Anatomy and Physiology of the Kidneys and Nephron

Anatomy of the Kidneys:

- Lie against the posterior wall of the abdomen at level T12 to L3.
- Right kidney lower than left due to the liver.
- Convex lateral surface and concave medial surface with hilum, where it receives the renal nerves, blood vessels, lymphatic vessels and urter.
- Protected by 3 layers; from superficial to deep- the fibrous renal fascia, the adipose capsule and the renal capsule
- Renal parenchyma (glandular tissue that forms the urine) appears C-shaped in frontal section and encircles a medial space, the renal sinus (occupied by blood and lymphatic vessels, nerves and urine-collecting structures)
- Parenchyma divided into outer cortex and inner medulla.
- Extension of cortex into the medulla called renal columns; divides the medulla into renal pyramids
- Each pyramid conical with a broad base facing the cortex and the blunt tip, called the papilla facing the sinus
- Each papilla is nestled in a minor calyx, which drains the urine and 2-3 minor calices converge to form a major calyx.
- 2-3 major calices then converge to form the renal pelvis which continues inferiorly as the ureter.

Functions of the kidney

- Filter blood plasma, separate metabolic wastes from the useful chemicals and eliminate the waste while returning the rest to the bloodstream
- Regulate blood volume and pressure by eliminating or conserving water as necessary
- Regulate osmolarity of body fluids by controlling the relative amounts of water and solutes eliminated
- Secrete enzyme rennin, which activates the hormonal mechanisms that control blood pressure and electrolyte balance
- Secrete hormone erythropoietin, which controls the RBC count and oxygen carrying capacity of the blood
- Function with the lungs to regulate PCO2 and acid-base balance of the body fluids
- Contribute to calcium homeostasis through their role in synthesizing clacitriol (active form of Vitamin D)
- Detoxify free radicals and drugs with the use of peroxisomes
- Carry out gluconeogenesis in times of starvation by deaminating amino acids

Anatomy of the nephron

- Functional unit of the kidney
- Each kidney contains 1.2 million nephrons
- Each nephron consists of 2 principal parts: the renal corpuscle (where blood plasma is filtered) and the long renal tube (that processes the glomerular filtrate into urine).
- Renal corpuscle consists of the glomerulus (ball of capillaries) enclosed in a two-layered glomerular capsule or Bowman’s capsule
- Renal tubule consists of the Proximal Convoluted Tubule, the Nephron Loop or the Loop of Henle, the Distal Convoluted Tube


NB: blue box: reabsorption/ pink box: secretion

Basic Physiology of the Different parts of the Nephron

Renal corpuscle

Glomerular filtration occurs here. It is a process in which water and solutes in the blood plasma pass from the capillaries of the glomerulus into the capsular space of the nephron to form the glomerular filtrate

Proximal Convoluted Tubule (PCT)

The PCT reabsorbs about 65% of the glomerular fluid back into the peritubular capillaries, while it also removes some substances from the blood and secretes them into the tubule for disposal in the urine.

Nephron Loop

The primary function of the nephron loop is to generate a salinity gradient that enables the collecting duct to concentrate the urine and conserve water.

Distal Convoluted Tubule (DCT)

Fluid reabsorption that is subjected to hormonal control (particularly by aldosterone, Atrial Natriuretic Peptide, Antidiuretic Hormone and parathyroid hormone) takes place here.

NB - Some terms to take note of:
Glomerular filtrate: Fluid in the glomerular capsular space
Tubular fluid: Fluid from the PCT to the DCT; different from the Glomerular filtrate due to substances removed or added by the tubule cells
Urine: fluid will be called urine once it enters the collecting ducts


Source: Saladin, Anatomy & Physiology 3rd Ed, Chap23

Contributed by: John Lee