Physiology Notes PDF

# Introduction to Physiology Physiology is the science of life. It is the branch of biology that aims to understand the mechanisms of living things, from the basis of cell function at the ionic and molecular level to the integrated behavior of the whole body and the influence of the external enviro...

# Introduction to Physiology Physiology is the science of life. It is the branch of biology that aims to understand the mechanisms of living things, from the basis of cell function at the ionic and molecular level to the integrated behavior of the whole body and the influence of the external environment. Research in physiology helps us to understand what the functions of each organ are, how the body works in health, how it responds and adapts to the challenges of everyday life, and the integration between different body organs. It also helps us to determine what goes wrong in disease, facilitating the development of new treatments and guidelines for maintaining human and animal health. ## Composition of the Human Body: As shown in Figure 1, the human body of a 70 kg adult man is composed of: | Component | Kg | % Body Weight | |---|---|---| | Water | 42.0 | 60% | | Protein | 12.6 | 18% | | Fat | 12.6 | 18% | | Carbohydrates | 0.4 | 0.6% | | Minerals | 2.8 | 4% | | **Total** | **70.0** | **100%** | **Figure 1:** Body composition of 70 kg Man in % of total body weight and corresponding Kilograms ### Water: 60% of the total body weight An adult male who is weighing 70 kg, has total body water (TBW) about 40 liters. It declines with age and in inverse relation with body fat. It is also affected by gender. Figure 2 illustrates the distribution of water in our body. | Compartment | Liter | |---|---| | Total Body Water | 40 | | Intracellular Fluid | 25 | | Extracellular Fluid | 15 | | Intravascular Fluid | 3 | | Interstitial Fluid | 12 | **Figure 2:** Distribution of total body water ### Proteins: 18% of the body weight Proteins are present in all tissues. The largest amount of protein is found in skeletal muscles. ### Fat: 18% of the body weight - The main energy stores of the body. - Helps in insulation from cold and heat. - Provides a cushion around soft organs. - Involved in the regulation of hunger and satiety. - Present around abdominal viscera, in subcutaneous tissues, and in the structure of the central nervous system. - Phospholipids are found in the structure of cell membranes. ### Minerals: 4% of the body weight Minerals and electrolytes distribution and concentrations in intracellular fluid (ICF) are different from extracellular fluid (ECF) as present in Figure 3. - Minerals present in small quantities EXCEPT: - Calcium: The total body calcium is about 1.2 kg in a young adult, the bulk of which is found in the bones. - Iron: it is about 3 to 4g. 70% of which is the hem part of hemoglobin and myoglobin. **Figure 3:** Distribution of Major Cations and Anions in Body Fluids (mEq/L)) # The Internal Environment (Extracellular Fluid (ECF)) ECF contain ions and nutrients needed by the cells to maintain life. It is in constant motion throughout the body to keep constant internal environment. There are narrow normal range values, not fixed values, for each component in ECF (ions & nutrients). Illness, injury, or major environmental challenges causes of abnormal ECF component values which cause disease in moderate dysfunction or death in extreme dysfunction. For example, the narrow range for acid-base balance in the body, the normal pH is 7.4, and lethal values are only about 0.5 above or below normal. ## Homeostasis Homeostasis is the ability of the body to maintain nearly constant conditions in its internal environment, despite changes in the surroundings, through homeostatic control mechanisms. Homeostatic Control Mechanisms are the regulatory mechanisms that tend to correct any deviation from normal in response to changes in the external or internal environment. ## The Control Systems of the Body: Normal body functions require the integrated actions of cells, tissues, and the multiple nervous, hormonal, and local control systems that contribute to homeostasis and good health. The body depends mainly on two major controllers: the nervous and endocrine systems. ### Components of Control Systems: **Figure 4:** components of control system of the body 1. **The receptor** is a sensor that detects a change in the surrounding 2. **Afferent pathway**: carry information from the receptor to the control centre. 