Mechanism of urine formation, Lecture notes of Physiology

Download Mechanism of urine formation and more Lecture notes Physiology in PDF only on Docsity!

Mechanism of urine formation

involving processes of filtration,

tubular reabsorption and

secretion , concentration and

diluting mechanism

Presented by :- Ritu Sharma

Introduction

• The constancy of body internal environment is maintained , in large part, by the continuous functioning of it’s about 2.6 million nephrons.
• Substance to excreted are removed by glomerular filtration and renal tubular secretion and pass into the urine.
• Substance that the body needs are retained by renal tubular absorption eg:-sodium, bicarbonate and returned to the body by reabsorptive processes.

1:- Pressure Gradients

• quality and degree of filtration are different through the filtration barrier. Like other capillary beds, the pressure gradients are due to operation of Starling forces that are hydrostatic and oncotic pressures
• Hydrostatic Pressure Gradient :- The hydrostatic pressure in glomerular capillary is considerably higher than the capillary pressure in other parts of the body. This is because the afferent arterioles directly arise from interlobular arteries and they are short and straight:-

Continue….

• 1. The glomerular capillary pressure at the afferent arteriolar end is same as at efferent arteriolar end, i.e. 45 mm Hg.
• 2. This pressure is opposed by the hydrostatic pressure in Bowman’s capsule, which is 10 mm Hg.
• 3. Therefore, the hydrostatic pressure gradient across the filtering membrane is 35 mm Hg (45–10 mm Hg) towards the capsule.
• 4. Thus, hydrostatic pressure strongly favors filtration across the entire glomerular capillary membrane from its afferent end to efferent end.

• The net filtration pressure:- The net filtration pressure (PUF) across the glomerular membrane depends on the difference between the hydrostatic pressure gradient and the glomerular capillary oncotic pressure (as the oncotic pressure of tubular fluid is negligible):- PUF = (PGC – PT) – ΠGC At the afferent end of glomerular capillary, the net filtration pressure is: Afferent PUF = 35 mm Hg – 20 mm Hg = 15 mm Hg outward At efferent end of glomerular capillary, the net filtration pressure is: Efferent PUF = 35 mm Hg – 35 mm Hg = 0 mm Hg Thus, the net filtration gradient is always from glomerular capillaries towards the tubule, which is high at afferent arteriolar end that slowly declines to almost nil at efferent arteriolar end.

2:- Filtration Coeffcient Filtration coefficient (Kf ) is the product of glomerular capillary wall permeability and the effective filtration surface area (size of the capillary bed). a. Capillary Permeability :- Glomerular capillaries are highly permeable, i.e. about 50 times that of capillaries in skeletal muscle. However, filtration depends on the many things:- Molecular size Shape Electrostatic Charge

Continue…..

b. Size of Capillary Bed :- The surface area for filtration of the capillary bed depends on the size of mesangial cells. The contraction of mesangial cells decreases the area available for filtration and relaxation of mesangial cells increases the area for filtration. Thus, mesangial cell contraction impairs and relaxation facilitates GFR.

• Factors that alter mesangial cell activity.
• A. Factors that produce contraction of mesangial cells
• − Angiotensin II − Norepinephrine − Endothelins − ADH − PDGF (platelet derived growth factor) − PAF (platelet activating factor) − Histamine − Thromboxane A2 − PGF − Leukotrienes
• B. Factors that produce relaxation of mesangial cells
• − Dopamine − ANP (atrial natriuretic peptide) − cAMP − PGE2 − Nitric oxide − Bradykini

Filtration Fraction

• Filtration fraction (FF) is the ratio of the GFR to renal plasma flow:
• 1. Normal FF ranges between 0.16 to 0.20. This is a better index of glomerular activity. For example, decreased blood pressure increases FF without much change in GFR.
• 2. In hypotension, renal plasma flow decreases. However, GFR decrease is less than the decrease in renal plasma flow as efferent arteriolar constriction maintains GFR. Thus, FF remains elevated.

Regulation of GFR

• Usually, factors that affect RBF also affect GFR. Thus, regulation of glomerular filtration involves :-
• neural mechanisms,
• hormonal mechanisms,
• myogenic mechanism
• tubuloglomerular feedback.

Hormonal Mechanism

• Various hormones affect GFR.
• Angiotensin , Histamine , Dopamine , Atrial Natriuretic , Endothelin , Nitric oxide , Bradykinin , Adenosine , Glucocorticoid , Prostaglandin.

Myogenic Mechanism

• Myogenic mechanism controls renal blood

flow and GFR. When arterial pressure

increases, the afferent arteriole is stretched

and stretch-induced contraction of the

smooth muscles of afferent arteriole

decreases GFR. Opposite mechanism

operates in hypotension