Human Physiology Part-1

1 Introduction to the Digestive System - Structure of the Alimentary Canal

From the moment you put a morsel of bread in your mouth, until the energy from it reaches your muscles – a long, highly organized journey begins. This journey takes place in your Alimentary Canal, which is a continuous, muscular, open tube about 8-10 meters long, starting from the mouth and ending at the anus – that is, a tunnel 5-6 times longer than your height, which coils and ends in your stomach! The function of the digestive system is to convert the complex nutrients and large molecules of food into simple, small and soluble substances with the help of various chemical reactions and enzymes – this process is called digestion.

The digestive system consists of two types of organs—first, the organs of the alimentary canal (mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus) through which food passes directly; second, the digestive glands (salivary glands, liver, pancreas) which release digestive juices into the alimentary canal through ducts. Digestion occurs primarily by hydrolysis—in which H+ and OH– ions of water bind to complex molecules (such as starch, protein, fat) and break them down into simpler molecules; enzymes act only as catalysts in this process; they are not destroyed themselves.

Four layers of the gut wall

The alimentary canal is not a simple hollow tube, but a highly organized, multilayered structure. From the inside out, there are four layers:

• Mucosa – The innermost layer, composed of simple columnar epithelium; it contains goblet cells that secrete mucus to lubricate and protect the wall.
• Submucosa – It contains blood vessels, lymph vessels and crypts of Lieberkuhn.
• Muscularis – Made up of two layers of circular muscle fibers and longitudinal muscle fibers; this layer produces peristalsis and propels food forward.
• Serosa – The outermost protective covering layer.

Three Major Functions of the Digestive System

• Digestion – breaking down complex molecules into simpler ones
• Absorption of digested food – transporting simple nutrients into the blood/lymph
• Propulsion – Propulsion of food from the mouth to the anus

2 Mouth, pharynx, esophagus and stomach – structure and function

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🖼️ Human digestive system – complete structure of the alimentary canal and attached glands from the mouth to the anus

Mouth

The first entrance to the digestive system is the mouth—a bowl-shaped organ with the hard palate above and the soft palate below. The tongue is connected to the floor of the mouth by the frenulum linguae. Taste buds on the tongue are arranged regionally—the front part of the tongue is most sensitive to sweet tastes, the front edge to salty tastes, the middle edge to sour tastes, and the back part to bitter tastes.

Teeth

The teeth in each jaw are located in a mould of the gums, a system called thecodont. In humans, two sets of teeth develop during a lifetime—hence the diphyodont system: first the milk teeth (20 in total), then the permanent teeth (32 in total).

Pharynx

A flask-shaped structure located behind the mouth that serves as a common passage for both the respiratory and digestive systems. It has three parts: the nasopharynx (air passage only), the oropharynx (air and food passage), and the laryngopharynx (laryngopharynx).

Oesophagus

A narrow muscular tube about 25 cm long, which passes through the neck and chest, crossing the diaphragm and enters the abdominal cavity. Its mucous glands keep the passage lubricated and juicy, so that the bolus of food easily reaches the stomach through peristaltic movement. While swallowing, the epiglottis (gill lid) momentarily closes the entrance to the trachea, so that food does not accidentally go into the windpipe – this is the reason why our breathing stops momentarily while swallowing.

Stomach

A muscular, J-shaped structure located between the esophagus and duodenum, behind the diaphragm in the left part of the abdominal cavity. It is a flexible organ that can hold 1 to 3 liters of food. The stomach has three parts:

• Cardiac Portion – The left large portion, where the esophagus enters.
• Fundus Portion – Located just below the diaphragm.
• Pyloric Portion – The right small portion where the small intestine joins.

The stomach wall contains two types of glands—the gastric gland, in which zymogen cells secrete pepsinogen and oxyntic/parietal cells secrete hydrochloric acid (HCl); and the pyloric gland, which secretes mucus. Gastric juice is a highly acidic fluid (pH 1-2) consisting of water (98%), HCl (about 0.5%), mucous mucin, and two enzymes—pepsin and lipase.

The stomach contains two sphincters—the cardiac sphincter, which prevents acidic food from flowing back into the esophagus; and the pyloric sphincter, which divides the stomach and small intestine. Food typically remains in the stomach for 3-4 hours, where pepsin and HCl combine to partially digest proteins.