3. **Control centre (integrator)**: It determines the exact set point and analyses the input it receives from the receptor and then determines the appropriate response. It presents in the hypothalamus. 4. **Efferent pathway**: carry information from the centre to the effector. 5. **The effector**: An organ, a gland, or a muscle that becomes active and carry specific activity in response to a stimulus. 6. **Feedback response**: There are two types of feedback: * **NEGATIVE FEEDBACK**: It depresses the response to a stimulus. Here, the output response is opposite to the stimulus. Interestingly, most homeostatic control mechanisms of the body act by negative feedback. * **POSITIVE FEEDBACK (cascades or vicious cycle)**: It enhances stimulus so that the reaction continues at an even or enhancing level. The response proceeds in the same direction of the initial disturbance, causing the variable to deviate further and further from its normal range e.g.: Childbirth, when uterine ## The Blood Blood is a connective tissue that circulates through the body. It is a unique fluid that circulates through the whole body to maintain a constant internal environment around the body cells. "Homeostasis". ### Blood Components: - **The fluid of blood called plasma**: forms 55% of blood volume (around 5 liters). - **The cellular elements**: form 45% include red blood cells, white blood cells and platelets. ### 1. The blood plasma: It is about 3 liters. **Constituents of plasma:** 1. **Water (90%)**: major component 2. **Inorganic matters (1%)**: Na+, Cl-, Ca++, K+ 3. **Organic constituents (9%) include:** * **Nutrients & waste products (2%)** * **Plasma proteins (7%)** = 7-9 g/dL. **Plasma proteins include:** - *Albumin ( = 4.8 g/dL)* - *Fibrinogen ( = 0.2 g/dL)* - *Globulins (a, b,y) ( = 2.7 g/dL)* - *Prothrombin ( = 0.02 g/dL)* ### Functions of plasma proteins: - Blood coagulation: prothrombin and fibrinogen. - Immunity: Antibodies (gamma globulins). - Regulation of blood volume by colloid osmotic pressure of plasma proteins ( = oncotic pressure) about 25 mmHg. ### 2. Red blood corpuscles (RBCs) : Not a true cell Red blood cells (erythrocytes) contain haemoglobin, which carry O2. They are formed mainly in the bone marrow, and normally the RBC count is from 4.5 – 5.5 million / mm³ of blood. They have a life span of about 120 days. They are anucleated and not considered true cells, so better named red blood corpuscle. ## Formation of RBC (Erythropoiesis): ### Factors affecting erythropoiesis: **Figure 5:** role of erythropoietin hormone in erythropoiesis 1. **Tissue oxygenation: role of erythropoietin** The most important factor to stimulate erythropoiesis is hypoxia, which increases production of erythropoietin. In the absence of erythropoietin, few red blood cells are formed by the bone marrow (negative feedback mechanism). 2. **Nutritional factors:** * **Proteins:** Animal proteins present in liver, kidney and muscles are superior in production of haemoglobin (globin fraction) compared to plant-derived proteins. * **Iron** * **Vitamins:** - Vitamin B12 and folic acid (hematinic factors) are essential building blocks of DNA. This means it is important for nuclear maturation and cell division of red cell precursors and are called maturation factors. - Vitamin C affects general metabolism and growth of body tissues, including the bone marrow. - Copper and Cobalt: act as catalysts in haemoglobin synthesis. ### 3. Hormones **Specific hormone:** erythropoietin **Non-specific hormones:** Thyroxine hormones are the most powerful stimulant of body metabolism, and their deficiency for a long time can cause anaemia. Male hormones (androgens) and growth hormone can stimulate body metabolism as well as erythropoiesis. ### 4. Liver: It manufactures globin, stores iron, copper, vitamin B12 and folic acid, and secretes a small fraction of erythropoietin. ### 5. Bone marrow: It is the factory in which red cells are formed. ### Fate of RBCs: RBCs can be recycled. Waste: Old RBCs are taken by cells of the reticuloendothelial system, which attack hemoglobin (Hb) hydrolyzing it: Hb `-->` hydrolysis `-->` globin + hem - Globin hydrolysis into amino acids. - Hem hydrolyzed into iron, biliverdin and bilirubin (bile