Anatomy of a Kidney

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T

he kidneys are part of the urinary system, serving as the starting point from which urine is collected and delivered to the bladder via the ureters. However, it has other functions as well, including controlling levels of water, electrolytes, and metabolic substances in bodily fluids. Let’s explore the anatomical features of this important organ…

What is a kidney?

There are two kidneys in the body, located dorsally (nearer to your back). Kidneys are typically associated with having a bean-shaped appearance; however, that is not the case for every animal e.g. cows and oxen for instance. Animals with the classic bean-shaped kidney include humans, pigs, dogs, cats, and some ruminants such as sheep, goats, camels, and giraffes.

The kidneys play an integral role in the urinary system, along with the ureters (connects each kidney to bladder), the bladder, and the urethra (through which urine is eliminated outside the body).

When you hear the term ‘renal’, it is referring to the kidney e.g. renal vein, renal failure.

The kidney can be unilobular or multilobular. We’ll examine what a renal lobe looks like shortly. These terms do not correlate with whether the kidney is bean-shaped or not. Both types can appear bean-shaped; it differs among species.

Unilobular – the medulla is not segmented. In dogs, cats, other carnivores, small ruminants

Multilobular – the medulla is segmented into lobes. In humans, pigs, cows, oxen

What’s in a kidney?

There are numerous important anatomical features within a kidney:

Kidney anatomy

Capsule – fibrous outer layer covering the surface of the kidney

Cortex – thick upper layer of the kidney under the capsule

Medulla – thick middle layer of the kidney under the cortex; is divided into lobes in multilobular kidneys

Renal lobe – one division of a multilobulated kidney comprising one renal pyramid and the cortical region above it

Renal pyramid – just the medullary region of the renal lobe (not including the cortical region); renal pyramid + cortex = renal lobe

Hilus – region where the renal artery, renal vein, and ureter attach to the kidney

Renal pelvis – large area at the hilus where the urine is collected from the collecting ducts of each renal lobe; is an extension of the ureter where it attaches at the hilus;

Calyx – cup-shaped extensions of the renal pelvis sitting under each renal lobe

Ureter – tube from renal pelvis in kidney where urine passes through to reach bladder

Nephron – functional unit of the kidney comprising renal corpuscle (glomerulus + Bowman’s capsule) and tubules (proximal and distal convoluted tubules) serving to filter blood; there are millions of nephrons in a kidney; two types: cortical nephron and juxtamedullary nephron

Juxtamedullary nephron – nephrons where the glomerulus is closer to the medulla and the tubules extend deeper down into the medulla

Cortical nephron – nephrons where the glomerulus is higher up and the tubules don’t extend as deeply into the medulla

Renal corpuscle – section of the nephron that’s located in the cortex; glomerulus + Bowman’s capsule

Glomerulus – globular network of capillary vessels serving as a filtration membrane for blood; blood enters via the afferent arteriole and is filtered, leaving the filtrate to enter the capsular space and continue into the tubules whilst the filtered blood exits via the efferent arteriole; located in the cortex; millions of glomeruli in each kidney

Bowman’s capsule – encapsulates the glomerulus

Afferent and efferent arteriole – afferent arteriole carries blood into the glomerulus (capillary network for filtration); blood is filtered and what’s remaining exits via the efferent arteriole

Capsular space – space between the glomerulus and Bowman’s capsule where the filtrate secreted from the glomerulus is located; blood is filtered through the glomerulus and the excess water, metabolic wastes, and unwanted solutes in the blood is excreted into the capsular space and then down the proximal convoluted tubules

Convoluted tubules – proximal and distal convoluted tubules located in the cortex; proximal and distal tubules are separated by the loop of Henle in the medulla

Loop of Henle – tubular section in the medulla; part of the tubule after the proximal convoluted tubule and before the distal convoluted tubule; comprises the thick descending limb of Henle, then thin descending limb, then thin ascending limb, then thick ascending limb; loop of Henle is longer in juxtamedullary nephrons.

Collecting ducts – takes urine from distal convoluted tubule to renal pelvis

Close-up of renal lobe

What does a kidney do?

As part of the urinary system, the kidneys play a key role in eliminating waste from blood. Through the afferent arteriole, blood enters the glomerulus where the blood – the plasma, to be exact – is filtered. Plasma is the fluid in blood excluding red blood cells, white blood cells, and platelets. This plasma is filtered and waste substances such as metabolic wastes and excess water are filtered out of the glomerulus and into the capsular space as filtrate. 

Through this process of filtration and eliminating wastes, the kidneys are able to fulfill their function of regulating water, metabolic substance and electrolyte concentrations in blood plasma.

The filtrate from the capsular space travels through the tubules and loop of Henle and eventually, all the waste is accumulated as urine in the renal pelvis where it will then enter the ureter and reach the bladder!

Hope that makes kidney anatomy a little bit clearer! See you in my next article Xx

Sources:

Hemsley, S 2019, AVBS2007 Animal Structure and Function, lecture: Urinary System Anatomy, lecture PowerPoint slides, Sydney School of Veterinary Science (The University of Sydney)

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