Physiology of excretion is the study of different metabolic activities occur inside our body and gives an idea of which organs and tissues facilitate the excretion mechanism. We sometimes get confused with the terms excretion and egestion.
Excretion is a mechanism of eliminating metabolic wastes from the body, while egestion is the process that removes the digestive waste or undigested food. Metabolic wastes includes sweat, urine and carbon dioxide released by the skin, kidneys and lungs, respectively.
In this article, we will mainly focus on the excretion of urine via kidneys. Urine contains toxic wastes like urea, uric acid and ammonia etc. that are formed by the metabolism of amino and proteins. Excretion relies upon the quantity of water that we intake.
Content: Physiology of Excretion in Human
Definition of Excretion
Excretion is a common process that occurs in all living organisms, which can define as the elimination of toxic metabolic nitrogenous wastes (urea, uric acid, ammonia etc.) that form by the metabolism of amino and nucleic acids. The process of excretion and the types of nitrogenous wastes that has to be eliminated varies greatly among the different species, depending upon the water availability.
Classification of Organisms Based on Excretion of Metabolic Waste
A kidney of human body discards the common excretory products like amino acids, ammonia, urea, uric acid etc through urine. Amino acids produce as a result of protein digestion, while ammonia and urea are the toxic metabolic waste that release as a result of amino acids deamination and metabolism. Uric acid releases in a crystalline form with small amount of water, in the animals those conserve water. Thus, the organisms broadly classify into the following groups, depending upon the type of metabolic waste eliminates.
- Ammonotelic organisms: These excrete ammonia as a metabolic waste, and it is quite common in aquatic animals. Its transpiration and excretion requires more water, as these are highly toxic. Ammonia readily solubilizes in water and release via oxidative deamination of amino acids.
- Ureotelic organisms: These excrete urea as a metabolic waste, and common in mammals, amphibians, few reptiles etc. It also requires much water for the excretion. Like ammonia, urea readily solubilizes in water and highly toxic. Urea releases as a result of ornithine cycle by the reaction of ammonia and carbon dioxide.
- Uricotelic organisms: These excrete uric acid as a nitrogenous waste, and it predominantly occurs in reptiles, birds and insects. Its excretion requires less water, as it is least toxic. Unlike ammonia and urea, uric acid is insoluble in water.
The urinary system in human includes kidneys, ureter, urinary bladder and urethra that collectively constitute the urine excretion. Kidney is a major organ that assist the separation of nitrogenous wastes to the urinary bladder through a long tube called ureter. Let us look into the external structure of our urinary system.
There are two kidneys in a human body, whose average weight is 120-170 grams. Its structure appears bean-shaped that is encircled by a layer of fat and connective tissue. A vertical section of kidney shows a renal capsule, cortex, medulla, pelvis and hilum.
- Renal capsule: It is a thin and tough outer covering of the kidneys that is composed of dense connective tissues.
- Renal cortex: It is found interior to the renal capsule. Renal cortex includes cluster of blood capillaries and a glomerulus.
- Renal medulla: It is found interior to the renal cortex. The renal medulla possess a radial appearance and comprises nephron tubule, vasa recta and collecting duct. It can be partitioned into an outer and inner medulla. An outer medulla comprises renal columns, which also refers as “column of Bertini”. Renal pyramids appear as cone shaped structures that constitute an inner medulla, as these extend out to form renal papillae.
- Renal pelvis: It resembles a funnel-shape that comprises around 8-18 minor and 2-3 major projections or calyces. Renal pelvis is inner to the hilum.
Ureters: It appears as two long slender tubes that originates from the region of renal pelvis and goes downwards to the urinary bladder.
Urinary Bladder: It is located in the lower part of abdominal cavity, and connected with the ureters and urethra. Urinary bladder acts like a hollow and muscular organ that comprises an elastic wall, which can expand or contract accordingly.
Nephron as a Excretory Unit
Nephrons are the functional units of the kidneys, which separates urine from the blood. In kidney, nephrons are generally categorized into cortical and juxtamedullary nephrons.