pigments). - Amino acids and iron are retained in the body to be used for the regeneration of new RBC while bile pigment are carried to the liver to be excreted to the intestine and then to the outside with the feces. ## 3-Leucocytes (white blood cells): These are about 6,000: 10,000/mm³. They are formed by the bone marrow and by the lymphoid tissue, and after a short life span (about one day), they are destroyed by the reticuloendothelial system. ### They are of 2 types: **Figure 6:** types of leukocytes 1. **Granular leukocytes (granulocytes)**: have granules in their cytoplasm and a lobed nucleus and are formed in the bone marrow. * According to the color of the granules, they are classified into 3 types: * **Neutrophils:** red and blue granules in the cytoplasm. Highly phagocytic and constitutes 50 - 70% of the leucocytes. * **Eosinophils:** red granules in the cytoplasm. Slightly phagocytic and constitute 1-4% of leucocytes. Increases in allergy and parasitic infestation. * **Basophils:** blue granules in the cytoplasm. Not phagocytic and constitute 0 - 1% of leucocytes. They release heparin in the blood. 2. **Non-granular Leucocytes (Agranulocytes)**: these have no granules in the cytoplasm and their nucleus is round and large. * They are formed in the lymphatic tissue and include 3 forms: * **Lymphocytes**: Not phagocytic and constitute about 25% of leucocytes; it acts with the immune system. * **Monocytes**: Slightly phagocytic and constitute 1% of leucocytes (tissue macrophage). * **Plasma cells**: 0-2% and are concerned with the formation of immune bodies. ## 4-Thrombocytes (blood platelets) : Not a true cell These are small, non-nucleated and oval cells. They are formed in the bone marrow from certain giant cells known as Megakaryocytes and circulate in the blood for about five days (life span), after which they are destroyed by the reticuloendothelial cells. Their number is about 150,000: 300,000 / cubic millimeter. ### Platelet functions: - **Vascular spasm**: by thromboxane A2 & serotonin. - **Platelet plug formation**: A.D.P. & collagen. - **Blood coagulation**: platelet factors. ## Haemostasis **Definition**: prevention of blood loss or stoppage of bleeding whenever a vessel is ruptured. **Mechanisms of haemostasis:** - **Vascular spasm**: This means contraction of vascular smooth muscle in response to blood vessel injury to reduce blood flow from the ruptured vessel. - **Formation of the platelet plug**: that is usually successful in stopping blood loss from a small wound. - **Formation of the blood clot (coagulation)** - **Growth of fibrous tissue in the blood clot.** ## Autonomic Nervous System There are two main control systems of our body: A. **Endocrine system** : slow control that lasts for a long time, through secreting hormones. B. **Nervous system**: exerts rapid control. ### Functional organization of the nervous system: 1. **The central nervous system (CNS)**: * **The brain**: The major parts of the brain are the cerebrum (cerebral cortex), cerebellum, basal ganglia, brain stem (midbrain, pons, and medulla), the thalamus, and the hypothalamus. * **Spinal cord**: The spinal cord segments are 31 segments, each segment gives a pair of spinal nerves. There are 31 pair of spinal nerves. 2. **The peripheral nervous system (PNS)**: * It includes cranial and spinal nerves; it is divided into: * **Somatic nervous system** mainly deals with the voluntary and conscious aspects of neurological control as skeletal movement and the various somatic sensory functions. * **The autonomic nervous system (ANS)**: deals with involuntary control mechanisms such as regulation of the heart, blood vessels, digestive functions. ### Autonomic ganglia Autonomic ganglia are an aggregation of cell bodies of neurons outside the CNS. ### Types of autonomic ganglia: - **Lateral ganglia (paravertebral or sympathetic chain)**: It is present on each side of the vertebral column. It is a site for relay of sympathetic fibres. - **Collateral ganglia (prevertebral)**: These are the celiac, the superior and inferior mesenteric ganglia. It is a site for relay of preganglionic sympathetic fibres that supply abdominal and pelvic viscera and is found midway between the sympathetic chain and the organ of supply. - **Terminal ganglia (peripheral)**: It is present near (or on the surface) of the innervated organs. It is a