Chapter 26: Urinary system

urinary system

consists of two kidneys, two ureters, one urinary bladder, and one urethra

urology

the branch of medicine that deals with the male and female urinary systems and the male reproductive system in called urology

urine

water and waste solutes removed from the body by the kidneys

8 Functions of the Kidneys

*(1) Regulation of blood ionic composition:*
- sodium and potassium
*(2) Regulation of blood pH:*
- excrete H+ and conserve bicarbonate
*(3) Regulation of blood volume:*
- conserve/ eliminate water
*(4) Enzymatic regulation of blood pressure:*
- *renin*, increases blood presure
*(5) Maintenance of blood osmolarity:*
- maintain balance of solutes dissolved in blood
*(6) Production of hormones:*
- Cacitriol (active Vit D) and Erythropoietin (RBC production) plus Renin (increases BP)
*(7) Regulation of blood glucose level:*
- amino acids for gluconeogenesis to release glucose into blood
*(8) Excretion of wastes and foreign substances:*
- excrete waste via urine

blood osmolarity

total concentration of all solute particles in blood

wastes

materials that have no functional use in the body
*Include:*
- ammonia and urea from deamination of amino acid
- bilirubin from catabolism of hemoglobin
- creatinine from breakkdown of creatine phosphate in muscles
- uric acid from catabolism of nucleic acids
- ingested *drugs and toxins*

kidneys

kidney-bean-shaped organs located just above the waist between the peritoneum and the posterior wall of the abdomen
- retroperitoneal (not inside peritoneum)
- *right kidney is lower* than left kidney because of displacement by liver

nephrology

scientific study of the anatomy, physiology, and pathology of the kidneys

renal hilum

a medial indentation where several structures enter or exit the kidney (ureters, renal blood and lymphatic vessels, and nerves)

3 Layers of Tissue in Kidneys

(1) fibrous capsule,

(2) perirenal fat capsule,

(3) renal fascia

fibrous capsule

a smooth, transparent sheet of dense irregular connective tissue

* continuous with outer layer of ureter
* barrier against trauma and helps maintain the shape of the kidney

perirenal fat capsule

a mass of fatty tissue surrounding the fibrous capsule

* protects from trauma
* holds kidney in place inside abdominal cavity

renal fascia

Collagenous and elastic dense irregular connective tissue

* anchors kidney to surrounding structures and the abdominal wall

2 Regions of the Kidneys

(1) Renal cortex
(lighter, superficial)

* renal columns

(2) Renal medulla (darker, deep)

* renal pyramids
* renal papilla

renal cortex

outer region of the kidney

renal medulla

inner region of the kidney

renal pyramid

cones-shaped mass of tissue within renal medulla

renal papilla

narrow, innermost tip of a renal pyramid

renal columns

extensions of the renal cortex in between the renal pyramids

renal lobe

consists of a renal pyramid, portion of the cortex at the pyramid base, and a portion of each adjacent renal column

parenchyma (of kidney)

renal cortex + renal pyramids of renal medulla;
the functional portion of the kidney

Nephrons

functional units of the kidneys.

nephrons

filtering units of the kidney that remove wastes from the blood
- form FILTRATE

papillary ducts

filtrate from nephron drains into this structure, which then drains into a minor calyx
- extend through renal papillae

minor calyx

cup that nestles the papilla of each pyramid
- collects filtrate from papillary duct
- delivers urine to major calyx

major calyx

cavity formed by the convergence of several minor calyces
- drain urine from minor calyx into the renal pelvis

Calyces

When does filtrate become urine?

renal pelvis

central collecting region in the kidney
- collects urine from major calyx
- drains urine into ureters and into urinary bladder

renal sinus

cavity within the kidney extended from hilum
*Contains:*
- calyces, renal pelvis, renal blood vessels, nerves, and lymphatics

renal arteries

two branches of the abdominal aorta that supply the kidneys
- further branch into segmental arteries

renal blood flow (RBF)

the blood flow through the kidneys
~ about 1200 mL per minute

segmental arteries

branches of the renal arteries as they enter the kidney
- supply different segments of the kidney
- further branch into interlobar arteries

interlobar arteries

branches of segmental arteries
- pass between renal lobes (through renal columns)
- further branch into arcuate arteries

arcuate arteries

branches of interlobar arteries that arch between the renal medulla and cortex
- pass through base (superficial) of renal pyramids
- further branch into cortical radiate arteries

cortical radiate arteries

branches of arcuate arteries which run upward through the renal cortex
- further branch into afferent arterioles

afferent arterioles

branches of the cortical radiate arteries that each supply ONE nephron
- branches into glomerulus inside the nephron (network of capillaries) which filters blood
- reconverges as an efferent arteriole

\-- thicker than efferent arteriole

efferent arterioles

blood vessel exiting the glomerulus
- convergence of blood capillaries inside glomerulus
- caries filtered blood away from glomerulus
- further branch into peritubular capillaries

