Download Physiology of kidney and excretion

Physiology of kidney and
excretion I
Romana Šlamberová, MD PhD
Department of Normal, Pathological and Clinical
Physiology
Urinary system
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Organ system that produces,
stores, and carries urine
Includes two kidneys, two ureters,
the urinary bladder, two sphincter
muscles, and the urethra.
Humans produce about 1.5 liters
of urine over 24 hours
This amount may vary according
to the circumstances:
 fluid intake
 perspiration (sweating) and
respiration
 Some medications interfere
directly or indirectly with urine
production, such as diuretics.
Function of urinary system
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Excretion
Keeping homeostasis
Keeping acid-base balance
Secretion (rennin, kallikrein, erytropoetin)
Excreted products:
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Product of the metabolism
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Water
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Hormones
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Vitamins
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Toxic substances
Composition of urine
Substance
urine
(urine/plasma)
900-930
950
--
proteins (g/l)
70
0
--
glukose (mmol/l)
5,5
0
--
Na+
130
152
1
Cl-
104
200
2
urea
5
325
65
Uric acid
0.23
2.9
12
Ca2+
1.9
2.7
2
phosphates
2.9
48.4
16
creatinin
0.08
8.8
100
K+
4.1
38.7
9
H2O (g/l)
Plasma
Kidneys
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Morphology
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It is paired organ (weight about 300 g)
Compound from two parts cortex (isotonic urine) and
medulla (hypertonic urine)
Cortex: Glomerular apparatus
Medulla: Divided: Outer and Inner
Consists of about 1 million filtering units termed
nephrons (basic structural and functional unit)
The kidney plays a crucial role in regulating
electrolytes in the human blood (e.g. Na+, K+, Ca2+).
It clears urea, a nitrogenous waste product from the
metabolism of amino acids.
Nephron 1
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A nephron (1-1.2 millions) is the basic structural and
functional unit of the kidney.
Function:
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regulate water balance and soluble substances
filtering the blood and reabsorbing what is needed
forming the final urine
Each nephron is composed of an initial filtering component
(the renal corpuscle) and a tubule specialized for
reabsorption and excretion (the renal tubule).
The renal corpuscle filters out large solutes from the blood,
delivering water and small solutes to the renal tubule for
modification.
Nephron 2
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Parts:
 Glomerular apparatus
 Proximal tubule
 Loop of Henle
 Distal tubule
Types of nephrons:
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Cortical nephrons (glomerular apparatus belong the surface and Loop
of Henle only to the outer part of the medulla)
Intermedial nephrons (in the middle)
Juxtamedullary nephrons (glomerular apparatus deep in cortex near
the medulla and Loop of Henle is going deep to the inner part of the
medulla)
Nephron 3
Renal (Malphigian) corpuscle
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nephron's initial filtering component.
Glomerulus capillary tuft with afferent
arteriole and efferent arteriole
 Efferent arterioles of juxtamedullary
nephrons (ie, the 15% of nephrons
closest to the medulla) send straight
capillary branches (vasa recta) that
deliver isotonic blood to the renal
medulla.
 The vasa recta = crucial role in the
countercurrent exchange system
Bowman's capsule - surrounds the
glomerulus
 visceral (inner) layer
 parietal (outer) layer
Glomerulus 2
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The glomerulus has several characteristics
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the endothelial cells of the glomerulus contain numerous
pores (fenestrae)
glomerular endothelium sits on a very thick basement
membrane
On the surface of the cells are negatively charged
glycosaminoglycans such as heparan sulfate.
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The negatively-charged basement membrane repels negativelycharged ions from the blood, helping to prevent their passage into
Bowman's space.
blood is carried out of the glomerulus by an efferent
arteriole instead of a venule.
Glomerular filter
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The filtration surface is 1.5 square meter
Amount of the filtered solution 180-200 l.
Final volume of urine is around (1.5 - 2 l per day)
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Glomerular filter:
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The rest (97 %) has to be reabsorbed in the tubules back to the body
the capillary endothelium
basal membrane
epithelium of the Bowman’s capsule (PODOCYTES)
Podocytes: special cells which have numerous of
pseudopodia (pedicles) that interdigitate to form filtration slits
along the capillary wall.
Glomerular filtration barrier
GLOMERULAR FILTRATION
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Depends on:
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Pressure gradient across the filtration slit
(endothelium, basal membrane, epithelium =
podocytes)
Blood circulation throughout the kidneys
Permeability of the filtration barrier
Filtration surface
The solution after filtration is very similar like
plasma, but should be WITHOUT PROTEINS
CLEARANCE
The ability of kidneys to clear plasma from different
products.