site for relay of preganglionic parasympathetic fibres. ### Functions of the autonomic ganglia * **Distributing centres** - ***In the sympathetic system***, the preganglionic fibber synapses and activates many postganglionic neurons. This allows for widespread distribution of nerve impulses over wide areas of the body; thus, producing generalized sympathetic effects. - ***In the parasympathetic***, the preganglionic fibre synapses and activates only few postganglionic neurons. Therefore, this arrangement produces localized and discrete parasympathetic activities. * **Site of relay:** relay station of preganglionic fibres coming from CNS. * **Site of Release of chemical transmitter:** - **Acetylcholine** is the mediator liberated at all preganglionic endings (sympathetic and parasympathetic). It is responsible for transmission of nerve impulse from preganglionic to postganglionic neurons (synaptic transmission). ### Functional organization of the autonomic nervous system: **Anatomically:** - Cranial outflow (cranial nerves III, VII, IX and X). - Thoraco-lumbar outflow (from Tl- L2). - Sacral outflow (sacral 2,3 and 4). **Functionally:** - **Sympathetic nervous system (thoraco-lumbar from Tl-L2).** - **Parasympathetic nervous system (cranio-sacral) including:** **The sympathetic nervous system:** Origin: preganglionic fibres exit the spinal cord from T1 to L2. **Functions of sympathetic nervous system:** **Functions of sympathetic supply to the head and neck:** - **Eye:** * Dilatation of eye pupil (mydriasis) for better far vision. * Retraction of the eyelids and widening of the palpebral fissure. - **Glands:** <br> * Lacrimal gland: low secretion of tears. * Salivary gland: trophic secretion of viscid saliva. - **Skin** VC of cutaneous BV, erection of hair, and copious secretion of sweat. - **The cerebral vessels**: weak vasoconstriction, which leads to increase cerebral blood flow. **Sympathetic supply to the thoracic viscera:** - **Heart:** * Excitation of all properties of the cardiac for better perfusion of vital organs and muscles. * Vasodilator to coronaries. - **Lungs:** * Bronchodilation for better lung ventilation and more 02 supply to the tissues. * Slight vasoconstriction of the pulmonary blood vessels. - **Sympathetic supply to the abdominal (splanchnic) viscera:** * GIT motility: decrease. * GIT secretion: inhibition of secretion **EXCEPT ADRENAL MEDULLA**. It is Secretory to adrenal medulla to potentiate of sympathetic. * Gall bladder: bile retention * Vasoconstriction to the blood vessels of the abdominal viscera. * Glycogenolytic to the liver to increase blood glucose. * Spleen: squeezing of blood rich in RBCs into the circulation. - **Sympathetic fibres to the pelvis:** * Vasoconstriction of the pelvic blood vessels * Retention of faeces and urine * Variable effects on uterus according to gravid state and hormones * Motor to muscles of male sex organs - **Sympathetic supply to the limbs, thoracic and abdominal parities:** * C to cutaneous blood vessels to limit bleeding in case of haemorrhage. * Secretory to the sweat glands for regulation of body temperature * Motor to the erector pilae muscle causing erection of hair (in animals). * Vasodilatation to the blood vessels of the skeletal muscles to increase blood flow. * Increase of muscle glycogenolysis to increase blood glucose. ### The overall role of the sympathetic in emergencies: - In **fight and flight**, the sympathetic system prepares the body for activities and increases its capacity and endurance for performing severe muscular efforts. - **Mass discharge** = large portions of the sympathetic nervous system discharges at the same time when the hypothalamus is activated by fear or severe pain. - **Alarm or stress response** (Figure 7) The widespread reaction throughout the body from sympathetic stimulation which activates reticular formation for more concentration. ## The Parasympathetic Nervous System The parasympathetic represent the cranio-sacral division of the autonomic nervous system because preganglionic neurons are located in several cranial nerve nuclei (III, VII, IX, and X) and in 2, 3 and 4 sacral spinal nerve. ### Functions of the parasympathetic outflow: - **The cranial autonomic:** * **The Oculomotor (3rd cranial) nerve:** * Pupillary constriction (i.e. miosis). * Accommodation for near vision can occur (increased