Excretion · reabsorption, tubular secretion ... Blood Supply of a Nephron –The glomerulus receives blood from a fairly large afferent arteriole and passes it to a smaller efferent

Excretion• Physiology of ultrafiltration,

reabsorption, tubular secretion

• Counter current theory

• Regulation of urine formation

• Method of urine formation

• Nitrogenous waste

• Regulation of acid0base balance

• Single Cell organism: waste discharge directly via cell surface

• Aquatic animal: amonia

• Terrestrial animal: Urea , Uric acd

Nitrogen is excreted as ammonia,

urea or uric acid

The ammonia, produced from the deamination of amino acids, enters the

ornithine cycle where it is converted into urea

flatworm earthworm

Fig. 49.17(TE Art)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Glomerulus

Renal cortex

Renal medulla

Bowman'scapsule Proximal

convoluted tubule

Descending limb of loop of Henle

Loop of Henle

Distalconvolutedtubule

Ascending limb of loop of Henle

Collecting duct

To ureter

Vasa recta

Peritubularcapillaries

Blood Supply of a Nephron

– The glomerulus receives blood from a fairly large afferent

arteriole and passes it to a smaller efferent arteriole.

– The efferent arteriole gives rise to the peritubular capillary

system, which surrounds the renal tubule.

Juxtaglomerular Apparatus

– At the point of contact between the afferent and efferent

arterioles and the distal convoluted tubule, the epithelial cells

of the distal tubule form the macula densa.

– Near the macula densa on the afferent arteriole are smooth

muscle cells called juxtaglomerular cells.

– The macula densa together with the juxtaglomerular cells

make up the juxtaglomerular apparatus.

Copyright © 2006 by Elsevier, Inc.

Structure of

the juxtaglomerular

apparatus: macula

densa

Figure 26-17;

Guyton and Hall

Copyright 2009, John Wiley & Sons, Inc.

Nephron• Parts:

– Glomerular apparatus

– Proximal tubule

– Loop of Henle

– Distal tubule

– Collecting ducts

• Types of nephrons:

– Cortical nephrons (glomerularapparatus belong the surface andLoop of Henle only to the outer partof the medulla)

– Intermedial nephrons (in themiddle)

– Juxtamedullary nephrons(glomerular apparatus deep in cortexnear the medulla and Loop of Henleis going deep to the inner part of themedulla)

Function of urinary system• Keeping homeostasis

• Keeping acid-base balance

• Secretion (rennin, kallikrein, erytropoetin)

• The kidneys also help control the rate of red blood cell formation by secreting erythropoietin, and regulate blood pressure by secreting renin.

• The kidneys function to regulate the volume, composition, and pH of body fluids and remove metabolic wastes from the blood in the process.

• Excretion of metabolic waste products: urea, creatinine, bilirubin,

hydrogen

• Excretion of foreign chemicals: drugs, toxins, pesticides, food additives

• Secretion, metabolism, and excretion of hormones

- renal erythropoetic factor

- 1,25 dihydroxycholecalciferol (Vitamin D)

- Renin

• Gluconeogenesis: glucose synthesis from amino acids

• Control of arterial pressure

• Regulation of water & electrolyte excretion

RENAL PHYSIOLOGY

Ultrafiltration – filteringblood under pressure

Selective reabsorption –reabsorbing the usefulsubstances

Production of an iongradient in the medulla – toallow production ofhypertonic urine if necessary

Adjustment of the water andion content of the blood – tomaintain homestasis

.

ULTRAFILTRATION

NET FILTRATION PRESSURE (NFP)

=GBHP – CHP – BCOP

= 55 mmHg 15 mmHg 30 mmHg

= 10 mmHg

GLOMERULAR BLOOD

HYDROSTATIC PRESSURE

(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

1

Proximal convoluted tubule




= 10 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD


(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

12





= 10 mmHg

BLOOD COLLOID

OSMOTIC PRESSURE

(BCOP) = 30 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD


(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

12

3


ultrafiltration occurs at the barrier between the blood and the filtrate in

the renal capsule or Bowman's capsule in the kidneys. The Bowman's

capsule contains a dense capillary network called the glomerulus. Blood

flows into these capillaries through the afferent arteriole and leaves

through the efferent arteriole.

The high hydrostatic pressure forces small molecules such as water,

glucose, amino acids, sodium chloride and urea through the filter, from

the blood in the glomerular capsule across the basement membrane of the

Bowman's capsule and into the nephron. This process is called

ultrafiltration. The fluid filtered in this way is called glomerular filtrate.

Glomerular pressure is about 75 millimeters of mercury (10 kPa). It is

opposed by osmotic pressure (30 mmHg, 4.0 kPa) and hydrostatic

pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This

difference in pressure is called effective pressure (25 mmHg, 3.3 kPa).

Physiology of ultrafiltration:

Movement of useful substances (water, salts, glucose)

back into the capillaries.

Filtrate from glomerular capsule flows into the renal

tubule.

Substances are reabsorbed into the surrounding

capillaries.

Sodium is actively pumped out of the epithelial cells of

the renal tubule and diffuses into capillaries.

The increased sodium in the interstitial space creates

high osmotic pressure, which draws water out of the

tubule by osmosis.

REABSORPTION

Physiology of Reabsorption:

Glucose, various amino acid and water are removed from the tubular

fluid and transported into the blood. It is called reabsorption (and not

absorption) because these substances have already been absorbed once

(particularly in the intestines).