\-- thinner than afferent arteriole

peritubular capillaries

branches of efferent arterioles that surround tubules of nephron in renal cortex
- directly receives substances from the tubular cells of nephron

vasa recta

present in some nephrons; capillary branches that supply tubular portions of the nephron in the renal medulla

peritubular venules

merging of peritubular capillaries

cortical radiate veins

transports blood from a peritubular venules to arcuate veins
- also receive blood from vasa recta

arcuate veins

receives blood that drains from the cortical radiate veins and feeds into the interlobar veins

interlobar veins

receives blood that drains from the arcuate veins before feeding it into the renal veins

renal vein

single blood vessel that carries blood away from each kidney towards heart
- exits renal hilum and drains into inferior vena cava

Flow of Blood Supply to Kidneys

abdominal aorta --\>
renal artery --\>
segmental artery* --\>
interlobar artery --\>
arcuate artery --\>
cortical radiate arteries --\>
afferent arterioles --\>
glomerulus (filtration) --\>
efferent arterioles --\>
peritubular capillaries --\>
peritubular venules --\>
cortical radiate veins --\>
arcuate veins --\>
interlobar veins --\>
renal vein --\>
inferior vena cava --\>
heart

Sympathetic part

Renal nerves are part of what in the autonomic nervous system

nephron

functional unit of the kidney
*Consists of:*
- renal corpuscle
- renal tubule

renal corpuscle

filters blood plasma
*Consists of two parts:*
- glomerulus
- Bowman's (glomerulus) capsule

renal tubule

tubular portion of the nephron that carries the filtrate away from the glomerular capsule
- site where tubular reabsorption and secretion occur

glomerulus

a network of capillaries surrounded by Bowman's capsule in the nephron
- the site of filtration in the kidney

Bowman's capsule

double-walled cup with a cavity between the two layers that encases and receives filtrate from the glomerulus

Proximal convoluted tubule

Nephron loop

Distal convoluted tubule

Three components of renal tubule

Flow of Filtrate through Renal Tubule

(1) proximal convoluted tubule
(2) Nephron loop
*also "Loop of Henle"
- descending limb
- ascending limb
(3) distal convoluted tubule
(4) collecting duct

\-- which drains into Papillary Duct

collecting duct

duct into which several distal convoluted tubules of several nephrons empty
- water reabsorption is regulated via antidiuretic hormone (ADH)

papillary duct

formed by merging of several collecting ducts
- empties into minor calyx

nephron loop (loop of Henle)

the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule

descending limb of nephron loop

contains fluid that becomes more concentrated as it moves down into the medulla

ascending limb of nephron loop

signals and monitors concentration of solute in the filtrate

Cortical Nephrons vs. Juxtamedullary Nephrons

*Cortical nephrons:*
- short loops of Henle
- blood supply from peritubular capillaries

*Juxtamedullary nephrons:*
- long loops of Henle
- blood supple from BOTH peritubular capillaries and vasa recta

cortical nephrons

85% of nephrons;
almost entirely located within cortex

juxtamedullary nephrons

- renal corpuscle deep in cortex
- long loops of Henle
*Two portions of the ascending limb:*
- thin ascending limb, followed by:
- thick ascending limb

\-- Enable the kidneys to excrete very concentrated urine

Histology of the Nephron & Collecting Duct

SINGLE layer of epithelial cells forms walls of entire tube (Bowman's capsule, renal tubule, ducts)
*Distinctive features due to function:*
- Bowman's capsule: podocytes (visceral layer) + simple squamous epithelium (parietal layer)
- Descending limb + Thin ascending limb: simple squamous epithelium
- Thick ascending limb: simple cuboidal to low columnar epithelium; *Macula densa*

Histology of Bowman'e Capsule

(1) Parietal layer:
- outer wall of capsule
- simple squamous epithelium
(2) Capsular space
- receives filtrate from glomerulus
(3) Visceral layer
- inner wall of capsule
- podocytes (wrap around glomerulus)

podocytes

modified simple squamous cells that form a porous membrane surrounding the endothelial cells of the glomerulus

What type of cells does the visceral layer of Bowman's capsule consist of?

*Podocytes.*
- These cells wrap around the capillaries of the glomerulus and help create filtrate that flows into the capsular space.
-Prevents too big molecules from passing through into filtrate.

What feature do cells of the Proximal Convoluted Tubule (PCT) have?

- The proximal convoluted tubule contains simple cuboidal cells with prominent *microvilli*.
- Function for *reabsorption and secretion.*

What structure does the ascending loop make contact with?