3 Small intestine, large intestine and rectum – structure and function

Small Intestine

The tube, approximately 7 meters long and 2.5 cm wide, begins at the pyloric portion of the stomach and ends at the large intestine—where most digestion and absorption occurs. It has three parts:

• Duodenum—The smallest C-shaped portion, connected to the stomach; bile from the liver and pancreatic juice from the pancreas meet here (the sphincter of Oddi is located at its opening). Brunner's glands are found in its wall, which secrete an alkaline fluid and protect the duodenum from acidic gastric juice.
• Jejunum—middle portion, about 2 meters long; here the main absorption occurs by enterocytes.
• Ileum—The last and longest part (about 4 meters), coiled; it absorbs bile salts and vitamin B12, which are not absorbed in the jejunum.

microscopic structure of the intestinal wall

The mucosa and submucosa fold to form a Kerckring ring, on which finger-like villi are found. At the base of the villi are the crypts of Lieberkuhn, which contain goblet cells (secreting mucus), intestinal cells (secreting water and enzymes), and Paneth cells (secreting the bactericidal lysozyme enzyme). Each villus contains a central lacteal and a network of blood capillaries. Microvilli, which are extensions of the plasma membrane, are also found on the epithelial cells of the villi.

Large Intestine

A tube approximately 1.5 meters long, connected to the small intestine, where specialized bacteria help break down undigested food through fermentation. Its main functions are the absorption of water and minerals and the excretion of undigested food. It has three main parts:

• Caecum—connected to the small intestine; from its first part arises the vermiform appendix, a vestigial organ in humans but large and functional in herbivorous mammals.
• Colon – Inverted 'U' shaped part, divided into four subdivisions – ascending, transverse, descending and sigma-shaped (S-shaped) colon.
• Rectum—The final part of the alimentary canal; its last 3 cm is called the anal canal, which opens into the anus. Here, two types of sphincters, internal and external, control bowel movements.

Egestion/Defaecation: Undigested parts (plant fibers) and unabsorbed digested matter are temporarily stored in the rectum, where increased water absorption forms semisolid stool. The defecation reflex relaxes the sphincter muscle and expels the stool—usually once or twice every 24 hours.

4 Digestive glands — salivary glands, liver, pancreas

Salivary Gland

Saliva is a mixture of serous fluid and viscous mucus—the liquid portion moistens food, while the mucous portion acts as a lubricant. Humans secrete approximately 1,000 to 1,200 milliliters of saliva daily—that's nearly a full water bottle full of saliva, every day!

There are three pairs of salivary glands:

• Parotid gland—largest, located anterior to the ear; secretes primarily aqueous, amylase-rich saliva.
• Sub-mandibular gland – Mixed gland (both water and mucus), at the angle of the lower jaw.
• Sublingual gland – Smallest gland under the tongue, secreting mainly mucus.

Functions of saliva: Partial digestion of starch (conversion into maltose by amylase/ptyalin enzymes), making food smooth and swallowable (bolus formation), cleaning of teeth, mouth and tongue, destruction of tooth decay causing bacteria by lysozyme, and stimulating the taste buds by dissolving food particles.

Liver

The largest and most important digestive gland in the human body, a triangular organ located in the upper right part of the abdomen, below the diaphragm. It is composed of approximately 100,000 tiny hexagonal structural-functional units—liver lobules—that is, a million mini chemical plants operating within the body!

Major functions of the liver:

• Bile formation – helps emulsify and digest fats
• Metabolism – the main center for the metabolism of carbohydrates, fats, proteins, and amino acids
• Glycogen storage—storing glucose as glycogen and regulating blood sugar
• Urea production – conversion of toxic ammonia into urea
• Plasma protein synthesis – prothrombin, fibrinogen (for blood clotting) and heparin
• Vitamin and mineral stores – vitamins A, D, E, K, B12 and minerals such as iron and copper
• Blood purification – Kupffer cells break down dead RBCs and hemoglobin.
• Detoxification – eliminating the effects of toxins and drugs in the blood

The liver has a tremendous regeneration capacity—it can regrow even if a large portion of its tissue is removed. Bile contains no digestive enzymes, yet it is essential for fat digestion—it is a yellow-green, alkaline fluid (pH approximately 8), whose main components are bile salts (sodium glycocholate and sodium taurocholate) and bile pigments—bilirubin and biliverdin. Bile is stored and concentrated in the gallbladder and released into the duodenum in response to food intake.

