Chapter 25

Chapter 25 Part A

The Urinary System

Function of Kidneys

  • Kidneys, a major excretory organ, maintain the body’s internal environment by:

  • Regulating total water volume and total solute concentration in water

  • Regulating ion concentrations in extracellular fluid (ECF)

  • Ensuring long-term acid-base balance

  • Excreting metabolic wastes, toxins, drugs

  • Producing erythropoietin (regulates blood pressure and renin (regulates RBC production)

  • Activating vitamin D

  • Carrying out gluconeogenesis, if needed

Kidneys are part of the urinary system, which also includes:

  • Ureters: transport urine from kidneys to urinary bladder

  • Urinary bladder: temporary storage reservoir for urine

  • Urethra: transports urine out of body

Gross Anatomy of Kidneys

Location and External Anatomy

  • Retroperitoneal, in the superior lumbar region

    • Located between T12 and L5

  • Right kidney is crowded by liver, so is lower than left

  • Adrenal (suprarenal) gland sits atop each kidney

  • Convex lateral surface

  • Concave medial surface with vertical renal hilum leads to internal space, renal sinus

    • Ureters, renal blood vessels, lymphatics, and nerves enter and exit at hilum

  • Three layers of supportive tissue surround kidney

    • Renal fascia

      • Anchoring outer layer of dense fibrous connective tissue

    • Perirenal fat capsule

      • Fatty cushion

    • Fibrous capsule

      • Transparent capsule that prevents spread of infection to kidney

Internal Gross Anatomy

Internal kidney has three distinct regions:

  1. Renal cortex: granular-appearing superficial region

  2. Renal medulla: deep to cortex, composed of cone-shaped medullary (renal) pyramids

    1. Broad base of pyramid faces cortex

    2. Papilla, tip of pyramid, points internally

    3. Renal pyramids are separated by renal columns, inward extensions of cortical tissue

    4. Lobe: medullary pyramid and its surrounding cortical tissue; about eight lobes per kidney

  3. Renal pelvis: Funnel-shaped tube continuous with ureter

    1. Minor calyces: Cup-shaped areas that collect urine draining from pyramidal papillae

    2. Major calyces: Areas that collect urine from minor calyces and Empty urine into renal pelvis

    3. Urine flow

    4. Renal pyramid → minor calyx → major calyx → renal pelvis → ureter

Blood and Nerve Supply

  • Blood Supply: kidneys cleanse blood and adjust its composition, so it has a rich blood supply

  • Renal arteries deliver about one-fourth (1200 ml) of cardiac output to kidneys each minute

  • Arterial flow: renal → segmental → interlobar → arcuate → cortical radiate (interlobular)

  • Venous flow: cortical radiate → arcuate → interlobar → renal veins

    • No segmental veins

  • Nerve supply: via sympathetic fibers from renal plexus

Nephrons: are the structural and functional units that form urine. There are > 1 million per kidney. Two main parts

  1. Renal corpuscle

  2. Renal tubule

Renal Corpuscle: Two parts of renal corpuscle, the glomerulus and the Glomerular capsule

Glomerulus

  1. Tuft of capillaries composed of fenestrated endothelium

  2. Highly porous capillaries

  3. Allows for efficient filtrate formation

  4. Filtrate: plasma-derived fluid that renal tubules process to form urine

Glomerular capsule: also called Bowman’s capsule: cup-shaped, hollow structure surrounding glomerulus

Consists of two layers

  1. Parietal layer: simple squamous epithelium

  2. Visceral layer: clings to glomerular capillaries; branching epithelial podocytes

    1. Extensions terminate in foot processes that cling to basement membrane

    2. Filtration slits between foot processes allow filtrate to pass into capsular space

Renal Tubule and Collecting Duct

  • Renal tubule is about 3 cm (1.2 in.) long

  • Consists of single layer of epithelial cells, but each region has its own unique histology and function

  • Three major parts

    1. Proximal convoluted tubule: Proximal, closest to renal corpuscle

    2. Nephron loop

    3. Distal convoluted tubule: Distal, farthest from renal corpuscle. The distal convoluted tubule drains into collecting duct.

Proximal convoluted tubule (PCT): Cuboidal cells with dense microvilli that form brush border. Increase surface area and also have large mitochondria.

  • Functions in reabsorption and secretion

  • Confined to cortex

Nephron loop: Formerly called loop of Henle. U-shaped structure consisting of two limbs.

  1. Descending limb

    1. Proximal part of descending limb is continuous with proximal tubule

    2. Distal portion also called descending thin limb; simple squamous epithelium

  2. Ascending limb

    1. Thick ascending limb

    2. Thin in some nephrons

    3. Cuboidal or columnar cells

Distal convoluted tubule (DCT): Cuboidal cells with very few microvilli. Function more in secretion than reabsorption. This is also confined to cortex.

