In the formation of urine, a series of events lead to the elimination of metabolic waste from body, regulation of total body water balance, control chemical composition of blood and control acid, base balance.
1. Filtration
Filtration takes placed at Malpighian body.
Urine formation begins with process of filtration.
Kidneys filter blood, when blood flows from afferent arteriole into glomerulus.
Afferent arteriole is under high pressure (60 mmHg) to filtered.
This pressure forces some of blood plasma (protein and blood cell) unable to pass through endothelial capsular membrane. It remain with in glomerulus and reabsorbed lated.
GFR (Glomerular filteration rate) controlled by JGA (juxtaglomerular apparatus).
Juxtaglomerular secret renin cell (present in afferent and efferent arteriole).
Macula densa cells (present in DCT) ; detect Na+, Cl-
Mesangial cells
Blood constitutent in glomerular filtration
Water
Mineral salt; Na+, K, Cl
Amino acid
Creatinine
Uric acid
Urea
Glucose
Some hormone/ some toxins/ some drugs
Blood constitutent which remains in glomerular capillaries.
WBC, RBC, Platelets, Plasma proteins and some drug.
2. Selective reabsorption
Reabsorption is the movement of substance out of renal tubules back into blood capillaries.
This process enables reabsorption into blood of filterate constituents needed to maintain fluid and electrolytes balance of the blood.
Sites of Reabsorption
Reabsorption begins in proximal convoluted tubules (PCT) and continues in loop of Henle, distal convoluted tubules (DCT) and collecting tubules.
1. Reabsorption in PCT
PCT have many microvilli that increases surface area for reabsorption.
Reabsorb;
100% of filtered glucose and amino acids.
80-90% of HCO3-
50% of Cl-
65% of water, Na+ and K+
2. Reabsorption in loop of Henle
10-20% of filtered HCO3-
15% of water
35% of Cl-
20-30% of filtered Na+, K+,and Ca+
3. Reabsorption in DCT
Reabsorb 10-15% of water
4. Collecting duct
Reabsorb water
Some constitutents of glomerular filtrate such as glucose, amino acid don't normally appear in urine because they are completely reabsorbed unless they are excessive in blood level.
Other substance reabsorbed by active transport and includes sodium, calcium, potassium, phosphate and chloride.
Some ions such as sodium and chloride can be reabsorbed depending on the site in the nephron.
Hormone helps in reabsorption
Parathyroid hormone
ADH hormone
Aldosterone
ANP (atrial natriuretic peptide)
1. Parathyroid hormone
Secreted from parathyroid gland. It regulates the reabsorption of calcium and phosphate.
2. ADH (antidiuretic hormone)
Secreted from pituitary gland and it increases the water reabsorption.
3. Aldosterone
Secreted by adrenal cortex. It increases the reabsorption of sodium and excretion of potassium.
4. ANP (atrial natriuretic peptide)
Secreted by atria of heart in response to stretching of atrial valve which decreases reabsorption of sodium and water in the collecting tubules to control in blood pressure.
Reabsorption of Nitrogenous waste product
Nitrogenous waste products such as urea, uric acid, ammonia, creatinine
3. Tubular secretion
Tubar secretion remove material from blood and adds them to filtrate.
Secretion is process by which substances move into distal and collecting tubules from blood into capillaries.
Tubular secretion is very important to maintain the fluid and electrolytes balance and to remove the waste product from body.
Secretion take place in PCT, DCT and collecting duct, allows kidney to increase their efficiency.
1. Secretion in PCT
H+, NH3, K+
2. Secretion in DCT
K+, H+, NH3
3. Secretion in collecting duct
K+, H+, PH balance
Substance into tubular secretion for removal from the body.
Transport of certain molecules out of blood and into the urine.
In cleaning certain substance from plasma and helps to maintain normal PH of blood.
Factors affecting GFR (Glomerular filteration rate)
Changes in renal blood flow
Changes in glomerular capillary hydrostatic pressure
Changes in systemic blood pressure
Changes in effective filtration surface area
Edema in kidney
Change in glomerular capillary permeability
Afferent and efferent arteriole constriction
Changes in hydrostatic pressure in Bowman's capsule
Changes in concentration of plasma protein
Ureteral obstruction
Blood supply to kidney
Kidney receives blood from renal arteries, left and right which branch directly from abdominal aorta.
Renal artery
Segmental arteries
Interlobar arteries
Arcuate arterioles
Interlobular arteries
Afferent arteries
Glomerular capillaries
Efferent arterioles
Pertitubular capillaries/or vasa recta
Interlobular veins
Arcuate vein
Interlobar vein
Renal vein (exiting kidney at renal Hilus).
Characteristic of Urine
Urine is clear, pale in color due to presence of bile pigment, which are reabsorbed and then excreted by kidney.
A healthy adult passes 1000-1500 ml urine per day.
Urine production is decrease during sleep and excercise.
Composition of urine
Water (96%)
Urea (2%)
Remaining 2% is uric acid, creatinine, ammonia, sodium, potassium, chloride, sulphate, phosphate, oxalate
Renin -Angiotensin System (RAS)
Renin-Angiotensin aldosterone system that regulates blood pressure and water (fluid) balance.
It is center of contrling blood pressure in the body.
Low renal blood flow or low blood pressure or law Na+ level
Tubules/ kidney renal blood flow pressure is low
Secrete renin by JG cells
Reach liver and excite to liver relaease
Angiotensin l
Reach in lung and convert (due to presence of ACE inhibitors)
Angiotensin ll ( angiotensin converting enzyme comming from lung)
Increased blood pressure (vasoconstriction; raised blood pressure)
(Start negative feedback mechanism here)
Go to kidney again and decrease the secretion of renin and maintain blood pressure normal.
Tubules (JG cell)
Decrease secretion of renin
Maintain normal Blood pressure
Angiotensin ll -- Vasoconstriction --- increase blood pressure --- -ve feed back mechanism start
Adrenal gland (adrenal cortex)
Aldosterone
Selective reabsorption of sodium and excretion of potassium
Due to reabsorption
Increase blood level of Na+
Blood pressure will be increase
(Start negative feedback mechanism here)
Go to adrenal cortex
Decrease the secretion of aldosterone
Bring blood pressure back to normal
1. Due to decreased renal blood flow or decreased blood pressure or decreased sodium level in kidney.
2. Renin will secreted in kidney
3. This secretion of renin will convert angiotensinogen (comming from liver and kidney) converted into angiotensin l.
4. Angiotensin l wiil be converted into Angiotensin ll
5. Due to formation of angiotensin ll vasoconstriction will increase blood pressure.
6. After vasoconstriction or after increase blood pressure, negative feed back mechanism is activated which will act in kidney and decrease the secretion of renin.
Onthe other hand,
1. Angiotensin ll will act on adrenal cortex and will cause secretion of aldosterone.
2. Secretion of aldosterone will lead to reabsorption of sodium and excretion of potassium from kidney tubules.
3. Due to reabsorption of sodium, blood sodium level will increase and ultimately will cause increase in blood pressure.
4. With the increasing blood pressure, negative feed back mechanism will be activated and it will act on adrenal cortex and reduce secretion of aldosterone.
5. Which will bring back blood pressure in normal.
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