power of lens). * **The Facial (7th cranial) nerve:** * Secretory and vasodilator to the lacrimal glands cause shedding of tears. * Secretory and vasodilator to the sublingual and submaxillary salivary glands to give copious watery saliva rich in electrolytes. * Vasodilatation of the blood vessels in the anterior 2/3 of the tongue. * **The Glossopharyngeal (9th cranial) nerve:** * Secretory and vasodilator to the parotid salivary gland giving copious watery saliva rich in electrolytes. * **The Vagus (10th cranial) nerve:** 75% of the parasympathetic nerve fibers in the body are in the vagus nerves * **Heart** * Inhibition of cardiac rhythmicity. * Vasoconstriction of coronary vessels * **Lungs**: * bronchoconstriction. * Secretory to the bronchial mucous glands. * **Gastrointestinal tract:** * Stimulation of motility and secretion from stomach, exocrine pancreas and liver. * Stimulates gall bladder causes secretion of bile. - **The sacral autonomic:** - The preganglionic neurons originate S2,3,4 segments, reaches the ganglia in the walls of the organs innervated (Terminal ganglia). **Function:** * It is the nerve of defecation. * It is the nerve of micturition. * Secretory to the seminal vesicles and prostate. ### General functions of the parasympathetic system: - **The parasympathetic called the anabolic nervous system.** - It prepares the body for recovery and repair. - Its activity continues and even increases during sleep and rest. - **The functions concern with the vegetative aspects of day to day living e.g.** - parasympathetic action favours digestion and absorption of food by increasing the activity of the intestinal musculature, increasing gastric secretion, and relaxing the pyloric sphincter. ## The Respiratory System Respiration means the different processes by which we can finally obtain energy from different food stuffs. ### Respiration generally includes the following steps: - **Ventilation**: i.e gas exchange between the alveoli and atmosphere. - **Diffusion**, i.e gas exchange between the alveolar air and the blood. - **Perfusion**: the flow of blood to alveolar capillaries. - **Gas transport**: which means transport of oxygen and carbon dioxide, in the blood and body fluids to and from the cells. **N.B: Internal respiration**: means the exchange of gases between the cells and blood; also, it includes the different intercellular chemical reactions that end by the releases of energy. ### Mechanics of breathing: - When the diaphragm contracts it becomes less convex and pushes the abdominal viscera downwards. This increases the vertical diameter of the thoracic cavity. - When the external intercostals muscles contract they tend to raise the upper ribs and sternum, increasing the antro-posterior diameter of the thoracic cavity, moreover, as the ribs are raised, they rotate outward, increasing the transverse diameter of the chest. - During normal resting inspiration, the diaphragm contributes about 70% of the increase in the volume of the thoracic cavity. ### Pressures in the respiratory system: - **Atmospheric pressure:** It is the pressure of the surrounding atmosphere at sea level; it is 760 mmHg (physiologically, we consider the atmospheric pressure is equal to zero pressure). - **Intra-alveolar (intra-pulmonary) pressure:** Pressure within the alveoli. At the end of expiration with the glottis open, it will be atmospheric. During inspiration, it tends to fall below atmospheric (-1mmHg) but with forcible inspiration it may fall much lower, to about (-70mmHg). During normal expiration as the lungs recoil back, the intra alveolar pressure rises above atmospheric (+1mmHg) but during the forceful expiration (Valsalva manoeuvre) it may rise up to 100 mmHg. - **Intrapleural (interathoracic) pressure:** The pleural space pressure is sub-atmospheric pressure (negative pressure) throughout the respiratory cycle during normal quiet breathing. The pressure will vary between 2mmHg at the end of normal expiration to about -6mmHg during inspiration. During forcible inspiration it may fall to-70mmHg. During forcible expiration (Valsalva maneuver) the Intrapleural pressure becomes positive rising to 50mmHg. ### I-Nervous control of Ventilation (The respiratory center): It is composed of several groups of neurons spread through the entire length of the medulla and pons. It can be divided into four