The human excretory system functions to remove waste from the human body. This system consists of specialized structures and capillary networks that assist in the excretory process.
i. Excretory function in human body
In human beings there are 5 organs involved in ridding the body of metabolic wastes. They are
‣ Skin
The skin's function of waste removal is essential to the body as part of a larger biological system of waste removal. Sweat glands help rid the body of harmful nitrogenous waste, such as urea, uric acid and ammonia. A build-up of these wastes could cause adverse affects on the body.
Some of the skin's excretions serve as protecting agents against outer elements. Sebaceous glands in the skin excrete sebum, a fatty substance that serves as a water repellent and helps the skin retain a smooth texture. Sebum also serves as a natural antibacterial and antifungal agent.
‣ Lungs
Although the lungs are part of the respiratory system, they also rid the body of wastes. They help in getting rid of unneeded carbon dioxide. The lungs also help in ridding the body of water and excess heat. We lose water each time we breathe out. If we exhale on a cold day, the “breath” we can see is water from our lungs forming in the air.
In the blood, oxygen is carried by the red blood cells as oxyhemoglobin. At the body cells, this oxygen leaves the hemoglobin by easily diffusing through the capillaries. The cells can now carry out cellular aerobic respiration at the mitochondria for energy. The end products (metabolic wastes) of cellular respiration, carbon dioxide and water) now diffuse back out of the cells into the blood. Carbon dioxide is carried in the blood plasma in the form of bicarbonate ion (CO3). These molecules then diffuse out of the blood through the walls of the capillaries which surround the alveoli and then are exhaled.
‣ Liver
The liver is a multipurpose, largest gland in vertebrates. It plays a vital role in excretion of many substances. Liver changes the decomposed haemoglobin of the worn out red blood corpuscles into bile pigments bilirubin and biliverdin. These pigments are passed into the alimentary canal with the bile for elimination in the faces.
The liver also excretes cholesterol, steroid hormones, certain vitamins and drugs through the bile. Urea is formed in the liver by a cyclic process called as urea cycle of Ornithine cycle or Knely - Henseleit cycle. The amino acids that are not needed in the body are deaminated by an enzyme oxidase, producing ammonia NH3. Ammonia, hung toxic, is quickly changed to urea.
Three amino acids participate in the process.
* Ornithine combines with ammonia and carbon dioxide to form citrulline combines with more ammonia to form water and arginine.
* Argeninine is then decomposed to form urea and Ornithine in the presence of enzyme arginase and water. Ornithine is set free for reuse in the urea cycle.
‣ Digestive Tract
It is responsible for ridding the body of certain salts, calcium, iron, magnesium and fats.
‣ Kidneys
Kidneys Rids the body of nitrogenous wastes especially urine and its constituents.
ii. Major Components of human urinary system The main excretory system in humans is the urinary system.
a) Kidneys
The urinary system includes a pair of bean-shaped kidneys located in the back of the abdominal cavity. Each day, the kidneys filter about 162 qt (180 L) of blood, enough to fill a bathtub. They remove urea, toxins, medications and excess ions and form urine. The kidneys also balance water and salts as well as acids and bases. At the same time, they return needed substances to the blood. Of the total liquid processed, about 1.3 qt (1.5 L) leaves the body as urine.
Functions of Kidneys:
* Main excretory organs are kidneys
* Osmo regulation (Osmo = Water)
* Mineral salt regulation
* Maintenance of B.P.
* Removes unwanted waste material
* Blood filtration takes place in kidneys
* Right kidney is smaller one and pushed down
* Renal artery supplies blood to kidneys
* Renal veins receive blood from kidneys
3 major parts of kidney are Renal Cortex, Renal Medulla and Renal pelvis
Renal Cortex:
Renal Cortex is the outer smooth, continuous layer of the kidney. The process of ultra filtration of blood is carried out in the renal cortex which is also known as high pressure filtration or the ultra filtration. Renal cortex is the external, outermost part of kidney and the filtration process is termed so, as this filtration can only be completed when the blood coming into the kidney through the renal artery is already carrying high pressure. Cortex consititues glomeruli in billions which are small tiny ball shaped capillaries.
