Renin-Angiotensin mechanism. The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. Angiotensin II also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the Renin-Angiotensin mechanism.
Atrial Natriuretic Factor (ANF). • An increase in blood flow to the atria of the heart can cause the release • of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of • blood vessels) and thereby decrease the blood pressure. ANF mechanism, • therefore, acts as a check on the renin- angiotensin mechanism
Reabsorption of nitrogenous waste products, such as urea and uric acid is very limited.
Negative feedback regulation of secretion of antidiuretic hormone (ADH).
Tubular secretion drugs including penicillin and aspirin, may not be cleared from the blood by filtration because of the short time it remains in the glomerulus. Such substances are cleared by secretion into the convoluted tubules and excreted from the body in the urine.
Composition of urine
Urine is clear and amber in colour due to the presence of urobilin healthy adult passes 1000 to 1500 ml per day. Urine production is decreased during sleep and exercise.
Urine is clear and amber in colour due to the presence of urobilin healthy adult passes 1000 to 1500 ml per day. Urine production is decreased during sleep and exercise.
The source of most body water is dietary food and fluid, and a small amount (called 'metabolic water') is formed by metabolic processes.
The balance between fluid intake and output is controlled by the kidneys. The minimum urinary output, i.e., the smallest volume required to excrete body waste products, is about 500 ml per day.
Urinary volume in excess of this is controlled mainly by antidiuretic hormone (ADH) released into the blood by the posterior lobe of the pituitary gland and control the volume of urine in feed back mechanism.
The feedback mechanism may be suppressed when there is an excessive amount of a dissolved substance in the blood.
In diabetes mellitus when the blood glucose level is above the transport maximum of the renal tubules, excess water is excreted with the excess glucose. This polyuria may lead to dehydration despite increased production of ADH but is usually accompanied by acute thirst and increased water intake.
When blood volume is increased, stretch receptors in the atria of the heart release atrial natriuretic hormone (ANP). This reduces reabsorption of sodium and water by the proximal convoluted tubules and collecting ducts, meaning that more sodium and water are excreted.
Changes in the concentration of electrolytes in the body fluids may be due to changes in: • the body water content, or • electrolyte levels. There are several mechanisms that maintain the balance between water and electrolyte concentration.
Sodium and potassium balance Sodium is the most common cation (positively charged ion) in extracellular fluid and potassium is the most common intracellular cation. Renin-angiotensin-aldosterone system regulates the sodium – potassium level in thebody
pH balance In order to maintain the normal blood pH (acid-base balance), the cells of the proximal convoluted tubuIes secrete hydrogen ions. In the filtrate they combine with buffers
Carbonic acid is converted to carbon dioxide (C02) and water (H20), and the CO2 is reabsorbed, maintaining the buffering capacity of the blood.
Hydrogen ions are excreted in the urine as ammonium salts and hydrogen phosphate.
The normal pH of urine varies from 4.5 to 8 depending on diet, time of day and a number of other factors. Individuals whose diet contains a large amount of animal proteins tend to produce more acidic urine (lower pH) than vegetarians.
Ureters The ureters are the tubes that convey urine from the kidneys to the urinary bladder. They are about 25 to 30 cm long with a diameter of about 3 mm.
The ureter is continuous with the funnel- shaped renal pelvis. It passes downwards through the abdominal cavity, behind the peritoneum in front of the psoas muscle into the pelvic cavity, and passes obliquely through the posterior wall of the bladder
Because of this arrangement, when urine accumulates and the pressure in the bladder rises, the ureters are compressed and the openings occluded. This prevents reflux of urine into the ureters (towards the kidneys) as the bladder fills and during micturition,
The ureters and their relationship to the kidneys and bladder.
The position of the ureter where it passes through the bladder wall
The ureters consist of three layers of tissue: • an outer covering of fibrous tissue, continuous with the fibrous capsule of the kidney • a middle muscular layer • an inner layer, the mucosa, composed of transitional epithelium
The ureters propel urine from the kidneys into the bladder by peristaltic contraction of the smooth muscle layer. This is an intrinsic property of the smooth muscle and is not under autonomic nerve control.
Organs associated with the bladder
Urinary bladder The urinary bladder is a reservoir for urine. It lies in the pelvic cavity and its size and position vary, depending on the volume of urine it contains. When distended, the bladder rises into the abdominal cavity.
The bladder wall is composed of three layers the outer layer of loose connective tissue, containing blood and lymphatic vessels and nerves, covered on the upper surface by the peritoneum • the middle layer, consisting of a mass of interlacing smooth muscle fibres and elastic tissue loosely arranged in three layers. This is called the detrusor muscle and when it contracts, it empties the bladder • the mucosa, composed of transitional epithelium
The bladder is distensible but when it contains 300 to 400 ml, awareness of the need to pass urine is felt. The total capacity is rarely more than about 600 ml.
The three orifices in the bladder wall form a triangle or trigone
Urethra The urethra is a canal extending from the neck of the bladder to the exterior, at the external urethral orifice. It is longer in the male than in the female.
The female urethra is approximately 4 cm long. It runs downwards and forwards behind the symphysis pubis and opens at the external urethral orifice just in front of the vagina. The external urethral orifice is guarded by the external urethral sphincter, which is under voluntary control.
Micturition The urinary bladder acts as a reservoir for urine. When 300 to 400 ml of urine have accumulated, afferent autonomic nerve fibres in the bladder wall sensitive to stretch are stimulated.
In the infant this initiates a spinal reflex and micturition occurs
Reflex control of micturition when conscious effort cannot override the reflex action.
Control of micturition after bladder control is established.
1. Describe the gross structure of kidney with the help of a diagram. 2. Draw and label the longitudinal section of human kidney. 3. Essay: explain the physiology of urine formation and its hormonal regulation. 4. Write a note on the normal composition urine. 5. Briefly explain Renin- angiotensin- aldosterone system. 6. Describe the structure and function of bladder.
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