Structural and physiological features of the kidney

2021-06-12 11:30 PM

The nephron is the structural, as well as functional, unit of the kidney, which is capable of producing urine independently of each other.

The kidney is bean-shaped and is located behind the peritoneum. Each kidney weighs about 130g. On the longitudinal plane, the kidney is divided into two distinct regions with different colors and structures:

Cortical region: located on the convex side of the kidney, in contact with the fibrous shell, red-pink with seed spots. This is where the glomerulus is mainly concentrated.

The medullary region: located on the concave side, pale pink with fringes. This is where the renal tubules are concentrated.

The basic structural unit of the kidney is the nephron.

Structure of the nephron

Figure. The cellular structure of the nephron.

The nephron is the structural as well as the functional unit of the kidney, which is capable of producing urine independently of each other. Both kidneys have more than 2 million nephrons.

Glomerulus

The glomerulus is the starting point of the nephron, located in the renal cortex. The glomerulus functions to filter plasma to form glomerular filtrate.

Each glomerulus is composed of two components: the glomerulus and the Bowman's capsule.

Glomerulus (Malpighian Platelet)

The glomerulus is a network of more than 50 parallel capillaries arising from the afferent arterioles, which are interconnected and enclosed in Bowman's capsule. These capillaries then regroup into the efferent arterioles, which are slightly smaller in diameter than the afferent arterioles and egress from the glomerulus.

Bowman capsule

Bowman's capsule is a hollow cavity that contains the glomerular filtrate and covers the glomeruli. Composed of 2 leaves:

Visceral: consists of podocytes that are adjacent to the capillaries in the glomerulus. These legged cells fuse with the basement membrane and glomerular capillary endothelial cells to form the glomerular filter. Through this membrane, plasma from capillary blood will be filtered into Bowman's capsule to form glomerular filtrate.

Parietal leaf: connects with proximal tubule of the renal tubule.

Renal tubules

Next to the glomerulus, the renal tubules have the function of reabsorption and excretion of some substances to convert the glomerular filtrate into urine.

The renal tubules consist of the proximal tubule, loop of Henle, distal tubule, and collecting duct.

Proximal tubule

Continued with Bowman's wall. The wall of the proximal tubule is composed of a layer of tall, cuboidal epithelial cells with a brush border on the lumen side. The brush brim has the effect of increasing the contact area many times. Because the cytoplasm contains many mitochondria, carrier protein molecules, and many Na+- K+- ATPase, proximal tubule cells have high metabolic activity and active transport occurs here very strongly.

Quai Henle

Next with the proximal tubule and go into the renal medulla. Cortical nephrons have short loops of Henle. In contrast, the proximal medullary nephrons have long loops of Henle and protrude deep into the medulla.

Each loop of Henle consists of 2 parallel U-shaped branches:

The branch that enters the renal medulla is called the descending branch. Epithelial cells of this segment are flattened, so they have thin descending branches, no brush border, few mitochondria in the cytoplasm, no carrier proteins. 

The branch facing the renal cortex is called the ascending branch. Epithelial cells at the beginning of the ascending branch are flattened, so the wall is also thin and similar in structure to the descending branch, so it is called the thin ascending branch. In contrast, the epithelial cells in the posterior segment of the ascending branch are thicker, cuboidal, with many mitochondria and carrier proteins, so they are called thick ascending branches.

The junction between the descending and ascending branches is called the apex of the loop of Henle.

Distal tubule

Next to the ascending limb of the loop of Henle and located in the renal cortex, the shape is curved. The epithelial cells of the distal tubule are cube-shaped, have few microvilli, so they do not form a brush border, the cytoplasm has many mitochondria, carrier protein molecules, many Na+-K+-ATPase and H+-ATPase, so cells Distal tubule cells also have a fairly strong active transport process.

The first part of the distal tubule fuses with the afferent arterioles to form a special structure called the para-glomerular organization. This organization plays a very important role in the process of blood pressure regulation.

Contribute, support

Not entirely nephron. In the renal cortex, about 8 distal tubules merge to form the cortical collecting ducts. The end of the collecting duct goes deep into the renal medulla and becomes the medullary collecting duct. Successive generations of collecting ducts coalesce to form larger collecting ducts that pass through the medulla, parallel to the loop of Henle, and empty into the renal pelvis.

Para glomerular organization

This is a special functional organization formed by the fusion of distal tubule epithelial cells and afferent arteriolar smooth muscle cells of the same nephron (figure).

Figure. Structure of the Para glomerular organization.

Epithelial cells at the distal tubule head when in contact with the glomerulus where the afferent arterioles enter, become denser called macula densa cells. These cells contain the Golgi apparatus and these secretory organs are directed into the lumen of the afferent arterioles. On the other hand, the medial smooth muscle cells of the incoming arterioles come into close contact with these dense cells and change shape: they swell, the cytoplasm contains many fine granules, which are precursors of Renin.

The para-glomerular complex plays an important role in the formation of the Renin-Angiotensin-Aldosterone system to regulate blood pressure.

Circulation of the kidney

Renal blood vessels

The renal artery originates from the abdominal aorta and enters the renal hilum and divides into branches of the interlobular artery, and the interlobular artery divides into arcuate branches that travel along the border between the renal cortex and the medulla. From the arcuate arteries, there are interlobular arteries that give off arterioles to enter the glomerulus to form a glomerular capillary network and then gather into arterioles leaving the glomerulus, which is the second capillary system Best.

The second capillary system exits the arterioles after exiting the glomerulus, forming a capillary network surrounding the renal tubule and finally emptying into the interlobular vein. This second capillary system plays a very important role in tubular reabsorption.

Particularly in the nephrons near the medulla, the afferent arterioles do not form a network of capillaries surrounding the renal tubules, but direct to the renal medulla to form a straight Vasa recta vessel that runs beside the loop of Henle and returns to the renal cortex and empties into the renal tubules. cortical veins. The straight Vasa recta play a very important role in the collection process of urine concentration.

Blood flow to the kidneys

The amount of blood entering the 2 kidneys in an adult, at rest is about 1,200 ml (equivalent to 20% of cardiac output). This is a huge flow because the kidneys make up only 0.4% of the body weight, which helps the kidney's blood filtration process to happen very strongly.

However, the blood flow in the renal cortex and the renal medulla is completely different: the blood flow in the renal cortex is very large, accounting for about 98-99%, and in the renal medulla is only about 1-2%. Therefore, blood flow in the Vasa recta of the proximal medullary nephrons is very little and very slow.

Nephron capillary pressure

The blood pressure in the glomerular capillaries is always stable and much higher than elsewhere in the body (about 60 mm Hg), which is very favorable for glomerular filtration. In contrast, in the peritubular capillary network, the pressure is very low (about 13 mm Hg), which is favorable for tubular reabsorption. The reason blood pressure in the glomerular capillaries is always kept stable is thanks to the regulatory mechanisms in the kidneys.

When blood pressure drops, the kidneys will have the following regulatory mechanisms to keep the blood pressure in the glomerular capillaries from falling:

Increases renin secretion to increase blood pressure.

Dilatation of the afferent arterioles.

Constrict the arterioles.

When blood pressure rises, the regulatory mechanisms reverse:

Decreased Renin secretion reduces blood pressure.

Incoming arteriolar constriction.

Thanks to those regulatory mechanisms, when the systemic blood pressure changes in the range of 80-170mmHg, the pressure in the glomerular capillaries remains stable, ensuring the normal functioning of the kidneys. However, when blood pressure changes beyond this range, these regulatory mechanisms are not able to regulate. At that time, the glomerular capillary pressure will change, affecting kidney function.