Question

Topic 11: Nephron physiology

Describe in detail the physical of all components of the nephron

• Glomerular capsule

• Proximal convoluted tubule

• Loop of Henle

• Distal convoluted tubule

• Collecting duct

In your discretion be sure to include the various permeability and movements of the following

• Water

• Potassium

• Sodium

• Glucose

• Hydrogen ions

Also, include

• How the vertical osmotic gradient is established and why this is significant

• The role of antidiuretic hormone (ADH) vasopressin and the renin-angiotensin-aldosterone system (RAAS) in urine formation.

Answer 1

The structure of nephron comprises two major portions:

A) Renal Tubule

B) Renal Corpuscle

Renal corpuscles, it consists of a glomerulus surrounded by a Bowman’s capsule. The glomerulus arises from an afferent arteriole and empties into an efferent arteriole.  A considerable amount of the blood plasma filters through the glomerulus into the nephron tubule. This results from the large amount of blood flow through the glomerulus, the large pores  in the glomerular capillaries, and the hydrostatic pressure of the blood. Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. Small molecules, including water, readily pass through the sieve-like filter into the nephron tubule. Both lipid soluble and polar substances will pass through the glomerulus into the tubule filtrate

Renal tubule : The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function.

a) Proximal Convoluted Tubule, (PCT) The blood brought by the renal artery is filtered by the glomerulus and then passed to the PCT, The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place, the surface area for reabsorption is facilitated by the lining of the simple cuboidal epithelium in them. Approximately 2/3rds of water in the nephron and 100% of the glucose and amino acids, 70% of sodium, phosphate, calcium, magnesium and 50% of urea in the nephron are reabsorbed in the proximal convoluted tubule.

b) Loop of henle, it  is a U-shaped tube that consists of a descending limb and ascending limb, Being parts of the same loop, both the descending and ascending limbs show different permeability. The descending limb is permeable to water but impermeable to an electrolyte, while the ascending limb is permeable to electrolytes but impermeable to water, this charecterstics causes an increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle’s loop is impermeable to water but highly permeable to ions, causing a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L. The PCT, selectively secretes ions such as hydrogen, ammonia, and potassium into the filtrate and absorbs hydrogen bicarbonate from it. Thus, PCT maintains the electrolyte and acid-base balance of the body fluids.

c) Distal Convoluted Tubule (DCT), The distal convoluted tubule (DCT) along with collecting duct (CD) are the final two segments of the kidney nephron, Once the filtrate passes through the thick ascending limb of Henle, it enters the distal convoluted tubule, which is a duct of the renal tubule located in the kidney's cortex, Similar to PCT, it reabsorbs calcium, sodium, and chloride and regulates the pH of urine by secreting protons and absorbing bicarbonate. Conditional reabsorption of sodium ions and water takes place in DCT. Thus, it maintains the pH and sodium-potassium level in the blood cells. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.

Collecting Duct : Collecting duct is a long, straight tube where hydrogen and potassium ions are secreted to maintain the electrolyte balance of the blood.The main role of the collecting duct is the reabsorption of water, through the action of anti-diuretic hormone (ADH). This is also the region where the maximum reabsorption of water takes place to produce concentrated urine.

Counter current multiplication, Repetitive reabsorption of sodium chloride by the thick asscending loop of henle and continued inflow of sodium chloride from the proximal tubule into the loop of henle is called counter current multiplication, this mechanism  is largely responsible for developing the osmotic gradients that are needed to concentrate urine, this generation of osmotic gradient is what is responsible for concentrating urine, otherwiswe kidneys would have produced, litres and litres of dilute urine every day

The Renin-Angiotensin-Aldosterone System (RAAS) is a hormone system within the body that is essential for the regulation of blood pressure, blood volume, sodium reabsorption, potassium excretion and fluid balance.The system is mainly comprised of the three hormones renin, angiotensin II and aldosterone. Renin, which is released primarily by the kidneys, stimulates the formation of angiotensin in blood and tissues, which in turn stimulates the release of aldosterone from the adrenal cortex.Aldosterone causes an increase in salt and water reabsorption into the bloodstream from the kidney thereby increasing the blood volume, restoring salt levels and blood pressure.