Collecting ducts: two cell types principal cella and intercalated cels. Collecting ducts receive filtrate from many nephrons, Run through medullary pyramids, Give pyramids their striped appearance. The ducts fuse together to deliver urine through papillae into minor calycesPrincipal cells

  1. Principal cells

    1. Sparse with short microvilli

    2. Maintain water and Na+ balance

  2. Intercalated cells

    1. Cuboidal cells with abundant microvilli

    2. Two types of intercalated cells

    3. A and B: both help maintain acid-base balance of blood

Classes of Nephrons: Two major groups of nephrons, cortical nephrons and juxtamedullary nephrons

  1. Cortical nephrons

    1. Make up 85% of nephrons

    2. Almost entirely in cortex

  2. Juxtamedullary nephrons

    1. Long nephron loops deeply invade medulla

    2. Ascending limbs have thick and thin segments

    3. Important in production of concentrated urine

Nephron Capillary Beds: Renal tubules are associated with two capillary beds, the glomerulus and capillaries. Juxtamedullary nephrons are associated with the vasa recta

Glomerulus: Capillaries are specialized for filtration. Different from other capillary beds because they are fed and drained by arteriole

  • Afferent arteriole enters glomerulus and leaves via efferent arteriole

  • Afferent arteriole arises from cortical radiate arteries

Efferent feed into either peritubular capillaries or vasa recta

Blood pressure in glomerulus high because:

  • Afferent arterioles are larger in diameter than efferent arterioles

  • Arterioles are high-resistance vessels

Peritubular capillaries: Low-pressure, porous capillaries adapted for absorption of water and solutes. Arise from efferent arterioles and cling to adjacent renal tubules in cortex. Empty into venules.

Vasa recta: Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons. Arise from efferent arterioles serving juxtamedullary nephrons. Function in formation of concentrated urine.

Juxtaglomerular Complex (JGC): Each nephron has one juxtaglomerular complex (JGC)

Involves modified portions of:

  1. Distal portion of ascending limb of nephron loop

  2. Afferent (sometimes efferent) arteriole

  3. Important in regulating rate of filtrate formation and blood pressure

Three cell populations are seen in JGC:

  1. Macula densa: Tall, closely packed cells of ascending limb, contain chemoreceptors that sense NaCl content of filtrate

  2. Granular cells (juxtaglomerular, or JG cells): Enlarged, smooth muscle cells of arteriole. Act as mechanoreceptors to sense blood pressure in afferent arteriole. Contain secretory granules that contain enzyme renin

  3. Extraglomerular mesangial cells: Located between arteriole and tubule cells and interconnected with gap junctions. May pass signals between macula densa and granular cells

Physiology of Kidney

  • 180 L of fluid processed daily, but only 1.5 L of urine is formed

  • Kidneys filter body’s entire plasma volume 60 times each day

  • Consume 20–25% of oxygen used by body at rest

  • Filtrate (produced by glomerular filtration) is basically blood plasma minus proteins

  • Urine is produced from filtrate

    • Urine

      • <1% of original filtrate

      • Contains metabolic wastes and unneeded substances

Three processes are involved in urine formation and adjustment of blood composition:

  1. Glomerular filtration: produces cell- and protein-free filtrate

  2. Tubular reabsorption: selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts

  3. Tubular secretion: selectively moves substances from blood to filtrate in renal tubules and collecting ducts

Step 1: Glomerular Filtration: is a passive process. Hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule. No reabsorption into capillaries of glomerulus occurs

Glomerular Filtration Rate (GFR): This is the volume of filtrate formed per minute by both kidneys (normal = 120–125 ml/min)

GFR is directly proportional to:

  • Net filtration pressure (NFP)

    • Primary pressure is glomerular hydrostatic pressure

  • Total surface area available for filtration

    • Glomerular mesangial cells control by contracting

  • Filtration membrane permeability

    • Much more permeable than other capillaries

Regulation of Glomerular Filtration

  • Constant GFR is important as it allows kidneys to make filtrate and maintain extracellular homeostasis

    • Goal of local intrinsic controls (renal autoregulation): maintain GFR in kidney

  • GFR affects systemic blood pressure

    • Increased GFR causes increased urine output, which lowers blood pressure, and vice versa

    • Goal of extrinsic controls: maintain systemic blood pressure

Extrinsic controls: These are both neural and hormonal mechanisms. The purpose of extrinsic controls is to regulate GFR to maintain systemic blood pressure. These will will override renal intrinsic controls if blood volume needs to be increased.

Renin-angiotensin-aldosterone mechanism: Main mechanism for increasing blood pressure

Three pathways to renin release by granular cells

  1. Direct stimulation of granular cells by sympathetic nervous system

  2. Stimulation by activated macula densa cells when filtrate NaCl concentration is low

  3. Reduced stretch of granular cells

Step 2: Tubular Reabsorption: Quickly reclaims most of tubular contents and returns them to blood. Selective transepithelial process where almost all organic nutrients are reabsorbed and water and ion reabsorption is hormonally regulated and adjuste. Includes active and passive tubular reabsorption

Substances can follow two routes:

  1. Transcellular route: Solute enters apical membrane of tubule cells. Travels through cytosol of tubule cells. Exits basolateral membrane of tubule cells. Enters blood through endothelium of peritubular capillaries

  2. Paracellular route: Between tubule cells. Limited by tight junctions, but leaky in proximal nephron. Water, Ca2+, Mg2+, K+, and some Na+ in the PCT move via this route

Sodium concentrates effects the water that is input.

