The Kidneys
Definition and Overview of the Kidneys
The kidneys are vital bean-shaped organs located in the abdominal cavity, positioned on either
side of the spine just below the ribcage. Each individual typically has two kidneys, which are
crucial components of the excretory system. Their primary functions include filtering blood to
remove metabolic waste products, regulating the body's fluid and electrolyte balance
(osmoregulation), maintaining blood pressure through the renin-angiotensin system, and
producing urine for waste excretion. Additionally, the kidneys play a role in producing
erythropoietin, a hormone that stimulates red blood cell production, and in activating vitamin D
for calcium absorption.
Functions of Kidney’s Structural Components
The kidneys are highly specialized organs composed of distinct anatomical regions, each
performing critical roles in blood filtration, waste removal, and fluid balance.
Renal Cortex
The outermost layer of the kidney, the renal cortex, contains the nephrons, the functional units
responsible for filtration. Within the cortex, the glomeruli and Bowman's capsules initiate blood
filtration by allowing water, ions, and small molecules to pass into the nephron while retaining
blood cells and proteins. The proximal convoluted tubules (PCTs) located here reabsorb essential
nutrients, including 65% of filtered water, glucose, amino acids, and electrolytes (Na⁺, K⁺, Cl⁻).
Additionally, the PCT secretes waste products such as creatinine and certain drugs into the
forming urine.
Renal Medulla
The inner renal medulla consists of renal pyramids, which contain the loops of Henle and
collecting ducts. The loop of Henle establishes a salt concentration gradient in the medulla,
crucial for water reabsorption. The descending limb of the loop is permeable to water, allowing it
to passively move out into the surrounding tissue, while the ascending limb actively transports
Na⁺ and Cl⁻ out, further concentrating the urine. The collecting ducts, influenced by antidiuretic
hormone (ADH), adjust water reabsorption to produce either concentrated or dilute urine based
on the body's hydration needs.
Renal Pelvis
The renal pelvis acts as a funnel, collecting urine from the nephrons and channeling it into the
ureter. This structure ensures the smooth passage of urine from the kidney to the bladder for
storage and eventual excretion.
Nephrons
Each kidney contains approximately one million nephrons, each consisting of a renal corpuscle
(glomerulus + Bowman's capsule) and a renal tubule (PCT, loop of Henle, DCT, and collecting
duct). The glomerulus filters blood under high pressure, while Bowman's capsule captures the
filtrate. The PCT reabsorbs vital nutrients and water, while the distal convoluted tubule (DCT)
fine-tunes electrolyte balance by secreting excess K⁺ and H⁺ ions and reabsorbing Na⁺,
regulated by aldosterone. The collecting duct finalizes urine concentration under ADH control.
Blood Vessels
The renal artery delivers oxygenated blood to the kidney, branching into smaller afferent
arterioles that supply the glomerulus. After filtration, blood exits through efferent arterioles,
which give rise to peritubular capillaries surrounding the nephron tubules, facilitating
reabsorption and secretion. The renal vein carries filtered blood back into systemic circulation.
Ureter
The ureter is a muscular tube that transports urine from the renal pelvis to the bladder, using
peristaltic contractions to ensure one-way flow.
Juxtaglomerular Apparatus (JGA)
Located near the glomerulus and DCT, the JGA monitors blood pressure and sodium levels. It
secretes renin, initiating the renin-angiotensin-aldosterone system (RAAS) to regulate blood
pressure and fluid balance.
Filtration: Glomerulus removes waste from blood.
Reabsorption: PCT and loop of Henle reclaim water and nutrients.
Secretion: DCT eliminates excess ions and toxins.
Hormonal Regulation: ADH (water balance), aldosterone (Na⁺/K⁺), and renin (blood pressure)
fine-tune kidney function.
How Kidneys Function: The Process of Urine Formation
The kidneys perform their filtration role through three key processes:
a) Glomerular Filtration
Blood enters the kidney via the renal artery, branching into smaller vessels that lead to the
glomerulus, a network of capillaries enclosed within Bowman’s capsule. Due to high blood
pressure, water, salts, glucose, and waste products (such as urea and creatinine) are forced out of
the blood into Bowman’s capsule, forming the filtrate. Larger molecules like proteins and blood
cells remain in the bloodstream.
b) Tubular Reabsorption
The filtrate moves into the proximal convoluted tubule (PCT), where essential substances
(glucose, amino acids, and about 65% of water and salts) are reabsorbed back into the blood. The
loop of Henle further concentrates the filtrate by creating a salt gradient in the medulla, allowing
water to be reabsorbed in the collecting duct.
c) Tubular Secretion
In the distal convoluted tubule (DCT) and collecting duct, additional waste products (such as
hydrogen ions, potassium, and drugs) are actively secreted into the filtrate. The final product,
urine, passes through the ureter to the bladder for storage and eventual excretion.
The Nephron: Structure and Its Role in Blood Filtration
The nephron is the functional unit of the kidney, with each kidney containing approximately one
million nephrons. A nephron consists of two main parts:
a) Renal Corpuscle (Glomerulus + Bowman’s Capsule)
The glomerulus is a high-pressure capillary network where blood filtration begins. Bowman’s
capsule surrounds the glomerulus and collects the filtrate.
b) Renal Tubule (PCT, Loop of Henle, DCT, Collecting Duct)
Proximal Convoluted Tubule (PCT): Reabsorbs nutrients (glucose, amino acids) and most water
and salts.
Loop of Henle: Establishes a salt gradient to facilitate water reabsorption.
Distal Convoluted Tubule (DCT): Fine-tunes electrolyte balance and pH by secreting excess
ions.
Collecting Duct: Adjusts water reabsorption based on the body’s hydration needs, influenced by
antidiuretic hormone (ADH).
The nephron’s structure ensures that essential substances are retained while waste is efficiently
removed.
The Kidney’s Role in Osmoregulation
Osmoregulation is the process by which the kidneys regulate water and electrolyte balance in the
body. This is primarily controlled by hormones:
a) When the Body is Dehydrated
The hypothalamus detects high blood solute concentration. The pituitary gland releases ADH
(Antidiuretic Hormone). ADH increases the permeability of the collecting ducts, allowing more
water reabsorption. This results in concentrated urine (dark yellow, low volume) and conserves
body water.
b) When the Body is Overhydrated
Low solute concentration reduces ADH secretion. The collecting ducts become less permeable to
water. More water is excreted, producing diluted urine (clear, high volume). Additionally, the
hormone aldosterone regulates sodium and potassium balance, further fine-tuning blood pressure
and electrolyte levels.
Conclusion
The kidneys are essential for maintaining homeostasis by filtering blood, removing toxins,
balancing fluids, and regulating blood pressure. The nephron is the key functional unit, where
filtration, reabsorption, and secretion occur. Through hormonal control (ADH, aldosterone), the
kidneys adjust water retention and electrolyte levels, ensuring the body remains in equilibrium.
Understanding kidney function is crucial for grasping broader concepts in physiology, including
blood pressure regulation and metabolic waste removal.