major groups of neurons: - **The dorsal respiratory group**: It comprises inspiratory neurons, which discharge rhythmically during resting and forced inspiration, hence the name "rhythmicity center". Thus, they are nearly entirely responsible for inspiration. - **Ventral respiratory group**: They are inactive during quiet breathing but become activated during increased pulmonary ventilation as in exercise. They are mainly expiratory neurons. At rest, when expiration is passive, they remain inactive. - The rhythmicity center sends excitatory impulses via the intercostals and phrenic nerves to the external intercostals muscles and the diaphragm, respectively. It receives impulses directly from higher brain centers, from other centers in the brain stem and from special receptors which give rise to respiratory reflexes. ### II-Chemical Control of Ventilation: The rhythmicity center is affected by chemical changes in the blood via two types of chemoreceptors: peripheral and central. 1. **Peripheral chemoreceptors**: These are located mainly in the carotid and aortic bodies. Peripheral chemoreceptors are highly sensitive to changes in arterial PO2 and to a lesser extent to PCO2 and pH. 2. **Central Chemoreceptors**: There is a diffusion barrier between the blood and the central receptors (blood-brain barrier) being freely permeable to CO2 but poorly permeable to hydrogen and bicarbonate ions. These central chemoreceptors are most likely located in the on the medulla oblongata bathed with cerebrospinal fluid. They are highly sensitive to the hydrogen ion concentration of the cerebrospinal fluid (C.S.F) evoked by arterial PCO2 since CO2 can freely cross the blood-brain barrier into the cerebrospinal fluid while the barrier is relatively impermeable to hydrogen and bicarbonate ions. Inside the C.S.F. the following reaction taking place: CO2 + H2O `-->` H2 CO3 `-->` HCO3 +H+ (C.A. = Carbonic Anhydrase enzyme.) Actually, the released H⁺ in C.S.F is the effective stimulus for central chemoreceptors. ## Gas Transport: ### Transport of oxygen: Oxygen is transported in the blood in two forms: 1. **The main mode of transport** is in the form of **oxyhaemoglobin**. Over 97% of the total oxygen carried in arterial blood is in this form. 2. **Less than 3% of oxygen in arterial blood is found in dissolved form.** ### Carbon dioxide transport Carbon dioxide is transported in the blood in two forms: 1. **Physically dissolved C02**: - About 5% of C02 is dissolved in the plasma and the RBCs. It is called free CO2. - It is very important because: * It is responsible for C02 tension (PC02) in the arterial blood (40mmHg) and the venous blood (45mmHg). * PC02 controls the breathing rate and depth. 2. **Chemically combined C02**: - It represents 95% of the C02 content. It is present in two forms: * **Carbamino compounds**: - About 10% is formed by combination of CO2 with the free amino group of plasma proteins and hemoglobin forming the carbamino compounds. * **Bicarbonate**: (Most Common form for CO2) - About 90% of CO2 content is present as bicarbonate. (KHCO3 in the RBCs and NaHCO3 in the plasma). Bicarbonate is about 43ml/100 in the arterial blood and about 46ml/100 in the venous blood. ## Hypoxia: Hypoxia refers to conditions in which there is deficient oxygen supply to the tissues. Hypoxia can be divided into four main categories: 1. **Hypoxic hypoxia**: results from any disturbance which interferes normal oxygenation of the arterial blood leading to low arterial P02. It can, therefore, be due to factors in the atmosphere such as high altitudes; it may be due to factors interfering with normal O2 diffusion in the lung, and it may be due to mixing of arterial blood with venous blood as in veno-arterial shunts as in congenital heart disease. 2. **Anemic hypoxia**: This is due to lowering of oxygen carrying capacity of the blood. It may, therefore, be caused by anemia, abnormal hemoglobins, or by rendering hemoglobin unavailable for oxygen uptake as in carbon monoxide poisoning. 3. **Stagnant hypoxia**: In this type of hypoxia there is slow or no circulation to the tissues. It may be a localized disturbance of circulation to a certain part of the body, e.g. the limbs, or it may be generalized, or central as in heart failure. 