The Urinary Tract

The Urinary Tract • Consists of four main parts paired kidneys & ureters, a single bladder & urethra • The urinary tract is a pathway for the elimination of metabolic by-products and toxic and other non-essential molecules all dissolved in a small amount of water (urine)

Kidney Structure & Function • Excretion: removal of metabolic by-products, toxic molecules & other non-essential molecules • Conservation of water, as necessary • Maintenance of acid-base balance in the blood

The Ureters & Kidney Stones • The ureters carry the urine formed in the kidneys to the bladder • Ureters are fibromuscular tubes, lined by transitional epithelium. • Their narrow structure makes them prone to being obstructed by mineralized concentrations (“stones”) from the kidneys. • If the stones are too large to pass naturally, they can be crushed using shockwave therapy.

Urinary Bladder & Urethra

• The urinary bladder is also a fibromuscular structure and the mucosa is lined with transitional epithelium • The bladder can hold as little as 50mL of urine or up to 7001000mL without injury • As it distends, it rises into the abdominal cavity and bulges posteriorly • The urethra is a fibromuscular glandular structure also lined with transitional epithelium • The male urethra is larger (20cm) than the female (4cm) – Males are more prone to urethritis (swelling & irritation of urethra due to viral or bacterial infection; painful urination, blood may be in the urine) – Females are more prone to cystitis – bladder inflammation, usually bacterial (UTI)

Kidney Structure I • The kidneys are about the size of a clenched fist and lie against the back abdominal wall just above the waistline • The kidney consists of filtering capsules, tubules, and blood vessels tightly packed together into what is called the parenchyma. • The outer covering of the kidney, called the capsule, is thin but tough and fibrous • When cut open, two regions appear and outer cortex and inner medulla. • A microscopic view will reveal the unit of kidney function, the nephron

Kidney Structure II • The cortex consists of convoluted tubules and filtering capsules • The apex of each medullary pyramid forms a papilla which fits into a cupshaped funnel called the minor calyx • These funnels open up into a major calaces which then open into the renal pelvis in the area called the renal sinus. The renal pelvis narrow to form the proximal ureter, sharing the area with the renal artery and vein

Renal Blood Flow • Renal blood flow (the amount of blood that flows through the kidney) is app 1300mL per minute (through both kidneys) • About 125-130mL of plasma is filtered into the renal tubular systems each minute. • Less that 1% of the filtered plasma (~0.7mL) is actually excreted as urine, demonstrating the important role kidneys play in water conservation.

The Nephron • The functional unit of the kidney is a nephron; There are about 1 million per kidney • A uriniferous tubule consists of a nephron and a collecting tubule

Uriniferous Tubule

Nephron Structures & Functions • Renal Artery: carries blood to the kidney • Glomerulus: cluster of capillary like vessels where filtrate leaves blood • Bowman’s (renal) capsul: surrounds the glomerulus; accepts/collects filtrate • Proximal Convoluted Tubule: where tubular reabsorption begins; NaCl, HCO3-, H2O, glucose & Aas, poisons & H+ filtered (secreted for removal) • Loop of Henle: H2O reabsorbed going down, NaCl reabsorbed going up; first passively than actively • Distal Convoluted Tubule: more NaCl and bicarbonate reabsorbed; some drugs & poisons filtered • Collecting Tubule: carries urine to ureter; also further reabsorption of H2O, NaCl & urea

A closer look at the Renal Capsule

The Loop of Henle

Summary Video Clip

Urine • Under normal resting conditions, the kidneys, comprising less than 0.5% of the body weight, receive 25% of the cardiac output. • Of the 1300mL of blood that enter through the renal arteries every minute, 1290-1299mL leave through the renal veins. The remaining 1-2mL leave as urine via the ureter. • Urine is primarily water, salt, small amounts of acid and a variety of waste products such as urea, however urine composition and volume change to compensate for any fluctuation in volume or composition of body fluids. • Kidneys are the “guardians” of the internal environment, reworking the body fluids 15x/day (hydration, pH etc.)

Normal Characteristics of Urine: Color & Transparency • Ranges from yellow to amber depending on urochrome (pigment resulting from hemoglobin destruction) concentration • The more concentrated the urine, the deeper the yellow color • Abnormal coloration (pink, brown, smoky tinge)may result from eating certain foods (beets) or presence of bile pigments or blood in the urine • Some drugs and vitamins alter urine color (riboflavin causes the bright “neon” yellow/green) • Cloudy urine may indicate infection in the urinary tract

Normal Characteristics of Urine: Odor & pH • Fresh urine should be only slightly aromatic • Left to stand, an ammonia odor will develop as bacteria metabolize urea solutes • Some drugs and vegetables alter odor (asparagus) • Diseases may also alter odor (diabetes mellitus can produce a fruity odor due to acetone content) • Normally urine is slightly acidic (pH ~6) • Metabolic or dietary changes can alter range (~4.5-8) – Diets high in protein and whole wheat lowers pH – Diets high in vegetables raise pH4 – Prolonged vomiting or urinary tract infection creates alkaline urine

Normal Characteristics of Urine: Specific Gravity

• Because urine is water plus solutes, it is more dense and weighs more than distilled water. • Specific gravity is the comparison of the weight of a substance to the weight of an equal volume of distilled water. • The specific gravity of distilled water is 1.0, the specific gravity of urine ranges from 1.001 to 1.035, depending on solute concentration • In extremely concentrated urine, solutes will precipitate out of solution

Normal Characteristics of Urine: Chemical Compostion

• Approximately 95% of the volume of urine is water, the remaining 5% is solutes • The largest component of urine, by weight, is urea, derived from the normal break down of amino acids • Other nitrogenous wastes include uric acid (from nucleic acid metabolism) and creatinine (from CP metabolism) • Normal solute constituents in order of decreasing concentration are: urea, sodium, potassium, phosphate and sulfate ions. Much smaller and variable amounts of calcium, magnesium & bicarbonate ions may also be found. • Abnormally high concentration of any solute may indicate pathology. • Certain diseases cause dramatic changes in composition: glucose, blood proteins, RBCs, hemoglobin, WBCs (pus) or bile pigments. Such substances are a sign of disease and useful in diagnosis.