BME 321: The Urinary System Quiz #6

Main Components of Urinary System & Basic Function

1. Kidneys - produce urine

2. Ureters - transport urine to urinary bladder

3. Urinary Bladder - temporarily stores urine prior to elimination

4. Urethra - conducts urine to exterior

Main Functions of Urinary System

1. Excretion - removal of organic waste products from body fluids

2. Elimination - discharge of wastes into the environment

3. Homeostatic Regulations - monitor volume and solute concentration of blood plasma by conserving or excreting substances

Other homeostatic functions of the urinary system

• regulation of blood volume/pressure via release of renin

• regulation of plasma concentrations of Na, K, Cl, and other ions

• Stabilize blood pH

• conserving valuable nutrients

• assist liver in detoxifying process

Hilum of the kidney

• prominent medial indentation of kidney

• point of entry for renal artery and renal nerves

• point of exit for renal vein and ureter

Position of kidneys

• between T12 and L3

• left kidney sits slightly higher than the right

• retroperitoneal

• protected by organs anteriorly and body muscle/ribs laterally and posteriorly

Maintaining position of kidneys

1. overlying peritoneum

2. contact with adjacent visceral organs

3. supporting connective tissues

Concentris layers of tissue surrounding kidney

1. Fibrous Capsule - layer of collagen fibres that cover outer surface of organ

2. Perinephric Fat Capsule - thick layer of adipose tissue that surround fibrous capsule

3. Renal Fascia - dense fibrous outer layer that anchors kidneys to surrounding structures. Collagen fibres extend out from Fibrous Capsule to this layer

Renal Pyramid

conical structure extending from cortex to a tip called the renal papilla

Renal column

band of granualr tissue that separates adjacent renal pyramids

Kidney lobe

• 6-8 per kidney

• includes renal pyramid, overlying renal cortex, and adjacent tissue of renal columns

Renal medulla

extends from renal cortex to renal sinus

Renal cortex

superficial portion of kidney in contact with fibrous capsule

Renal sinus

fibrous capsule stabilized positions of ureter, the renal blood vessels, and renal nerves

Fibrous capsule

covering outer surface of kidney

Minor calyx

collects urine produced by single kidney lobe

Major calyx

forms by fusion of 4-5 minor calyces

Renal pelvis

large, funnel-shapped structure that collects urine from major calyces and is continuous with the ureter

Nephron (+ types and locations)

Nephron: microscopic structure that performs the essential functions of the kidney

2 types:

1. Cortical nephrons (85%) - located in superficial cortex

2. Juxtamedullary nephrons (15%) - long, nephron loops extend deep into medulla

Cortical Nephrons

Function

• Most abundant

• Responsible for most of the regulatory functions of the kidney

Juxtamedullary Nephrons

Function

• Small percentage of total nephrons

• Crucial role in establishing conditions in renal medulla that are needed for water conservation and production of concentrated urine

Anatomy of Nephron

Components and functions

1. Proximal convoluted tubule (PCT) - reabsorb nutrients from filtrate

2. Renal corpuscle - spherical structure for production of filtrate

3. Nephron loop - establishes osmotic gradient in renal medulla

4. Distal convoluted tubule (DCT) - adjusts composition of tubular fluid via secretion and absorption

Anatomy of Nephron

Collecting system

1. Collecting duct - carries tubular fluid through osmotic gradient in renal medulla

2. Papillary duct - collects tubular fluid from multiple collecting ducts and delivers to minor calyx

Circulation in Kidneys

Arterial supply

Renal artery >

segmental arteries >

interlobal arteries >

arcuate arteries >

cortical radiate arteries >

afferent arterioles

Circulation in Kidneys

Nephron steps

1. Afferent arteriole delivers blood to glomerulus (filtration occurs)

2. Efferent arteriole carries blood from glomerulus to peritubular capillaries

3. Peritubular capillaries collect water and solutes reabsorbed by the nephron and deliver other solutes to nephron for secretion

Circulation in Kidneys

Juxtamedullary nephron steps

Peritubular capillaries are connected to vasa recta

Vasa recta - long straight capillaries that are parallel to the nephron loop

Capillaries of vasa recta collect and transport water and solutes within the renal medulla

