Kidney Function and Nephron Structure

• Excretion is the biological process where the body removes harmful metabolic wastes like urea, uric acid (nitrogenous wastes), and salts.

Excretion in Different Organisms

• Unicellular organisms like amoebas remove waste through simple diffusion across their body surface into the surrounding water.
• Surplus water in Amoeba is expelled via contractile vacuoles.
• Lower multicellular organisms such as flatworms use flame cells.
• Earthworms use nephridia for excretion.
• Insects use Malpighian tubules to excrete metabolic waste.
• Aquatic amphibians eliminate nitrogenous waste as ammonia across their skin, since ammonia is highly soluble in water.
• Higher multicellular organisms, including fish, land-dwelling frogs, lizards, birds, and humans, use kidneys for excretion.

Classification of Animals Based on Nitrogenous Waste

• Ammonotelism describes the process of eliminating ammonia from the body.
• Organisms that do so are called ammonotelic, including protozoans, echinoderms, poriferans, crustaceans, amphibians, and bony fishes.
• Aquatic animals excrete ammonia directly into their environment because ammonia is diluted quickly; it is also very toxic to tissues.
• Ureotelic organisms excrete urea.
• In these animals, ammonia is converted to urea in the liver and released into the blood.
• Kidneys filter the urea and expel it from the body
• Some urea is retained in the kidney's matrix to maintain osmolarity
• Urea is less toxic than ammonia; examples of ureotelic organisms include sharks, alligators, other mammals, and humans.
• Uricotelic animals remove nitrogenous wastes as uric acid in paste or pellet form.
• This process is costly, but it minimizes water loss and uric acid is the least toxic of the nitrogenous wastes.
• Uric acid isn't readily soluble in water
• Excrement forms a pasty white suspension; most reptiles, birds, and insects are uricotelic.
• Osmolarity refers to the total solute concentration within a specific volume of solvent, measured in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).

Human Excretory System

• The human body produces various toxic waste products from metabolic processes.
• Waste products: Carbon dioxide (eliminated by the lungs during respiration).
• Nitrogenous metabolic wastes like urea and uric acid (produced in the liver from protein consumption and eliminated by the kidneys).
• Bile pigments like bilirubin are derived from the breakdown of hemoglobin in erythrocytes.
• Excess salts, water, and water-soluble vitamins are eliminated by the kidneys.
• Sweat, contains 99% water and traces of urea and uric acid.
• After heavy exercise, lactic acid becomes a major component of sweat.
• Sweat glands in the skin allow excess water and salts to evaporate.
• Sweating is vital for thermoregulation.
• The human excretory system (or urinary system) consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

Kidneys

• The kidneys are reddish-brown, bean-shaped organs in the abdomen on either side of the backbone.
• The right kidney sits slightly lower than the left due to the liver above it.
• A longitudinal section of the kidney shows two regions: the outer renal cortex and the inner renal medulla.
• The cortex contains randomly arranged tiny tubules called nephrons.
• Each kidney has 1.25 million nephrons, which are the functional units of the kidney.
• The hilum is the concave part of the kidney where blood vessels, nerves, and the ureters enter and exit.
• Kidneys play a vital role in homeostasis by: Excreting nitrogenous waste products, regulating water balance, acid-base balance, eliminating substances like drugs and pigments, and regulating blood pressure with hormones.
• Homeostasis represents a steady state of internal chemical and physical conditions maintained by living systems.

Ureters

• Ureters are a pair of long, narrow tubes extending from the kidneys to the urinary bladder.
• The upper end forms the renal pelvis by connecting to the kidney, while the lower end connects to the urinary bladder.
• Valves prevent urine backflow as the ureters enter the bladder through an aperture.
• Ureters transport urine from the kidneys to the urinary bladder.

Urinary Bladder

• The urinary bladder is a muscular sac that temporarily stores urine, holding 0.5 to 1 liter.
• It sits in the lower abdomen.
• Sphincter muscles guard the opening of the bladder into the urethra and remain closed until urination.

