Tubular functions of nephrone, Lecture notes of Physiology

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Tubular Function

Presented by:- Ritu Sharma

• (^) tubular transports are selective processes as solvent and different solutes are either reabsorbed or secreted by mechanisms specific to their transport.
• (^) Tubular exchange of water and electrolytes finally determines the volume and composition of urine.
• (^) Thus, tubular mechanisms are most important processes in determination in urine volume and composition. By modulating the reabsorption and secretion of substances of its luminal fluid, renal tubule plays an important role in the control of composition, osmolality, pH and volume of ECF

Common Principle of Tubular

Function

• (^) The major function of tubule is to reabsorb water and solutes from the tubular fluid, which is crucial for water and electrolyte homeostasis of the body. These transport mechanisms for various substances are different in different parts of the kidney tubule.
• (^) Transport Mechanism:-
• (^) 1:- Passive Transport Mechanism
• (^) 2:- Active Transport Mechanism

Passive Transport Mechanisms

• (^) Diffusion
• (^) Facilitated Diffusion
• (^) Solvent Drag
• (^) Osmosis

Facilitated Diffusion

• (^) In this transport mechanism , a specific carrier protein in the membrane facilitates the process of diffusion.
• (^) Reabsorption of glucose via glucose transporter is an example of facilitated diffusion.
• (^) There are two mechanisms of coupled transports: symport mechanism, and antiport mechanisms.
• (^) Symport Mechanism =
• (^) The symport mechanism is the process of coupled transport of two or more solutes in same direction by a carrier protein. The examples are the transport of Na+ -glucose, Na+ -amino acid, etc. Antiport Mechanism.

• (^) Antiport Mechanism =
• (^) Antiport mechanism is the process of coupled transport of two or more solutes in opposite direction by a carrier protein.
• (^) An example is the Na+ –H+ exchange in the proximal tubule that reabsorbs Na+ from the tubular fluid in exchange for secretion of H+ into it.

Osmosis

• (^) When a considerable amount of osmotically active solute is transported, water is reabsorbed along with it to maintain the osmotic balance.
• (^) This is the major mechanism for reabsorption of water from the tubular lumen.
• (^) For example, water reabsorption follows reabsorption of Na+ and Cl– from the tubular fluid.
• (^) Conversely, increased osmolality of tubular fluid increases water excretion, known as osmotic diuresis

Active Transport Mechanism

• (^) Transport of solutes is considered to be active when ATP is utilized in the process.
• (^) In this mechanism, solutes are transported from the area of lower concentration to the area of higher concentration.
• (^) The best example of active transport mechanism is the Na+ K+ pump.
• (^) 1. The Na+ -K+ ATPase is mainly located in the basolateral membrane of the tubular epithelial cells. This pumps sodium out of the tubular epithelial cells and potassium into the cell.

Tubule Function

• (^) Proximal Tubular Functions:- The proximal tubule is the most important part of the nephron as it reabsorbs about 67% of the filtered water, Na+ , Cl− , K+ , and HCO3 − and almost all the filtered glucose and amino acids.
• (^) 1. The proximal tubule has convoluted and straight portions. Though the convoluted part (PCT) comprises 70% of the tubule, functionally both the parts are similar with few minor differences.
• (^) 2. The primary focus of reabsorption process in the proximal tubule is directed at the Na+ reabsorption, which is usually secondary to electrochemical gradient created by Na+ K+ pump located on the basolateral membrane of the epithelial cells.
• (^) 3. Reabsorption of water and most of the solutes is directly or indirectly linked with this pump.

Water Reabsorption

Glucose Reabsorption

Functions Loop of Henle

• (^) Transport of Solutes: LOH reabsorbs about 25% of the filtered NaCl and K+ , 30% of filtered Ca++, and 65% of filtered Mg++. The descending limb of LOH is permeable to water but not to the solutes, whereas the ascending limb is impermeable to water. Na+ , K+ , and Cl− are cotransported out of the thick segments of the ascending limb.
• (^) Descending Limb Descending limb is permeable to water, but not to solutes. Therefore, fluid in terminal part of descending limb becomes hypertonic as water moves out of the tubule to enter the interstitial space.
• (^) Thin Ascending Limb In the thin part of ascending limb of LOH, transport mechanism is predominantly a passive phenomenon, and permeability to water is less. Though passive transport of NaCl occurs to some extent, active transport of Na+ is almost nil.
• (^) Thick Ascending Limb In the thick ascending limb of LOH, the fluid becomes more dilute (hypotonic) as Na+ and Cl− are taken out of the tubular fluid.

Transport of Solutes