|
. ..The urinary system is composed of the kidneys, bladder and accessory structures. The kidneys produce urine, a fluid waste product whose composition and volume vary.
????The six functions of the kidneys are regulation of extracellular fluid volume, regulation of osmolarity, maintenance of ion balance, homeostatic regulation of pH, excretion of wastes and foreign substances, and production of hormone. The most important function of the kidneys is the homeostatic regulation of the water and ion content of the blood.
????I. Structure of the kidneys
????Each kidney has about 1 million nephrons. Each nephron in the kidneys consists of a renal corpuscle and a tubule.
????1. Each renal corpuscle comprises a capillary tuft, termed a glomerulus, and a Bowman's capsule, into which the tuft protrudes.
????2. The tubule extends out from Bowman's capsule and is subdivided into many segments, which can be combined for reference purposes into the proximal tubule, loop of Henle, distal convoluted tubule and collecting duct. Beginning at the level of the collecting ducts, multiple tubules join and empty into the renal pelvis, from which urine flows through the ureters to the bladder.
????3. Each glomerulus is supplied by an afferent arteriole,and an efferent arteriole leaves the glomerulus to branch into peritubular capillaries, which supply the tubule.
????II. Basic Renal processes
????1.The three basic renal processes are glomerular filtration, tubular reabsorption, and tubular secretion. In addition, the kidneys synthesize and /or catabolize certain substances. The excretion of a substance is equal to the amount filtered plus the amount secteted minus the amount reabsorbed.
????2. Urine formation begins with glomerular filtration - approximately 180L/day - of essentially protein-free plasma into Bowman's space.
????(1) Glomerular filtrate contains all plasma substances other than proteins and substances bound to protein.
????(2) Glomerular filtration is driven by the hydrostatic pressure in the glomerular capillaries and is opposed by both the hydrostatic pressure in Bowman's space and the osmotic force due to the proteins in the glomerular capillary plasma.
????3. As the filtrate moves through the tubules, certain substances are reabsorbed into the peritubular capillaries.
????(1) Substances to which the tubular epithelium is permeable are absorbed by diffusion because water reabsorption creates tubule-interstitium concentration gradients for them.
????(2) Tubular reabsorption rates are generally very high for nutrients, ions,and water, but are lower for waste products. Reabsorption may occur by diffusion or by mediated transport.
????(3) Many of the mediated-transport systems manifest transport maximums, so that when the filtered load of a substance exceeds the transport maximum, large amounts may appear in the urine.
????4. Tubular secretion (movement from the peritubular capillary into the tubules), like glomerular filtration, is a pathway for entrance of a substance into the tubule.
????Ⅲ. Renal regulation
Renal function is regulated by neural and hormonal influences. The most important of these are:
????1. renal sympathic nerves
????2. renin-angiotensin system
????3. aldosterone
????4. atrial natriuretic peptide
????5. antidiuretic hormone
????6. prostaglandins
????7. parathyroid hormone
????Ⅳ. Clearance
????Clearance is an abstract concept that describes what volume of plasma passing through the kidneys has been totally cleared of a substance in a given period of time. For substances such as inulin which are neither actively absorbed nor secreted by the kidneys, clearance is equivalent to the glomerular filtration rate (GFR). In clinical settings, creatinine is used to measure GFR.
????If a person's GFR is known, then it is possible to measure the filtration rate of a substance.
????If less substance appears in the urine than was filtered, then some was reabsorbed by the nephrons. If more substance appears in the urine than was filtered, then there is net secretion of the substance. If the same amount of the substance is filtered and excreted, then the substance is neither reabsorbed nor secreted.
????Clearance values are also used to determine how the nephron handles a substance filtered into it. If the clearance of a substance is less than the inulin or creatinine clearances, then the substance has been reabsorbed. Conversely, if the clearance rate of the substance is greater than inulin or creatinine then it has been actively secreted into the nephron.
V. Micturition
????Urine is stored in the bladder until released by urination, also known as micturition.
????1. In the basic micturition reflex, bladder distention stimulates stretch receptors that trigger spinal reflexes; these reflexes lead to contraction of the detrusor muscle, mediated by parasympathetic neurons, and relaxation of the external urethral sphincter, mediated by inhibition of the motor neurons to this muscle.
????2. Voluntary control is exerted via descending pathways to the parasympathetic nerves supplying the detrusor muscle and the motor nerves supplying the external urethral sphincter.
|
|