Pancreas

It is a mixed gland—endocrinely, it regulates blood sugar by producing insulin and glucagon hormones (from the islets of Langerhans); exocrinely, it produces pancreatic juice, which helps in the digestion of proteins, fats, and carbohydrates in the intestines. It is a 6-8 inch long, U-shaped gland, surrounded by the liver, duodenum, and spleen. Its secretions reach the duodenum through the pancreatic duct.

5 Stepwise digestion of carbohydrates, proteins and fats

The three main nutrients in food—carbohydrates, proteins, and fats—are broken down step by step by different enzymes in different parts of the digestive tract until they are reduced to their simplest absorbable units. The general sequence of the entire process is: Ingestion → Digestion → Absorption → Assimilation → Egestion.

(A) Digestion of carbohydrates

Final absorbable form: Monosaccharides — glucose, fructose, galactose.

(B) digestion of proteins

Final absorbable form: Amino acids.

(C) Digestion of fat

Final absorbable form: Fatty acids and glycerol.

6 Absorption, assimilation and hormones and disorders

Mechanism of Absorption

In the mouth, water, water-soluble vitamins, and small amounts of simple sugars are absorbed. In the stomach, water, glucose, alcohol, some minerals, and drugs are absorbed by osmosis, diffusion, and active transport. However, the main site of absorption is the small intestine—where the villi and microvilli increase the absorptive surface area by approximately 1000 times.

The large intestine absorbs mainly water (mostly in the colon), and some mineral salts are also absorbed by active transport. Assimilation involves the conversion of fatty acids and glycerol back into fat and storing them in adipose tissue. Excess monosaccharides are converted into glycogen, and amino acids into body proteins.

neural and hormonal control

Major disorders of the digestive system

• Vomiting — the forceful ejection of stomach contents; the most common cause is gastroenteritis (intestinal infection from a virus/bacteria).
• Diarrhea—loose stools 3 or more times a day; caused mainly by rotavirus and E. coli in developing countries. Spread through contaminated food and water.
• Constipation – due to insufficient fiber, irregular diet, less exercise and stress.
• Indigestion – due to overeating, spicy food, stress, caffeine, tobacco, alcohol.
• Jaundice – Yellowing of the skin and eyes due to excess bilirubin in the blood; caused by liver inflammation or bile duct blockage.

7 Respiratory system – journey from the nose to the lungs

Just as the digestive system extracts energy from food, the respiratory system draws oxygen from the atmosphere and delivers it to every cell in the body, expelling carbon dioxide produced by cellular activity. The respiratory system is divided into three parts—the upper respiratory tract, the lower respiratory tract, and the respiratory muscles.

Upper Respiratory System

• Nose—The first respiratory organ; divided into two parts by the nasal septum. Nasal hairs, mucus, and cilia work together to filter dust, pollen, and mold from the inhaled air, making it pure, warm, and moist.
• Mouth—Secondary respiratory tract; air taken in through the mouth is not as pure as air taken in through the nose.
• Pharynx – muscular, chimney-like tube; three parts – nasopharynx, oropharynx, and larynx.
• Larynx (voice box) — A structure made of nine layers of cartilage; it contains the vocal cords, which vibrate with the flow of air to produce sound. The epiglottis closes the glottis during swallowing.

Lower Respiratory System

• Trachea—A tube about 5 inches long, lined with pseudostratified ciliated columnar epithelium and composed of C-shaped cartilage rings that keep it open and prevent adhesions. In the thoracic cavity, it divides into the right and left primary bronchi.
• Bronchi & Bronchioles – The primary bronchi divide into secondary bronchi, then into tertiary bronchi, and finally into smaller bronchioles and terminal bronchioles – together form a tree-like structure (Bronchial Tree).
• Alveoli — Cup-shaped structures found at the ends of the marginal bronchioles, surrounded by numerous capillaries. The walls of both alveoli and capillaries are composed of a single layer of flat epithelial cells, which facilitate gas exchange. There are approximately 30 million alveoli in each lung.

8 Lungs, Pleural Integuments and Volume Capacities

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🖼️ Human respiratory system – complete structure from the nose to the lungs and alveoli

The lungs are flexible, soft, and pale pink organs located as a pair in the chest, just above the diaphragm. Interestingly, the right lung is slightly smaller but wider than the left—because the left lung is divided into two lobes while the right lung is divided into three (the left lung is smaller to allow room for the heart).

The lungs are surrounded by a double membrane—the pleura—filled with pleural fluid. This fluid reduces friction during lung movement, much like oil reduces friction between machine parts.