Urethra

• The urethra is a short muscular tube extending from the urinary bladder to the exterior.
• In females, it opens directly to the outside and only serves as a passage for urine
• In males, it opens into the penis and serves as passage for both urine and semen.
• A ring of sphincter muscle guards the opening of the urethra.
• The muscle relaxes during urination.

Blood Supply to the Kidneys

• The renal artery, a branch of the aorta, enters the kidney at the hilum, bringing oxygenated blood with nitrogenous waste.
• Inside the kidney, the renal artery branches into thin vessels called afferent arterioles.
• An afferent arteriole enters each Bowman's capsule and forms a network of capillaries called the glomerulus, which then unites to form an efferent arteriole.
• After leaving Bowman's capsule, the efferent arteriole branches into the vasa recta (capillary network around the renal tubule).
• The capillaries combine to form renal venules, which then unite to form the renal vein that drains into the inferior vena cava.

Nephron

• The nephron is the kidney's basic structural and functional unit, composed of a renal corpuscle and a renal tubule.

Structure of the Nephron

• The renal corpuscle lies in the kidney's cortex and consists of a glomerulus surrounded by a Bowman's capsule.
• Bowman's capsule is a thin, double-walled cup-like structure that contains a network of capillaries called the glomerulus.
• It collects incoming fluid from the glomerular capillaries.
• The glomerulus is a network of capillaries inside Bowman's capsule, formed by incoming afferent and outgoing efferent arterioles.
• The renal tubule extends from Bowman's capsule and is a highly coiled part of the nephron further divided into three parts.
• The proximal convoluted tubule (PCT) is nearest to Bowman's capsule in the renal cortex.
• The loop of Henle is a U-shaped part of the tubule in the medulla region with ascending and descending limbs.
• The distal convoluted tubule (DCT) is highly coiled, found in the cortex, and opens into the collecting duct.
• The collecting duct is a wider tube formed by the open ends of many nephrons that extends from the cortex to the medulla.
• It collects contents from nephrons and empties urine into the renal pelvis.

Working of the Nephron

• The nephron's main function is to form urine, involving three main processes: ultrafiltration, selective reabsorption, and tubular secretion.
• The kidney carries out these processes in the steps of urine formation:
• Glomerular ultrafiltration/Ultra-filtration occurs in the glomerulus.
• Blood is filtered, allowing minerals, waste, and water to pass through, while red blood cells, proteins, and large molecules remain in the blood.
• Blood enters through the afferent arteriole (larger diameter) .
• Blood leaves through the efferent arteriole (smaller diameter).
• Pressure increases in the glomerulus (hydrostatic pressure) .
• Liquid and small substances pass through the capillary walls to create glomerular filtrate (GF) in Bowman's capsule.
• Glomerular filtrate (GF) contains water, urea, inorganic salts, amino acids, glucose, and plasma solutes.
• Only blood cells, large proteins, and fat molecules remain in the efferent arteriole blood.
• Selective tubular reabsorption reabsorbs useful substances like glucose, amino acids, salts, and water from the initial filtrate.
• These substances are reabsorbed from the renal tubule at various locations (PCT, Loop of Henle, and DCT) into the blood capillaries surrounding the tubules (vasa recta.)
• Tubular secretion occurs mainly in the DCT and the CT (collecting duct) of the nephron, where cells of the cuboidal epithelium lining the tubules actively transport additional wastes.
• During tubular secretion substances like potassium, hydrogen, creatinine, and drugs such as penicillin are directly excreted by the tubular cells into the filtrate.
• Urine, the fluid that flows through the last parts of the tubule, is 95% water and 5% wastes.
• Wastes are ions of sodium, potassium, and calcium, and nitrogenous wastes like creatinine, urea, and ammonia.