4. **Histotoxic hypoxia**: The disturbance is in the uptake of oxygen by the tissues either due to poisoning of cellular enzymes (e.g. cyanide poisoning) or due to a long diffusion distance between the capillaries and cells as may occur in tissue edema. ### Cyanosis: Cyanosis is a blue coloration of the skin and mucous membranes. It starts to show when the level of reduced hemoglobin in capillary blood is more than 5gm/100ml of blood. Reduced hemoglobin is dark in color, and when its concentration exceeds 5g%. ## Digestive System The digestive system consists of: 1. **Alimentary canal**: from the mouth to the anal canal. 2. **Associated glands**: salivary gland, liver and pancreases. 3. **Accessory organs**: tongue, teeth, and muscle of mastication. ### The main physiological functions of GIT are: - **Digestion**: Is the breakdown of food materials into small particles, ready for absorption and utilization by the body cells Digestion mostly occurs in the proximal part of the small intestine. - **Absorption**: Is the transfer of the contents of the gut from lumen to the blood vessels in the mucosa. - **Secretion of digestive juice**: About 7500 ml of secretions added daily to the lumen of the GIT by the associated glands. - **GIT serves also in the body's immune system**: Humoral antibodies and cellular immune system (lymphocytes in the Peyer's patches of the intestine) protect the body against micro-organisms in the lumen of the gut. - **Endocrine function**: e.g. release of somatostatin, intestinal glucagon, gastrin, secretin, CCK, hormones. ### Salivary gland secretion: There are three main pairs of salivary glands, which are the parotid, submandibular and sublingual salivary glands. The volume of salivary secretion ranges between 1000 and 1500 ml/day. ### Salivary amylase - It acts at an optimum pH of 6.9-7.1. - It hydrolyses cooked starch only because raw starch granules have a cellulose covering. #### Types of salivary secretion: | Gland | Type of Secretion | Percentage of Total Salivary Secretion in Human | |---|---|---| | Parotid | Serous | 25% | | Submandibular | Mixed | 70% | | Sublingual | Mucous | 5% | ### Mastication (chewing) Mastication is the mechanical processing of food, breaking it into pieces and grinding it. At the same time, it is mixed with saliva, softened, and made ready to swallow. ### Deglutition (swallowing) Deglutition is the transfer of food from the mouth to the stomach. ### Gastric secretion: - Human gastric secretion is about 3 liters/day. - It is the most acidic secretion in the body, pH approximately 1. #### The functions of the stomach are: - Storage of large quantities of food until they can be accommodated in the lower portion of the GIT. - Mixing of food with gastric secretions. - Regulation of the rate of evacuation of chime into the duodenum, via the pyloric pump, to prevent rapid passage of food into the small intestine. - The acidic gastric juice facilitates the absorption of various substances from the upper intestine such as calcium and iron. - Digestive functions by gastric juice. - Release intrinsic factor for Vit B12 absorption. - Secretion of mucin, which is protective for mucosa. #### Functions of gastric secretion: - **Functions of Hydrochloric acid (HCI)**: * Activation of the inactive proteolytic enzyme (pepsinogen) to the active form (pepsin). * Antibacterial action. * Facilitates the absorption of calcium and iron. * Hydrolysis of some food materials as disaccharides. - **Functions of the enzyme:** * **Pepsin**: It is secreted in an inactive form called pepsinogen. Pepsinogen is activated by HC1 to pepsin. Any minute amounts of pepsin formed can activate the rest of pepsinogen (autoactivation). Pepsin converts proteins into acid metaproteins, proteoses, and peptones, no amino acids are formed. * **Mucin:** It provides a major shell of protection for the stomach wall as well as contributing to lubrication of food transport and neutralization of considerable amount of HCl. * **Intrinsic factor**: It is necessary for absorption of cyanocobalamin (Vit. B12) from the small intestine, and it prevents pernicious anemia. ### Exocrine pancreatic secretion: The exocrine pancreatic secretion is considered as the most important source for the digestive enzymes of the gastrointestinal tract. It is poured into the lumen of the small intestine. Pancreatic secretion contains enzymes for digesting the three major types of food: proteins, carbohydrates and fats. It