Pulmonary Volumes & Capacities

Various volumes and capacities have been defined to measure lung function, which are measured with a spirometer:

9 Respiratory mechanisms — inhalation and exhalation

Breathing occurs in three stages—pulmonary ventilation (inhalation/exhalation), gas exchange between the alveoli and tissues, and the transport of O₂ and CO₂ through the blood. Let's understand the mechanics of breathing.

Inspiration – the active process

This is an active process, initiated by contraction of the diaphragm and external intercostal muscles. When the diaphragm contracts, it flattens and descends; simultaneously, contraction of the external intercostal muscles pulls the ribs upward and outward. Both of these actions increase the volume of the thoracic cavity—causing the air pressure in the lungs to fall below atmospheric pressure, and air rapidly fills the alveoli.

Expiration – passive process

At rest, this is a passive process. The diaphragm and external intercostal muscles relax—the ribs descend under their weight, and the diaphragm rises, forming a dome again. This reduces the volume of the chest cavity, air pressure exceeds atmospheric pressure, and air is forced out of the alveoli. In forced breathing, additional intercostal and abdominal muscles also play an active role.

The normal resting respiratory rate is about 12–16 (some sources state 16–20) times per minute, and about 600–800 mL of air is exchanged per minute. This entire mechanism is based on a pressure gradient—where higher-pressure air (the atmosphere) flows naturally toward lower-pressure areas (the lungs), much like water flows from a height downward.

10 Exchange and transport of gases

Gas Exchange

Exchange of gases occurs by diffusion due to difference in partial pressure. In the alveoli, the partial pressure of O₂ (Po₂) is approximately 105 mm Hg and that of CO₂ (Pco₂) is approximately 40 mm Hg, whereas in blood (coming from veins), Po₂ is approximately 40 mm Hg and Pco₂ is approximately 46 mm Hg. Due to this pressure difference, O₂ diffuses from the alveoli to the blood and CO₂ from the blood to the alveoli – this is called external respiration. The opposite exchange occurs in the tissues, which is called internal respiration.

Transport of oxygen

Approximately 97% of O₂ is conjugated with hemoglobin and transported as oxyhemoglobin (Hb₄(O₂)₄), leaving only 3% dissolved in plasma. The higher the Po₂, the greater the saturation of hemoglobin—the curve representing this relationship is S-shaped (sigmoid).

Transport of carbon dioxide

CO₂ is transported in three forms:

• As bicarbonate (about 70–75%)—the largest portion; the carbonic anhydrase enzyme in RBCs converts CO₂+H₂O to carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻.
• As carbamino-hemoglobin (about 20-23%) – CO₂ binds to the NH₂ group of hemoglobin.
• In plasma in dissolved form (about 5-7%).

Chloride Shift: When HCO₃⁻ moves out of the blood cell into the plasma, Cl⁻ ions from the plasma enter the blood cell to balance it. Haldane Effect: More CO₂ is removed from the blood than O₂-Hb combination in the lungs.

11 Regulation of respiration and diseases of the respiratory system

Respiration is an involuntary action controlled by respiratory centers located in the medulla oblongata and pons—the dorsal respiratory center (controls expiration), the pneumotaxic center (controls expiration and limits expiration), and the ventral respiratory center (assists in forced breathing). An increase in CO₂ and H⁺ ions in the blood increases the respiratory rate.

Major diseases of the respiratory system

12 Circulatory system – composition of blood, blood groups and coagulation

So far, we've seen how food is digested and oxygen enters the body—but who delivers these two nutrients to every corner of the body? This responsibility is performed by the circulatory system, whose central pump is the heart, and whose carrier fluid is blood.

Composition of Blood

Blood is a red, thick, slightly alkaline fluid connective tissue, which is made up of two parts – plasma (55%) and formed cells (45%).

Plasma — A pale yellow liquid composed of water (~90%), proteins (albumin, globulin, fibrinogen), and salts. Its functions include transporting digestive products and hormones, transporting waste products to the excretory organs, evenly distributing body heat, and supporting blood clotting and immunity.

RBCs are formed in the red bone marrow and are destroyed in the spleen and liver after a lifespan of approximately 120 days – hence the spleen is also known as the 'graveyard of RBCs'. WBCs are of two types – granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes); they serve as the body's immune system.

Blood Groups – ABO System

There are four blood groups – A, B, AB and O, based on the antigens (A, B, both, or none) on the RBC membrane and the presence of antibodies in the plasma.

