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فیزیولوژی تکمیلی Advanced Physiology (part 6, Urinary system) By: A. Riasi (PhD in Animal Nutrition & Physiology) Animal Sci. Dep. Isfahan University of Technology http://riasi.iut.ac.ir Respiratory system Reference: Urinary Systems Vertebrate kidneys come in a variety of morphologies and functional capacities. Urinary Systems Mammalian kidneys have four major functions: Regulating major inorganic solutes Regulating plasma volume Regulating of harmful or unneeded organic molecules Control the osmotic balance Urinary Systems In mammals, kidneys have the following additional functions Secretion of erythropoietin Secretion of renin, a hormone important for salt conservation Conversion of vitamin D into its active form Excretion of pheromones for sexual signaling, marking territories, and so forth Urinary Systems Nephrons are functional unit of each kidney: Tubular component Vascular component Urinary Systems Urinary Systems Urinary Systems The three layers for glomerular filtration: The glomerular capillary wall consists of a single layer of flattened endothelial cells. The basement membrane, a non-cellular gelatinous layer composed of collagen and glycoproteins The collagen provides structural strength Thee glycoproteins discourage the filtration of small plasma proteins The inner layer of Bowman’s capsule. It consists of podocytes (podo, “foot”), octopus-like cells that encircle the glomerular tuft. Urinary Systems Bowman’s capsule podocytes Urinary Systems Three physical forces for glomerular filtration Urinary Systems Autoregulation of GFR Urinary Systems Urinary Systems Two intrarenal mechanisms involve in autoregulation The myogenic mechanism A common property of arteriolar vascular smooth muscle, which contracts inherently in response to the stretch accompanying increased pressure within the vessel. The tubuloglomerular feedback (TGF) mechanism Involves the juxtaglomerular apparatus (JGA). Urinary Systems Within the wall of the afferent arteriole, the smooth muscle cells in the JGA are specialized. The distal tubule at JGA are specialized and known as macula densa. The macula densa detect changes in the salt level of the fluid flowing past them through the tubule. Urinary Systems In response to the resultant rise in salt delivery to the distal tubule, the macula densa cells release ATP and adenosine. These are paracrines, which act on the adjacent afferent arteriole, causing it to constrict and thus reducing glomerular blood flow and returning GFR to normal. Urinary Systems In the opposite situation, when less salt is delivered to the distal tubule because of a spontaneous decline in GFR accompanying a fall in arterial pressure, the macula densa releases less paracrines. The resultant afferent arteriolar vasodilation increases the glomerular flow rate, restoring the GFR to normal. Urinary Systems To exert even more exquisite control over TGF, the macula densa cells also secrete the vasodilator nitric oxide. By means of the TGF mechanism, the tubule of a nephron is able to monitor the salt level in the fluid flowing through it and adjust its own GFR accordingly to maintain fluid and salt delivery in the distal tubule. Urinary Systems The myogenic and TGF mechanisms work in unison to autoregulate the GFR during the transient changes in blood pressure that accompany daily activities unrelated to the need for the kidneys to regulate H2O and salt excretion. Urinary Systems If autoregulation did not occur, the GFR could increase and H2O and solutes would be lost needlessly whenever arterial pressure rises during intense activity. If the GFR were too low, the kidneys could not adequately eliminate unneeded materials. Urinary Systems Urinary Systems The GFR can be influenced by changes in the filtration coefficient (Kf). Research indicates that Kf is subject to physiological control. Two factors affect the Kf: The surface area The permeability of the glomerular membrane Urinary Systems The surface area available within the glomerulus is represented by the extent of the capillary surface. Each tuft of glomerular capillaries is held together by mesangial cells. Contraction of the mesangial cells reduces the radius of the filtering capillaries, which reduces the surface area available. Sympathetic stimulation causes the mesangial cells to contract, thus providing a second mechanism (besides afferent arteriolar vasoconstriction) by which sympathetic activity can decrease the GFR. Urinary Systems Podocytes also possess actin like contractile filaments. The contraction or relaxation can, respectively, decrease or increase the number of filtration slits open in the inner capsule. membrane of Bowman’s Urinary Systems Tubular reabsorption Urinary Systems Reabsorption of most substances occurs in the proximal tubule. Tubular reabsorption involves two transepithelial transport methods, and act highly selective: Passive transepithelial transport Active transepithelial transport Urinary Systems Mammalian tubules typically reabsorb 99% of the filtered H2O and salt and 100% of the filtered glucose and amino acids. The tight junctions largely prevent substances except H2O from moving between the cells. So materials must move transcellularly to leave the tubular lumen and gain entry to the blood. Urinary Systems Substances that are actively reabsorbed against a gradient and are of particular importance to the body include: Organic nutrients such as glucose and amino acids Electrolytes such as Na+and PO43− Urinary Systems The waste products remaining in the tubular fluid become highly concentrated. Urea molecules passively reabsorbed in peritubular capillaries as a result of this concentrating effect. About 40% of the filtered urea is passively reabsorbed and only 60% of the fitered urea is eliminated. Urinary Systems Urinary Systems Effect of the renin–angiotensin–aldosterone system (RAAS) Angiotensinogen is a plasma protein synthesized by the liver and always present in the plasma in high concentration. Once renin secreted into the blood convert angiotensinogen into angiotensin I. In the lungs, angiotensin I is converted into angiotensin II by angiotensin-converting enzyme (ACE). Urinary Systems Angiotensin II is the primary stimulus for the secretion of the hormone aldosterone from the adrenal cortex. These do the following effects: Na+ reabsorption Water retention Vasoconstriction Thirst and salt hunger Urinary Systems Urinary Systems The second route of entry selected substances into the tubules is tubular secretion. The most important substances secreted by the tubules are: Hydrogen ion (H+) Potassium (K+) Organic anions and cations Urinary Systems Hydrogen ion can be added to the filtered fluid by being secreted by the proximal, distal, and collecting segments, using proton-pumping. The extent of H+ secretion depends on the acidity of the body fluids. Urinary Systems Potassium secretion is controlled by aldosterone. Changes in the plasma K+ concentration can be dangerous and causes heart arrhythmia. Potassium ion is selectively moved in opposite directions in different parts of the tubule. It is actively reabsorbed in the proximal tubule It can also actively secreted by principal cells in the distal and collecting tubules Urinary Systems Urinary Systems During periods of K+ depletion, K+ secretion in the distal portions of the nephron is reduced to a minimum. The K+ secretion, not the filtration or reabsorption of K+, is varied in a controlled fashion to regulate the rate of K+ excretion and maintain the desired plasma K+ concentration. Urinary Systems Several factors can alter the rate of K+ secretion, the most important being aldosterone. An elevation in plasma K+ concentration directly stimulates the adrenal cortex to increase output of aldosterone. Urinary Systems The manifestations of ECF K+ depletion are: Skeletal muscle weakness Diarrhea Abdominal distention Abnormalities in cardiac rhythm and impulse conduction