Each kidney contains over 1 million tiny structures called nephrons. Each nephron has a glomerulus, the site of blood filtration. The glomerulus is a network of capillaries surrounded by a cuplike structure, the glomerular capsule (or Bowman’s capsule). As blood flows through the glomerulus, blood pressure pushes water and solutes from the capillaries into the capsule through a filtration membrane. This glomerular filtration begins the urine formation process.Inside the glomerulus, blood pressure pushes fluid from capillaries into the glomerular capsule through a specialized layer of cells. This layer, the filtration membrane, allows water and small solutes to pass but blocks blood cells and large proteins. Those components remain in the bloodstream. The filtrate (the fluid that has passed through the membrane) flows from the glomerular capsule further into the nephron.The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste, but also other substances the body needs: essential ions, glucose, amino acids, and smaller proteins. When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule. As it moves, the needed substances and some water are reabsorbed through the tube wall into adjacent capillaries. This reabsorption of vital nutrients from the filtrate is the second step in urine creation.The filtrate absorbed in the glomerulus flows through the renal tubule, where nutrients and water are reabsorbed into capillaries. At the same time, waste ions and hydrogen ions pass from the capillaries into the renal tubule. This process is called secretion. The secreted ions combine with the remaining filtrate and become urine. The urine flows out of the nephron tubule into a collecting duct. It passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.The nephrons of the kidneys process blood and create urine through a process of filtration, reabsorption, and secretion. Urine is about 95% water and 5% waste products. Nitrogenous wastes excreted in urine include urea, creatinine, ammonia, and uric acid. Ions such as sodium, potassium, hydrogen, and calcium are also excreted

1. URINE FORMATION
SAKSHI NAYAK
BSC 3RD YEAR
INST.OF BIOMEDICAL SCIENCES
Bundelkhand University,Jhansi

2. INTRODUCTION
 The urinary system consists of two kidneys that filter the
blood.
 It consists of two ureters, a urinary bladder, and a urethra
to convey waste substances to the outside.

3. KIDNEY
 The kidney is a reddish brown, bean-shaped organ 12 centimeters
long.
 It is enclosed in a tough, fibrous capsule.
 The two kidneys lie on the posterior wall of the abdomen ,outside
the peritoneal cavity.
 Each kidney of the human weighs about 150grams.
 Each kidney is about the size of a clenched fist.

4. GENERAL ORGANIZATION OF THE KIDNEY
AND THE URINARY SYSTEM

5. ANATOMY OF KIDNEY

6. FUNCTIONS OF KIDNEY
 Excretion of metabolic waste products and foreign chemicals.
 Regulation of water and electrolyte balances .
 Regulation of body fluid osmolality and electrolyte concentration.
 Regulation of arterial pressure.
 Regulation of acid- base balance .
 Secretion, metabolism , and excretion of hormones.
 Gluconeogenesis.

7. NEPHRON
 It is the functional unit of the kidney .
 Each kidney in the human contains about 1 million nephrons, each capable of
forming urine.
 It contains a tuft of glomerular capillaries called the glomerulus, through which
large amounts of fluid are filtered from the blood, and a long tubule in which the
filtered fluid is converted into urine on its way to the pelvis of the kidney.
 In each kidney, there are about 250 of the very large collecting ducts, each of which
collects urine from about 4000 nephrons.

8. NEPHRON

9. TYPES OF NEPHRON
cortical nephron
 Location Cortex
 Proportions(%) 85
 Function Filtration, reabsorption
 Length of loop of henle Short
 Ascending limb of loop Thin segment only
 Vasa recta Absent
 Peritubular capillary network Large
Juxtamedullary nephron
Medulla – cortical junction
15
Filtration, reabsorption , creation of
medullary osmotic gradient
Long
Thin and thick segment
Present , dipping into medulla
Small

10. RENAL BLOOD SUPPLY

11. URINE FORMATION
 It involves three major process:
I. Glomerular filtration.
II. Tubular reabsorption.
III. Tubular secretion.

12. OVERVIEW OF URINE FORMATION

13. GLOMERULUS FILTRATION
 It is the first step in urine formation.
 It is non – selective passive process.
 It is the process by which kidneys filter the blood ,removing
excess wastes and fluids.
 Glomerular filtration rate (GFR) is a calculation that
determines how well the blood is filtered by the kidneys.
 The GFR is about 125ml/min (180L/day)
 It is also used to measure kidney function.

14. GLOMERULAR CAPILLARY MEMBRANE
 It is made up of three layers :
• Endothelium: This has relatively large
pores called fenestrae (70-100
nanometers) which hinder the passage
of plasma proteins.
• Basement membrane : It consists of
meshwork of collagen and proteoglycan
fibrillae that allow passage of water and
small solutes.
• Epithelial cells layer: These cells have
foot like structures (podocytes).These
are separated by gap called slit pores
and do not allow plasma proteins to
filter out.

