Learning Outline

Urinary System

A&P 2

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IMPORTANCE OF THE URINARY SYSTEM

Urinary system filters blood and thus helps to maintain:

Fluid/electrolyte balance and

Acid/base balance

Urinary system produces urine, but as a side-effect —NOT the primary function of the urinary system!

Fluid/Electrolyte homeostasis

Fluid compartments

"Compartments" are a convenient way to picture the fluids of the body

Extracellular fluid compartments (about a third of all body fluids)

Interstitial fluid (IF) — fluid between tissue cells

Plasma — fluid portion of the blood

Lymph — fluid in lymph nodes and vessels

Other: CSF, joint fluid, eyeball fluid, and so on

Intracellular fluid compartment (about 2/3 of all body fluids)

Includes cytosol of all cells

Fluid homeostasis

Balance is maintained for good health

Leaks prevented by blood clotting

Excesses or deficiencies of fluids in one tissue are simple "moved around" or "rationed" among all the other tissues by the plasma

Thus, no one tissue is very far out of balance (under- or overhydrated)

Instead, ALL tissues are out of whack just a little bit

Overall input and output of water to/from the body can be adjusted

Input

Primarily adjusted by the thirst mechanism

Liquid ingestion

Ingestion of moist solids

Osmoreceptors in the brain work with the hypothalamus to regulate the thirst mechanism

Subfornical organ (SFO) in roof of 3rd ventricle has osmoreceptors

Additional osmoreceptors in ADH-producing cells of hypothalamus

Metabolism (H₂O is a product of cellular respiration) supplies some water, but is not really adjusted to maintain water balance

Output

Primarily adjusted by changing output (volume) of urine from the kidneys

Also affected by these other outputs that are not adjusted to maintain water balance:

Respiratory (loss during expiration)

Digestive (feces)

Skin (sweating)

Usually easier to adjust output than input because input requires availability of water (which is not under physiological control)

Fluid balance regulated by hypothalamus

Electrolyte homeostasis

Electrolytes dissociate to form ions when dissolved in water

Cations are positive ions

Anions are negative ions

Distribution of various electrolytes differs between the intracellular fluid compartment and the extracellular fluid compartment

Cellular mechanisms of balance: sodium-potassium pump, calcium pump, other ion pumps

Extracellular mechanism of balance: urinary system

Functional anatomy of the urinary system

Macroscopic (gross) anatomy

Kidneys

Location: back of abdominal cavity at top of lower back (lumbar region)

Usually paired

Retroperitoneal — behind the parietal peritoneum

At level of T12-L1 (right kidney is slightly lower than left kidney)

Structure: bean-shaped paired organs covered with fibrous capsule

Size: from approx. 7.5 cm x 2.5 cm up to about 11.25 cm x 5 cm

Hilum

Medial "notch" where vessels/tubing enter/exit the kidney

Capsule

Outer wall of fibrous tissue

Cortex (a.k.a. renal cortex)

Outer region of kidney tissue

Medulla (a.k.a. renal medulla)

Inner (deeper) region of kidney tissue

Renal pyramids are cone-shaped pieces of kidney tissue that point toward the medial opening of the kidney

The tips of the pyramids are called renal papillae and have many tiny openings for the release of urine from the pyramids

Tissue between the pyramids is called "renal columns"

Plumbing

Urine from renal papillae is collected in branchlike tubes that drain into a basin called the renal pelvis

Branches that lead into the basin are called (major and minor) calyces (singular calyx, literally "cup")

Ureter drains urine from the pelvis of the kidney

Smooth muscle in wall; mucous lining; fibrous outer coat

Uses peristalsis to pump urine away from kidney

Ureter is retroperitoneal

Urinary bladder collects urine from ureters

Stores urine until a socially acceptable time and place to urinate

Lined with transitional epithelium capable of stretching greatly without damage

Trigone: 3-cornered floor of bladder

Posterior corners: left and right ureters enter

Anterior corner: urethra exits

Capacity: about 150 cc (more or less)

Urethra drains urine from bladder to outside of body

Urethra is longer in males (where it is also used to conduct semen) than in females

Urinary meatus is opening of urethra to the outside of body

Plumbing issues [choose "Urinary" then "Bladder..."]

