Physiology exam prep 4

Nephron and Kidney Function

• The nephron is the basic structural and functional unit of the kidney.
• There are two types of nephrons:
  • Cortical nephrons (80%): located primarily in the renal cortex, with short nephron loops that either dip into the superficial part of the renal medulla or never leave the cortex.
  • Juxtamedullary nephrons: located close to the boundary between the renal cortex and the renal medulla, with long nephron loops that burrow deeply into the renal medulla.

Blood Circulation in the Kidneys

• The kidneys receive about 20% of the cardiac output, with 90% going to the kidney cortex and 10% to the medulla.
• The kidneys receive blood from the renal artery, which branches from the abdominal aorta.
• The renal artery has a large diameter, few branches, and a short length, with pressure almost the same as in the aorta.

Intestinal Juice

• Intestinal juice has a pH of 7.5 and contains various enzymes, including:
  • Proteases (e.g., enterokinase, aminopeptidases, dipeptidases)
  • Lipases (e.g., lipase, alpha-amylase, lactase, sucrase)
  • Nucleases
• Intestinal juice is produced by glands in the small intestine and plays a crucial role in chemical processing of food.
• The small intestine contains around 10,000 glands per 1cm³.

Blood Circulation

• Blood circulation is provided by the heart, which pumps blood through the circulatory system.
• There are two main circuits: the pulmonary circuit (right side of the heart to lungs to left side of the heart) and the systemic circuit (left side of the heart to the body to right side of the heart).
• Blood flow is regulated by the heart, with capillaries supplying blood to tissues and veins regulating blood flow to the heart.

Thyroid Hormones

• Calcitonin (thyrocalcitonin) is produced by the thyroid gland and regulates calcium and phosphorus levels in the blood.
• Triiodothyronine (T3) and thyroxine (T4) are produced by the thyroid gland and play a crucial role in cell metabolism.
• T3 is more active than T4 and increases the intensity of cell metabolism, leading to increased ATP production.

Mineralocorticoids

• Mineralocorticoids, such as aldosterone, are produced in the zona glomerulosa of the adrenal gland cortex.
• Aldosterone regulates the reabsorption of electrolytes (mainly Na+ and K+) from nephron tubules, which helps maintain blood pressure and pH.

Carbohydrate Metabolism

• Carbohydrates are an energy source, cell component, and necessary for muscle contractions and relaxation, as well as the action of the nervous system.
• Carbohydrate metabolism involves the breakdown of carbohydrates into glucose, which is then absorbed into the bloodstream and used by the body.
• Glycogen can be synthesized from glucose, glycerin, fatty acids, amino acids, and volatile fatty acids.

Adrenal Glands and Hormones

• The adrenal glands produce corticosteroids, which include mineralocorticoids (zona glomerulosa), glucocorticoids (zona fasciculata), and sex steroids (zona reticularis).
• Aldosterone is the most active mineralocorticoid, regulating electrolyte reabsorption from nephron tubules.
• Glucocorticoids, such as cortisol, stimulate glucose release from the liver, suppress allergic reactions, and reduce inflammation.

Absorption in the Gastrointestinal Tract

• Absorption is the transfer of substances from the GI tract to the blood or lymph through epithelial cells
• Nutrients, water, inorganic salts, hormones, and drugs are absorbed
• Factors affecting absorption:
  • Degree of nutrient splitting
  • Absorption surface area
  • Physical processes (filtration, osmosis, diffusion)
  • Active cellular processes involving epithelial cells
• Protein absorption:
  • As amino acids
  • Phosphorylated by intestinal enzymes
  • Animal proteins are better absorbed
• Carbohydrate absorption:
  • As monosaccharides (glucose, galactose)
  • In ruminants, glucose is absorbed in small amounts in the intestines
• Fat absorption:
  • Mainly as glycerol and fatty acids
  • Only well-emulsified fats can be absorbed whole
  • Glycerol is well-absorbed due to its water solubility
  • Fatty acids are absorbed in complex with bile acid
• Water and mineral absorption:
  • Most intense in the large intestine
  • Depends on osmotic pressure of food

Physiological Meaning and Regulation of Protein Exchange

• Proteins are necessary for:
  • Plastic processes (growth, development, tissue restoration)
  • Defense reactions (immune substances, antibodies)
  • Enzymes and hormone function
  • Transporting chemical substances (e.g., hemoglobin)
  • Energy source (4.1 kcal/g)
• Cows receive 25% of their daily protein requirement from ruminal microorganisms
• The value of proteins is determined by:
  • Presence of all essential amino acids
  • Mutual ratio of essential amino acids
• Liver is essential for protein exchange, and impaired liver function can disrupt protein exchange
• Growing animals and those in lactation require well-balanced food due to increased protein formation

Cough and Sneezing Reflex

• Coughing:
  • Similar to sneezing reflex, but with irritation of receptors in the respiratory tract
  • Reflex involves sensory nerve activation, brainstem processing, and motor neuron activation
  • Phases: inspiratory, compressive, and expiratory
• Sneezing:
  • Occurs when irritant receptors in the nasal cavity are stimulated
  • Consists of nasal/sensitive phase and efferent/respiratory phase
  • Results in explosive exit of air through the mouth and nose