Proximal convoluted tubule: Site of most reabsorption

  • All nutrients, such as glucose and amino acids,are reabsorbed

  • 65% of Na+ and water reabsorbed

  • Many ions

  • Almost all uric acid

  • About half of urea (later secreted back into filtrate)

Nephron loop

  • Descending limb: H2O can leave, solutes cannot

  • Ascending limb: H2O cannot leave, solutes can

  • Thin segment is passive to Na+ movement

  • Thick segment has Na+-K+-2Cl– symporters and Na+-H+ antiporters that transport Na+ into cell

  • Some Na+ can pass into cell by paracellular route in this area of limb

Distal convoluted tubule and collecting duct: Reabsorption is hormonally regulated in these areas

  1. Antidiuretic hormone (ADH): Released by posterior pituitary gland and an increased ADH levels cause an increase in water reabsorption

  2. Aldosterone: Targets collecting ducts (principal cells) and distal DCT and will cause little Na+ to leave the body. Without aldosterone, daily loss of filtered Na+ would be 2%, which is incompatible with life. Functions to increase blood pressure and decrease K+ levels.

  3. Atrial natriuretic peptide: Reduces blood Na+, resulting in decreased blood volume and blood pressure. Released by cardiac atrial cells if blood volume or pressure elevated

  4. Parathyroid hormone: Acts on DCT to increase Ca2+ reabsorption

Step 3 Tubular Secretion: Tubular secretion is reabsorption in reverse and it occurs almost completely in PCT. Selected substances are moved from peritublar capillaries through tubule cells out into filtrate. K+, H+, NH4+, creatinine, organic acids and bases

Two types of countercurrent mechanisms

  1. Countercurrent multiplier: interaction of filtrate flow in ascending/descending limbs of nephron loops of juxtamedullary nephrons

  2. Countercurrent exchanger: blood flow in ascending/descending limbs of vasa recta

Gradient in the Kidney: runs from 300 mOsm in cortex to 1200 mOsm at bottom of medulla. This gradient allows more water to move in the kidneys the deeper it gets into the kidneys.

Urine’s chemical composition:

  • 95% water and 5% solutes

  • Nitrogenous wastes

  • Urea (from amino acid breakdown): largest solute component

  • Uric acid (from nucleic acid metabolism)

  • Creatinine (metabolite of creatine phosphate)

  • Other normal solutes found in urine

    • Na+, K+, PO43–, and SO42–, Ca2+, Mg2+ and HCO3–

Urine’s physical characteristics

Color and Transparency:

  • Should be clear

    • Cloudy may indicate urinary tract infection

  • Pale to deep yellow from urochrome

    • Pigment from hemoglobin breakdown

    • Yellow color deepens with increased concentration

  • Abnormal color (pink, brown, smoky)

    • Can be caused by certain foods, bile pigments, blood, drugs

Odor of Urine

  • should be slightly aromatic when fresh.

    • Develops ammonia odor upon standing as bacteria metabolize urea

    • May be altered by some drugs or vegetables

  • Disease may alter smell

    • Patients with diabetes may have acetone smell to urine

pH of Urine

  • Urine is slightly acidic (~pH 6, with range of 4.5 to 8.0)

  • Acidic diet (protein, whole wheat) can cause drop in pH

  • Alkaline diet (vegetarian), prolonged vomiting, or urinary tract infections can cause an increase in pH

Specific gravity

  • Ratio of mass of substance to mass of equal volume of water (specific gravity of water = 1)

  • Ranges from 1.001 to 1.035 because urine is made up of water and solutes

Ureters: slender tubes that convey urine from kidneys to bladder. These are retroperitoneal and enter base of bladder through posterior wall. As bladder pressure increases, distal ends of ureters close, preventing backflow of urine

Urinary bladder anatomy: Muscular sac for temporary storage of urine. Retroperitoneal, on pelvic floor posterior to pubic symphysis

  1. Males: prostate inferior to bladder neck

  2. Females: anterior to vagina and uterus

Trigone: Smooth triangular area outlined by openings for ureters and urethra. Infections tend to persist in this region

Internal urethral sphincter: Involuntary (smooth muscle) at bladder-urethra junction and contracts to open

External urethral sphincter: Voluntary (skeletal) muscle surrounding urethra as it passes through pelvic floor

Female urethra (3–4 cm): tightly bound to anterior vaginal wall

  • External urethral orifice: anterior to vaginal opening; posterior to clitoris

Male urethra carries semen and urine and has three named regions

  • Prostatic urethra (2.5 cm): within prostate

  • Intermediate part of the urethra (membranous urethra) (2 cm): passes through urogenital diaphragm from prostate to beginning of penis

  • Spongy urethra (15 cm): passes through penis; opens via external urethral orifice

Micturition: also called urination or voiding