Dysuria — difficulty or pain in urination

Kidney/bladder stones caused by "precipitation" of chemicals in the urine to form crystals [choose "Urinary" then "Kidney stones"]

Anuria — no urine output

Retention — retaining urine in the bladder

Suppression — failure of the kidney to form urine

Microscopic anatomy

Nephron

Bowman's capsule

Proximal (convoluted) tubule

Nephron loop (loop of Henle)

Distal (convoluted) tubule

Renal tubule

Nephron

Collecting duct (shared by several different nephrons)

Renal corpuscle: Bowman's capsule and glomerulus

Glomerulus

Ball-like network of capillaries

Supplied by an afferent arteriole

Drained by an efferent arteriole (which leads to a second network of capillaries after the glomerulus)

Endothelial cells of capillary wall has fenestrations or pores (like White Castle hamburgers)

Acts as a filter, keeping blood cells and proteins in blood and allowing water and small solutes to filter out of blood

Mesangial cells — "support cells" between the capillaries

Bowman's capsule

Surrounds glomerulus like a hollow cup

Inner wall of capsule adheres to outer walls of glomerular capillaries

Cells are spider-like cells called podocytes (lit. "foot cells")

Podocytes have pedicels or "toes" that interlock like a zipper to form a filtration membrane with slits (filtration slits)

Filtration slits are covered with a thin fibrous membrane (slit diaphragm)

Acts as second layer of filter

Filtration membrane

Glomerular capillary wall

Basement membrane

Inner wall of Bowman's capsule

Proximal [convoluted] tubule

Convoluted means it has a lot of twists and turns

Proximal refers to the fact that it is close to the beginning of the nephron

Nephron loop (Loop of Henle)

Hairpin turn of the nephron, dipping far down into the medulla (from the cortex, where most of the nephron is located)

Has a descending limb followed by an ascending limb

The ascending limb has a thick-walled region

Distal [convoluted] tubule

Drains filtrate from the loop of Henle

Collecting tubule (duct)

Drains filtrate from distal tubules of several different nephrons

Many collecting ducts converge at the renal papillae and release urine from the kidney tissue

Blood supply: Afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries (includes vasa recta)

Peritubular capillaries surround the entire nephron (except the Bowman's capsule, which has the glomerulus instead)

Vasa recta (lit. "vessels at a right angle") conduct blood down, then up, the outside of the loop of Henle

Two types of nephron

Cortical nephrons are further to the outside and have short nephron loops that do not reach into the medulla

Juxtamedullary nephrons are mostly in the cortex close to the medulla and have long nephron loops that dip far into the medulla (from the cortex)

URINARY PHYSIOLOGY

The basics

The basics—balancing of blood plasma & formation of urine [choose "Urinary" then "Urination"]

Importance: adjusts fluid and electrolyte balance of blood (thus, entire body)

Three essential functions:

Filtration

Reabsorption

Secretion

Bowman's capsule

Ultrafiltration (from glomerulus)

About 20% of plasma flow (most is later reabsorbed)

Adds up to about 50 gallons of filtrate per day (that's not a mistake —50 gallons that can potentially be released as urine!) (125 ml/min)

Glomerular filtration rate (GFR) influenced by blood pressure

Effective filtration pressure (EFP) is needed to maintain sufficient GFR

Proximal tubule

Reabsorption and secretion

Reabsorption of most of Na⁺ , Cl^- and H₂O

Sodium is transported actively

Chloride and water follow passively

Reabsorption of other solutes (passive - or actively cotransported with Na⁺)

Glucose transport maximum (Tmax)

Co-transported with Na⁺

Na⁺ is pumped from back of tubule cell, drawing more Na⁺ in through front of tubule cell (by way of passive carriers)

Carrier mechanism carry glucose at the same time

Thus, the active-transport driven movement of Na⁺ brings glucose "along for the ride"

Largest amount of glucose that can be transported at once

Determined by how many passive carriers for glucose you have (the more carriers, the higher the transport maximum)

pH adjustment (H⁺ secretion)

About half of the urea is reabsorbed passively here

Nephron loop (of Henle)

