Human Physiology: Digestive, Respiratory, and Cardiovascular Systems

Posted on Dec 17, 2024 in Biology

Digestive System: Enzymes and Hormonal Regulation

The digestive system involves processes such as digestion, motility, absorption, and secretion. Key enzymes and their functions include:

  • Pepsin: Digests proteins; released as pepsinogen by chief cells.
  • Lipase: Digests fats into two fatty acids and monoglycerides.
  • Trypsin, Chymotrypsin, Elastase: Digest proteins into peptide fragments.
  • Carboxypeptidase: Splits off the terminal amino acid from the carboxyl end of a protein.
  • Amylase: Breaks polysaccharides into glucose and maltose.

Key regulatory substances and their roles:

  • HCl: Dissolves stomach contents, kills harmful bacteria; produced by parietal cells.
  • Gastrin: Released by G cells in the lower part of the stomach.
  • Histamine: Produced in enterochromaffin-like (ECL) cells.
  • Acetylcholine (ACh): Neurotransmitter.
  • Somatostatin: Produced in the stomach, intestine, and pancreas; travels in the bloodstream.

Control Mechanisms of the Digestive System

Control mechanisms of the gastrointestinal (GI) system are governed by the volume and composition of luminal contents. Luminal stimuli include:

  • Distension of the wall by the volume of lumen contents
  • Chyme osmolarity
  • Chyme acidity
  • Chyme concentration of specific digestion products

Neural regulation involves the central nervous system (CNS) and the enteric nervous system (ENS), which includes the submucosal plexus and myenteric plexus. The CNS contributes to neural control through the regulation of the sympathetic and parasympathetic nervous systems. Hormonal regulation also plays a crucial role.

Liver Functions and the Hepatic Portal System

The liver is a secretory organ that processes and stores nutrients, filters old red blood cells, and synthesizes plasma proteins. The hepatic portal system is a large vein in the abdominal cavity that drains blood from the spleen and the GI tract into the liver. This ensures that ingested food is processed by the liver before entering systemic circulation, allowing for detoxification and nutrient absorption.

Structure of the GI Tract

The GI tract consists of several layers:

  • Mucosa: A thin layer of epithelium, highly folded to increase surface area (SA) for absorption. It contains tubular exocrine glands (secreting mucus, enzymes, water) and endocrine glands (secreting GI hormones). The lamina propria supports the epithelium.
  • Submucosa: A connective tissue layer containing blood and lymphatic vessels. The submucosal plexus regulates local contraction, absorption, and intestinal secretion.
  • Muscularis Externa: Composed of two smooth muscle layers, circular and longitudinal. The myenteric plexus controls peristalsis. An inner oblique layer helps churn stomach contents.
  • Serosa: A smooth membrane consisting of a thin layer of connective tissue and cells that secrete serous fluid to lubricate internal structures.

Food processed by the stomach is called chyme. The cephalic phase stimuli include the sight, smell, taste, or thoughts of food. Fat-soluble vitamins include A, D, E, and K.

True or False: After absorption, the products of protein digestion are carried by blood directly to the liver. (True)

Which pathway is activated during the cephalic phase of gastrointestinal control? Parasympathetic nerves to the enteric nervous system.

Following the removal of a large portion of the pancreas due to cancer, which of the following will NOT be affected in the patient’s GI tract? Trypsinogen.

Respiratory System: Functions and Mechanisms

The respiratory system has several vital functions:

  • Provides oxygen (O2) for metabolic processes
  • Eliminates carbon dioxide (CO2)
  • Regulates hydrogen ion concentration ([H+])
  • Defends against microbes
  • Influences arterial concentration of chemical messengers
  • Traps dissolved blood clots from systemic veins

Steps of Respiration

  1. Ventilation: Exchange of air between the atmosphere and alveoli by bulk flow.
  2. Exchange of O2 and CO2 between alveolar air and blood in lung capillaries by diffusion.
  3. Transport of O2 and CO2 through pulmonary and systemic circulation by bulk flow.
  4. Exchange of O2 and CO2 between blood in tissue capillaries and cells in tissues by diffusion.
  5. Cellular utilization of O2 and production of CO2.

