The Human Excretory System

The excretory systems are central to homeostasis because they dispose of metabolic wastes and control body fluid composition through the regulation of solute movement between internal fluids and the external environment. Today we will overlook the basic structures of the human excretory system.

Excretory organs

Kidneys: a pair of organs that functions in both osmoregulation and excretion.

Ureter: the duct in which urine produced by each kidney exits and drains into the urinary bladder.

Urinary bladder: a sac where urine is collected from the ureters.

Urethra: the tube in which urine is expelled from the urinary bladder during urination; empties to the outside near the vagina in females and through the penis in males.

Sphincter muscles near the junction of the urethra and bladder regulate urination.

Kidney structure

Renal cortex: the granular outer section of a dissected kidney; contains the glomeruli and convoluted tubules of nephrons.

Renal medulla: the smooth, striated inner section of a dissected kidney; contains the loops of Henle and the collecting tubules of nephrons.

In both the renal cortex and medulla, blood is supplied by a renal artery and drained by a renal vein. Within them lie tightly packed excretory tubules and blood vessels. It is these tubules that carry out the process of filtration of blood entering the kidney. Fluids in the filtrate are reabsorbed into the surrounding blood vessels and exit the kidney in the renal vein.

Renal pelvis: an inner cavity formed by the ureter as it enters the kidney; urine collects into the pelvis from the collecting tubules and exits the kidney via the ureter.

Types of nephron

Nephrons: the functional units of a kidney; exist between and across the renal cortex and medulla.

Cortical nephrons: nephrons that only reach a short distance into the medulla; represents 85% of total nephrons in a human kidney.

Juxtamedullary nephrons: nephrons that extend deep into the medulla; are essential for the generation of concentrated urine (hyperosmotic) due to its long loop of Henle.

Nephron

Glomerulus: a ball of capillaries that is surrounded by the Bowman’s capsule; blood pressure forces fluid from the blood in the glomerulus into the lumen of the Bowman’s capsule.

Bowman’s capsule: a cup-shaped swelling at the end of tubule; passes filtrate collected from the glomerulus to the proximal tubule.

Proximal tubule: the section of the tubule in which reabsorption of ions, water, and nutrients takes place.

Loop of Henle: functionally divided into two sections, the descending limb and the ascending limb. The descending limb consists mainly of aquaporin and is responsible for the removal of water. The ascending limb is impermeable to water and further divided into two regions, the thin and thick segment. In the thin segment, NaCl diffuses out of the permeable tubule into the interstitial fluid, whereas in the thick segment NaCl is actively transported.

Distal tubule: plays a key role in regulating the K⁺ and NaCl concentration of body fluids, and contributes to the regulation of pH by controlling the secretion of H⁺ and reabsorption of HCO3^-.

Collecting duct: processes the filtrate into urine and carries it to the renal pelvis; hormonal control of the permeability of the collecting duct ultimately determines the concentration of the urine.

Blood enters the nephron through the afferent arteriole and leaves through the efferent arteriole. The capillaries of this arteriole form the branched peritubular capillaries, which surround the proximal and distal tubule. Other branches extend downward and form the vasa recta, surrounding the loop of Henle.

An overview of the human excretory system

Nephron organization

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

The Editors of Encyclopaedia Britannica. “Kidney.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 23 Jan. 2019, www.britannica.com/science/kidney#ref287943.