Summary of the Anatomy of the Kidney
Kidneys are a part of the renal system. The reddish-brown kidneys weigh 120 to 170 grams and are 10-12 cm long, 5-7 centimeters (cm) wide, and 2-3cm thick. It is covered by three layers, which are fibrous capsule, perirenal fat, and renal fascia. Internally the kidney consists of three parts, outer cortex, inner medulla, and renal sinus (empty cavity). Nephrons are the structural and functional units of the kidneys.
Kidneys are one of the essential organs of our body. Humans can live with one kidney, but if both the kidneys are not working correctly, it will cause severe problems in our body. It is essential for everyone (we believe) to learn about the anatomy of organs so that we can know our organs better and can take proper care of them. So, today let us learn about the anatomy of the kidney. Are you ready?
Before learning the anatomy of the kidneys, let us learn about the renal system.
Kidneys help in collecting the waste products of the blood.
Urinary bladder helps in storing urine for a small period of time.
Urethra is the narrow passageway that passes urine out from the urinary bladder.
Ureters are the connecting tubes between kidneys and urinary bladder and helps in transporting urine from kidneys to urinary bladder.
Kidneys are also known as renes (the word renal is derivative of renes) and nephros (root word for nephrons, nephritis, etc.).
They are retroperitoneal organs (organs that are present behind the peritoneum*).
Kidneys are bean-shaped structures. They are reddish-brown. Each kidney weighs only 120 to 170 grams. Each kidney has a length of 10-12 centimeters, a width of 5-7 centimeters, and a thickness of 2-3 centimeters.
Kidneys are kept in place by the renal fascia (connective tissue which covers kidneys and adrenal glands), perirenal fat (adipose or fat tissue which is present between renal fascia and renal capsule), and pararenal fat (adipose or fat tissue which is present over the renal fascia).
They have two surfaces – the anterior surface, which is irregular, and the smooth posterior surface. However, it is difficult to distinguish just by looking at the kidneys.
They have two borders. One is a lateral border (close to the midline), which is convex, and the other is the medial border (farther from or away from the midline), which is concave.
Anatomy of the Kidney: Coverings/Capsules of the Kidneys
There are three coverings of the kidneys which are briefly described below:
It is the innermost layer and is also called the renal capsule. It is a thin membrane that wraps the kidneys and the renal sinus (the cavity of the kidney).
Perirenal or Perinephric Fat
It is a layer of adipose tissue that lies outside the fibrous capsule. It is thickest at the borders, and it fills any space present in the renal sinus.
It is the connective tissue that covers kidneys and adrenal glands.
The kidney’s middle part has a depression, which is called hilum or hilus. It is through hilum or hilus, the ureters, blood vessels, nerves, etc. enter and exit the kidneys.
Inner to the hilum, there is renal pelvis, which has projections known as calyces (plural word for calyx). The renal pelvis continues as ureter as it exits the hilum.
There are two types of calyces. The first type is the major calyces, which are nearly 2 to 3 in number and minor calyces, which are 7 to 13 in number.
Anatomy of the Kidney: Regions of the kidney
The kidneys consist of three regions – outer cortex (reddish-brown) and inner medulla (pale color) and renal sinus (cavity of the kidney).
The medulla has near about 10 conical masses, which are usually known as medullary pyramids or renal pyramids. Their tips form renal papillae, which indent the minor calyces.
The part of the cortex which enters into the medullary pyramids is called renal columns or Columns of Bertini.
Kidneys, ureters, pelvis all have smooth muscles, and therefore, the muscles contract and relax rhythmically without our voluntary help.
The calyces collect and empty the urine in the ureter. Urine reaches urinary bladder (where it is stored temporarily) through the ureter and then is expelled through the urethra.
Anatomy of the Kidney: Structure of the Nephron
Each kidney consists of nearly 1-3 million uriniferous tubules. These uriniferous tubules consist of two parts which are the nephron and collecting tubule
These two structures are embryologically different from each other.
Nephrons are the structural and functional units of kidneys. Nephrons consist of two parts which are the renal corpuscle or malpighian corpuscle and renal tubule
The renal corpuscle also has two parts. They are glomerulus and bowman’s capsule.
The glomerulus is a tuft of capillaries which is formed by afferent arteriole (which is a branch of the renal artery). Blood moves out of the glomerulus through afferent arteriole. Its endothelium has many pores called fenestrae.
Bowman’s capsule encloses glomerulus. It is a double-walled cup-like structure. Renal tubule starts with the Bowman’s Capsule.
The outer layer of Bowman’s Capsule is made up of squamous epithelium, whereas the inner layer is made up of podocytes (a type of epithelial cells).
Podocytes have cytoplasmic extensions, and they eventually end as foot processes on the basement membrane of glomerulus.
The openings present between these foot processes are known as filtration slits.
Renal tubule further continues to form proximal convoluted tubule (PCT) and then as the loop of Henle or Henle’s loop. This is a hairpin or U-shaped structure.
Henle’s loop consists of thin descending limb and thick ascending limb. The ascending limb further continues as the coiled region called the distal convoluted tubule (DCT).
Distal convoluted tubules of many nephrons make the collecting duct. These collecting ducts pass through the medullary pyramids in the pelvis region.
Collecting ducts converge and form a papillary duct. It is through these ducts that urine reaches the calyces. The calyces open into the renal pelvis (which is the inflated upper part of the ureter).
Malpighian Corpuscle, PCT, and DCT are present in the cortical region of the kidney. In contrast, the Henle’s loop is present in the medulla region.
Depending on how deep Henle’s loop is present in the medulla region, the nephrons are categorized into two groups. They are:
Cortical nephrons – Nephrons have short Henle’s loop, and hence, Henle’s loop doesn’t go deep into the medulla region.
Juxtamedullary nephrons – Some nephrons have a long loop of Henle and are deeply embedded in the medulla.
There are two capillary beds of a nephron. The first one is the glomerulus, and the other one is peritubular capillaries. Afferent arteriole supplies the blood, and efferent arteriole drains the blood from the glomerulus. The efferent arteriole that leaves glomerulus forms an excellent network of capillaries around the renal tubule, which is nothing but a peritubular capillary bed.
Fun Fact: The efferent arterioles of the juxtamedullary nephrons form vasa recta and runs parallel to the loop of Henle. Vasa recta are either completely absent or reduced in cortical nephrons.
Fun Fact: Losing fat quickly has its impact on the kidneys as well. When we lose fat drastically, the kidneys droop down, which leads to the kinking of the ureter. If the ureter gets kinked, the urine enters the kidney again, and the kidney swells up.
Fun Fact: The fetus consists of lobulated kidneys. There can be nearly 12 lobules. After conception, the lobules fuse to form a smooth kidney. However, in some cases, the evidence of lobulation persists even after fusion.
Functions of the Kidneys
Kidneys help in the homeostasis of the body. It keeps the amount of fluid in the body in the normal ranges.
It produces calcitriol and erythropoietin hormones.
It helps in maintaining the normal pH of the blood (7.4) by either increasing or decreasing the excretion of hydrogen ions.
Peritoneum*: a serous membrane that lines the abdominal cavity and abdominal organs.
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