Reabsorption is a two-step process beginning with the active or passive

extraction of substances from the tubule fluid into the renal interstitium

(the connective tissue that surrounds the nephrons), and then the

transport of these substances from the interstitium into the bloodstream.

These transport processes are driven by Starling forces, diffusion, and

active transport.

Some key regulatory hormones for reabsorption include:

❖ aldosterone, which stimulates active sodium reabsorption (and

water as a result).

❖ antidiuretic hormone, which stimulates passive water reabsorption.

❖ both hormones exert their effects principally on the collecting ducts.

Tubular reabsorption

Tubular secretion

At the same time that the “good”

substances are being reabsorbed,

wastes (urea) still in the blood are

actively secreted from the capillaries.

SECRETION

Method of Urine formation

Urine Formation

• Proximal convoluted tubule and Peritubularcapillary

• Na+ goes down gradient and brings glucose, amino acids, etc. back into blood stream (cotransport).

• Reabsorbs about 65% of filtrate.

Urine Formation

• Descending limb

• Goes into medulla - increasing salt gradient

• Water leaves

• Fluid concentrates

• Ascending limb

• Goes up toward cortex - decreasing salt gradient

• Na+ pumped out

• Fluid relatively diluted

Countercurrent Multiplication

in the Nephron Loop

Nephron Loop

Collecting duct

COUNTERCURRENT

vs

COUNTERCURRENT EXCHANGER

Tow function types of countercurrent systems are recognized:

Active system and passive system

• The active system are countercurrent multiplier.

• The passive system are countercurrent exchanger.

(countercurrent diffusion exchanger)

In active system metabolic energy is used within countercurrent

system Itself to induce to flux of commodities into or out of the

fluid stream (energy is used to transport NaCl out of the

ascending limb)

The loop of Henle multiplies the sodium concentration within medulla by

retaining the new sodium ions coming from the glomerular filtrate. It is

called Counter Current Multiplier

Urine Composition

– Urine composition varies from time to time and reflects the amounts of

water and solutes that the kidneys eliminate to maintain homeostasis.

– Urine is 95% water, and also contains urea, uric acid, a trace of amino

acids, and electrolytes.

Urine Elimination

• After forming in the nephrons, urine passes from the collecting ducts

to the renal papillae, then to the minor and major calyces, and out the

renal pelvis to the ureters, urinary bladder, and finally to the urethra,

which conveys urine to the outside.

Regulation of Urine Concentration and

Volume

– Most of the sodium ions are reabsorbed before the urine is excreted

under the direction of the hormone, aldosterone

– Normally the distal convoluted tubule and collecting duct are

impermeable to water unless the hormone ADH is present.

- A countercurrent multiplier system is a mechanism that expends

energy to create a concentration gradient. It can refer to the process that is

underlying the process of urine concentration, that is, the production

of hyperosmotic urine by the mammalian kidney

Renal auto control: myogenic, Tuboglomerular

Neuronal control:

Hormonal control:


1. Sympathetic Nervous System

RA + RE GFR + RBF

Control of Glomerular Filtration

3. Angiotensin II

RE GFR + RBF

(prevents a decrease in GFR)

2. Catecholamines ( norepinephrine)

RA + RE GFR + RBF

(Renal Afferent, Renal Efferent, Glomerulus Filtration Rate, Renal Blood Flow)



5. Endothelial-Derived Nitric Oxide (EDRF)

RA + RE GFR + RBF

4. Prostaglandins

RA + RE GFR + RBF

6. Endothelin

RA + RE GFR + RBF



7. Autoregulation of GFR and Renal Blood Flow

• Myogenic Mechanism

• Macula Densa Feedback

(tubuloglomerular feedback)

• Angiotensin II ( contributes to GFR but

not RBF autoregulation)


Myogenic Mechanism

Stretch of

Blood Vessel

Cell Ca++

Permeability

Arterial

Pressure

Intracell. Ca++Blood Flow Vascular

Resistance


Macula densa

feedback

mechanism

for GFR

autoregulation

Figure 26-18;

Guyton and Hall


Regulation of GFR by Ang II

GFR Renin

AngII

Efferent Arteriolar

Resistance

Macula Densa NaCl

BloodPressure

When the sodium chloride concentration in the tubular fluid decreases, the macula densa senses

these changes and causes the juxtaglomerular cells to secrete renin.

The heart can also increase filtration rate when blood volume is high.

Glomerular filtration rate is relatively constant, although sympathetic impulses may

decrease the rate of filtration.

ACID-BASE REGULATION

Maintenance of an acceptable pH range in the

extracellular fluids is accomplished by three

mechanisms:

1) Chemical Buffers (blood)

React very rapidly

(less than a second)

2) Respiratory Regulation (lung)

Reacts rapidly (seconds to minutes)

3) Renal Regulation (kidney)

Reacts slowly (minutes to hours)

Kidneys role in acids and bases control• Bicarbonate is filtered and enters nephron at Bowman’s capsule

• Proximal convoluted tubule

– Can reabsorb all bicarbonate (when need it to neutralize excessive acids in body)

OR

– Can reabsorb some or NONE of the bicarbonate (have too much base in body and it needs to be eliminated)

How can the kidneys control acids and

bases?

• Acidosis

• Intercalated cells

– Secrete excessive hydrogen

– Secreted hydrogen binds to buffers in the lumen (ammonia and phosphate bases)

– Secretion of hydrogen leads to formation of bicarbonate

HPO4-

NH3

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Excretion · reabsorption, tubular secretion ... Blood Supply of a Nephron –The glomerulus receives blood from a fairly large afferent arteriole and passes it to a smaller efferent