Diffusion of gas across the alveolar membrane increases with:

  • Increased surface area of the membrane
  • Increased alveolar pressure difference (PA2 – PA1)
  • Increased solubility of the gas
  • Decreased membrane thickness

Respiratory Zones

  • Conducting Zone: Moves air from the atmosphere into the lung; no respiratory function (dead space).
  • Respiratory Zone: Thin-walled and in close proximity to blood vessels to allow for gas diffusion (respiration).

Hemoglobin-Oxygen vs. Myoglobin-Oxygen Dissociation

Hemoglobin exhibits a sigmoid (S-shaped) curve due to cooperative oxygen binding, while myoglobin shows a hyperbolic curve because it binds oxygen non-cooperatively. Myoglobin has a much higher affinity for oxygen at lower partial pressures, making it better suited for oxygen storage in muscle tissue. Hemoglobin is designed for efficient oxygen transport in the blood.

Oxygen Transport Steps

  1. Delivery of O2 to the lung.
  2. Diffusion into the blood (reversible binding to hemoglobin within red blood cells).
  3. Transportation to various tissues through blood flow.
  4. Diffusion into tissue and mitochondria of cells.

Pleural Pressure and Lung Compliance

The visceral pleura is attached to the lungs, while the parietal pleura is attached to the thoracic wall and diaphragm. Intra-pleural pressure is always around 4 mmHg or less. If it exceeds alveolar pressure, the lungs will collapse.

Lung compliance is the inverse of stiffness. The greater the lung compliance, the easier it is to expand the lungs. Determinants include lung stretchability and surface tension of water-air interfaces within alveoli. Surfactant, produced by Type II epithelial cells, wets the surface of the alveoli, reducing surface tension and increasing lung compliance.

The pleural sac surrounds each lung and contains a tiny volume of lubricating fluid. Subatmospheric intrapleural pressure is most responsible for keeping the lung surface and the thoracic wall from separating.

During exhalation, which of the following is NOT true? Alveolar pressure is greater than atmospheric pressure. (False)

Inhalation/inspiration is caused by: Flattening (downward movement) of the diaphragm.

True or False: Intrapleural pressure is lower than alveolar pressure. (True)

The oxygen-hemoglobin dissociation curve demonstrates that: At normal resting systemic arterial PO2, hemoglobin is almost 100% saturated with oxygen.

Which of the following would cause a decrease in the binding affinity of hemoglobin for oxygen? Increased blood temperature.

True or False: Lung compliance is a measure of how quickly lung volume increases as transpulmonary pressure increases. (True)

Cardiovascular System: Functions and Regulation

The cardiovascular system performs several critical functions:

  • Circulates oxygen and nutrients
  • Removes waste products of metabolism
  • Circulates cells that protect against disease and infection
  • Clotting to stop bleeding after injury
  • Transports hormones to target cells and organs
  • Helps regulate body temperature

Poiseuille’s Equation

Blood flow requires a pressure difference (ΔP) to overcome vessel resistance (R). Flow increases with greater ΔP and smaller R. Flow = ΔP/R.

Sequence of Excitation in the Heart

Activity across cardiac cells is continuous due to gap junctions, allowing direct ion transfer between cells. The sequence is as follows:

  1. The sinoatrial (SA) node initiates action potentials that contract the atria (P wave of the electrocardiogram (ECG)).
  2. The signal travels to the atrioventricular (AV) node, which delays firing to ensure full atrial contraction and relaxation.
  3. The AV node fires, and the signal travels down the bundle of His, branching to Purkinje fibers (QRS complex of the ECG).
  4. Purkinje fibers excite the ventricles to contract.
  5. Ventricles relax and repolarize (T wave of the ECG).