The ascending limb makes contact with the afferent arteriole of the renal corpuscle.

macula densa

receptor cells in ascending limb that monitor filtrate osmolarity
and filtration rate
- Drop in osmolarity: dilate the afferent arteriole (increase the blood pressure in the glomerulus and increase filtration)
- stimulates the juxtaglomerular (JG) cells to secrete *renin* (to raise systemic blood pressure)

juxtaglomerular (JG) cells

Modified smooth muscle cells in afferent arteriole across from ascending limb
- stimulated by macula densa (change in osmolarity of filtrate)
- dilate or constrict the arterioles (modify blood pressure)

juxtaglomerular apparatus

*macula densa* (ascending limb) plus *JG cells* (afferent arteriole) - helps regulate blood pressure within kidneys

What 2 types of cells are present in the Distal convoluted Tubule and Collecting Duct?

(1) principal cells
(2) intercalated cells

principal cells

contain receptors for anti-diuretic hormone (ADH) and aldosterone
- Regulate ion (sodium and potassium) balance

intercalated cells

help to manage blood pH

3 Functions of Nephrons

(1) Glomerular filtration
(2) Tubular reabsorption
(3) Tubular secretion

glomerular filtration

blood plasma and dissolved substances (smaller than most proteins) get filtered from glomerulus capillary walls into the glomerular capsule (then into renal tubule)

tubular reabsorption

RETURNS 99% of water and useful solutes from the filtrate in renal tubule into the blood
- water and solutes returned to blood via peritubular capillaries and vasa recta

tubular secretion

REMOVES substances (wastes, drugs, excess ions) from blood in the peritubular capillary into the renal tubule

\---\> molecules too big for glomerulus to filter get dumped into filtrate here via Secretion

Where does glomerular filtration occur?

Renal corpuscle
- Bowman's capsule
- Glomerulus

Where does tubular reabsorption and secretion occur?

(1) Renal tubule
(2) Collecting duct

What is urine?

- Solutes and the fluid that drain into the minor and major calyces and renal pelvis.
- They are excreted via ureters.

What do the 3 functions of the Nephron collectively do?

By filtering, reabsorbing, and secreting, nephrons help *maintain homeostasis of blood* volume/ composition.

glomerular filtrate

fluid that enters capsular space
(has been filtered by glomerulus)

Filtration fraction

Fraction of blood plasma in the afferent glomerular arterioles of the kidneys that becomes glomerular filtrate

* 0.16-0.20 typical value

filtration membrane

a 3 layer leaky barrier that permits filtration of water and small solutes into the capsular space (then into renal tubule)

3 Layers of Filtration Membrane

(1) Glomerular endothelial cells

(2) Basement membrane of glomerulus

(3) Pedicels (extensions) of podocytes

Glomerular endothelial cells

permit all solutes in blood plasma to exit the glomerulus but prevents filtration of blood cells and platelets

mesangial cells

contractile cells that help regulate glomerular filtration
- relaxation provides maximal surface area for filtration
- contraction decreases surface area (and glomerular filtration)

Basement membrane

porous layer of acellular material between the endothelium and the podocytes

pedicels of podocytes

extensions of podocytes that wrap around glomerulus;
contain filtration slits that permit only small molecules to pass through

Why is the volume of fluid filtered by the renal corpuscle much larger than the volume of other capillaries of the body?

(1) Glomeruli provide large surface area for filtration (mesangial cells control how much surface area is available).
(2) Filtration membrane is thin and porous.
(3) Blood pressure in glomeruli is high, which produces more filtrate. Thinner efferent arteriole compared to afferent arteriole.

Three main pressure of glomerular filtration

(1) Glomerular blood hydrostatic pressure

(2) Capsular hydrostatic pressure

(3) Blood colloid osmotic pressure

Glomerular blood hydrostatic pressure

blood pressure in glomerular capillaries

* 55 mmHg
* promotes filtration by forcing water and solutes in blood plasma through the filtration membrane

Capsular hydrostatic pressure

hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule

* opposes filtration
* “back pressure” of about 15 mmHg

Blood colloid osmotic pressure

due to the presence of proteins such as albumin, globulins

* fibrinogen in blood plasma, also opposes filtration.
* The average BCOP in glomerular capillaries is 30 mmHg

edema

Abnormally high volume of interstitial fluid.

Glomerular filtration rate

The amount of filtrate formed in all renal corpuscles of both kidneys each minute

* 125 mL/min in males and 105 mL/ min in females

(1) Renal autoregulation

(2) Neural regulation

(3) Hormonal regulation

Three mechanisms that regulate glomerular filtration rate

Myogenic mechanism

__Increased stretching of smooth muscle fibers__ in afferent glomerular arteriole walls due to increased blood pressure.

Decreases GFR

Tubuloglomerular feedback

__Rapid delivery of Na+ and Cl− to the macula densa__ due to high systemic blood pressure

Decreases GFR

Neural regulation

Increase in activity level of __renal sympathetic nerves__ releases __norepinephrine__.

Decreases GFR