O group = universal donor (no antigen); AB group = universal recipient (no antibodies). The Rh factor is also important—it determines whether the fetus is Rh+ or Rh–; incompatibility between a pregnant Rh– mother and an Rh+ fetus can lead to serious complications.

Blood clotting system

Haemophilia – A genetic disease, mainly in males, in which the blood does not clot due to a deficiency of coagulation factors and bleeding continues.

13 Structure of the heart, cardiac cycle and motor system

The heart is a muscular, hollow, red organ made of muscular tissue—about the size of a clenched fist. It is surrounded by a double-membranous sac called the pericardium, within which pericardial fluid protects the heart from external shocks and reduces friction.

four chambers and valves of the heart

The heart has two atria (small, blood-receiving chambers) at the top and two ventricles (large, powerful, pumping chambers) at the bottom. The interatrial septum and interventricular septum keep the right and left lobes completely separate—the right carries impure blood, while the left carries pure blood.

Valves allow blood to flow in only one direction—the bicuspid valve/mitral valve (left atrium-ventricle), the tricuspid valve (right atrium-ventricle), and the semilunar valves (aortic and pulmonary valves, at the ventricle-artery openings). Closing of the valves produces a "lub-dub" sound, which can be heard with a stethoscope.

Cardiac Cycle

One cardiac cycle is completed in about 0.8 seconds, consisting of diastole (relaxation) and systole (contraction). The normal adult heart rate is about 72 beats/minute, and the heart pumps about 70 mL of blood per beat (stroke volume). Thus, cardiac output (total blood pumped in one minute) = 72 × 70 ≈ 5000 mL, or about 5 liters per minute.

Conducting System of the Heart

The sequence of heartbeats is determined by a special electrical conduction system:

• Sino-atrial node (SA Node) – Located in the upper wall of the right atrium; called the natural pacemaker, it is where the heartbeat begins.
• Atrioventricular node (AV Node) — Located between the right atrium and ventricle; transmits impulses from the SA node to the ventricles.
• Bundle of His and Purkinje Fibres – They rapidly transmit impulses to the ventricular muscles and produce coordinated contractions.

If the SA node becomes faulty, an artificial pacemaker is implanted in the chest. An ECG (Electrocardiogram) is a device that records the conduction of the heartbeat, which is helpful in detecting heart-related disorders.

14 Blood Vessels and Types of Circulation

Blood Vessels

Types of circulation

• Systemic Circulation—The aorta carries purified blood from the left ventricle to the rest of the body; the great veins return impure blood to the right atrium.
• Pulmonary Circulation—Impure blood from the right ventricle goes to the lungs; the pulmonary veins bring pure blood to the left atrium.
• Coronary Circulation – The blood supply to the heart's own muscles is through the coronary arteries.
• Hepatic Portal System – Nutrient-rich blood from the small intestine travels directly to the liver through the hepatic portal vein, allowing the liver to absorb and process nutrients.

In humans, blood passes through the heart twice, hence the name Double Circulation—first through the body-heart (somatic) and then the heart-lung-heart (pulmonary) cycle. This dual system ensures that pure and impure blood never mix, keeping oxygen distribution highly efficient—much like a city has separate pipelines for drinking water and wastewater.

15 Diseases of blood pressure, lymphatic system and circulatory system

Blood Pressure

The pressure that blood exerts on the walls of arteries during ventricular contraction (systole) is called blood pressure—systolic pressure (higher) and diastolic pressure (lower). The normal reading for a healthy adult is 120/80 mm Hg, measured with a sphygmomanometer. When systolic pressure ≥140 and diastolic pressure ≥90 mm Hg remain persistent, it is called hypertension—it can adversely affect the heart, brain, and kidneys.

Lymphatic System

The lymphatic system is a complementary system to the circulatory system. It consists of lymph (a colorless fluid containing plasma and WBCs but devoid of RBCs), lymph vessels, lymph nodes, and lymphoid organs (spleen, thymus, bone marrow). The main functions of the lymphatic system are to collect excess fluid from tissues and return it to the blood, aid in the absorption of fats (via lacteals), provide immunity, and prevent the spread of infection.

Lymph flows in only one direction—from the organs to the heart, never the other way around—and has no pumping mechanism; it is propelled forward by muscle movement.