Functions of the Spinal Cord

• Functions:
  • Reflector function (associated with nerve centers in gray matter)
  • Conduction function (provides communication between CNS parts)
• White matter:
  • Made of myelin (proteins and lipids) and axon bundles
  • Conducts, processes, and sends nerve signals up and down the spinal cord
  • Damage can affect movement, sensory faculties, and reflexes
• Spinal shock:
  • Disappearance of reflexes regulated through the brain
  • Atony (lack of tone) and loss of sensations below the damaged area

Brainstem

• The brainstem consists of three parts: medulla oblongata, pons, and midbrain
• Medulla oblongata:
  • Performs conduction and reflex functions
  • Centers for life support, defense reflexes, gastrointestinal regulation, and muscle tone regulation
• Pons:
  • Regulates reflector movements and muscle tone
  • Acts as a conduction pathway between higher and lower brain centers
• Midbrain:
  • Functions include vision, hearing, and precise motion regulation
  • Contains the reticular formation, responsible for nonspecific functions of the CNS

Solutions and Osmolarity

• Solutions are classified into three categories based on osmolarity: isotonic, hypertonic, and hypotonic
• Isotonic solutions:
  • Have the same osmolarity as the cytoplasm of the cell
  • No net movement of water into or out of the cells
• Hypertonic solutions:
  • Have a higher osmolarity than the cytoplasm of the cell
  • Water moves out of the cells, causing them to shrink or crenate
• Hypotonic solutions:
  • Have a lower osmolarity than the cytoplasm of the cell
  • Water moves into the cells, causing them to swell and potentially burst (lyse)

Gastric Juice Secretion

• Gastric activity is divided into three stages: cephalic, gastric, and intestinal phases
• Cephalic phase:
  • Responds to sight, taste, or thought of food
  • About 30% of total acid secretion occurs before food enters the stomach
  • Involves the nervous system and neurotransmitters such as acetylcholine
• Gastric phase:
  • Occurs during eating and accounts for 50-60% of total gastric acid secretion
  • Involves direct contact of food with the stomach wall and the release of gastrin and histamine
• Intestinal phase:
  • Occurs after eating and accounts for 5-10% of total gastric acid secretion
  • Involves signals from the small intestine about stretchiness, pH, and osmotic concentration

Renal Functions and Hormones

• Humoral regulation of renal functions involves the renin-angiotensin-aldosterone system (RAAS)
• RAAS:
  • Primary function: maintaining systemic blood pressure
  • Secondary function: regulating glomerular filtration rate (GFR)
  • Impacts tubular reabsorption, electrolyte balance, and blood volume
• Hormones involved:
  • Renin (enzyme)
  • Angiotensin (hormone)
  • Aldosterone (hormone)
  • ADH (antidiuretic hormone)
  • Parathormone and calcitonin (regulate blood Ca levels)

Placental Transfer of Respiratory Gases

• The placenta is responsible for transporting materials and heat between the blood of the fetus and the mother
• Placental transfer occurs through diffusion and active transport
• Transfer of oxygen is limited due to the reduction in maternal pO2 along the maternal blood vessels
• Adaptations in the fetus include:
  • Higher affinity for O2 in fetal hemoglobin
  • Increased hemoglobin content in fetal blood
  • Higher cardiac output relative to body mass

Phospholipids

• Structure: glycerol + 2 fatty acids + phosphate group
• Functions:
  • Structural components of cell membranes
  • Emulsifiers, bringing water and fat together
  • Involved in lipid absorption and transport
  • Prevent fatty liver
  • Form surfactant in lungs
  • Play a role in blood coagulation

Regulation of Ventilation

• pCO2 (CO2 partial pressure) is the most important factor regulating ventilation
• Increased pCO2 leads to:
  • Deeper and more rapid breathing to normalize pCO2 levels
  • Increased nerve impulse frequency to inspiratory neurons
  • Increased H+ concentration, but H+ does not cross the blood-brain barrier
• Both peripheral and central chemoreceptors are highly sensitive to pCO2 changes (pH changes)
• Peripheral chemoreceptors have a 40% effect on ventilation in response to pCO2 changes
• Central chemosensor responds to pH changes in the surrounding fluid, not directly to pCO2
• A mere 2-3 mmHg increase in pCO2 doubles pulmonary ventilation

Regulation of Ventilation by pO2

• pO2 (O2 partial pressure) is monitored by peripheral chemoreceptors
• In normal circumstances, pO2 is NOT important in regulating ventilation (a paradox, considering O2's importance to the body)
• Arterial pO2 must fall significantly for pO2 to play a role in ventilation regulation

Milk Ejection

• Milk ejection occurs through the contraction of myoepithelial cells, which surround each individual alveolus and milk ducts.
• The process is induced by a neuroendocrine reflex, which involves the stimulation of sensory nerve fibers in the teats, leading to the release of oxytocin from the posterior pituitary gland.
• Oxytocin binds to receptors on myoepithelial cells, causing contraction and increasing hydrostatic pressure in the alveoli.
• This contraction decreases the resistance of excretory ducts, relaxes the sphincter muscle in the teat canal, and allows milk to move from the alveoli down the ducts, resulting in milk ejection.
• The entire process takes around 60-90 seconds from initial stimulation to milk ejection.
• Oxytocin secretion lasts for a few minutes, and prolactin from the anterior pituitary gland promotes milk production in the mammary glands.
• Oxytocin secretion can be stopped by epinephrine, which is released in response to stress, pain, fear, or anxiety.