Circulation in Kidneys

Venous system

From glomerulus >

Cortical radiate veins >

Arcuate veins >

Interlobal veins >

Renal vein

Nervous Supply to the Kidneys

Innervation via renal nerves from the superior mesenteric ganglion

Mostly sympathetic fibres

Enters kidney at hilum

Follows arterial supply to each nephron

Parasympathetic innervation via vagus nerve

Nervous Innervation

Function

Can adjust rates of urine formation by changing blood flow and pressure at nephron

Stimulate release of renin, which restricts losses of water and salt in urine by stimulation reabsorption at kidney

Overview of Renal Physiology

Goal of urine production

Maintain homeostasis by regulating volume and composition of blood

Involves secretion of solutes and metabolic waste products

3 Main Organic Waste Products

1. Urea - most abundant, by-product of breakdown of amino acids in the liver

2. Creatinine - generated in skeletal muscle tissue through breakdown of creatine phosphate

3. Uric Acid - waste product formed during recycling of nitrogenous bases of RNA molecules

Basic Steps of Urine Formation

1. Filtration

2. Reabsorption

3. Secretion

Filtration

process

Blood pressure forces water and solutes across wall of glomerular capillaries and into capsular space

Small solutes pass through filtration membrane carried by water molecules

Based on size of molecule

Reabsorption

process

Removal of water and solutes from filtrate

Solutes move across tubular epithelium into peritubular fluid

Involving simple diffusion or activity of carrier proteins in tubular epithelium

Secretion

process

Transport of solutes from peritubular fluid across tubular epithelium and into tubular fluid

Necessary because filtration does not force all the dissolved materials out of the plasma

Renal Threshold

Plasma concentration at which a specific compound or ion starts to appear in urine

Due to saturation of transport mechanism

Transport maximum determines renal threshold

Varies with substance

Anatomy of Renal Corpuscle

• Capsular space - separates parietal and visceral layers of glomerular capsul

• Juxtaglomerular complex - specialized cells that secrete renin when glomerular blood pressure falls

• Mesangial cells - special supporting cells, lie between adjacent glomerular capillaries

  • contract or relax to control capillary diameter and rate of capillary blood flow

Filtration at Renal Corpuscle

Filtration membrane

Capillary knot of glomerulus projects into the capsular space

Visceral layer of glomerular capsule consists of large cells “podocytes” that had feet that wrap around the glomerular capillaries

Materials passing out of blood must be small enough to fit through the filtration slits of adjacent pedicels

Glomerular capillaries are fenestrated (pores)

Basal lamina of capillaries is a specialized dense layer that has a negative charge to repel other negative charged molecules

Components of Filtration Membrane

Fenestrated capillaries

Dense layer

Filtration slits

Glomerular Filtration

Primary factor is balance between hydrostatic and osmotic colloid pressures

Glomerular Hydrostatic Pressure (GHP)

Blood pressure inside the glomerular capillaries

Tends to push water and solutes out of the blood

Averages about 50 mm Hg

Blood Colloid Osmotic Pressure (BCOP)

Tends to draw water out of filtrate and into plasma

Averages about 25 mm Hg

Capsular Hydrostatic Pressure (CsHP)

Opposes GHP

Due to resistance of filtrate already present in the nephron that must be pushed towards the renal pelvis

Averages 15 mm Hg

Filtration Pressure

Definition and formula

Net pressure acting across glomerular capillaries

Represents sums of hydrostatic pressures and colloid osmotic pressures

Averages 100 mm Hg out of capillaries

FP = GHP - BCOP - CsHP

Glomerular Filtration Rate (GFR)

Definition, how it’s measured, formula

Rate the kidneys produce filtrate at each minute

Roughly 10% of fluid to kidneys via the renal arteries leaves the bloodstream and enters the capsular space

Creatinine clearance test used to estimate the GFR

GFR = (creatinine/hour)/(plasma concentration)

Factors Affecting GFR

Affected by filtration pressure across glomerular capillaries

Seen with drop in renal blood pressure

i.e. kidneys are much more sensitive to changes in blood pressure (filtration pressure) than other organs

Control of Glomerular Filtration Rate

3 processes

Autoregulation

Hormonal regulation

Autonomic regulation

Autoregulation Control

Hormonal and Neural Regulation

Hormones involved due to BP changes

Decreased BP: renin released from the juxtaglomerular complex, resulting in angiotensin II

Increased BP: natriuretic peptides (ANP and BNP)

