Excretory Products and their Elimination

Chapter 6

Excretory Products and their Elimination

Excretion

• Excretion is the process of elimination of substances from the body.

• Different waste products like urea, uric acid, creatinine, bilirubin and ammonia are formed during various reactions taking place in cells.

• These waste products should be eliminated from the body, accumulation of the same may lead to different disease conditions.

• Kidneys,large intestine and skin are the important organs of excretion.

•    Waste products are excreted by different modes are as follows:

Urine: Kidneys help to excrete urea, uric acid, creatinine, ammonia through urine. Excess amount of ions like Phosphates, Ca**, Na* etc. are also excreted through urine. Many drugs, toxins and even excess water are excreted through urine.

Feces: Small amount of water, inorganic salts, bacteria, products of bacterial decomposition, unabsorbed digested materials and indigestible parts of food are excreted in feces. Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces

Sweat: Very small amounts of salts, carbon dioxide, urea and ammonia are excreted through sweat.

• Human excretory system
• The human urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra. The kidneys are important organs of this system, the remaining parts are mainly connecting ducts or passageways and storage spaces.

Structure of Kidney

• A pair of kidneys is present just above the waist in posterior wall of the abdomen. The kidneys are reddish, kidney-bean-shaped organs with length of 10-12, width of 5—7 cm and thickness of 3 cm.

•  The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters.

• A frontal section through the kidney reveals two distinct regions: Renal cortex andrenal medulla.

1.         The outer, superficial area called the renal cortex.

2.        The deep, inner region called the renal medulla.

• The renal medulla consists of several cone-shaped regions called renal pyramids. The base of each pyramid faces the renal cortex. The apex of renal pyramid is called renal papilla. Renal papilla points toward the renal hilum.

• Renal columns are portions of the renal cortex that extend between renal pyramids.

•  A renal lobe consists of a renal pyramid, its overlying area of renal cortex, and one- half of each adjacent renal column.

• The functional portion of kidney is called as parenchyma. Parenchyma is composed of the renal cortex and renal pyramids.

Structure of Nephron

Each nephron is made up of two parts:

1. Renal corpuscle

2. Renal tubule 

Renal corpuscle:    

The Renal corpuscle is composed of two components:

(i)  Glomerulus (capillary network) and

(ii) Glomerular (Bowman’s) capsule.

• Glomerular (Bowman‘s) capsule is a double-walled epithelial cup that surrounds the glomerular capillaries.
• Blood plasma is filtered in the glomerular capsule, and then the filtered fluid passes into the renal tubule, which has three main sections.

Renal tubule:

The renal tubule is composed of three parts:

(i)   Proximal convoluted tubule: This part is attached to glomerular capsule.

(ii)  Loop of Henle (nephron loop): This is the tightly coiled part of renal tubule.

(iii) Distal convoluted tubule: This is the last part of renal tubule.

• The renal corpuscle and both convoluted tubules lie within the renal cortex. 

• The loop of Henle extends into the renal medulla, makes a hairpin turn, and then returns to the renal cortex.

• The distal convoluted tubules of several nephrons empty into a single collecting duct. Collecting ducts then unite and join into several hundred large papillary ducts, which drain into the minor calyces.

• The collecting ducts and papillary ducts extend from the renal cortex through the renal medulla to the renal pelvis.

Functions of Excretory System

• Kidneys plays the following roles:

(a) Regulate blood volume and composition

(b) Help to regulate blood pressure

(c) Regulate pH

(d) Regulate glucose levels

(e) Produce two hormones (calcitriol and erythropoietin)

(f) Excrete wastes in urine

• A pair ureters transport urine from kidneys to urinary bladder. 
•  Urinary bladder stores urine and expels it into urethra.
•  Urethra discharges urine out of the body.

Urine Formation

There are three steps in urine formation: glomerular filtration, tubular reabsorption, and tubular secretion

1. Glomerular filtration: This is the first step of simple filtration. In this step, water and most solutes in blood plasma are filtered into the glomerular capsule and then transferred to the renal tubule.

2. Tubular reabsorption: In the second step, as filtered fluid flows along the renal tubule and through the collecting duct, tubule cells reabsorb about 99% of the filtered water and many useful solutes.

3. Tubular secretion: As fluid flows alonp the renal tubule and through the collecting duct, the tubule and duct cells secrete other materials, such as wastes, drugs, and excess ions, into the fluid. Tubular secretion takes away a substance from the blood.

  Renin Angiotensin Aldosterone System

• Dehydration, Na* deficiency, or haemorrhage leads to decrease in blood volume.

• Decrease in blood volume decreases the blood pressure. This decreased blood pressure leads to decreased stretching of the walls of the afferent arterioles. This leads to secretion of the enzyme renin by the juxtaglomerular cells of kidney into the blood.