Abnormal Types of Urine
Abnormal Types of Urine
Proteinuria—Protein content in urine, often due to leaky or damaged glomeruli.
Oliguria—An abnormally small amount of urine, often due to shock or kidney damage.
Polyuria—An abnormally large amount of urine, often caused by diabetes.
Dysuria—Painful or uncomfortable urination, often from urinary tract infections.
Hematuria—Red blood cells in urine, from infection or injury.
Glycosuria—Glucose in urine, due to excess plasma glucose in diabetes, beyond the amount able to be reabsorbed in the proximal convoluted tubule.
The nephron of the kidney is involved in the regulation of water and soluble substances in blood.
A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity.
It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman's capsule.
A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.
The Bowman's capsule surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers.
Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.
Proximal Convoluted Tubule
The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps that actively transports sodium across the basolateral membrane.
Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.
Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.
The Loop of Henle
The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle's loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.
Distal Convoluted Tubule and Collecting Duct
The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.
Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.
The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.
An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.
The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.
ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.
After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.
Micturition (urination) is a process, by which the final urine is eliminated out of the body .
After being drained into the ureters, urine is stored in urinary bladder until being eliminated.
Bladder is a hollow muscular organ, which has three layers:
- epithelium : Composed of superficial layer of flat cells and deep layer of cuboidal cells.
- muscular layer : contain smooth muscle fibers, that are arranged in longitudinal, spiral and circular pattern . Detrusor muscle is the main muscle of bladder. The thickening of detrusor muscle forms internal urinary sphinctor which is not an actual urinary sphincter. The actual one is the external urinary sphincter, which is composed of striated muscle and is a part of urogenital diaphragm.
- adventitia: composed of connective tissue fibers.
So: There are two phases of bladder function that depend on characterestics of its muscular wall and innervation :
1. Bladder filling : Urine is poured into bladder through the orifices of ureters. Bladder has five peristaltic contraction per minute . These contraction facilitate moving of urine from the ureter to the bladder as prevent reflux of urine into the ureter.. The capacity of bladder is about 400 ml. But when the bladder start filling its wall extends and thus the pressure is not increased with the increased urine volume.
2. Bladder emptying : When bladder is full stretch receptors in bladder wall are excited , and send signals via the sensory branches of pelvic nerves to the sacral plexus. The first urge to void is felt at a bladder volume of about 150 ml. In sacral portion of spinal cord the sensory signals are integrated and then a motor signal is sent to the urinarry blader muscles through the efferent branches of pelvic nerve itself.
In adult people the neurons in sacral portion could be influenced by nerve signals coming from brain ( Micturition center in pons ) that are also influenced by signals coming from cerebral cortex.
So: The sensory signals ,transmitted to the sacral region will also stimulate ascending pathway and the signals be also transmitted to the micturition center in the brain stem and then to the cerebrum to cause conscious desire for urination.
If micturition is not convenient the brain sends signals to inhibit the parasympathetic motor neuron to the bladder via the sacral neurons.
It also send inhibitory signal via the somatomotor pudendal nerve to keep external urinary sphincter contracting.
When micturition is convenient a brain signal via the sacral neurons stimulate the parasympathetic pelvic nerve to cause contraction of detruser muscle via M-cholinergic receptors and causes relaxation of external urinary sphincter and the micturition occurs.
Sympathetic hypogastric nerve does not contribute that much to the micturition reflex. It plays role in prvrntion reflux of semen into urinary bladder during ejaculation by contracting bladder muscles.
Factors , affecting glomerular filtration rate :
Factors that may influence the different pressure forces , or the filtration coefficient will affect the glomerular filtration rate .
1. Dehydration : Causes decrease hydrostatic pressure , and thus decreases GFR
2- Liver diseases that may decrease the plasma proteins and decrease the oncotic pressure , and thus increases glomerular filtration rate .
3- Sympathetic stimulation : will decrease the diameter of afferent arteriole and thus decreases glomerular filtration rate.
4- Renal diseases : Nephrotic syndrome for example decreases the number of working nephrons and thus decreases the filtration coefficient and thus decreases the glomerular filtration rate.
Glomerulonephritis will causes thickening of the glomerular basement membrane and thus decreases the glomerular filtration rate by decreasing the filtration coefficient too.
Regulation of glomerular filtration :
1. Extrinsic regulation :
- Neural regulation : sympathetic and parasympathetic nervous system which causes vasoconstriction or vasodilation respectively .
- Humoral regulation : Vasoactive substances may affect the GFR , vasoconstrictive substances like endothelin ,Angiotensin II , Norepinephrine , prostaglandine F2 may constrict the afferent arteriole and thus decrease GFR , while the vasodilative agents like dopamine , NO , ANP , Prostaglandines E2 may dilate the afferent arteriole and thus increase the filtration rate .
2. Intrinsic regulation :
- Myogenic theory ( as in the intrinsic regulation of cardiac output) .
- Tubuloglomerular feedback: occurs by cells of the juxtaglomerular apparatus that is composed of specific cells of the distal tubules when it passes between afferent and efferent arterioles ( macula densa cells ) , these cells sense changes in flow inside the tubules and inform specific cells in the afferent arteriole (granular cells ) , the later secrete vasoactive substances that affect the diameter of the afferent arteriole.