Creates/maintains osmotic gradient between medulla and cortex

Medulla's IF maintained at high saltiness

"saltiness" is measured as osmolality (units: mOsm)

high osmolality = high saltiness = high osmotic pressure (tendency to gain water by osmosis) = hypertonic

Most body fluids are isotonic to each other at about 300 mOsm

Medullary IF goes up to about 1200-1400 mOsm

Salt actively removed by ascending limb

This is the countercurrent multiplier mechanism

A "countercurrent mechanism" simply implies that fluid is flowing in opposite directions right alongside each other (as does highway traffic)

Makes IF hypertonic (1200 mOsm)

That is, IF has osmolality or high salt content

Urea from collecting duct adds to high osmolality of IF

Also makes filtrate hypotonic (low osmolality) 100 mOsm

Vasa recta also has a countercurrent flow

This reduces removal of solutes from interstitium

Compare to straight-line flow of blood, which would remove all the salt added to the IF by the loop of Henle

This mechanism is called "countercurrent exchange"

HINT: there are two different "countercurrent mechanisms"

Countercurrent multiplier mechanism in the loop of Henle

Increases saltiness of medullary IF (reduces saltiness of the filtrate)

Countercurrent exchange mechanism of the vasa recta

Reduces the rapid removal of salt from the medullary IF

Distal and collecting tubules

Secretion and adjustment of final urine osmolality

ADH (antidiuretic hormone from posterior pituitary gland)

Promotes tubule wall's H₂O permeability

H₂O can diffuse out of tubule into hypertonic IF

Adjusts final osmolality of urine

You can now have a range of osmolality of urine from 100 mOsm (hypotonic) to 300 mOsm (isotonic) to 1200 mOsm (hypertonic)

Depends on how much water the body needs to save (conserve) or get rid of to achieve balance (homeostasis)

Aldosterone (hormone from adrenal glands)

Increases K⁺ secretion

Thus, increases Na⁺ reabsorption (K⁺ is "traded" for Na⁺)

Makes urea a more dominant solute

Indirectly increases H₂O reabsorption (permitted by ADH)

Conserves plasma volume

Renin-angiotensin mechanism (see figure in textbook for details)

Involves renin from the juxtaglomerular (JG) cells at junction of distal tubule and afferent arteriole of the glomerulus

ANH (atrial natriuretic hormone from atrial walls of heart)

Triggered by increase in blood plasma volume, which stretches the atrial wall beyond normal

Increases Na⁺ loss by plasma

This in turn causes osmosis of H₂O out of blood and into the filtrate

Loss of water from blood tends to lower plasma volume

ANH opposes the action of aldosterone

Allows for fine-tuning of water content of body

Urine composition

Water (about 95%)

Ions (mostly sodium and chloride, along with some others including H⁺)

Urochromes (pigments)

Mostly bile pigments from breakdown of old RBCs in spleen, etc.

Could be some beta-carotenes from food / supplements

Wastes

Nitrogenous waste

Urea — waste of breaking down amino acids so they can be used for cellular respiration (in place of glucose)

Excess drugs, hormones, toxin

ACID - BASE BALANCE

Normal pH range

Blood plasma: 7.35 - 7.45

Changes as small as 0.1 pH unit can have profound effects on cellular functions

Acidosis

pH 7.34 - 6.80

Respiratory acidosis (if caused by respiratory mechanism)

Metabolic acidosis (if caused by anything else)

Alkalosis

pH 7.46 - 8.00

Respiratory alkalosis (if caused by respiratory mechanism)

Metabolic alkalosis (if caused by anything else)

pH-balancing mechanisms

Buffer mechanisms

In blood plasma

Several pairs of buffers

Bicarbonate system

Phosphate system

protein (Hb) system

Act to neutralize additions of acids or bases

Respiratory mechanism

Hyperventilation:

Decreases carbon dioxide (Pco₂)

Thus, raises pH

Hypoventilation:

Increases carbon dioxide (Pco₂)

Thus, lowers pH

Renal mechanisms

H⁺ secretion

Compensation

"Compensated acidosis" and "compensated alkalosis" refer to conditions where the body's mechanisms for balancing pH are attempting to maintain normal pH despite an abnormal disturbance to the usual pH scenario