The Urinary System

Objectives: Describe location of kidneys in body Identify the following regions of a kidney: hilum, cortex, medulla, medullary pyramids, calyces, pelvis, and renal columns Recognize a nephron and describe its anatomy Describe process of urine formation, identify areas of the nephron that are responsible for filtration, reabsorption and secretion Describe the function of the kidneys in excretion of nitrogen- containing wastes Define polyuria, anuria, oliguria, and diuresis Describe the composition of normal urine List abnormal urinary components

Helpful Word Parts: -Continence …… to hold Neph- …… kidney Pyel …… renal pelvis Ren- …… kidney -ur- …… urine

Location & Structure Soap Extend from T12 to L3 Small, dark red organs Kidney-bean shape Lie against dorsal body wall

Basic Function Maintain the purity and constancy Bear the major responsibility of excreting nitrogenous wastes, toxins and drugs from the body Regulate the blood’s volume and chemical makeup Produces enzymes renin and erythropoietin

Simplified Structure: Medial indentation - renal hilum Ureters, renal blood vessels and nerves Adrenal gland A transparent fibrous capsule The perirenal fat capsule surrounds each kidney The renal fascia (outermost capsule) anchors the kidney

Homeostatic Imbalance The fat surrounding kidneys is vital If the fat diminish kidneys may drop to a lower position called ptosis Ptosis becomes a problem if the ureters, -which do what again?- become kinked Once kinked, the urine can no longer pass through, causing back up and pressure on the kidney tissue called hydronephrosis

More On Structure Renal cortex (cortex – bark) Renal medulla: has many triangular regions with a striped appearance Renal or medullary pyramids: triangular regions with striped appearance Separated by renal columns Renal pelvis

Structure Continued… Calyces: extensions of the pelvis, Collect urine which constantly drains from the pyramids Drain it into the renal pelvis. Renal pelvis to the ureter Bladder for temporary storage

Blood Supply 25% each minute The arterial supply = renal artery Divides into segmental arteries Interlobar arteries Interlobar arteries give off the arcuate arteries The arcuate arteries then branch off into cortical radiate arteries Venous blood draining from the kidney flowes through veins that trace the pathway, but run in an opposite direction: cortical radiate veins to arcuate veins to interlobar viens to the renal vein

Nephrons & Urine Formation Over a million nephrons Nephrons are the structural and functional units of kidneys Two main structures: a glomerulus and a renal tubule A glomerulus is a knot of capillaries Has a closed end which is enlarged and cup-shaped to completely surround the glomerulus Called the glomerular (glom = little ball) The inner layer of the tubule is made of highly modified octopus-like cells called podocytes.

Podocytes Long branching processes….foot processes Intertwine and cling to the glomerulus “Filtration slits” between extensions Podocytes form a porous membrane around the glomerulus As the tubule extends, it coils and twists before forming a hairpin loop

More on Podocytes… Coils and twists again Collecting tubule called the collecting duct These regions of the tubule have specific names: • Proximal convoluted tubule (PCT) • Loop of Henle • Distal convoluted tubule - The lumen surfaces (exposed to filtrate) of the tubule cells in the proximal convoluted tubules are covered with dense microvilli, which increase surface area

Nephrons: Cortical nephrons are located almost entire in the cortex Few times nephrons are called juxtamedullary nephrons which are located close to the cortex-medulla junction - Their loops of Henle dip deep into the medullary pyramids - This is what gives the pyramids their striped appearance - Deliver the final urine product into the calyces and renal pelvis

More On Nephrons Associated with two capillary beds: The glomerular. Fed and drained by arterioles. The afferent arteriole is the feeder vessel The efferent arteriole receives blood The glomerular is specifically different from other capillary beds

How? Both fed and drained by arterioles Diameter of afferent arteriole > diameter of efferent arteriole Blood pressure is higher in the glomerular capillaries High pressure forces fluid and solutes out of the blood 99% of this filtrate is reclaimed by the renal tubule cells and returned to the blood

Capillary Beds Peritubular capillaries arise from the efferent arteriole that drains the glomerulus Capillaries are low-pressure, porous vessels that are adapted for absorption Cling closely to the whole length of the renal tubule Receive solutes and water from the tubule cells These substances are reabsorbed from the filtrate The peritubular capillaries drain into interlobular veins leaving the cortex

Urine Formation (1 of 3) Glomerular filtration Nonselective, passive process - fluid passes from blood into glomerular capsule The fluid is then filtrated Both proteins & blood cells = to large Systemic blood pressure is normal, filtrate will be formed If arterial blood pressure drops too low, glomerular pressure will also

Urine Formation (2 of 3) Tubular Reabsorption a) Begins as soon as filtrate enters the proximal convoluted tubule b) Tubule cells act as transporters c) Absorbed into the capillary blood by active transport processes d) Some reabsorption is done passively (Water – osmosis) e) Active transport involves membrane carriers which are very selective f) Abundance of carriers for substances that will be reused g)Needed substances will be entirely removed from the filtrate h)Nitrogenous waste products are poorly reabsorbed i) Urea which is formed by the liver and an end product of protein breakdown j) Uric acid is released when nucleic acids are metabolized k) Creatinine associated with creatinine metabolism in muscle tissue l) These tend to be found in high concentrations in urine m) Most reabsorption occurs in the proximal convoluted tubules

Urine Formation (3 of 3) Tubular secretion a) The opposite of tubular reabsorption b) Some substances move from the blood of the peritubular capillaries through the tubule cells c) This process is important to rid the body of substances not previously in the filtrate

Homeostatic Imbalance Oliguria – abnormally low urinary output Between 100 and 400 ml per day Anuria – extremely low urinary output Less than 100 ml per day Usually indicate glomerular blood pressure is too low to cause filtration Anuria a can also be a result from transfusion reactions and acute inflammation of the kidney

Characteristics of Urine: -Filtrate contains everything that blood plasma does except proteins Urine remains, containing nitrogenous wastes and unneeded substances Urochrome is a pigment More solutes in urine call for a deeper yellow color Urine may be strange colors at times Sterile pH is slightly acidic Specific gravity is the term used to compare how much heavier urine is than distilled water Chronic renal failure is a condition in which the kidney loses its ability to concentrate urine Kidney inflammation is pyelonephritis

Homework: Read pages 517-527 “Did you get it?” Questions 1-7 … In class Self Test 15.1

Citations: Marieb, E. N. (2009). Essentials of human anatomy & physiology. San Francisco, CA: Pearson Education, Inc.