively by all nephrons in both kidneys per minute, averaging $120\text{--}125\,\text{mL/min}$.

Mesangial Cells

Contractile, supportive cells situated within the cleft between glomerular arterioles and capillaries that can adjust filtration surface area and regulate capillary blood flow.

Antidiuretic Hormone (ADH)

A neurohypophysial hormone released by the posterior pituitary that stimulates water reabsorption and permeability to urea in the collecting ducts.

Aldosterone

A mineralocorticoid secreted by the adrenal cortex that acts on the distal convoluted tubules and collecting ducts to increase $\text{Na}^+$ reabsorption (obligating water reabsorption) and promote $\text{K}^+$ secretion.

Parathyroid Hormone (PTH)

A peptide hormone produced by the parathyroid glands that acts on distal convoluted tubule cells to enhance $\text{Ca}^{2+}$ reabsorption.

Countercurrent Multiplier

The mechanism occurring in the loops of Henle of juxtamedullary nephrons where active $\text{NaCl}$ extrusion by the thick ascending limb drives osmotic water exit from the thin descending limb, generating a hyperosmotic medullary gradient.

Renal Autoregulation

The intrinsic ability of the kidneys to maintain a relatively constant GFR and renal blood flow across systemic mean arterial pressures between $80\,\text{mm Hg}$ and $180\,\text{mm Hg}$.

Myogenic Mechanism

An autoregulatory response in which increased systemic arterial pressure stretches vascular smooth muscle in afferent arterioles causing vasoconstriction, whereas reduced pressure causes vasodilation to stabilize GFR.

Tubuloglomerular Feedback

A renal autoregulation mechanism where macula densa cells monitor the composition of downstream tubular fluid and send paracrine signals to modulate afferent arteriole diameter and GFR.

Juxtaglomerular Apparatus (JGA)

A specialized neuroendocrine and sensory structure located at the vascular pole of each nephron where the ascending limb of the loop of Henle contacts the afferent and efferent arterioles.

Granular Cells (Juxtaglomerular Cells)

Enlarged, smooth muscle mechanoreceptors in the wall of the afferent arteriole that sense blood pressure changes and synthesize, store, and release renin.

Macula Densa

A group of tall, tightly packed chemoreceptor cells in the thick ascending limb of the nephron loop that detect changes in the solute concentration and flow rate of tubular fluid.

Vasa Recta

Long, straight hairpin capillary loops that arise from efferent arterioles of juxtamedullary nephrons, follow the nephron loops deep into the medulla, and drain into interlobular or arcuate veins.

Detrusor Muscle

The middle muscular layer of the urinary bladder consisting of three interlaced layers of smooth muscle that contract under parasympathetic control during micturition.

Trigone

A smooth, triangular region located at the floor of the urinary bladder outlined by the two ureteric orifices and the internal urethral orifice, noted for persistent clinical infections.

Internal Urethral Sphincter

An involuntary smooth muscle ring located at the bladder-urethra junction that keeps the urethra closed, regulated by autonomic sacral nerves.

External Urethral Sphincter

A ring of voluntary skeletal muscle surrounding the urethra as it passes through the urogenital diaphragm (pelvic floor), innervated by the somatic nervous system.

Prostatic Urethra

The proximal segment of the male urethra ($2.5\,\text{cm}$ in length) that passes through the prostate gland.

Membranous Urethra

The short intermediate region of the male urethra ($2\,\text{cm}$ in length) that passes through the pelvic floor / urogenital diaphragm.

Spongy Urethra (Penile Urethra)

The longest segment of the male urethra ($15\,\text{cm}$ in length) that traverses the corpus spongiosum of the penis to open at the external urethral orifice.

Micturition

The process of emptying the urinary bladder, requiring coordinated detrusor contraction (parasympathetic), internal urethral sphincter opening, and voluntary relaxation of the external urethral sphincter.