GLOMERULAR FILTRATION RATE (GFR)
 measuring the excretion and plasma level of a substance
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that freely filtered through the glomeruli
neither secreted nor reabsorbed by the tubules
such as INULIN (polymer of fructose)
GFR = U x V/P
U = concentration of inulin in urine
V = volume of the urine
P = concentration of inulin in plasma
 Normal GFR is around 125 ml/min (7.5 l/h)
Juxtaglomerular apparatus 1
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The juxtaglomerular cells - synthesize, store, and
secrete the enzyme renin
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Specialized smooth muscle cells in the wall of the afferent
arteriole that are in contact with distal tubule.
Have mechano-receptors for blood pressure
The macula densa
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specialized cells lining the distal convoluted tubule (lies next
to the juxtaglomerular apparatus)
Cells of macula densa are taller and have more prominent
nuclei than surrounding cells.
Sensitive to the concentration of sodium ions in the fluid.
Juxtaglomerular apparatus 2
Proximal tubule
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Morphology: 15 mm long and 55 µm in diameter
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Function: Reabsorption of the largest volume of solution
filtered in glomerular apparatus.
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epithelium cells have a striate brush border (projections)
enlarge the surface for the reabsorption
75 - 80 % water
Na+, Cl-, HCO3-, K+, Ca2+, Mg2+, HPO42Glucose
Results in ISOOSMOTIC SOLUTION
Fluid in the filtrate entering the proximal convoluted tubule is
reabsorbed into the vasa recta, including approximately 2/3 of
the filtered salt and water and all filtered organic solutes
(primarily glucose and amino acids).
Lining of tubules
Trojan: Fyziologie
Glucose transport
Loop of Henle
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A U-shaped tube
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descending limb (thin part)
ascending limb (thin and thick part)
Begins in the cortex and extends into the
medulla and then returns to the cortex.
Its primary role is to concentrate the salt in
the intersticium.
Loop of Henle - Descending
limb
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Permeable to water and salt
hypertonic intersticium - water flows freely out of
the descending limb by osmosis
Longer descending limbs allow more time for
water to flow out of the filtrate
Longer limbs make the filtrate more hypertonic
than shorter limbs.
Results in HYPERTONIC SOLUTION
Water and ion transport
Loop of Henle - Ascending
limb
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Impermeable to water, permeable for salts.
Actively pumps sodium out of the filtrate,
generating the hypertonic intersticium
Important for counter-current exchange
Results in HYPOTONIC SOLUTION
Counter-current exchange
Water and ion transport
Distal tubule 1
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Morphology:
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Reabsorption:
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direct part.
Convolute part – Juxtaglomerular apparatus (the part of distal tubule
near the glomerular apparatus) = special cells = MACULA DENSA (thin
cells very tight next to each other) Large nucleus, secretion of RENIN
Water
Na+
Results in ISOOSMOTIC SOLUTION
After traveling the length of the distal convoluted tubule, only
3% of water remains, and the remaining salt content is
negligible.
97.9% of the water in the glomerular filtrate enters the
convoluted tubules and collecting ducts by osmosis.
Distal tubule 2
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The distal convoluted tubule = similar to the proximal
convoluted tubule in structure and function.
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regulated by the endocrine system
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Cells lining the tubule have numerous mitochondria, enabling active
transport to take place by the energy supplied by ATP.
parathyroid hormone = reabsorbs more Ca2+ and excretes more
phosphates
aldosterone = more Na+ is reabsorbed and more K+ excreted
Atrial natriuretic peptide = excretes more Na+
In addition, the tubule also secretes hydrogen and
ammonium to regulate pH.
Collecting duct
1
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Collects about 10 distal tubules
Continues as medullary pyramides (about 2700
nephrons).
begin in the renal cortex and extend deep into the
medulla.
Final adjustment (hypertonic intersticium) – loosing
water
Results in HYPERTONIC SOLUTION
Collecting duct 2
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The collecting duct is normally impermeable to water
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becomes permeable under the actions of antidiuretic
hormone (ADH).
As much as 3/4 of the water from urine can be
reabsorbed as it leaves the collecting duct by osmosis
The levels of ADH determine whether urine will be
concentrated or dilute.
Dehydration results in an increase in ADH, while water
sufficiency results in low ADH allowing for diluted
urine.
Collecting duct 3
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Lower portions of the collecting duct are also permeable to
urea - helps to maintain its high ion concentration.
Urine leaves the collecting duct through:
 the renal papilla
 the renal calyces
 the renal pelvis
 The ureter
the collecting duct is not part of nephron – different
development
 the collecting duct from endoderm
 the nephron is from mesoderm