urinary system part 1

Kidneys

Filter blood to remove wastes and regulate water volume

Urinary system structures

Two kidneys

Retroperitoneal

Located behind the peritoneum and outside the peritoneal cavity.

Renal cortex

Superficial outer region of the kidney.

Renal medulla

Inner region of the kidney containing cone-shaped renal pyramids.

Renal columns

Extensions of cortical tissue that separate renal pyramids.

Renal pelvis

Funnel-shaped structure that collects urine from the calyces and drains into the ureter.

Urine flow through the kidney

Collecting duct → renal papilla → minor calyx → major calyx → renal pelvis → ureter.

Nephron

Structural and functional unit of the kidney responsible for filtering blood and forming urine.

Renal corpuscle

Consists of the glomerulus and glomerular (Bowman's) capsule; site of filtration.

Glomerulus

Network of highly porous capillaries where blood plasma is filtered.

Renal tubule

Consists of the proximal convoluted tubule

Filtrate pathway

Glomerular capsule → PCT → descending limb → ascending limb → DCT → collecting duct.

Afferent arteriole

Carries blood into the glomerulus.

Efferent arteriole

Carries blood away from the glomerulus.

Glomerular filtration

Passive process in which blood pressure forces water and small solutes into the glomerular capsule

Tubular reabsorption

Movement of useful substances such as glucose

Tubular secretion

Movement of substances from blood into filtrate to eliminate wastes and regulate blood composition.

Filtration membrane

Consists of fenestrated capillary endothelium

Glomerular hydrostatic pressure (HPgc)

Main outward filtration force caused by blood pressure in glomerular capillaries; approximately 55 mmHg.

Capsular hydrostatic pressure (HPcs)

Inward pressure exerted by filtrate in the glomerular capsule that opposes filtration; approximately 15 mmHg.

Glomerular colloid osmotic pressure (OPgc)

Inward force caused by blood proteins pulling water into capillaries; approximately 30 mmHg.

Net filtration pressure (NFP)

Outward pressure minus inward pressures: 55 − (15 + 30) = 10 mmHg.

Glomerular filtration rate (GFR)

Volume of filtrate formed by both kidneys per minute; normally 120–125 mL/min.

Renal autoregulation (intrinsic control)

Maintains a nearly constant GFR despite blood pressure changes using myogenic and tubuloglomerular feedback mechanisms.

Myogenic mechanism

Increased blood pressure stretches and constricts afferent arterioles to protect glomeruli; decreased pressure causes dilation.

Extrinsic regulation of GFR

Neural and hormonal mechanisms that regulate GFR to maintain systemic blood pressure

Sympathetic nervous system response to low BP

Norepinephrine and epinephrine cause vasoconstriction of afferent arterioles

Renin-angiotensin-aldosterone mechanism (RAAS)

Hormonal mechanism that helps increase blood pressure.

Micturition

Urination or emptying of the bladder; involves the pontine micturition center and relaxation of urinary sphincters.