- Cortical nephrons: It constitutes about 80-85% of nephrons. The renal corpuscles lie within outer renal cortex. Here, the loop of Henle runs very little to the medulla. It maintains the ionic balance of blood.
- Juxtamedullary nephrons: In this the renal corpuscles lie close between the junction of renal cortex and medulla. Unlike cortical nephrons, the loop of Henle run deep into the medulla. It primarily concentrates the urine.
Malpighian or renal corpuscle and the coiled uriniferous tubules are the structural elements of nephron.
It comprises two components, namely glomerulus and Bowman’s capsule. A glomerulus is the capillary network of afferent arterioles, which is surrounded by the double layered epithelial cup called Bowman’s capsule. A glomerulus is composed of three layers:
- Visceral layer of epithelial cells (podocytes) and basement membrane: The epithelial cells link with the basement membrane via pedicels, and that’s why also called as “podocytes”. Over the basement membrane, the pedicles are arranged in a sequence leaving a narrow space in between that are called as “filtration slits”. A basement membrane lies within the visceral and parietal layer. It is a thin, middle layer that retains the plasma proteins from being filtered out.
- Parietal layer of squamous endothelial cells: It possesses large pores that permits the passage of solutes, plasma proteins etc.
A visceral layer participates in the urine filtration, and passes the filtrate to the capsular space of parietal layer via the capillaries.
Coiled Uriniferous Tubules
It consists of proximal, nephron and distal tubule that performs specific tasks inside the kidney. The proximal tubule is a 15 mm long convoluted tube that originates from the capsular space of parietal layer, and extends downwards to the medulla to form loop of Henle.
Henle’s loop or nephron tubule originates from the proximal tubule descends into a thin limb (2-14 mm long) and goes upward forming a thick limb (12 mm long). The ascending loop reaches glomerulus and passes close to its afferent and efferent arteriole forming macula densa (a part of juxtaglomerular apparatus).
Distal convoluted tubule originates from the macula densa cells of the juxtaglomerular apparatus that measures 5 mm length. It joins to the collecting duct. The filtrate from collecting tubule reaches renal pelvis, from where the urine discharge into the urinary bladder through a pair of ureter.
The kidneys filter the undesirable metabolic wastes from the blood in the form of urine. The whole process facilitates urine excretion. Nephrons along with the collecting duct facilitates the urine formation. Its formation includes three stages:
It is a filtration site that filters the water and particulate solutes from the blood. When the blood passes from a glomerulus, the water and solutes goes into the bowman’s capsule across the filtration membrane by the force of blood pressure in the blood capillaries. The filtrate obtained by this process called as “glomerulus filtrate”.
The filtration membrane allows the passage of fluid and solutes, but blocks the substantial particles like blood cells and large proteins that retains in the bloodstream. The filtrate flows from the glomerular capsule to the uriniferous tubules.
It is the secondary phase that helps in retaining the vitals nutrients from the glomerular filtrate. The filtrate contains waste, but also includes essential ions, glucose, amino acids etc. that is reabsorbed by the wall of uriniferous tubules into the adjacent capillaries.
Waste and hydrogen ions goes into the lumen of the nephron tubule by the capillaries called tubular secretion. The secreted ions along with the filtrate forms urine that flows out of the distal tubule into a collecting duct. Finally, the urine goes down from the renal pelvis, into the ureter, and lastly to the bladder.
The nephrons forms urine by processing the blood via sequential stages like filtration, reabsorption, and secretion. Urine is 95% water and the remaining 5% is metabolic wastes like urea, creatinine, ammonia, and uric acid. Dissolved salts and Ions (Na+, K+, H+, and Ca2+) also excrete along with the urine.
Urine excretion in human includes specialized organ (kidney) that comprises complex capillary network and coiled tubules. The kidneys assist excretion and osmoregulation by their functional unit (nephron). Thus, we can conclude that the excretion of urine is mediated by a specialized organ (Kidney) and hormones assist the urine formation and excretion.