Human Physiology | Module 4 | Calicut University First Semester BSc. Zoology Notes PDF |

 Hello everyone! In this study material, I’ve compiled easy-to-follow notes for Module 4: Digestive and Excretory Systems from the First Semester B.Sc. Zoology (Calicut University) syllabus. This unit explains how the digestive system breaks down food for absorption and how the excretory system removes metabolic wastes and maintains fluid balance. The topics have been written in a concise and exam-oriented format, following the University pattern.

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Summary of what’s Covered in This Study Material?

🍽 Physiology of Digestion

Digestion is the biochemical and mechanical process by which complex food substances are broken down into absorbable forms. The digestive system performs ingestion, digestion, absorption, assimilation, and egestion in a coordinated sequence.

Structurally, the alimentary canal extends from the mouth to the anus and includes accessory glands such as salivary glands, liver, and pancreas. Each segment of the gastrointestinal tract performs specialised yet interdependent functions.

🦷 Buccal Cavity and Initial Digestion

Digestion begins in the buccal cavity, where food undergoes mastication and mixing with saliva. The stratified squamous epithelium protects the lining from mechanical abrasion, while the tongue assists in manipulation and swallowing.

Saliva, secreted by parotid, submandibular, and sublingual glands, contains salivary amylase that initiates carbohydrate digestion by converting starch into maltose. The moistened mass of food, known as a bolus, is then propelled into the pharynx.

Teeth are structurally adapted for mechanical breakdown. Enamel provides hardness, dentine supports the structure, and the pulp cavity houses nerves and blood vessels essential for vitality.

🫁 Oesophagus and Peristalsis

The oesophagus is a muscular conduit that transports the bolus from the pharynx to the stomach through coordinated peristaltic movements. Upper and lower oesophageal sphincters prevent reflux and ensure one-way movement.

Stomach 

The stomach is a J-shaped muscular organ that temporarily stores food and initiates protein digestion. Gastric glands in the mucosa secrete hydrochloric acid, pepsinogen, and mucus.

Hydrochloric acid creates an acidic environment, converting pepsinogen into active pepsin for protein breakdown. Mucus protects the stomach lining from self-digestion. Rhythmic contractions churn food into a semi-liquid substance known as chyme.

The pyloric sphincter regulates controlled release of chyme into the small intestine.

🧬 Small Intestine 

The small intestine, comprising duodenum, jejunum, and ileum, the main site of enzymatic digestion and nutrient absorption.

The duodenum receives bile from the liver and pancreatic juice from the pancreas. Bile emulsifies fats, increasing surface area for enzymatic action. Pancreatic enzymes digest carbohydrates, proteins, and lipids into simpler molecules.

The inner lining of the ileum contains villi and microvilli that significantly increase absorptive surface area. Nutrients enter blood capillaries or lacteals for systemic distribution.

🏭 Liver and Pancreas

The liver is the largest gland in the body and performs both digestive and metabolic functions. It secretes bile, regulates carbohydrate metabolism (glycogenesis, glycogenolysis, gluconeogenesis), deaminates amino acids, synthesizes plasma proteins, and detoxifies harmful substances.

The pancreas is a dual gland with exocrine and endocrine functions. Its exocrine portion secretes pancreatic juice rich in digestive enzymes, while the endocrine islets regulate blood glucose through insulin and glucagon secretion.

🔄 Hormonal Control of Digestion

Digestive processes are precisely regulated by gastrointestinal hormones.

• Gastrin stimulates gastric acid secretion.
• Secretin promotes bicarbonate-rich pancreatic secretion and neutralizes acidic chyme.
• Cholecystokinin stimulates pancreatic enzyme secretion and gallbladder contraction.
• Gastric inhibitory peptide reduces gastric activity and enhances insulin release.

⚖ Disorders Related to Digestion

Peptic ulcer results from erosion of gastric or duodenal mucosa due to excessive acid or infection by Helicobacter pylori.

Obesity represents excessive adipose tissue accumulation, often linked to metabolic imbalance and lifestyle factors.

Eating disorders such as anorexia nervosa and bulimia nervosa involve psychological and metabolic disturbances affecting nutritional status.

Flatulence, vomiting, and hiccups are physiological responses that may indicate transient or pathological digestive disturbances.

🚰 Physiology of Excretion

Excretion is the removal of nitrogenous wastes and maintenance of water, electrolyte, and acid–base balance. The kidneys are the principal excretory organs.

Each kidney contains approximately one million nephrons, the functional units responsible for urine formation.

🧪 Urea Cycle – Detoxification of Ammonia

Ammonia produced from amino acid metabolism is highly toxic. The liver converts ammonia into urea through the ornithine cycle (urea cycle). Urea is then transported to the kidneys for excretion.

This biochemical pathway links nitrogen metabolism with energy metabolism, preventing ammonia toxicity.

🩸 Formation of Urine

Urine formation occurs through three coordinated processes: glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular filtration produces approximately 180 liters of filtrate daily. Essential substances are selectively reabsorbed in renal tubules, while additional wastes are secreted into tubular fluid. The final urine volume averages 1–1.5 liters per day.

🧠 Hormonal Regulation of Urine Formation

• Antidiuretic hormone increases water reabsorption.
• Aldosterone enhances sodium reabsorption.
• Atrial natriuretic peptide promotes sodium excretion.
• Renin initiates the renin–angiotensin–aldosterone system, regulating blood pressure and filtration rate.

🧫 Clinical Conditions of Urinary Abnormalities

Glycosuria indicates glucose presence in urine when renal threshold is exceeded.

Albuminuria suggests increased glomerular permeability.

Haematuria reflects red blood cells in urine due to urinary tract damage.

Ketonuria occurs during excessive fat metabolism.

Uraemia represents accumulation of nitrogenous wastes due to renal failure.

This article is a structured academic interpretation of the prescribed syllabus for Calicut University BSc Zoology.

You may use the linked PDF for personal study and exam preparation.

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