The glomoeruli present inside the renal cortex, filters plasma fluids. With the help of inter pyramidal extensions, the renal cortex penetrates in between the two kidneys which are termed as renal columns, thereby acting as support for blood vessels. Renal cortex actively takes participation in kidney metabolism by creating ammonia for titrating urine acidity and thus help in acid-base regulation. Cortex is highly involved in the regulation of metabolic wastes like uric acid, urea and creatinine.
Renal Medulla:
The renal medulla is a term used for the innermost portion of the kidney. It is divided into several sections known as renal pyramids because of their cone-like appearance. Inside are structures known as nephrons and they have the responsibility of maintaining the balance of water and salt within the blood.
The renal artery provides the blood supply to the kidney then branches into what are known as arcuate arterioles. These blood vessels are located at the border of the renal medulla and are responsible for renal or kidney, circulation. The arcuate arterioles are so named due to the fact that they are shaped like arcs due.
The arcuate arterioles branch even farther into interlobular arterioles, which are smaller arteries that branch off at right angles. These are small capillaries responsible for starting the process of filtering toxins from the blood and forming urine. Damage to the kidney can occur due to traumatic injury or a naturally occurring disease process and when the renal medulla is damaged, the results can be devastating.
Renal Pelvis:
Renal pelvis enlarged upper end of the ureter, the tube through which urine flows from the kidney to the urinary bladder. The pelvis, which is shaped somewhat like a funnel that is curved to one side, is almost completely enclosed in the deep indentation on the concave side of the kidney, the sinus. The large end of the pelvis has roughly cuplike extensions, called calyces, within the kidney these are cavities in which urine collects before it flows on into the urinary bladder.
The renal pelvis is lined with a moist mucous-membrane layer that is only a few cells thick; the membrane is attached to a thicker coating of smooth muscle fibres, which, in turn, is surrounded by a layer of connective tissue.
Structural and functional units of Kidney are Nephrons.
Nephron:
There are normally approx. one million (0.8 - 1.5 million) kidney nephrons in each of the two kidneys in the body. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine.
A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
The Glomerulus
First filtration takes place in Glomerulus. This type of filtration called ultra filtration. Due to ultra filtration primary urine forms.
The glomerulus is a tiny blood vessel or capillary, which looks like a ball of yarn. Actual filtering of blood occurs in the glomerulus. Each of glomeruli acts like a sieve that helps keep normal proteins and cells in bloodstream and allows wastes, excess fluid and other substances to pass.
Blood pressure forces small molecules from the glomerulu (a blood capillary bed) into the glomerular capsule (part of the nephron of the excretory system). These small molecules are called glomerular filtrate and include water, salts, nutrients and wastes. Glomerular filterate includes about 20% of the blood. Glomerular filterate passes from glomerula capsule into proximal convoluted tubule. 80% of the blood, including serum proteins and blood cells are not filtered from the blood, but pass on through the circulatory circuits out of the kidneys.
Bowman’s Capsule:
The Bowman's capsule is an expansion at the closed end of a renal tubule. A kidney's renal corpuscle is composed of tangled clusters of blood capillaries, called a glomerulus, and a thin-walled, saclike structure called the Bowman's capsule, which surrounds the glomerulus.
The Bowman's capsule is composed of two layers of cells. An inner layer that closely covers the glomerulus. An outer layer that is continuous with the inner layer and with the wall of the renal tubule. The renal tubule leads away from the Bowman's capsule and becomes highly coiled.
The Tubule
The tubule, also called renal or kidney tubule, is a tiny tube where the wastes, extra fluid and other recyclable substances like sodium and potassium filtered out from the glomerulus pass through. Your kidneys measure out chemicals like sodium, phosphorus and potassium and release them back to the blood to return to the body when need arises. In this way, your kidneys regulate body’s level of these substances. The right balance is necessary for you to function properly.