Path of Urine Formation

• Renal Artery → Kidney → Afferent Arteriole → Glomerulus → Bowman's Capsule → (Ultrafiltration) → Proximal Convoluted Tubule (PCT) (Reabsorption) → Henle's Loop (Reabsorption) → Distal Convoluted Tubule (DCT) (Reabsorption & Secretion) → Collecting Duct → Ureter → Stores Urine in Urinary Bladder → Urethra (urethral peristalsis).
• Micturition: As urine forms in the nephrons the urinary bladder stretches.
• Receptors in the wall of the bladder send signals to the Central Nervous System, thereby bladder contracts and urethral sphincter muscles relax to release urine.
• Urine is a transparent, pale-yellow, acidic fluid comprised of 95% water and 5% solid wastes including:
  • Hormones, medicines such as antibiotics, and excess water-soluble vitamins.
  • Organic compounds like Urea, Creatinine, Uric acid, and Ammonia.
  • Inorganic compounds like NaCl and KI.
• Typically, a person excretes 1000 - 1700 ml of urine daily. The amount depends on water intake, diet, climate, mental state, and physiological condition.
• Tea, coffee, alcohol, and other beverages increase urine production and are called diuretics.
• Osmoregulation maintains the osmotic concentration (water, minerals, and salt) of blood in a living being.
• In humans, the kidneys, hypothalamus, and pituitary gland regulate osmoregulation.
• Osmoregulation is important for cell function and survival in all living organisms.
• If there is more water outside the cell, the water moves into cells by endosmosis, causing them to swell and burst.
• If there is less water outside the cells, the water moves out of cells by exosmosis, causing cells to shrink.
• Disrupted cellular metabolism occurs in both instances above.
• When the body contains a large amount of water, kidney nephrons reabsorb very little water from the filtered urine and dilute urine is produced.
• When the body contains less water, kidney nephrons reabsorb more water from the filtered urine and concentrated urine is produced.
• Kidneys filter nitrogenous wastes like urea and assist in osmoregulation.
• Antidiuretic hormone (ADH) regulates the amount of water reabsorbed by the kidneys.
• It is produced by the pituitary gland.
• When the body needs to conserve water, ADH is released, increasing the permeability of the DCT and CT.
• This process allows more water to be reabsorbed back into the blood.
• Reduced ADH secretion increases urine production because less water is reabsorbed.

Nephron vs. Alveoli

• Alveoli and nephrons both remove wastes from the blood and use capillaries for blood filtration.
• Alveoli in the lungs are balloon-shaped to increase the surface area for gas exchange and removes CO2, and uptake fresh oxygen in the air via blood capillaries
• Nephrons are the working units of the kidneys and have a Bowman's capsule with an extensive capillary network and blood supply
• They remove nitrogenous wastes, i.e., urea along with filtered water, salts, glucose etc.
• Important materials are reabsorbed before the urine is released from the nephron.
• Alveolar capillaries have lower blood pressure than the capillaries in the nephrons.
• High blood pressure in the nephron capillaries helps remove water and dissolved wastes from the blood plasma.

Artificial Kidney

• Hemodialysis, or dialysis, purifies the blood of people whose kidneys don't function normally.
• An anticoagulant like heparin is added to blood drawn from an artery which is then pumped into a dialyzing unit.
• The unit contains a coiled cellophane tube surrounded by dialyzing fluid that has the same composition as plasma but lacks nitrogenous wastes.
• The porous cellophane membrane allows molecule passage based on concentration gradient
• As nitrogenous wastes are absent in the dialyzing fluid, they move out freely.
• The blood is returned to the body through a vein after adding anti-heparin.
• For permanent kidney damage dialysis must be performed for about twelve hours, twice a week.
• Some patients with chronic kidney failure have survived more than 12 years on dialysis
• Diseased kidneys can be replaced via kidney transplantation
• A healthy person can live a typical lifestyle with only one kidney and donate the other.

Excretion in Plants

• Oxygen produced during photosynthesis diffuses out of the leaf stomata.
• During the day, carbon dioxide produced in respiration is used up in photosynthesis.
• Excess water is removed by transpiration through the leaves.
• Some waste products are stored as resins and gums in old xylem.
• Latex (white fluid) is stored in stems and leaves of some plants.
• Elephant Yam tubers contain calcium oxalate crystals (raphides).
• Many waste products are stored in vacuoles.
• Some waste products may be stored in the leaves that fall off.
• Plants excrete some waste materials into the soil around them.