Renal Physiology Cheat Sheet by Ajita

Each nephron consists of two parts:

2. Renal Tubules: into which the filtered fluid passes. Consists of Proximal Convoluted Tubule, Loop of Henle, Distal Convoluted Tubules. Loop of Henle has an ascending limb and a descending limb.

15-20% nephrons are juxtam­edu­llary nephrons, which have corpuscles deep in the cortex and long loop of Henle that extends into the medulla. Long loops of Henle receive blood supply from peritu­bular capill­aries and vasa recta that arise from efferent arteri­oles.

1. Glomerular Filter­ation: water and most solutes in the blood plasma move across the wall of glomerular capill­aries into the glomerular capsule and then into renal tubule.

3. Tubular Secretion: As the fluid flows through the renal tubule and collecting ducts, the tubule and ducts secrete other materials, such as wastes, drugs and excess ions, into the fluid.

When blood volume and blood pressure decrease, walls of the afferent arterioles are stretched less and Juxtag­lom­erular apparatus secretes renin.

Angiot­ens­inogen --renin--> angiot­ensin I --ACE--> angiot­ensin II

Within principal cells are tiny vesicles containing many copies of a water channel protein known as aquapo­rin-2. ADH stimulates insertion of the aquapo­rin­-2–­con­taining vesicles into the apical membranes via exocyt­osis.

These effects increase the excre- tion of Na􏱩 in urine (natri­uresis) and increase urine output (diure­sis), which decreases blood volume and blood pressure.

A lower-­tha­n-n­ormal level of Ca2⁺ in the blood stimulates the parath­yroid glands to release parath­yroid hormone (PTH). PTH in turn stimulates cells in the early distal convoluted tubules to reabsorb more Ca2⁺ into the blood.

The fraction of blood plasma in the afferent arterioles of the kidney that becomes glomerular filtrate- filtration factors

Endoth­elial cells of the glomerular capill­aries and the podocytes (which completely encircle the capill­aries) form a leaky barrier know as the Filtration Membrane. This assembly permits filtration of water and solutes but prevents filtration of plasma proteins, platelets and blood cells.

1. Glomerular endoth­elial cells are leaky because they have large fenest­rat­ions. Located among the glomerular capill­aries and in the cleft between the afferent and efferent arterioles are the mesangial cells. These contra­ctile cells help regulate glomerular filtration rate.

3. Extending from each podocyte and thousands of footlike projec­tions termed pedicels which wrap around glomerular capill­aries. The spaces between pedicels are the filtration slits. A thin membrane extends across the slits called the filtration membrane that permits the passage of smaller molecules such as water, glucose, vitamins, amino acids, ammonia, urea and ions.

1. Glomerular capill­aries present a large surface area for filtration because they are long and extensive. When mesangial cells are relaxed, the surface area is maximal and the glomerular filtration is very high. Contra­ction of mesangial cells reduces the available surface area, and the glomerular filtration decreases.

3. Glomerular capillary blood pressure is higher than in capill­aries elsewhere in the body because the efferent arteri­ole's diameter is much smaller than the afferent arteri­ole's diameter. Hence, resistance to outflow of blood from the glomerulus is high.

Glomerular filtration depends of three main pressures:

2. Capsular Hydros­tatic Pressure: hydros­tatic pressure exerted against the filtration membrane by the fluid already in the capsular space and renal tubule. It opposes filtration and exerts a pressure of 15mmHg.

Net filtration pressure: (GBHP-­CHP­-BCOP) = (55-15-30) =10mmHg.

The amount of filtrate formed in all the renal corpuscles of both kidneys in a minute, is the Glomerular Filtration Rate. Average GFR of males- 125 ml/min and of females- 105 ml/min.

Three mechanisms control GFR:

~ Myogenic Mechanism occurs when stretching of afferent arteriole due to elevated blood pressure triggers the smooth muscle fibres in the wall to contract, which narrows the arteri­ole's lumen, thereby decreasing renal blood flow and restoring GFR.