The collecting Tubule
The collecting tubule is part of the kidney. It is also known as the arcuate renal tubule, or the CNT. It is a tube-shaped portion of the kidney's collecting duct system; this system links the distal convoluted tubule to the cortical collecting duct. The collecting tubule helps maintain the proper levels of electrolytes and water in the body at all times
The loop of Henley
The Loop of Henle is the most inferior and middle portion of the nephron. It is responsible for the resorption of water, chloride and sodium. The urea from the collecting duct enters the medullary interstial fluid and diffuses into the loop of henle. As it passes back up the ascending limb of the loop of henle and reabsorption of other ions occurs the urea becomes even more concentrated. This recirculation can occur several times and steadily increases urea concentration in the medullary tissue until equilibrium is reached.
If water conservation is important this equilibrium is not reached until the urine is more concentrated and urea makes up 40% of the interstial osmolarity. The overall aim of this recirculation is to excrete a high concentration of urea in very little water.
b) Ureters
In human anatomy, the ureters are tubes made of smooth muscle fibers that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 25–30 cm (10–12 in) long and ~3–4 mm in diameter. Histologically, the ureter contains transitional epithelium and an additional smooth muscle layer in the more distal one-third to assist with peristalsis.
A kidney stone can move from the kidney and become lodged inside the ureter, which can block the flow of urine, as well as cause a sharp cramp in the back, side or lower abdomen. The affected kidney could then develop hydronephrosis, should a part of the kidney become swollen due to blocked flow of urine. There are three sites where a kidney stone will commonly become stuck -
* at the ureteric junction of renal pelvis
* as the ureter passes over the iliac vessels
* where the ureter enters into the urinary bladder
c) Urinary Bladder
The urinary bladder is a muscular sac in the pelvis, just above and behind the pubic bone. When empty, the bladder is about the size and shape of a pear.
Urine is made in the kidneys, and travels down two tubes called ureters to the bladder. The bladder stores urine, allowing urination to be infrequent and voluntary. The bladder is lined by layers of muscle tissue that stretch to accommodate urine. The normal capacity of the bladder is 400 to 600 ml.
Bladder Conditions:
Cystitis: Inflammation or infection of the bladder causing acute or chronic pain or discomfort or urinary frequency or hesitancy.
Urinary stones: Stones (calculi) may form in the kidney and travel down to the bladder. If calculi block urine flow to or from the bladder, they can cause severe pain.
Bladder cancer: A tumor in the bladder is usually discovered after blood is noticed in the urine. Cigarette smoking and workplace chemical exposures cause most cases of bladder cancer.
Urinary incontinence: Involuntary urination, which may be chronic. Urinary incontinence can result from a variety of causes.
Overactive bladder: The bladder muscle (detrusor) contracts involuntarily, causing some urine to leak out. Detrusor overactivity is a common cause of urinary incontinence.
Hematuria: Blood in the urine. Hematuria may be benign, or may be caused by infection or a serious condition like bladder cancer.
Urinary retention: Urine does not exit the bladder normally due to obstruction or suppressed bladder muscle activity. The bladder may swell to hold more than a quart of urine.
Cystocele: Weakened pelvic muscles (usually from childbirth) allow the bladder to press on the vagina. Problems with urination can result.
Bed-wetting (nocturnal enuresis): Bed-wetting is defined as a child age 5 or older who wets the bed at least one or two times a week over at least three months.
Dysuria (painful urination): Pain or discomfort during urination due to infection, irritation, or inflammation of the bladder, urethra or external genitals.
d) Urethra
During urination, the bladder muscles contract, and two sphincters (valves) open to allow urine to flow out. Urine exits the bladder into the urethra, which carries urine out of the body. Because it passes through the penis, the urethra is longer in men (8 inches) than in women (1.5 inches).
The urethra has a sphincter that is normally closed to keep urine inside the bladder. When the bladder fills with urine, there are both voluntary and involuntary controls to open the urethral sphincter to allow urine to come out.
Any inflammation of urethra can result in scarring, which then can lead to a stricture or a narrowing of the urethra. Trauma, infection, tumors, surgeries, or any other cause of scarring may lead to urethral narrowing or stricture. Mechanical narrowing of the urethra without scar formation (developmental causes or prostate enlargement) can also narrow the urethra.
