The Urinary System

Chapter 5

The Urinary System 

Introduction

• For body cells to function effectively, their environment should be maintained relatively constant (homeostasis). Continuous working of cells necessitates utilization of nutrients (glucose) and oxygen and results into production of certain waste substances. Thus, the concentration of substances present in the internal environment of the body (extracellular fluid) varies. To avoid such variations and to maintain homeostasis, the waste products of cell metabolism should be removed from the body. Amongst the body systems involved in maintaining homeostasis are the digestive system, respiratory system, skin and the urinary system. The urinary system is, however, the main system maintaining the homeostasis of internal environment. It eliminates variety of cellular metabolic products such as urea, uric acid and creatinine and maintains water and electrolyte balance of the body.

• Following are the main excretory organs in the human body

Kidneys 

• They excrete water, inorganic salts, and nitrogenous wastes from protein catabolism.

Lungs: They excrete carbon dioxide, water vapour.
Skin: It excretes heat, water, carbon dioxide, salts and urea.
Alimentary canal: It excretes undigested waste, water, salts and heat.

• Two kidneys, two ureters, one urinary bladder and one urethra constitute the urinary system. The kidneys form the urine, which passes through the ureter to the bladder for excretion.

• This helps in maintaining the fluid and electrolyte balance and disposal of waste material from the body. The ureter propels urine from the kidneys into the bladder by peristaltic contraction of their muscular walls. Peristalsis is stimulated by presence of urine. The urinary bladder acts as a reservoir of urine. The accumulated urine is excreted through urethral opening. The urethra is long in male and short in female.

• Each kidney is covered by a thin smooth fibrous coat called as the renal capsule. A mass of adipose tissue surrounds the renal capsule that helps to hold the kidney firmly in place, and is called as adipose capsule. The outermost covering over adipose capsule is a layer of dense irregular connective tissue called renal fascia. The renal fascia anchors the kidney to the posterior abdominal wall. 

• Through the hilum the renal artery enters the kidney and renal vein, and ureter leaves it. The cavity in the medullary portion of the kidney in which the renal calyces open and from where the renal pelvis starts is called as renal sinus.

• The substance of kidney in the outer reddish area is called as the cortical substance (cortex) and in the inner reddish-brown area is called as medullary substance (medulla). 

• The tips (Papillae) of the pyramids project into funnel shaped minor calyx. Several minor calyces join together to form a major calyx.

• The major calyces join together to form renal pelvis, which is an expanded upper end of the ureter. 

Nephron 

• The microscopic functional unit constituting the parenchyma (functional portion) of kidneys is known as Nephron. In each kidney there are about one million nephrons. Each nephron consists of two parts, i.e. a renal corpuscle and a tubule. The proximal end of the closed tubule is indented to form a double walled cup shaped structure known as glomerular capsule (Bowman's capsule). There is a fine network of blood capillaries in close contact within the cup shaped glomerular capsule, called as glomerular capillaries.

• The glomerular capsule with glomerular capillaries is known as the renal corpuscle (Malpighian body). Beyond glomerular capsule, the nephron forms a long tube, with convolutions and a loop, this part of nephron is known as tubule. 

Physiology of kidneys

• Kidneys form urine by filtration and secretion of waste materials from the blood. In addition the selective reabsorption by tubular cells contributes to maintenance of homeostasis (regulatory activities of kidneys). 

•  Formation of Urine 5.7 The Urinary System The nephrons of the kidney perform this function. Primarily, three processes are involved in the formation of urine, the glomerular filtration, tubular reabsorption and tubular secretion.
•  Glomerular filtration: This occurs in the glomerular capsule. The barrier between internal portion of Bowman's capsule and the blood present in the glomerular (the blood capillaries) act as an ultra-filter. This barrier is composed of endothelial cells of capillaries, its basement membrane and the podocytes that form the inner portion of Bowman's capsule. The ultra-filter is highly permeable and allows free passage of water and solutes in blood plasma into the capsular space, forming the glomerular filtrate. The pore size of ultra-filter is the only limitation for passage of solutes in blood plasma into the filtrate. Thus, the blood cells and plasma proteins are not filtered; hence except these constituents the composition of filtrate is same as that of plasma. 

• Three different pressures are acting at this ultra-filter. These are of different magnitude and the sum total of these is the net filtration pressure. The major with highest magnitude of these is the capillary blood pressure (hydrostatic pressure). It is about 60 mm Hg. The diameter of efferent arteriole is smaller than that of the afferent arteriole (See Fig. 5.4) and this generates the hydrostatic pressure under the influence of which the filtration occurs. Two other pressures oppose this hydrostatic pressure; the oncotic pressure of blood plasma in capillaries; and the glomerular filtrate pressure. The plasma proteins are not filtered, and they exert an oncotic (osmotic) force that opposes the filtration. This force is of about 27 mm Hg. The glomerular filtration formed exerts pressure on the ultra-filter in opposite direction and is about 15 mm Hg. The resultant net filtration pressure is thus, 60 - (27 + 15) = 18 mm Hg.

Urinary Bladder

• It is a hollow muscular organ situated in the pelvic cavity posterior to the symphylids pubis, anterior to rectum in male and anterior to neck of uterus and vagina in female. Its shape depends on amount of urine it contains; when empty it collapses and when accumulation of urine occurs it goes on distending. When it is full of urine, it is ovoid in shape and rises considerably up in the abdominal cavity. 

• The urethra is a muscular tube that extends from inferior surface of urinary bladder and carries urine to the exterior of the body. At the pelvic floor when it passes through the muscle, (urogenital diaphragm) it is surrounded by the external urethral sphincter. In females, it is a narrow membranous canal extending from bladder to external orifice the meatus, and about 03.8 cm in length. 

Micturition

• The act of passing the urine to the exterior is called as micturition. It is carried out by both voluntary and involuntary nerve impulses. When urine accumulates in the bladder, sufficiently (about 300 ml) it results into stretching of bladder wall and stimulates the nerves to initiate sensory impulses transmitted to the lower portion of the spinal cord. These impulses by way of sensory tracts to the cortex, initiate a conscious desire to expel urine. By way of micturition reflex, of which the centre is in sacral spinal cord; the bladder muscles are stimulated to contract and the internal urethral sphincter to relax. By way of sensory tracts to the cortex the conscious desire to expel urine is felt. These higher centers in the brain can voluntarily control contraction or relaxation of external urethral sphincter. When voluntarily external urethral sphincter is relaxed, the micturition occurs.

Pathophysiology of Renal Diseases

• Glomerulonephritis is an inflammatory condition, possibly of allergic origin. Glomeruli are the main targets. Oedema may be present in both the acute and sub-acute stages of nephritis. This stage is called dry nephritis. The cause of oedema is as follows: Two, more or less equal but opposite, forces are always at work in the fine capillaries. (a) Blood pressure, which tries to force the fluid part of the blood through the vessel wall into the tissues.

• A urinary calculus is a stone that is formed in the urinary tract. It may be formed either in the kidney or in the bladder. The symptoms include pain, the passage of pus and blood through urine and is evidence of renal failure.