2. Neural Regula­tion: blood vessels of kidney are supplied by sympat­hetic ANS fibres that release norepi­nep­hrine. Norepi­nep­hrine causes vasoco­nst­riction through activation of alpha 1 receptors, which are plentiful in smooth muscle fibres of afferent arteri­oles. With great sympat­hetic stimul­ation, blood flow into the glomerular capill­aries is decreases and GFR drops. This reduces urine output, which helps conserve blood volume and it permits greater blood flow to other body tissues.

Epithelia cells along renal tubule and duct and PCT cells carry out reabso­rption.

2. Transc­ellular Reabso­rption: Reabso­rption of fluid through the apical membrane, cytosol ad basola­teral membrane of a tubule cell.

Primary Active Transport: Energy derived from hydrolysis of ATP is used to pump a substance across a membrane. Eg. Na⁺/K⁺ ATPase.

Sympor­ters: membrane proteins that move two or more substances across a membrane in the same direction.

Transport Maximum (Tm): maximum rate at which a system is able to transport a solute.

Facult­ative Water Reabso­rption: Reabso­rption regulated by antidi­uretic hormone. Occurs in the collecting ducts.

Solute reabso­rption in the proximal convoluted tubules involves Na⁺. Na⁺ transport occurs via symporters and antipo­rters.

Na⁺/ H⁺ antipo­rters carry filtered Na⁺ down its concen­tration gradient into a PCT cell as H⁺ is moved from cytosol into the lumen, causing Na⁺ to be reabsorbed into the blood and H⁺ to be secreted into the tubular fluid.

Solute reabso­rption in PCT promotes water reabso­rption by osmosis. Reabso­rption of solutes creates a concen­tration gradient that promotes osmosis.

In the second half of PCT, electr­och­emical gradients for Cl^-, K⁺, Mg²⁺, Ca²⁺ and urea promote their passive diffusion into peritu­bular capill­aries.

Ammonia is a poisonous waste product derived from the deamin­ation of various amino acids, a reaction occuring in hepato­cytes.

Ascending limb of the Loop of Henle is relatively imperm­eable to water.

Movement of positively charged K into the tubular fluid leaves a relative negativity in the inters­titial fluid, which promotes reabso­rption of cations like sodium, calcium, magnesium.

Early or initial part of DCT reabsorbs 10-15% filtered water, 5% Na and 5% Cl. Reabso­rption of Na and Cl happens through Na⁺/Cl^- sympor­ters.

In late DCT, two types of cells are present: principle cells and interc­alated cell

1. Osmolarity of the inters­titial fluid of the medulla becomes progre­ssively greater, more and more water is reabsorbed by osmosis as tubular fluid moves along descending limb of loop of Henle. Hence, fluid in the lumen becomes more and more concen­trated.

3. Although solutes are being reabsorbed in the thick ascending limb, the water permea­bility is quite low

5. Principle cells in the DCT and collecting ducts are imperm­eable to water when ADH levels are low. Thus tubular fluid becomes for diluted as it flows onwards.

Ability of ADH to cause excretion of concen­trated urine depends on the presence of an osmotic gradient of solutes in the inters­titial fluid of medulla.

1. difference in solute and water permea­bility and reabso­rption in different sections of the long loops of Henle and collecting ducts.

Counter current flow refers to the flow of fluid in opposite direct­ions.

Counte­rcu­rrent multip­lic­ation is the process by which a progre­ssively increasing osmotic gradient is formed in the inters­titial fluid of the renal medulla as a result of counte­rcu­rrent flow.

1. Symporters in the thick ascending limb of the loop of Henle cause a buildup of Na⁺ and Cl^- in the renal medulla. Water is not reabsorbed in this section as it is imperm­eable to water.

3. Cells in the collecting ducts reabsorb more water and urea with increase in ADH. With loss of water, urea left behind in the collecting ducts becomes increa­singly concen­trated and diffuses from the fluid in the duct into the inters­titial fluid of the medulla.

Constant transfer of urea between segments of the renal tubule and inters­titial fluid is termed urea recycling.

Counte­rcu­rrent exchange is the process by which solutes and water are passively exchanged between the blood of the vasa recta and inters­titial fluid of the renal medulla.