Question

Difference between cortex and medulla of kidney under microscope

Answer 1

In mammals, the kidney has a granular outer most part known as the renal cortex. It forms continuous smooth outer zone with a number of projections known as cortical columns. The cortical columns are extending down between the renal pyramids. It contains renal corpuscles (glomerulus and Bowman’s capsule) as well as renal tubules except in the loop of Henle. It also contains blood vessels and cortical collecting ducts.

The renal cortex is the part of the kidney where ultrafiltration of blood takes place. The blood flows into the glomerular capillaries in the Bowman’s capsule through afferent arterioles and leaves out from the efferent arterioles. The hydrostatic pressure forces the smaller molecules in a tubular fluid such as amino acids, water, glucose, sodium chloride, urea through the filter. These things are flowing from the blood in the glomerular capsule across the basement membrane of the Bowman’s capsule into the renal tubules. This process is known as ultrafiltration. The glomerular filtrate or ultrafiltrate is free from large proteins and blood cells. The glomerular filtrate later becomes more concentrated due to the reabsorption of water and solutes. The solutes like glucose and amino acids leave the glomerular filtrate and combine again.

Water and salts also return again to the circulatory system. And glomerular filtrate is further modified by the process of secretion where blood removes waste materials into the urine. In this way, urine produces and excretes through the urethra.  Urine excretion can be measured as follows,

Urinary Excretion = Filtration + Secretion – Reabsorption

The erythropoietin which triggers producing red blood cells is synthesized in the renal cortex.

Renal medulla is the innermost part of the kidney which is divided into smaller sections known as renal pyramids. The renal medulla contains the parts of the structures of the nephron which are responsible for maintaining the water and salt balance of the blood. These structures include vasa rectae, venular rectae, medullary capillary plexus, the loop of Henle and the collecting tubule. The renal medulla is hypertonic to the filtrate in the nephron that aids in maintaining water balance by the reabsorption of water.It is believed that the inner substance the medullar contains high concentration Na+ ions. Due to this, the water will be extracted through the tubule walls into the medulla. It happens until the concentration of Na+ equals in tubes and outside them. This process conserves most of the water in the body. So, the renal medulla is very important to maintain the salt and water balance in the body.

Answer 2

differences between the renal cortex and medullaDescribe the structure of the filtration membraneIdentify the major structures and subdivisions of the renal corpuscles, renal tubules, and renal capillariesDiscuss the function of the peritubular capillaries and vasa rectaIdentify the location of the juxtaglomerular apparatus and describe the cells that line itDescribe the histology of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts

The renal structures that conduct the essential work of the kidney cannot be seen by the naked eye. Only a light or electron microscope can reveal these structures. Even then, serial sections and computer reconstruction are necessary to give us a comprehensive view of the functional anatomy of the nephron and its associated blood vessels.

Nephrons: The Functional Unit

Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).

Renal Corpuscle

As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowman’s (glomerular) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowman’s capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowman’s capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries (Figure 1).



Figure 1. Podocytes interdigitate with structures called pedicels and filter substances in a way similar to fenestrations. In (a), the large cell body can be seen at the top right corner, with branches extending from the cell body. The smallest finger-like extensions are the pedicels. Pedicels on one podocyte always interdigitate with the pedicels of another podocyte. (b) This capillary has three podocytes wrapped around it.

These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowman’s capsule and funneled to the PCT. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of Bowman’s capsule is their shared basement membrane (Figure 2). These three features comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule though pores that are only 70 nm in diameter.



Figure 2. Fenestrations allow many substances to diffuse from the blood based primarily on size.

The fenestrations prevent filtration of blood cells or large proteins, but allow most other constituents through. These substances cross readily if they are less than 4 nm in size and most pass freely up to 8 nm in size. An additional factor affecting the ability of substances to cross this barrier is their electric charge. The proteins associated with these pores are negatively charged, so they tend to repel negatively charged substances and allow positively charged substances to pass more readily. The basement membrane prevents filtration of medium-to-large proteins such as globulins. There are also mesangial cells in the filtration membrane that can contract to help regulate the rate of filtration of the glomerulus. Overall, filtration is regulated by fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The result is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.