SA Node and Rhythmic Cardiac Cycling

The SA node is a pacemaker that generates activity without afferent input. Neurotransmitters and hormones can change the firing rate, thus altering heart rate (HR). Rhythmic cardiac cycling involves:

  1. At hyperpolarized potentials, voltage-gated funny Na+ channels open, producing a depolarizing current.
  2. Transient (t-type) Ca2+ channels open, producing a depolarizing current.
  3. Depolarization to threshold activates long-lasting (L-type) Ca2+ and K+ channels.
  4. L-type Ca2+ channels close while K+ channels remain open, re-polarizing the cell.
  5. At hyperpolarized potentials, voltage-gated funny Na+ channels open again.

Cardiac Cycle

The general blood flow pathway is: veins → atria → ventricles → aorta/arteries. The cardiac cycle has four phases:

  1. Diastole 1: Ventricular Filling
    • Patria increases as atria fill with blood.
    • Patria > Pventricle, so the AV valves open, and blood flows into the ventricles.
    • Pventricle < Paorta, so the aortic valve is closed.
    • Atria contract (P-wave on ECG), ejecting blood to the ventricles (atrial kick).
    • Vventricle increases, reaching end-diastolic volume.
  2. Systole 2: Isovolumetric Ventricular Contraction
    • Ventricles depolarize and contract (QRS complex on ECG).
    • Pventricle > Patria, so the AV valve closes.
    • Pventricle < Paorta, so the aortic valve remains closed.
    • All valves are closed, resulting in no volume change.
  3. Systole 3: Ventricular Ejection
    • Pventricle > Paorta, so the aortic valve opens.
    • Blood is ejected into circulation via the aorta and arteries.
    • Vventricle and Pventricle decrease.
    • The AV valve remains closed.
  4. Diastole 4: Isovolumetric Ventricular Relaxation
    • Ventricles relax after blood ejection.
    • Pventricle < Paorta, so the aortic valve closes.
    • Elastic recoil of the aorta creates the dicrotic notch.
    • Pventricle > Patria, so the AV valve remains closed.
    • All valves are closed, resulting in no volume change.

Frank-Starling Law

Stroke volume (SV) increases as end-diastolic volume (EDV) increases. Increased EDV leads to greater ventricular expansion, stretching cardiac cells closer to optimal length, increasing force output and SV.

DPG binds more strongly to deoxygenated hemoglobin, causing an allosteric effect that changes the shape of the hemoglobin molecule.

Artery and arteriole diameter are regulated by:

  • Extrinsic signals: Sympathetic nervous system stimulation and hormones (epinephrine and norepinephrine) cause vasoconstriction.
  • Intrinsic signals: Local chemical factors (H+, O2, CO2, K+) typically cause vasodilation.

Cardiac output = stroke volume × HR.

Funny Sodium Channels (Na+(F)) and Transient Calcium Channels (Ca2+(T)) open when the cell is negative (repolarizes) and close when the cell becomes more positive (depolarizes), causing the cell to cycle through depolarization and repolarization, creating the”pacemakin” function.

Erythropoiesis is the production of red blood cells (RBCs).

Based on F = ΔP/R, the rate of fluid flow in a tube will increase if the pressure at the beginning is increased while the pressure at the end of the tube stays the same or is decreased.

Binding of ACh receptors decreases HR.

Depolarization of ventricles corresponds to the QRS complex.

An electrode partner with a heart will use the electrode less than one with skeletal muscle.

Semilunar valves open at the beginning of the QRS complex.

In a person at rest, the duration of diastole is greater than that of systole.

Heart valves open due to a pressure difference on the two sides of the valve.

The Frank-Starling mechanism relates length and tension in cardiac muscle cells, and stroke volume increases with higher venous return.

Arterial systolic pressure occurs during the middle of the phase of ventricular ejection, while arterial diastolic pressure occurs just before the semilunar valve opens.