contains large quantities of bicarbonate ions, which has an impotent role in neutralizing the gastric acid chime in the duodenum. #### Composition of pancreatic secretion: Pancreatic secretion is the most alkaline juice in the human body. About 1200ml/day of pancreatic secretion are secreted per day, its pH is 8.3. It contains sodium bicarbonate in a concentration about 5 folds that of plsam. | Proenzyme | Active Enzyme | |---|---| | Trypsinogen | Enterokinase `-->` trypsin | | Chymotrypsinogen | trypsin `-->` chymotrypsin | | Procarboxypolypeptidas | trypsin `-->` carboxypolypeptidase. | - **Pancreatic amylase**: `-->` cooked & un cooked starch * This enzyme hydrolyses uncooked starches, glycogen, and most other carbohydrates except cellulose to form disaccharides. - **Lipolytic enzymes**: pancreatic lipase is the main enzyme for digestion of fat (neutral fat into glycerol and fatty acid). ## Liver and Biliary system: The liver has many complex functions: - **Digestive function**: secretion of bile - **Nondigestive functions**: * It acts as a blood reservoir. Normally contains about 350 ml of blood. An additional liter may be stored in the liver and transferred to systemic circulation. * Filtering of the portal blood occurs through the activity of phagocytic reticuloendothelial cells (Kupffer cells), found in the liver sinusoids. * Production of red blood cells is a normal function of fetal liver. * Destruction of aged red blood cells by the activity of kupffer cells. * Excretion of bile pigments. * The liver detoxifies materials that might be harmful to the body by oxidation, reduction or combination with other materials. * Metabolic functions including metabolism of carbohydrates, lipids and proteins. * Storage of iron, vitamins. * Endocrine function: synthesis of erythropiotin, somatomedins, * Essential for formation of blood clotting factor (synthesis of fibrinogen, prothrombin,...) ### Bile secretion: - The volume of bile is about 700ml/day. - In the gall bladder bile is acidified by the reabsorption of its NaHCO3 content, the acidic medium prevents the precipitation of calcium and gall stone formation. #### Functions of bile salts: - **Digestion**: Bile salt has direct digestive effects. It helps fat digestion by their ability to reduce the surface tension and emulsification of fat to give a good surface area exposed to the action of pancreatic lipase. Bile salts have the ability to dissolve the insoluble fatty acid in water; thus, the fatty acids formed are removed from the medium, and new molecules of neutral fat are exposed to the action of pancreatic lipase. Bile salts also activate the lipase enzyme. - **Absorption**: bile salts greatly enhance fatty acids absorption by the intestinal mucosa, they combine with fatty acids to form micelles, water-soluble complexes from in cells of the intestine, where they split again into fatty acids and bile salts, bile salts reach the liver via the portal circulation and resecreted again. Bile salts facilitate the absorption of fat-soluble vitamins (D, E, K,A). Absorption of fatty acids also help indirectly the absorption of calcium and iron by prevention of their precipitation in an insoluble form. - **Solvents**: bile salts prevent the precipitation of cholesterol and fatty acids by keeping them in solution and guard against gall stones formation. - **Choleretic**: Bile salts are the best choleretic (stimulants for bile secretion by liver cells). Absorption of any amount of the secreted bile salts in the intestinal mucosa, passes via the portal circulation to the liver, leading to stimulation of bile secretion (entero-hepatic circulation of bile salts). - **Stimulants of peristalses**: bile salts stimulates the motility of the intestine and colon and prevent constipation. - **Anti-putrefactive**: bile salt have no direct antiseptic effect, but they prevent putrefaction by absorption of fats, in its absence undigested fat covers the protein particles and hinder their digestion, the undigested protein particles in the large intestine are attacked by bacteria leading to putrefaction. ## The Small Intestine: The small intestine performs both endocrine functions by secretion of hormones, and exocrine functions by secretion of digestive enzymes. In the lumen of the small intestine, the intestinal contents are mixed with the secretions of the mucosal cells