Definitions [12]
Define the following term:
Kidney
Kidneys are the primary excretory organs, eliminating nitrogenous wastes (chiefly urea) from the blood and throwing it out in the form of urine.
Define the following:
Excretory organs
During different metabolic activities taking place in our body, the body produces many substances of which some are useful and some are useless.
If retained in the body the unwanted substances may become poisonous and cause much harm and in severe cases, even death. The organs which remove these unwanted and toxic substances from the body are called excretory organs.
Define the following:
Excretion
During different metabolic activities taking place in our body, the body produces many substances, of which some are useful and some are useless.
The process of removing useless and harmful metabolic waste substances is called excretion.
The process of removal of chemical wastes (Mainly Nitrogenous) from the body is known as ''excretion''. It plays an important role in maintaining the homeostatic (steady-state) condition of the body.
Define the following:
Dialysis
The artificial process which cleans and filters the blood in a person where one or both the kidney may stop working properly is called dialysis.
Define the following:
Nephron
Inside the kidney, there are millions of microscopic tubes called renal tubules or nephrons. It is the structural and functional unit of the kidney.
Ultrafiltration is the process in which blood is filtered under high pressure in the glomerulus, allowing water and small solutes to pass into the Bowman’s capsule.
Glomerular filtrate is the fluid formed after ultrafiltration that enters the renal tubule and contains water, urea, salts, glucose, and other small molecules.
Selective absorption is the process by which only useful substances like glucose, some salts, and water are absorbed from the renal tubule back into the blood, without disturbing its normal concentration.
Tubular secretion is the active transfer of certain substances, such as ions and drugs, from the blood into the renal tubule during urine formation.
Define the following term:
Ultrafiltration
The blood flows through the glomerulus under great pressure which causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under high force is called Ultrafiltration.
Define the Glomerular Filtration Rate (GFR).
Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both kidneys per minute. In a healthy individual, it is about 125 ml/minute.
The separation of small molecules from large molecules in a solution by interposing a semipermeable membrane between the solution and water using artificial machine known as haemodialyser.
Key Points
| Feature | Ammonotelism | Ureotelism | Uricotelism |
|---|---|---|---|
| Waste excreted | Ammonia | Urea | Uric acid |
| Water needed | Large amount | Moderate amount | Very less amount |
| How excreted | Diffusion across body surface as NH₄⁺ ions | Through kidneys | As pellets/paste, minimum water loss |
| Toxicity | Most toxic | Less toxic than ammonia | Non-toxic, almost insoluble in water |
| Conversion | Direct | Ammonia converted to urea via ornithine/urea cycle (3 ATP used) | Ammonia converted to uric acid via inosinic acid pathway (in liver of birds) |
| Examples | Aquatic invertebrates, bony fishes, aquatic amphibians, aquatic insects | Mammals, cartilaginous sharks, rays, aquatic reptiles, most adult amphibians, terrestrial animals, marine fishes | Birds, reptiles, land snails, insects |
- The human excretory system consists of a pair of kidneys, two ureters, a urinary bladder and a urethra.
- Kidneys are dark red, bean-shaped, retroperitoneal structures located from the 12th thoracic to the 3rd lumbar vertebra. Size: 10-12 cm × 5-7 cm × 2-3 cm; weight: 150 g (males), 135 g (females).
- Ureters are narrow tubular structures made of transitional epithelium that carry urine from the kidneys to the urinary bladder.
- The urinary bladder is a pear-shaped, hollow, muscular organ in the pelvic cavity, lined by transitional epithelium, and acts as a reservoir of urine.
- Two sphincters exist between the bladder and the urethra: the internal sphincter (involuntary, detrusor muscles) and the external sphincter (voluntary, striated muscles).
- The urethra is a canal-like structure that opens to the exterior via the urethral orifice, much longer in males than in females.
- The aorta supplies oxygenated blood to the kidneys; the inferior vena cava carries deoxygenated blood away from the kidneys.
- Kidney function is regulated by hormonal feedback involving the hypothalamus, JGA and the heart. Osmoreceptors detect changes in blood volume and ionic concentration.
- Low body fluid → osmoreceptors activated → hypothalamus → neurohypophysis releases ADH → DCT and collecting duct reabsorb more water → prevents diuresis.
- High body fluid → osmoreceptors are suppressed → ADH decreases → more water is excreted in urine.
- Low GFR → JGA releases renin → angiotensinogen → angiotensin I → angiotensin II (vasoconstrictor) → increases BP and GFR.
- Angiotensin II → adrenal cortex releases aldosterone → increases Na⁺ and water reabsorption → raises blood volume and BP.
- Increased blood flow to the heart atria → releases ANF → vasodilation → decreases BP.
- ANF acts as a check on the renin-angiotensin mechanism - both work in opposite directions to maintain BP balance.
- Micturition is the process of urine release, controlled by the micturition reflex and the central nervous system (CNS).
- When the bladder is full, the CNS signals the bladder muscles to contract and the urethral sphincter to relax, allowing urine to pass out.
- An adult excretes about 1–1.5 L of urine per day.
- Urine is light yellow, slightly acidic (pH ~6), and contains about 25–30 g of urea daily.
- The presence of glucose or ketone bodies in urine indicates disorders like diabetes mellitus.
| Disorder | Cause | Types / Characteristics |
|---|---|---|
| Kidney Stones (Renal calculi) | High protein diet, lack of water, bacterial infection, and genetic disorder | Calcium stones (calcium oxalate/phosphate), Struvite stones (infection), Uric acid stones, Cystine stones |
| Uremia | Increase in the urea level in the blood | Urea level rises above 0.05%; may lead to kidney failure |
| Nephritis | Increased permeability of the glomerular capsular membrane | Inflammation of the kidney, protein loss in urine, and oedema |
| Renal Failure | Severe bleeding, obstruction of ureters, nephrotoxic drugs, and chronic glomerulonephritis | Acute: sudden decrease in function, low urine, high creatinine; Chronic: progressive decline in GFR, reduced kidney size |
| Albuminuria | Increased blood pressure, toxins, and injury to kidney cells | Excess albumin (protein) in urine |
| Other Indicators | Diabetes mellitus, starvation, low-carbohydrate diet, infection | Ketone bodies in urine; the presence of leucocytes indicates infection |
- Haemodialysis is an artificial method of filtering blood when kidney function falls below 5–7%.
- In this process, blood is taken out of the body and passed through a semipermeable membrane (cellophane tube) in a dialysis machine.
- The tube is placed in dialysate fluid, which is similar to normal blood plasma, allowing only excess salts and wastes to diffuse out.
- Waste substances move from blood into dialysate, and purified blood is returned to the body.
- Anticoagulant (heparin) is added to prevent clotting, and the process is slow as blood flows gradually through the tube.