Types of Receptors

• Receptors can be classified based on the type of irritation they perceive and their location.
• Examples of receptors include:
  • Photoreceptors: sensitive to visible wavelengths of light, found in rods and cones in the retina.
  • Mechanoreceptors: sensitive to mechanical energy, found in touch receptors in the skin and baroreceptors in blood vessels.
  • Thermoreceptors: sensitive to heat and cold, found in Krause end bulbs and Ruffini endings in the skin.
  • Osmoreceptors: detect changes in the concentration of solutes in body fluids and changes in osmotic activity, found in cells in the supraoptic nucleus.
  • Chemoreceptors: sensitive to specific chemicals, found in smell and taste receptors, receptors in the blood for O2 and CO2 concentrations, and receptors for the chemical content of the digestive tract.
  • Nociceptors: pain receptors sensitive to tissue damage or distortion, found in the skin.
• Agonists and antagonists are ligands that bind to receptors, causing a response or blocking the response, respectively.
• Types of agonists and antagonists include:
  • Full agonists: activate receptors, producing a strong biological response.
  • Partial agonists: partially activate receptors, producing a partial response.
  • Antagonists: bind to receptors but do not activate them, blocking the response.
  • Inverse agonists: reduce the activity of receptors by inhibiting their constitutive activity.

Centres of Respiration

• The centres of respiration are located in the medulla oblongata, pons, hypothalamus, limbic system, and cerebral cortex.
• The medulla oblongata contains inspiratory and expiratory neurons, which are inhibited by each other.
• The preBötzinger complex, a cluster of interneurons in the medulla oblongata, generates the respiratory rhythm.
• The spinal cord regulates respiratory muscles through cervical and thoracic motor neurons.
• The pons regulates the change from inspiration to expiration, with the pneumotaxic and apneustic centers modulating the length of inspiration and expiration.
• The hypothalamus regulates changes in respiration due to autonomic functions, such as thermoregulation and metabolism.
• The limbic system regulates changes in respiration during different emotions and pain.
• The cerebral cortex regulates conditioned reflexes.

Chemical Signaling between Cells

• Chemical signaling between cells can occur through autocrine, paracrine, endocrine, neurocrine, and neuroendocrine signaling.
• Autocrine signaling: a cell secretes chemical messengers that stimulate the same cell or nearby cells of the same type.
• Paracrine signaling: a cell secretes chemical messengers that affect surrounding cells of a different type.
• Endocrine signaling: chemical messengers are secreted into the bloodstream by endocrine glands and cells, affecting distant cells.
• Neurocrine signaling: neurotransmitters are secreted into a synaptic cleft, affecting adjacent neurons, muscles, or glandular cells.
• Neuroendocrine signaling: hormone is released both locally and distantly, affecting both adjacent neurons and distant cells.

Types of Hormones

• Hormones can be classified based on their place of synthesis, including:
  • Classical hormones: synthesized in the cells of endocrine glands, secreted directly into the bloodstream.
  • Tissue hormones (parahormones): synthesized in various types of cells, including nerves, and secreted into the interstitial fluid.
  • Neurohormones: synthesized in neuroendocrine cells and secreted into the blood, synapses, or magnocellular bodies.

Composition of Bile

• Bile is composed of:
  • Bile acids (cholic acid, deoxycholic acid, etc.)
  • Mucin (mucus)
  • Bile pigments (bilirubin, biliverdin)
  • Cholesterol
  • Lecithin (phosphatidylcholine)
  • Fatty acids
• Bile acids, cholesterol, and lecithin are the three major lipid constituents in bile.

Function of Bile

• Bile emulsifies fats, making them more accessible for lipase.
• Bile mixes fatty acids with bile acids, forming a water-soluble complex that can be absorbed.
• Bile activates lipase and inactivates gastric pepsin.
• Bile promotes the absorption of lipid-soluble vitamins (D, E, K, A).
• Bile neutralizes excess stomach acid before it enters the ileum.
• Bile promotes intestinal peristalsis.
• Bile acids inhibit the development of microbes in the intestines.

Neurohumoral Regulation of Bile Secretion

• Bile secretion is regulated by both neural and humoral mechanisms.
• Humoral regulation: secretin and cholecystokinin (CCK) are stimulated by the presence of chyme in the duodenum, leading to the secretion of bile from the liver and the contraction of the gallbladder.
• Neural regulation: parasympathetic nerves stimulate the excretion of bile, while sympathetic nerves inhibit it.