Major diseases of the circulatory system

• Atherosclerosis – Plaque buildup due to cholesterol deposits on the artery walls, gradually narrowing and hardening the arteries.
• Heart attack — a clot (thrombosis) in a coronary artery that blocks blood supply to the heart muscle; obesity, smoking, high blood pressure, and high cholesterol are major risk factors.
• Stroke – Blockage or clot in the arteries of the brain.
• Anemia – a deficiency of hemoglobin; caused by a deficiency of iron and vitamin B12.
• Varicose Veins – The veins in the legs become stretched and crooked in people who stand for long periods of time.
• Leukemia (blood cancer) — uncontrolled production of WBCs.

16 Excretory System – Nitrogenous Wastes and the Structure of the Kidneys

The digestive, respiratory, and circulatory systems together provide the body with nutrition and energy—but who cleans up the waste generated during this process? This responsibility lies with the excretory system, which maintains homeostasis by removing waste products produced by the body's cells.

Three types of nitrogenous waste

parts of the human excretory system

Two kidneys (Kidneys — main organs, filter blood and form urine), two ureters (Ureters — carry urine to the bladder), a urinary bladder (Temporary storage of urine) and a urethra (Urethra — the way for urine to exit the body).

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🖼️ Structure of the nephron – the functional unit of urine formation

structure of the kidney

The kidney is a bean-sized, red organ—10–12 cm long, 6 cm wide, and 3–4 cm thick. It is located in the abdominal cavity, beneath the diaphragm, on either side of the vertebral column. The kidney has a groove—the hilum—on its medial surface, through which the ureters, nerves, and blood vessels enter.

In a vertical section of the kidney, two parts are clearly visible – the cortex (Cortex, the outer part, appears granular due to Malpighian corpuscles) and the medulla (Medulla, the inner part, divided into triangular pyramids, between which the renal columns of Bertini are embedded).

17 Nephron and the three-step process of urine formation

Each kidney contains 10-12 lakh fine, long and coiled tubes called nephrons – this is the structural and functional unit of the kidney.

Parts of the nephron (in order)

• Malpighian Corpuscle — Bowman's Capsule + Glomerulus. The afferent arteriole brings blood, the efferent arteriole takes it back. The inner wall of Bowman's capsule is made up of podocytes, which act as a highly permeable sieve.
• Proximal convoluted tubule (PCT)—Long, thick tubule; most reabsorption occurs here.
• Loop of Henle—U-shaped portion located in the medulla; composed of a descending and ascending arm.
• Distal convoluted tubule (DCT) — Located in the cortex, opens into the collecting duct.
• Collecting Duct—Located in the pyramids of the medulla; several collecting ducts join to form the duct of Bellini, which ultimately opens into the renal pelvis.

three stages of urine formation

• Ultrafiltration – The glomerulus has numerous tiny pores in its walls; the hydrostatic pressure of the blood (~70 mm Hg) filters water, glucose, amino acids, and urea into Bowman's capsule (blood cells and larger proteins are not filtered). This fluid is called the glomerular filtrate.
• Selective Reabsorption—Approximately 65–80% of the PCT (glucose, amino acids, salts) is returned to the blood by active transport; water-salt balance is maintained by a countercurrent mechanism in the loop of Henle; final water reabsorption is achieved in the DCT.
• Tubular Secretion — The cells of the PCT and DCT release K+, H+, ammonia, uric acid, and some drugs into the tubules through active transport to maintain blood homeostasis. The filtrate is then called urine.

Composition of urine: Color light yellow (due to urochrome pigment), quantity 1-2 liters per day, pH slightly acidic (about 6), water 96%, urea about 2%, and other organic and inorganic salts.

18 Hormonal regulation, dialysis and diseases of the excretory system

Hormonal regulation (osmoregulation)

Kidney function is primarily controlled by two hormones:

• Aldosterone—secreted from the adrenal cortex; increases Na+ reabsorption in the renal tubules, maintaining sodium and water balance in the body.
• Vasopressin/ADH (Anti-Diuretic Hormone) — secreted by the posterior pituitary gland; determines urine concentration by controlling water reabsorption in the collecting duct — when water is deficient, more ADH is secreted, resulting in less and more concentrated urine.

Dialysis

When the kidneys do not function properly, urea in the blood increases (uremia). The process of removing urea from the blood using an artificial kidney is called hemodialysis. The patient's blood is passed through tubes made of cellophane membrane, which allows small molecules (urea, uric acid) to pass through but blocks larger protein molecules. The concentration gradient forces harmful substances from the blood into the dialyzing fluid, and the purified blood is then returned to the patient's vein.

Major diseases of the excretory system