15. DETERMINANTS OF THE GFR
 It is determined by :
 The sum total of hydrostatic and colloid osmotic forces across the
glomerular membrane , which gives the net filtration pressure.
 The glomerular capillary filtration coefficient.
GFR = KF x net filtration pressure
 The forces includes : PG, PB, ΠB , ΠG
GFR = KF [(PG – PB)-(ΠG- ΠB)]
= 12.5 x [(60 + 0)-(32+ 18)]
= 12.5 x 10
= 125 ml/min

16. NET FILTRATION PRESSURE
OPPOSING PRESSURE
 GHP (Glomerular hydrostatic pressure)
=60 mm Hg
 Net filtration pressure :
= [GHP- (BCOP + CHP)]
= [60-(30+20)]
= 10 mm Hg
FAVORING PRESSURE
 BCOP(Blood colloidal osmotic pressure )
= 30mm Hg
 CHP (capsular hydrostatic pressure) =
20 mm Hg

17. REGULATION OF GFR AND RENAL
BLOOD FLOW
 Autoregulation:
 Myogenic regulation
 Tubuloglomerular regulation (macula densa
mechanism)
 Angiotensin II (contributes to regulation of
GFR only )
 Hormonal regulation

18. AUTOREGULATION
 Increased Nacl concentration in the
filtrate is sensed by the macula
densa of juxtaglomerular complex.
 Increased GFR tends to inhibit the
release of vasodilator Nitric oxide
,and stimulates renin release that
leads to Angiotensin
II(vasoconstrictor).
 Afferent arterioles constrict leading
to a decreased GFR.

19. MACULA DENSA FEEDBACK MECHANISM

20. REGULATION OF GFR BY ANGIOTENSIN II

21. HORMONAL REGULATION
HORMONES
 Norepinephrin
 Epinephrin
 Endothelin
 Angiotensin II
 Nitric oxide
EFFECT ON GFR
↓
↓
↓
↓
↓

22. TUBULAR REABSORPTION
 It is highly selective process.
 It includes active and passive mechanism.
 Substances which are reabsorbed are of three types
:
i. High threshold substances: water , glucose , amino
acids , ions
ii. Low threshold substances : urea , uric acid
iii. Non – threshold substances : creatinine , inulin

23. TUBULAR REABSORPTION IN VARIOUS PARTS
OF NEPHRON

24. REGULATION OF TUBULAR
REABSORPTION
Glomerulotubular
balance
Hormonal control

25. GLOMERULOTUBULAR BALANCE
 It is the ability of the tubules to increase reabsorption rate in response to
increased tubular load .
 It helps to prevent overloading of the distal tubular segments when GFR
increases.
 It acts as second line of defence to buffer the spontaneous changes in GFR on
urine output .(The first line of defence was tubuloglomerular feedback, which
prevent changes in GFR ).

26. HORMONAL CONTROL
HORMONES
 Aldosterone
 Angiotensin II
 Antidiuretic hormone
 Atrial natriuretic peptide
EFFECTS
 ↑ Nacl ,H2O reabsorption , ↑ k+ secretion
 ↑ Nacl ,H2O reabsorption ,↑ H+ secretion
 ↑ H2O reabsorption .
 ↓ Nacl reabsorption .

27. TUBULAR SECRETION
 Essentially reabsorption in reverse , where substances moves from peritubular
capillaries or tubular cells to filtrate.
 It is important for :
 Disposing of the substances which are not present in the filtrate .
 Eliminating undesirable substances such as urea and uric acid .
 To get rid off excess potassium ions.
 Controlling blood pH.

28. CONCENTRATION AND DILUTION
OF URINE
 Body has a powerful feedback mechanism system for regulating plasma osmolarity and
sodium concentration that operates by altering renal excretion of water .
 A feedback effector for this system is ADH, also called vasopressin.
 When osmolarity in the body fluids increases above normal , water reabsorption
increases and urine volume decreases.
 When there is excess water in the body and extracellular fluid osmolarity is reduced ,
water reabsorption decreases (ceases) and more dilute urine excreted.

29. MECHANISM FOR EXCRETING DILUTE
URINE
 Tubular fluid remains isosmotic in the
Proximal Tubule .
 Tubular fluid is diluted in the Ascending
Loop of Henle .
 Tubular fluid in the Distal Tubule and
Collecting Tubules is further diluted.

30. MECHANISM OF URINE
CONCENTRATION
 The main requirements for forming a concentrated urine
are:
I. A high level of ADH ,which increases the permeability of
the distal tubules and collecting ducts to water ,thereby
allowing these tubules to reabsorb water .
I. A high osmolarity of the of the renal medullary interstitial
fluid, which provide the osmotic gradient necessary for
water reabsorption to occur in the presence of high levels
of ADH .

31. ACTION OF ADH

32. COUNTERCURRENT MECHANISM
 It is the interaction between two adjacent
tubules whose contents flow in opposite
direction.
 Loop of Henle :countercurrent multiplier.
 Vasa recta : countercurrent exchanger .

33. DISORDERS OF THE URINARY SYSTEM
 Urolithiasis:A condition marked by the formation of calculi in the urinary tr
act.
 Kidney failure: A condition in which the kidneys lose the ability to remove
waste and balance fluids.
 Urethritis: inflammation of urethra .
 Hydronephrosis: Distension of kidney due to obstruction in blood flow .
 Bladder cancer :Cancer that begins in the bladder.
 Nephroptosis : abnormal condition in which the kidney drops down into
the pelvis when the patient stands up (floating kidney).

34. ABNORMAL URINARY CONSTITUENTS
s
SUBSTANCE NAME OF DISEASE POSSIBLE CAUSES
• Glucose
• Proteins
• Pus (WBCs ,
Bacteria)
• RBCs
• Hemoglobin
• Bile pigment
Glucosuria
Proteinuria
Pyuria
Hematuria
Hemoglobinuria
Bilirubinuria
Excessive intake of sugary food ,
Diabeties mellitus .
Glomerulonephritis
Urinary tract infection
Bleeding in the urinary tract
Hemolytic anemia
Liver diseases (hepatitis)