GENERAL FUNCTIONS OF THE KIDNEYS
AND EXCRETORY SYSTEM.
LEARNING OBJECTIVES
At the end of Lecture, students will be able to know:
1) Introduction to the Excretory System.0
2) The parts of the Excretory System.
3) The Histology of the Functional unit —Nephron.
4) The Primary functions of the Kidneys.
5) The process achieving the functions.
Gross Anatomy of the Kidney.
Major Functions of the Kidneys:
1. Regulation of:
body fluid osmolarity and volume
electrolyte balance
acid-base balance
blood pressure
2. Excretion of
metabolic products
foreign substances (pesticides, chemicals etc.)
excess substances (water, etc)
3. Secretion of
erythropoitin
1,25-dihydroxy vitamin D3 (vitamin D activation)
renin
prostaglandin
Physiological Anatomy of Kidney:
1)Two on either side ,each weighing 150 gms in an adult.
2)Bean shaped indentation in the middle of its medial side - ―HILUM‖
3)Renal artery ,vein ,ureter,lymphatic and nerves enter/exit via the hilum.
LONGITUDINAL SECTION:
1) It shows two zones 1)cortex 2)medulla
MODULLARY ZONE:
1) It shows pyramid shaped structure called Pyramids. The narrow apex fits into a small tube called minor calyx.
2) Several minor calyces join to form major calyces.
3) Three to four major calyces join to form renal pelvis.
4) Renal pelvis →ureter→urinary bladder.
URINARY TRACT:
1)Components are ureter →urinary
bladder→urethra.
1.
Nephron and Collecting Duct:
Nephron: The functional unit of the
kidney
Each kidney is made up of about 1
million nephrons
Each nephron has two major
components:
1) A Bowman ‗s capsule
2) A Renal tubule.
•
Bowman’s capsule:
–
Glomerulus(tuft of capillaries)
–
Renal tubule:
–
proximal convoluted tubule (pct)
–
loop of Henle
–
•
•
descending limb
•
ascending limb
distal convoluted tubule
many nephrons connect to collecting duct.
TYPES OF NEPHRONS
Cortical nephron – glomeruli in outer cortex & short loops of Henle that extend only short distance into medulla1)Blood flow through cortex is rapid.
2) Majority of nephrons are cortical .
3)Cortical interstitial fluid - 300 mOsmoles.
4)Efferent arteriole form‖Peritubular arteries‖.
Juxtamedullary nephron – glomeruli in inner part of cortex & long loops of Henle which extend deeply into
medulla.
1)Blood flow through vasa recta in medulla is slow . 2)Medullary interstitial fluid is hyperosmotic .
3)This nephron maintains osmolality in addition to filtering blood and maintaining acid-base balance.
The Renal Corpuscle
Composed of Bowman‘s capsule and Glomerulus.
Renal
and
Tubules
Collecting
dut:
The
Juxtaglom
erular
apparatus
Including macula
densa, extraglumerular
mesangial cells, and
juxtaglomerular
(granular cells) cells.
JUXTA-GLOMERULAR CELLS
1) Modified myo-epithelial cells of the efferent arteriole which comes in contact with the ascending limb of Henle.
2)Contain granules of ―RENIN‖.
MACULA DENSA :
1)Modified living epithelial cells of the thick ascending limb of loop of Henle.
MESANGIAL CELLS:
1)Located in between the loops of glomerular capillares .
2)Have contractile power.
3)Phagocytic Power.
4)Behave as Immune – cell.
2.
Blood Supply to the Kidney :
3.
The renal artery -- segmental arteries -- interlobar
arteries that communicate with one another via
arcuate arteries.
4.
The arcuate arteries give off branches called
interlobular arteries that extend into the cortex.
5.
Venous return of blood is via similarly named veins.
6.
The interlobular arteries --afferent arterioles -- glomerulus - efferent arterioles --capillary network
surrounding the tubule system of the Nephron.
Organ
7.
Approx. blood flow
(ml/min/g of tissue)
A-V O difference
The interlobular veins are then the collecting vessel of the
nephron capillary system.
Characteristics of the renal blood flow:
1, high blood flow. 1200 ml/min, or 21 percent of the cardiac output. 94% to the cortex
2, Two capillary beds
High hydrostatic pressure in glomerular capillary (about 60 mmHg) and low hydrostatic pressure in
peritubular capillaries (about 13 mmHg).
Blood flow in kidneys and other organs:
2
(ml/L)
Kidney
4.00
12-15
+)
(depends on reabsorption of Na
Heart
0.80
96
Brain
0.50
48
Skeletal muscle (rest)
0.05
-
Skeletal muscle (max. exercise)
1.00
-
HUMAN RENAL PHYSIOLOGY
•
Four Main Processes:
– Filtration
– Reabsorbtion
– Secretion
– Excretion
•
Functions of the Kidney:
•
Filtration:
– First step in urine formation
– Bulk transport of fluid from blood to kidney tubule
•
Isosmotic filtrate
•
Blood cells and proteins don‘t filter
– Result of hydraulic pressure
– GFR = 180 L/day
•
Reabsorbtion:
•
Process of returning filtered material to bloodstream
•
99% of what is filtered
•
•
May involve transport protein(s)
•
Normally glucose is totally reabsorbed
Secretion:
– Material added to lumen of kidney from blood
– Active transport (usually) of toxins and foreign substances
•
•
Saccharine
•
Penicillin
Excretion:
– Loss of fluid from body in form of urine
Amount
of Solute
= Amount + Amount
Filtered
Secreted
-
Amount
Reabsorbed
Excreted
Reabsorption and Secretion
Concept of Reabsorption and Secretion.
GFR  125 ml/min (180L/day)
•
(about 1% is excreted)
Filtration, reabsoption, and excretion rates of substances
by the kidneys
Filtered
Reabsorbed
Excreted
(meq/24h)
(meq/24h)
Reabsorbed
(meq/24h)
(%)
Glucose
(g/day)
Bicarbonate (meq/day)
> 99.9
Sodium
99.4
(meq/day)
180
4,320
25,560
180
0
4,318
25,410
100
2
150
Chloride
180 99.1
(meq/day)
19,440
Water
99.1
(l/day)
169
167.5
Urea
(g/day)
48
24
Creatinine
(g/day)
1.8
Mechanism of Transport
1. Primary Active Transport
2. Secondary Active Transport
3. Pinocytosis
4. Passive Transport
Primary Active Transport
19,260
0
1.5
24
1.8
50
0
Secondary active transport
Secondary active transport
Tubular
Tubular Cell
lumen
co-transport
(symport)
out
Interstitial
Tubular
Fluid
lumen
in
Na+
glucose
Co-transporters will move one
moiety, e.g. glucose, in the same
direction as the Na+.
Tubular Cell
Interstitial
Fluid
counter-transport
(antiport)
out
in
Na+
H+
Counter-transporters will move
one moiety, e.g. H+, in the
opposite direction to the Na+.
Pinocytosis:
Some parts of the tubule, especially the proximal tubule, reabsorb large molecules such as proteins by pinocytosis.