Triggers for Renin Release

1. Decline in blood pressure at glomerulus

2. Stimulation juxtaglomerular cells by sympathetic stimulation

3. Decline in osmotic concentration of the tubular fluid at Macula Densa

Role of Angiotensin II

At Nephron: constriction of efferent arteriole, elevating glomerular pressure and filtration rates

At Suprarenal Glands: stimulates secretion of aldosterone

In CNS: sensation of thirst, trigger ADH release, increase sympathetic motor tone, mobilizing venous reserve, and increasing cardiac output

In Peripheral Capillary Beds: brief but powerful constriction of arteriole and percapillary sphincters

Natriuretic Peptides

ANP and BNP

Natriuretic peptides released from heart when walls become stretched with increased venous returns (acts opposite of angiotensin II)

ANP and BNP promote loss of Na+ and water at kidneys, inhibit renin release, and secretion of ADH and aldosterone

Trigger dilation of afferent glomerular arterioles and constriction of efferent arterioles (elevates glomerular pressure and increase GFR)

Autonomic Regulation

& it’s effect on GFR

Innervation is mostly sympathetic postganglionic fibres

Effect on GFR: powerful vasoconstriction of afferent arterioles, decreasing GFR and slowing production of filtrate

Reabsorption at PCT

PCT reabsorbs 60-70% of the volume of filtrate produced at the renal corpuscle

Reabsorbed materials enter peritubular fluid, diffuse into peritubular capillaries, and returned to circulation

Functions of the PCT

1. Reabsorption of Organic Nutrients

2. Active Reabsorption of Ions (ion pumps)

3. Reabsorption of Water (due to changes in osmotic concentrations)

4. Passive Reabsorption of Ions (passive diffusion)

5. Secretion

Nephron Loop Reabsorption

50% of water and ⅔ of Na and Cl ions will be reabsorbed from the tubular fluid

Done via countercurrent exchange

Reason for Countercurrent Exchange

Thin descending limb and thick ascending limb of nephron loop lie very close together, so exchange occurs between them, “Countercurrent Multiplication”

Occurs due to different permeabilities of ascending/descending limbs

“Countercurrent” because the exchange occurs between fluids moving in opposite directions

“Multiplication” because the exchange increases as movement of fluid continues

Descending vs. Ascending Limb

Nephron Loop

Thin descending limb permeable to water, but relatively impermeable to solutes

Thick ascending limb impermeable to water and solutes, but has active transport mechanisms for pumping sodium and chloride ions from tubular fluid to peritubular fluid

Steps of Countercurrent Multiplication

1. Na and Cl pumped out of thick ascending limb into peritubular fluid

2. Pumping elevates osmotic concentration of peritubular fluid around thin descending limb

3. Results in osmotic flow of water out of thin descending limb and into peritubular fluid, increasing solute concentration in thin descending limb

4. Arrival of highly concentrated solution in thick ascending limb accelerates transport of Na and Cl ions out

Rate of Ion Transport

Across thick ascending limb

Proportional to ion concentration in tubular fluid

More Na and Cl ions pumped into peritubular fluid in the medulla than seen at the cortex (concentration gradient)

Maximum solute concentration in turn of loop ~1200 mOsm/L, ⅔ due to Na and Cl ions, rest due to urea

Urea concentration rises as water is reabsorbed

Benefits of Countercurrent Multiplication

1. Efficiently reabsorbs solutes and water before tubular fluid reaches DCT and collecting system

2. Establishes a concentration gradient that permits passive reabsorption of water from the tubular system in collecting system (regulated by ADH)

How does Countercurrent Exchange Occur?

1. Blood flowing through vasa recta deep in renal medulla

   • Alongside the ascending limb, water diffuses out of capillaries by osmosis.

2. Salts in interstitial fluid enter vasa recta by diffusion along concentration gradients (blood losing water and gaining salts)

   • Total salt concentration in blood increases.

3. Blood flow ascends in vasa recta up alongside descending limb of nephron loop back towards the cortex

   • Salts go into area of reversed osmotic and solute gradients, so salts diffuse back out of the blood into the intersitital fluid, and water moves back into the vasa recta

Maintaining Ion Concentrations

Countercurrent Exchange System

Concentration gradients of solutes are maintained in the interstitial fluid as blood flows through vasa recta