How and Why are Kidneys Important? 

Kidneys are the primary organs that eliminate all the waste products from within the body. The acids produced in the cells as a result of metabolism are removed by the kidneys. Kidneys help maintain the salt, water, and minerals like potassium, calcium, and sodium concentration in the blood. Only when the waste is removed, all other organs in the body can function properly. 

Structure of the Kidney 

The middle portion of the back is where the kidneys are positioned in the human body. These are superior to the rib cage. Each of the two kidneys has a size of a fist of an adult. They are major filtration organs that eliminate all the waste from the blood. Around one-quarter of the blood is filtered out every minute by the kidney, which sums up to about 150 to 170 litres of the blood per day. This organ has numerous functions, apart from the excretory function. Some of these are controlling the blood pressure (BP), production of red blood cells, controlling the acid level of the blood, and regulation of calcium. There are certain special cells in the filtering tubules of the kidney which can sense the sodium levels. When there is a pressure drop, the sodium levels also fall. When there are high sodium levels in the body, more water will be retained inside the body, which in turn increases the BP. There is yet another mechanism of regulating the BP by the kidneys which is through the hormone renin. Renin is associated with angiotensin I and II and will result in the constriction of the blood vessels, thereby increasing BP. 

"Anatomy of kidney "
CC BY 3.0 | staff (2014)

Working of the Kidneys 

The major filtering unit inside the kidneys is the nephrons. Every nephron filters a large blood volume and extends into structures called tubule and glomerulus. There are two steps by which the filtration takes place: 

  • Filtration by Glomerulus 
  • Re-absorption by tubule 

Filtration and Re-absorption 

Glomerular filtration is the filtration of the blood through the pores of glomeruli. The glomerulus structure is rich in blood vessels and has thin walls which makes it permeable to the small molecules and wastes, majorly water to travel across the tubule. Some of the larger molecules such as the blood cells and proteins remain inside the blood without filtering. All the essential substances to the body are returned by the tubule. This happens through the reabsorption of all the excess water by the blood vessels along with the minerals and nutrients. The rest of the non-essential fluid coming from the tubule becomes urine by reaching the urinary system.  

The renal tubule is a structure that is convoluted and long. This emerges from the glomerulus and is further classified into 3 parts depending on its function. They are, namely, the distal convoluted tubule (DCT), the loop of Henle, and the proximal convoluted tubule (PCT). DCT is referred to as the neuron’s last part that generally connects and empties its content into the microscopic passage called collecting ducts. The main role of the collecting duct is water reabsorption. This is via the action of aquaporins and anti-diuretic hormone (ADH). 

"Process of Reabsorption"
CC BY 3.0 | | Holly Fischer

Blood Flow Through the Kidneys 

This flow happens through major blood vessels such as the renal artery. This renal artery further branches into the tiniest of the nephrons where the blood is filtered out. Nephrons comprise the glomerulus. Later, the filtered blood tends to flow out through the veins. 

Blood keeps circulating through the kidneys all throughout the day. However, per day, around 150 quarts of blood are filtered out. But only one or two quarts of that filtered substance become urine. 

Blood flow through renal artery 

As and when the blood arrives at the kidney, the filtration process starts. The blood gets into the kidney at the hilum via the renal artery. This renal artery later divides into two arteries. These are arcuate arteries that later branch into interlobular arteries. These branches further run inside the renal pyramids and renal column and, finally, into the afferent arteriole. 

Blood flow through afferent arteriole 

The afferent arteriole serves as the incoming blood vessels which supply blood to the glomerulus. This helps to regulate the BP inside the kidneys. The afferent arteriole is wider than the efferent arteriole. This results in the blood rushing inside the glomerulus and exit through a narrow pathway. When the blood enters the narrow blood vessels inside the kidneys, they are pressurized. This pressure creates plasma, which is a yellow fluid of the blood consisting of electrolytes, water, and proteins, to pass through the glomerulus. The plasma that remains after filtration is called the filtrate. 

Blood flow through efferent arteriole 

This flow occurs after exiting the glomerulus. This efferent arteriole is the outgoing blood vessel that carries the blood away from the nephrons comprising the glomerular capillaries. The blood later flows into the individual vessels known as the peritubular capillary. This peritubular capillary surrounds the distal and proximal convoluted tubule inside the kidney which later descends to the vasa recta and travels through a venule network that converges into the cortical radiate and arcuate veins. 

Blood flow through renal veins 

When the blood reaches the arcuate veins, it pushes the blood towards interlobar veins which finally converges with the large renal veins. These veins transport the blood away from the kidney. After the blood exits the kidneys, the filtrate travels through the kidneys to the ureters, which is developed from the funnel-shaped structure termed renal pelvis. This descends towards the bladder and remains stored unless it is excreted from the body. 

"Representation of blood supply"
CC-BY-SA 4.0 | OpenStax CNX

Healthy kidneys tend to work throughout the day to filter the blood and remove all the waste substances from them. However, they are parallelly involved in several other regulatory mechanisms as well.  

When there is any malfunction in the kidney organ and any component of it, it can lead to diseases like hypertension and diabetes which can further damage the kidney over a while. This happens due to the damage of the fine arteriolar network present in the organ. When the arterioles running inside the kidneys become narrow or hardened due to various reasons, the filtration process does not take place properly and the blood does not get free from the metabolic waste which turns out toxic.  

Certain early symptoms of kidney failure will not be exhibited in some cases. But over time it can get chronic and fatal.  

Weakness, lethargy, confusion, swelling, and shortness of breath are some of its common symptoms. Renal artery stenosis (narrowing of arteries), glomerulonephritis (inflammation of glomeruli) and urinary tract infections are some examples of significant kidney diseases. 

Context and Applications 

This concept is very essential in professional coursework of both undergraduate and graduate programs such as  

  • Bachelor of Science in Biology 
  • Bachelor of Science in Zoology 
  • Bachelor of Science in Medicine 
  • Master of Science in Biological science 
  • Master of Science in Human anatomy and physiology 
  • Masters in Biotechnology 

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