Introduction to Kidneys

I'm currently doing my Nephrology rotation. So I will be talking about different topics from nephrology for my next few posts on medicine. The first is of course an introduction to our kidneys. In the coming days, I will try to explain how urine is formed in the kidneys, some other important functions of the kidneys and some important diseases in nephrology.

I will try my best to keep these posts as simple as possible!!

Lets get started!!

Everyday our body produces a large number of metabolic waste products. The main waste products are water, carbon dioxide and nitrogenous wastes. It is absolutely essential to get rid of these wastes to avoid damaging healthy tissues in the body.

Carbon dioxide is mainly removed via the lungs. The Urogenital system is responsible to remove the other wastes, namely water and nitrogenous waste products.

The Kidneys play the most important role in excreting these wastes.

Image from google.

We have two kidneys. Human kidneys are usually bean shaped. each kidney weighs about 120-170 grams. The kidneys are located on either side of our vertebral column, between the 12th thoracic vertebra and the 3rd lumbar vertebra.

The basic functional unit of the kidneys is known as Nephron. Each kidney has about 1 million nephrons.

Source : biologydiscussion.com

Each nephron is made of glomerulus, proximal convoluted tubules (PCT), loop of henle (descending part, hairpin loop, ascending part), distal convoluted tubules (DCT) and the collecting ducts.

Each kidney has two major parts, the outer cortex and the inner medulla. The glomeruli and convoluted tubules lie in the cortex. The medulla consists of the rest of the tubes and ducts, which project toward the inner cavity of the kidney known as the pelvis.

The Glomerulus is a network of Afferent and Efferent arterioles. Afferent arterioles enters the glomerulus and the efferent arteriole leaves the glomerulus.

Mnemonic : A for Afferent, A for Arriving. E for Efferent, E for Exiting.

This glomerular capillary network is surrounded by a double walled epithelial sac called the Bowman's Capsule (BC). The BC leads filtrate from the glomerular capillaries into the tubules of the nephron mentioned above, PCT, loop of henle and DCT and eventually into collecting ducts which drains into the renal pelvis.

The lumen of PCT is lined by epithelial cells containing something called Brush Border. Brush borders are basically just like microvilli and functions to increase the surface area immensely which helps reabsorption (more on reabsorption in the next post).

All cells that line any tube/lumen has two distinct sides. The part of the cell which is immediately adjacent to the lumen is called the Luminal Side. The other side is the Basolateral side. The basolateral side of the cells lining the lumen of the entire nephron has Na+-K+ ATPase channels which takes the sodium from inside the cells and throws them out of the cell to the interstitial space on the basolateral side (NOT into the lumen).

The luminal side has special characteristics, channels and transporters which are different in different regions of the nephron.
(These special features of different regions of the nephron will be discussed in the next post where I will be talking about how urine is produced in the kidneys.)

The loop of henle as mentioned above has 3 parts : a descending limb, a hairpin loop (labelled as segment in the diagram of nephron) and an ascending limb. the descending limp is freely permeable to water. The ascending limb has a thin part and a thick part, both of which are completely impermeable to water.

The ascending limb continues into the DCT, which then opens into a collecting tubule or duct, which in turn drains the urine into the renal pelvis. Urine from the renal pelvis continues to the bladder via the ureters.

There are two types of nephrons : Cortical (80%) and Juxtamedullary (20)%.

The loop of henle of the juxtamedullary nephrons is long and goes deep into the medullary tissue. the glomeruli of these nephrons lie close to the medulla.

The loop of henle of the cortical nephrons are short and dips only slightly into the medullary tissue. The glomeruli of these nephrons lie closer to the outer surface of the kidney.

Image from google.

Blood Supply of the Kidneys :

Blood enters the kidney via the renal arteries which originate from the abdominal aorta. The renal artery branches several times till it forms the Afferent arteriole. The afferent arteriole enters the glomerulus (remember A for afferent, A for arriving) and breaks down into the glomerular capillary network.

The glomerular capillaries now does something very interesting and unique. Everywhere else in our body, arteriole breaks down into capillaries, then thye cappilaries rejoin to form venules. But in the glomerulus, these capillaries rejoin to for another arteriole, the Efferent arteriole, which exits the glomerulus.

The efferent arteriole now breaks down into another capillary network, known as the peritubular capillary network. This network surrounds the PCT, loop of henle and DCT and is involved in reabsorption of substances from and secretion into the lumen of the nephron. These capillaries rejoin to form venules which drain into and join with further venules and eventually becomes the Renal vein, via which blood exits the kidneys into the inferior vena cava.

![Human-kidney-adjusted-27543047.jpg]
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Image from google

Alright guys, that's it for now!! Hope you learned something today!!

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